Chapter 6 Formwork

6.1 Introduction

As a rule, formwork constitutes one third of the costs of reinforced concrete, but can be more expensive than both reinforcement and concrete work and, in particularly complicated constructions, it may be more than the cost of the aforementioned put together. To this must be added the time that formwork takes, which is considerably longer than the total time for both reinforcement work and filling the formwork with concrete. Formwork has, therefore, the greatest influence on the total execution time and, as a consequence, must be well planned and prepared.

A distinction must be made between traditional formwork and system formwork. Traditional formwork is produced from boards and timber, where the formwork is constructed on site. System formwork on the other hand is industrially prefabricated formwork, which can be bought or ren- ted and, with an outset in larger units, can be assembled together on site.

There are a number of different formwork systems on the market today. As a rule they are expensive to acquire, however, they can sustain considerable reuse. Furthermore, they require considerably less time to assemble and dismantle than traditional formwork – especially when the formwork system is used in connection with cranes, which enable work to be done with larger units of formwork.

6.2 Loads on the formwork

6.2.1 Form pressure on formwork for walls

When concrete has just been mixed, it is a mixture of liquids and solids. In this condition, the concrete in wall formwork will exert pressure, which can be considered to be hydrostatically distributed. During the setting of the concrete, whose start and termination is heavily temperature dependent, the inner cohesion forces will increase until the concrete becomes self- supporting, after which the pressure will not increase anymore. The wall formwork can be calculated on the basis of the pressure distribution as shown in Figure 6.01. The maximum concrete pressure, Pmax, will depend on a number of factors:

• The concrete’s vertical pouring speed in the formwork. The maximal pressure is increased with the vertical pouring speed as hmax, the distance down to the self-supporting concrete, becomes greater. • The formwork’s height. The greatest pressure can occur when the formwork is filled in one go, as, for example, with columns and thin walls, and before the concrete starts to set. The bottom of the formwork will, therefore, have a pressure corresponding to the weight of the concrete in its full height (hform). • The concrete mass´ consistency. The consistency of the concrete has significance because stiff concrete (concrete with a low slump test figure) puts less pressure on the formwork than a very thin concrete. • The concrete’s density. There is insignificant variance in the density of concrete with normal aggregate. It is approximately 23.50 KN/m³, but is often inserted into calculations as 24 KN/m³. For very heavy concrete, e.g., for reactor protection or ballast-blocks, consideration should be taken to the higher density, and the calculation sum can be made smaller if aggregate such as Leca or Fibro expanded clay aggregate is used. • The concrete’s temperature during casting. This has special significance for small vertical pouring speed s. The casting-temperature affects the concrete’s setting time. For a given vertical pouring speed , the formwork pressure varies in such a way that a concrete with high casting temperature givers a lower pressure than a concrete with a low temperature.

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• The temperature of the surroundings. This has significance for the concrete’s heat exchange with its surroundings and consequently with the concrete’s setting-temperature. For example, casting in winter can result in heating the formwork and/or insulating the formwork after casting concrete. • The roughness and tightness of the formwork has significance because tight and smooth formwork results in higher concrete pressure. Modern vibration techniques have, however, resulted in unevenness in the form having less importance for the magnitude of concrete pressure. • Vibration effect. The inner friction forces are eliminated when concrete is vibrated and the full concrete pressure is, therefore, exerted on the formwork. • Cement types. The use of slow setting cement, the addition of retardants or pozzolana (volca- nic ash) gives higher formwork pressure. • The construction’s width. A broad concrete construction, e.g. the wall of a dam, results in the vertical pouring speed being lower and the vibration effect on the formwork being limited. The pressure on the formwork sides is therefore lower than in a narrow construction that is filled faster.

Calculation of the formwork As there is no research that links all above factors into one formula, a number of methods have been developed for the calculation of the pressure in formwork. The three most common methods are reviewed below.

Method 1 This method was developed in England on the basis of a great amount of data where the actual formwork pressure was measured in a great number of constructions by using pressure sensors.

From a conservative posture, the method determines the maximal formwork pressure in such way that it is not underestimated.

Formula 6.01

If the calculated maximal formwork pressure exceeds the hydrostatic pressure distribution (D · casting-height), the hydrostatic pressure distribution is presumed. This distribution should also be used if

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Hydrostatic pressure

Figure 6.01: Concrete pressure on wall.

Material

Rapid-, low alkaline sulphate resistant and white Portland-Cement with/without plasticization 0.30 substances and concrete aeration substances.

As above, but with retardant-substances (also for high levels of plasticization substances). 0.45

Concrete, where the binder comprises up to 40 % pozzolana with/without plasticization and 0.45 aeration substances.

As above, but only with retardants (also for high dosage levels of plasticization substances). 0.60 Table 6.01: Material parameters

Method 2

ACI (The American Concrete Institute) recommends the following formulas for determination of the maximal formwork pressure:

Walls

Formula 6.02

Formula 6.03

Columns

for all values of v Formula 6.04

p = the maximal pressure in kN/m² v = vertical pouring speed in m/h t = temperature in °C

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Method 3

This method, apart from taking consideration to the temperature and vertical pouring speed , also takes consideration to the slump test figure. The maximum concrete pressure here is:

Formula 6.05 where the slump test s is inserted in millimetres.

Formula 6.05 is expressed as the nomogram in Figure 6.02. To exemplify the use of the nomogram, a concrete pour with a vertical pouring speed of 8 m/h, a pour temperature of 25°C and a slump figure of 110 mm is observed. The maximum concrete pressure is 80 kN/m², and this occurs 3.3 m under the concrete surface.

Formulas 6.01 – 6.05 give the maximum concrete pressure in walls and columns presuming that the formwork is tall enough to develop the calculated pressure. A greater pressure than corresponding to the wall’s height multiplied by the concrete’s density should not be expected. This is illustrated in the nomogram when it states at which depth the maximum concrete pressure will occur.

Figure 6.01 shows the concrete pressure’s distribution for a wall whose height, hform is greater than necessary to develop the maximal concrete pressure; here, hmax gives the depth under the surface where the maximal concrete pressure begins to occur, such that hmax · 24 = concrete pressure according to the formula

Wall piece BC must, therefore, be calculated for the maximal concrete pressure, while piece AB can be calculated for a decreased pressure. However, it is not unusual to find the wall formwork calculated for the same concrete pressure all the way up. Possibly, the distance between coupler- rods is increased higher up in the formwork structure.

The formulas are based on concrete with a density of 24 kN/m³. If concrete of a different density is cast, the calculated concrete pressure can be equally proportioned with regard to the concrete densities.

Modern concrete constructions use crushed stones as aggregate and a low v/c ratio is sought. Fly ash and micro silica is added, as well as super plasticization substances, that together have a retardant effect, and even though accelerator is added, great casting pressures are realised. During the work on the West Bridge in the Great belt project, up to 14 hours elapsed before the concrete stiffened.

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Correction for concrete temperature T in oC

Characteristic form pressure kN/m2 at slump = 150 mm

Correction for slump different from 150 mm

Figure 6.02: Nomogram for the determination of the maximal characteristic formwork pressure.

6.2.2 Formwork pressure on formwork for slabs

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The formwork pressure on the formwork of slabs hails from the concrete’s dead load of 2400 kg/m³ multiplied by the slab thickness, as well as other loads, which can be set at 360 kg/ m³ unless otherwise stated. A specification of other loads can look like the following:

Casting an excess height of 50 mm 120 kg/ m2 Live load: manpower, concrete-bucket, etc. 200 kg/ m2 Dead load of formwork 40 kg/ m2 360 kg/ m2 ~ 3.6 kN/ m²

Sometimes, to the “other loads” must be added the load of, for example, driving with a concrete dumper on the formwork and an addendum for shocks that occur due the free-fall of concrete from a crane-bucket or concrete pump.

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Breaking forces

When concrete dumpers are used on slab formwork, apart from the addendum for vertical dead loads, consideration must be given to their breaking effect. If a concrete dumper weighs W kg with concrete, and it is decelerated a m/s², the horizontal force is:

Formula 6.06

If a dumper weighting 1500 kg and having a speed of 16 km/h is stopped during the course of 5 seconds, the horizontal force is 1330 N. If the dumper is stopped in 3 seconds, the breaking effect is 2220 N. If several dumpers can be expected to stop simultaneously, the breaking forces can be added up.

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6.3 Materials

Materials that are used for formwork can be timber, boards, plywood, oil hardened wood-fibre boards, plastic, steel and aluminium. Important characteristics are strength, stiffness, surface structure and economy seen from a general perspective re: assembly, dismantling, material cost (cost of hiring), general use, equipment input, etc.

6.3.1 Wood and veneer

Wood, which is the most commonly used material for formwork, is traded in different forms. Figure 6.04 shows how it can be cut-out and processed from timer logs. Pine and spruce is used for formwork.

Logs are debarked tree trunks where the dimension is given for the middle diameter section because there is a narrowing of normally 10 mm per metre.

(straight edge striker)

Figure 6.03: Choice of formwork surfaces

Half-section timber is often used in the dimension 100 · 100 mm – the so-called “four by four”, named after the traditional timber dimension system. If the log is cut without a waney edge, i.e., with sharp edges (corners), the resulting piece of timber is called square section timber.

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Logs Square section Half-section Quarter section timber timber timber

Figure 6.04: Some trading dimensions for timber

Half-section timber, which is a longitudinally cut through square section timer, looses 2 mm because of the saw-cut on the short sides. The illustrated cross cut cannot be recommended because of the risk of uneven deformation.

Timber is sorted in accordance with traditional cabinetmaker’s rules (after appearance). Often in Denmark, Kvinta (VTA) is used for formwork, and for more secondary areas, cull.

Plywood is produced from glued veneer where the fibres are alternately shifted in different directions. Therefore, it is important to know the direction of the outer veneer when using plywood for load bearing structures as the load bearing capacity is naturally greatest in the direction of the fibres. Special water resistant plywood has been developed for formwork. They are, as a rule, covered with a resistant surface film on both surfaces, which makes both surfaces suitable for use in formwork. These can be reused up to 100 times. After reuse 15 times on both sides they will probably be downgraded for use in foundation formwork and other constructions with lesser demands to surface-quality.

Dimension Use 25 · 100 mm Panel formwork (wall, slab, column and beam) 32 · 100 mm Formwork walers 25 · 125 mm Braces and horizontal shoring 32 · 125 mm

Formwork soldiers 25 · 100 mm Joists and formwork bearers (stringers) 50 · 100 mm Braces and horizontal shoring 32 · 125 mm Column moulding frame and column formwork walers

32 · 150 mm Formwork bearers (stringers)

Studs (rounded – waney edged) 100 · 100 mm Folding wedges (Fox wedges) Table 6.02: Common timber dimensions and their uses

The corners are damaged and the surface film has begun to crack, which will mark the finished surface of the concrete. The nice, smooth surface that the plywood leaves on the concrete structure is often acknowledged as the finished surface that is ready for painting. One of the disadvantages of large continuous plate casting surfaces in comparison to board surfaces is that the air in the concrete cannot escape during vibration and, therefore, leaves small blisters of air on the surface of the finished concrete structure. Furthermore, the connections between the formwork plates become very visible and unattractive. This can, however, be avoided by using tape over the connections and joints. These water resistant plywood panels are found in thicknesses from 6 to 27 mm, and panel sizes as standard 1200 · 2440 mm and 1500 · 3000 mm.

There are many plywood products on the market. For example, a casting-plywood that has an outer birch veneer and a core of alternating birch and pine. These are used as direct casting panels in thickness from 12 to 25 mm and panel sizes of 1220 · 2440 mm. It is possible to bend these panels by using steam, but it must be born in mind that thick dimensions exert great forces that make it fairly difficult to maintain the curved shape.

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Hard oil-hardened wood fibre boards (Masonit) are used in 3 mm thicknesses to line board cladding where a smooth concrete surface is desired. These are available in 610 · 1220 mm panels and are extremely flexible, as the bending radius is 220 mm.

Plastic. Plastic’s ability to be shaped gives great freedom when forming concrete elements. It is a light material that gives an attractive surface. It neither corrodes nor rusts. Furthermore, the joints in plastic formwork can be made tight on site so that their impressions cannot be seen on the finished concrete surface. Plastic is also used as separators and bushing-pipes for coupler- rods and as rebate pipes.

6.3.2 Metal

Steel is used first and foremost in connection with timber as profile steel for shoring timber constructions in formwork elements. A whole steel element, where the casting surface is also made of steel panels, is only economical in situations of great reusability (100-200 times). In contrast to timber, it is greatly heat conducting, i.e., it is not very good at protecting the concrete from heat loss in the winter time. For steel type S235, it is reasonable in formwork calculations to set the calculator yield stress at 201 N/mm² and the coefficient of elasticity at 210,000 N/ mm². Otherwise, steel is used for steel-pipe shoring and steel girders from suppliers of formwork systems, and special clamp-steel is produced in different steel qualities.

Aluminium has, despite its lightweight advantages, been a long time coming onto the scene as a formwork material. It is available as alloys in strengths corresponding to Steel S 235, but has an elasticity module that is one third of steel’s. Aluminium is now marked as supports and girders for slab formwork. Formwork material of aluminium should be cleaned carefully after each casting task, as concrete usually sticks well to aluminium.

6.3.3 Glass fibre reinforced polyester

Formwork made of glass fibre reinforced polyester has, just like steel formwork, the advantage of being strong and durable.

From a price point of view they are relatively expensive, as they have to be built-up around a steel skeleton in order to have the necessary stiffness; the plastic formwork comprises only the surface against which the concrete is cast.

Plastic formwork has an advantage when there are special demands to the concrete’s surface structure and appearance, e.g., in the form of requirements for the surface’s pattern.

If a surface pattern corresponding to the appearance of rough boards is required, formwork of glass fibre reinforced polyester will be well suited for the task.

A formwork surface is built up on a background of sand-blasted timber boards( in order to bring out the structure of the wood), which time and again gives the impression of rough boards in comparison to traditional timber formwork, which only gives that impression the very first time casting is done, as the roughness decreases with repeated use.

6.3.4 Nails

Nails are often used for joining timber formwork. They may be round, quadratic, and smooth or profiled with ribs. Double-headed nails are especially used for formwork, as it is easier to extract them with the claw of a carpenter’s hammer. The nails´ dimensions are given by two figures. The first figure gives the edge length or diameter in a tenth of a millimetre, and the second figure

428 gives the length in millimetres, e.g., ø20/40 is a nail that is 2 mm in diameter and 40 millimetres long. Nails give the greatest strength when they are driven into the wood perpendicular to the fibres´ direction. Their strength consists partly of the load bearing capacity for the perpendicular load on their cross section, and partly on the extraction force necessary to pull them out.

The permissible cross-sectional load bearing capacity in pine and fir can be calculated as:

for quadratic nails Formula 6.07

for round nails Formula 6.08 where d is the edge length/diameter in millimetres and Fd is the cross-sectional load bearing capacity per section in N.

The formula is valid, as mentioned, for the most common cases, i.e., nailing perpendicular to the direction of the fibres. Furthermore, the anchorage length for smooth nails is presumed to be 12 d, and the anchorage length for ribbed nails must be 8 d.

The permissible (calculator) extraction force Fd in N for nails in pine and fir can be calculated as the lesser of the following values:

for smooth nails

for ribbed nails Formula 6.09 where d is the nail thickness in mm, h the length in mm of the nail in the part of the wood that contains the nail head, l the nail’s anchorage length in mm where the pointed part of the length is neglected (approx 1.5 d), and for ribbed nails from the smooth part of the nail shaft.

For determination of edge distance, bolt and screw joints, etc., reference is made to DS 413.

6.3.5 Formwork release agents

For most concrete casting tasks, formwork release agents are used as accessories to get the formwork to loosen from the concrete surface without damaging either the former or the latter.

There are many different types of release agent available, but none can be said to be ideal in every situation, so trials have to be implemented to find the right release agent for the job at hand.

Release agents can be subdivided into the following:

• Mineral oil based release agents without additives • Mineral oil based release agents with additives • Emulsion oils (oils in water, or water in oil) • Chemical release agents • Wax-based release agents

Mineral oil based release agents without additives and oil-in-water emulsions can partly give many air bubbles, partly discolouration due to lime precipitation and uneven hybridisation of the cement - which is why its use is not recommended where great demands are placed on the appearance of the finished product.

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Before release agents are applied to formwork, the formwork must be carefully cleaned. The release agent is applied in a thin, even layer and must cover the whole surface area of the formwork, but surplus release agent is undesirable and must be removed as it can damage the concrete or the appearance of the concrete surface.

Chemically acting release agents are usually acids that react with the concrete’s alkalines and form lime-soaps. As these are chemical compositions, the result is usually a heavy oil-film. In cases of overdoses, the chemical reaction can however result in the surface being soft and dusty.

Formwork release agents must be applied to the formwork surface before reinforcement is mounted into place in order not to minimise the concrete’s adhesion to the reinforcement. Because of the same reason, spraying release agent onto or letting it drip down to the formwork skeleton must be avoided.

Chemical release agents can be supplemented with water-displacing additives so that the drying of wet formwork can be avoided before the release agent is applied, as this enables the oil to seep in under the water layer. Similarly, the oil is not washed off if rain occurs after the release agent has been applied to the formwork.

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6.4 Dimensioning

6.4.1 Definitions

Formwork is dimensioned on the basis of resistance to bending (bending strength), resistance to shear, its compressive strength and its permissible deflection/bending. It must be observed that the variable load on the concrete formwork belongs to the load-group short time load.

6.4.2 Dimensioning formulas and curves

Figure 6.05 shows the commonly occurring load types and beam systems for calculating formwork with information about the maximal moments, resistance to shear forces and deflection.

With regard to formwork timber (K14), the following calculations must be implemented:

Table 6.03: Calculator values for formwork timber K 14

The following shows, especially with regard to timber, calculations and drawings of curves that give a guideline for calculation of formwork.

The values for beams over several bays are taken as the highest occurring for a complete calculation of 3 or several bay beams.

Figure 6.06 and 6.07 are load bearing curves for use in choosing the joists and formwork soldiers that have a uniformly distributed load from the shutter (boards or ply against the casting). The sharp dents in the curves are transitions from the demand for deflection of maximal 1/400 to the demand of a maximal resistance to shear of 1.6 N/mm2.

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6.05 A

6.05 B

6.05 C

6.05 D

6.05 E

6.05 F

6.05 G

6.05 H

6.05 I

6.05 J

6.05 K

Figure 6.05: Beam formulas (moment of inertia is stated as J)

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Figure 6.06: Formwork boards with simple supports based on a permissible deflection of of the span.

Figure 6.07: Formwork boards (shuttering). Span in outer bay for through boards, based on a permissible deflection of 1/400 of the span.

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Figure 6.08: Joists and formwork bearings for simple supports Supports (propping-up) types

Figure 6.09: Joists and formwork bearings for through boards

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Clamp/coupler Clamp (“She Permissible Clasp surface Necessary system bolt”)/Tie Bar Load for formwork clasp surface at couplers maximal loading kN mm² mm² R7 8 3,200 Traditional R8 11 2,600 4,400 R10 12 4,800 Fast alignment R7 8 3,200 coupler system R8 11 4,400 4,400 (Geku) R10 12 5,600 SP 100 R10 12 3,300 4,800 Dywidag Anchor nut 100 8,500 (15 mm rod) Anchor nut 230 91 28,500 44,000 Table 6.06: Coupling system

The curves are also useful for other timber dimensions than the ones shown. If the load is, e.g., 15 kN/m, it is possible to enter with half the load 7.5 kN/m to the curve 25 · 100 and from this choose 50 · 100 with a span of 0.52 m. With regard to Tie Bars, it is possible to use the values given in Table 6.04, where the necessary clasp-surface has a permissible pressure on the end- grain (end face of wood) of 2.5 N/mm².

6.4.3 Procedure for dimensioning

Figure 6.10 shows a sketch of formwork for a wall with horizontal formwork boards and vertical studs, which are again supported with double vertical coupling-boards and Tie Bars.

The procedure for calculating the formwork can be the following:

• The formwork pressure p in kN/m² is determined on the basis of the casting speed. • A thickness of formwork shutter is chosen, and the span is determined on the basis of the figures. • The Formwork Soldier dimension, for fixing the formwork shutter, is chosen and the distance b can be determined on the basis of the figures. • Based on the load bearing capacity of the wall ties, the maximal distance c can now be determined between coupler-rods. • The Formwork Walers are dimensioned on the basis of the individual forces (reaction from the Formwork Soldiers), and evaluation is made of the bending and shear forces, as well as the deflection. • Pressure perpendicular to the wood grain is controlled at the contact areas between the Soldiers and the Walers.

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Formwork Shutter

Formwork Soldier

Formwork Waler

Wall ties

Figure 6.10 Formwork for wall Slab dimensioning is done in the same way as for walls. However, there will be additional components like studs and soleplates. Table 6.05 gives load bearing capacities for wooden studs of pine and fir. For waney-edge timber, the values are reduced by 10 %. If the props are shored, the free column length is reduced. Today, steel columns (props) are often used, but if wooden columns (studs) are used, 100 · 100 mm will be the preferred dimension without this dimension ever being fully exploited, but small dimensions will not be used as there is a need for having space into which nails can be hammered.

In order to distribute the pressure down into the ground, the studs/columns are mounted on soleplates; see Figure 6.11.

Dimension Load bearing capacity in kN for free column lengths in metres

Table 6.05: The permissible load bearing capacity of wooden columns (props) of pine and fir.

Wooden or Steel Prop

Soleplate

Figure 6.11: Props and soleplate adjacent to ground

If the props pressure is distributed evenly over the soleplate’s length a, we have the following:

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6.5 The formwork’s design

6.5.1 Foundation formwork

Foundation formwork commonly occurs in one of two types: either as strip foundation trenches for walls, or point foundations for, e.g., columns.

Trench foundations in firm soil can be executed as shown in Figure 6.12, where the soil is excavated leaving vertical sides corresponding to the desired foundation width and where only the plinth part of the foundation needs formwork. Here, the formwork for the plinth is made beforehand with boards and braces before assembly on the trench. The side boards for the plinth are not produced any larger than they can be borne by two men, which means that they are no bigger than 5 m². In order to create a tight vertical joint in the formwork, the outermost brace is placed so that it protrudes 10-20 mm beyond the edge of the formwork boards. One side of the foundation formwork is shored to a longitudinal lath, which is held in place by driven piles (boards); the other side of the formwork is held in place by a longitudinally positioned board at the bottom, and Tie Bars, or nailed-on boards, at the top of the formwork. Formwork boards of up to 600 mm in height are used.

Plinth formwork

Kicker Stake

Figure 6.12: Foundation trench

Otherwise, the trench foundation is executed on top of an open trench as shown in Figure 6.13, which for heights greater than 600 mm are assembled by using either tie bars or 3 mm annealed tie-wire. The actual dimensioning is as for walls, but the foundations are no higher than can be filled in one go so that the maximal casting pressure is the casting height multiplied by 24 kN/m³.

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Bushing pipe with conus works as a spacer to keep Figure 6.13: Strip foundation formwork distance between the 2 sides of the formwork

Figure 6.14: Tie bar with snap ties

Figure 6.15: Column foundation formwork

Tie Bars, which are used for vertical formwork, are found in many variants; Figure 6.14 shows one type (“lightning form tie”). Tie Bars are pressed here against the formwork with a tightening tool and held in place with a saw-toothed lock mechanism, which can be loosened with a single blow of a hammer. The round steel Tie Bars that are used in dimensions of 7 and 10 mm are surrounded inside the formwork by bushing pipes of plastic. The locking system can also be based on fixing with bolts, wedge-action (SP 100), or on screw-action on the Tie Bars (Dywidag). System formwork uses fewer Tie Bars in special steel and greater dimensions with special Snap Ties (“She Bolts”). Fixing can, for example, be achieved with the aid of screw-threads that are profiled into the Tie Bars.

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Figure 6.14 shows a bushing-pipe (spacer) with conical ends where the Tie Bars are extracted after casting, the conical end is removed and the hole is plugged with cement mortar - if the placing of the Tie Bar hole is not exploited architecturally. This connection is not watertight. If a watertight connection is desired, permanent conical ends can be used with “drain caps” (perfo- rated surface). After casting, the Tie Bar hole is plugged with a plastic plug. The plastic bushing pipe can be omitted by making the Tie Bar (coupling) so stiff that it can act as a spacer, which can be retained in the construction after casting.

Simple rectangular forms for column foundations can be formed as shown in Figure 6.15. They are constructed of 4 pieces, 2 side sections and 2 end sections. The end sections are built corresponding to the foundation dimensions, while the sides are made a little longer and are equipped with vertical sides for supporting the end-formwork. The Tie Bars fix the form parts together. With larger foundations, the use of Tie Bars through the foundation structure cannot be avoided. Furthermore, the figure shows a template for placing cast-in bolts or starter-bars.

Often, the foundation has to be cast together with a column element; hence the rebate at the top of the foundation. The rebate box, which is placed with great accuracy, is formed with a 10 % bevel (slanting bottom), lifting-fittings and is struck (removed) 1 - 2 hours after pouring the concrete.

For the execution of column foundations, it must be recollection that this can be achieved by drilling out the hole with subsequent fast pouring of concrete, or by using manhole frame rings.

6.5.2 Formwork for walls

The choice of formwork for walls will depend on several factors that must be considered:

• Demands to the finished surface • Reusability • Watertight coupling-solutions • Couplings placed in patterns • Crane capacity • The workforce’s and manager’s abilities • The company’s available formwork material • The time available for execution

Formwork for walls can be distributed into three categories:

• The traditional that is erected on site, board for board. • Panel cassette systems with plywood lining • Large panel wall systems

The traditional wall formwork shown on Figure 6.16 can basically be used everywhere. It is especially used where:

• The form amount and degree of reusability is small • The formwork is complicated • There are special demands to the rustic surface, or where the boards have to be placed in special patterns.

The number of working hours necessary for traditional formwork is great and the possibility for progression is therefore limited. On the other hand, corner solutions can easily be implemented and costs due to waste in connection with starter bars is low. Finally, it must be mentioned that costs for formwork is dependent on the pouring sequence, as traditional formwork entails a situation where materials are bought and formwork is not hired.

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Rules of thumb and facts regarding traditional formwork can be summarized as follows:

• The formwork gives uneven walls when using quarter-sawn and tangentially sawn timber for boards and studs. • The casting surface, which is in direct contact with the concrete, is executed in timber boards or plywood. Normal dimensions are 25 · 100 mm or 32 · 125 mm. When insulation is incorporated in the construction it is mounted in the form against the formwork sides as spread shuttering can be used, i.e., the casting boards are not placed closely next to each other. • Studs are the vertical timber members that the boards are fixed to, which can also be fixed horizontally (in which case they should be called noggins instead of studs) if vertical board contours are desired in the finished concrete surface. Usual dimensions are 32 · 125 mm, 50 · 100 mm or 50 · 150 mm. • Coupling-boards are normally executed as a double construction with the coupling-rod sandwiched in-between the boards. They are aligned with each other on either side of the formwork perpendicular to the vertical studs and keep the latter in position. They ensure that the wall is aligned and transfer the forces of the Tie Bars to the formwork. The coupling- boards, which are also known as stretch or locking-boards, are normally executed as 32 · 125 mm, 50 · 100 mm or 50 · 150 mm. • Tie Bars protrude 0,5 m beyond the coupling-boards in order to make space for the ratchet device that tightens the forms together. They are dangerous when placed at eye-height, which make it easy to collide with them, so they should be bent or cut to size after they have been tightened. Casting boards = Formwork shutter

Studs (horizontal) = Formwork Walers Tie Bars Studs (vertical) = Formwork Soldiers) Locking boards

Figure 6.16: Traditional formwork for walls.

• Shoring boards (Kickers), which are not shown on the illustration, are nailed (with a nail gun) to the under layer as side support and fixed to the vertical studs during the erection of the formwork. Typical dimensions are 32 · 125 mm, 50 · 100 mm. • The sloped shoring is fixed to one side of the formwork per 2.5 - 3 m in order to hold the formwork in place during casting of concrete. If the shoring boards are not sufficiently fixed to the under layer, shoring should also be established at the ends of the formwork. Normal dimensions for shoring can be 50 · 100 mm. • Tie Bars (couplings) are normally common 7 - 10 mm round steel rebar.

Even though the use of the traditional formwork is in rapid decline, it forms the basis for development of formwork systems where, for example, much of the timber is replaced by steel and alu-

441 minium profiles or girders. Today there is a great abundance of formwork systems where each manufacturer has their own system, which are not identical, but which are very similar to one another. Examples of these systems will be reviewed later.

Panel wall formwork, as seen in Figure 6.17, is a special form of cassette formwork that has won favour in the industry. The idea behind it is smaller and, therefore, lighter cassettes with a number of adaptable pieces that make them suitable for many purposes. Furthermore, they are joined with simple clasps to connecting locking devices that are attached to the edges of the cassette sides. Panel formwork comprises 15 mm thick plywood that is mounted onto 75 mm steel profile skeletons. The steel profiles have rebates for Tie Bars, which are 15 mm threaded dywidag steel rods. The cassettes are built to sustain a maximal form pressure of 35 kN/m².

The table only shows adaptable pieces for the basic element 1000 · 750, however, corresponding adaptable pieces are also available for the two other basic elements. The system incorporates corner solutions, both the fixed one for the 90°bend and the hinged one for more obtuse or acute angles. Special multi-edged equalization pieces that are used for round walls with radii from approximately 1.2 m and greater are also available. The formwork elements can be displaced in height in relationship to each other because of the many slit-holes at the edge. As can be seen in the table, there are length adaptable pieces down to 50 mm, and under this dimension adaptable wooden pieces of 10, 20, 30 and 40 millimetres thickness are used.

Large panel wall formwork is the term for formwork elements that are so large that they require transporting with a crane. They can be produced on site by using system girders, or they can be bought/hired as prefabricated large elements. Figure 6.18 shows a cross-section through a large element formwork, which is built using system girders. Characteristic for this is that there is only one row of Tie Bars, at the top and at the bottom.

If visible coupling-rod holes are not desired in the finished wall, the top row can be placed above the casting-height and the bottom row can be hidden by the wooden floor, which will be established later on joists. With only two rows of Tie Bars, these are exerted to great pressure – even though they are placed adjacent to two girders. Dywidag-rods, which are made of special steel with = 900 N/mm², can for example be used.

Adaptable Basic element element piece

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Weight in kg M2 Description Size in mm 30.0 0.75 Basic element 1000 x 750 49.5 1.25 1000 x 1250 1.50 1000 x 1500 24.1 0.56 Adaptable element piece 750 x 750 19.1 0.45 600 x 750 16.8 0.38 500 x 750 14.1 0.34 450 x 750 13.1 0.30 400 x 750 12.1 0.26 350 x 750 11.1 0.23 300 x 750 10.1 0.19 250 x 750 9.1 0.15 200 x 750 8.1 0.11 150 x 750 7.1 0.08 100 x 750 5.4 0.04 50 x 750 10.6 Internal corner (hinged) 3.8 External corner 1.0 Equalization piece 50 x 750 0.3 Equalization piece 10 x 750 1.5 Tightening rod DW 15 0.5 Nut 3.5 Spacing rod 50 x 750 Figure 6.17: Panel wall formwork

The AZ girder is, as described in Figure 6.18, symmetrically formed in steel and lined with a wooden strip so there is something to nail into. Between the girders´ openings, where the girders have a mutual distance of 0.5 – 1.0 m, are placed minimum 2 nos. wooden planks of 75 · 125 mm, INP12, or special tension elements. These are wedged into place in order to ensure a straight and stable large panel formwork element. The coupling system’s anchors are placed and lined through the slit-formed end openings. When 32 mm boards are used for the gangplank, support brackets are mounted at mutual distances of 1.4 m.

The casting-surfaces, girders, tension elements and gangplank are mounted into a fixed unit in the desired size and is lifted and transported into place by crane. It is, furthermore, possible to allow slanted shoring to be included into the fixed unit large panel formwork element. An addendum of 10 % must be included for the price of the girders, tension elements and the casting- surfaces for miscellaneous items (such as fixing materials), and a further 10 % for consumption of other goods (e.g., the coupling system, etc.).

As mentioned earlier, storey-high large panel formwork elements can be delivered prefabricated; an example is shown in Figure 6.19. It is necessary to have an addendum to the price of this for miscellaneous consumption goods.

Similarly, Rasto is a formwork system that is executed in lightweight steel that can both be used for foundation and wall formwork purposes. The system is light enough to mount using unassis- ted manpower, and strong enough to be mounted using a crane.

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Form Weight m Kg

2.40 · 2.70 294 1.20 · 2.70 148 1.05 · 2.70 135 0.90 · 2.70 122 0.75 · 2.70 108

Figure 6.19: Prefabricated large panel wall formwork

Rasto is delivered in heights of 1.5 and 2.7 metres, and widths from 450 to 900 mm. The formwork skeleton is galvanised, which ensures a long lifetime of use. A form width of 750 mm weighs 60 kg and can be borne by 2 men. Rasto has a form thickness of 120 mm and can withstand a form pressure of 60 kN/m².

Rasto XXL is a supplement to Rasto, and is a large cassette measuring 2.4 x 2.7 metres for lifting by crane. Rasto XXL can be combined with the ordinary Rasto formwork elements.

Figure 6.20 Rasto formwork

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Figure 6.21 Rasto XXL wall formwork

Manto is a large panel formwork system for big assignments involving high concrete form pressure of up to 80 kN/m². Manto is delivered in several different sizes, which can all be combined with each other. With a form height of 2.7 m and a framework thickness of 140 mm, the form needs to be lifted into place by crane.

Form Size in metres Weight in kg

Large cassette 2.40 x 2.70 305.5 Cassette 1.20 x 2.70 155.0 Cassette 1.05 x 2.70 141.3 Cassette 0.90 x 2.70 126.9 Cassette 0.75 x 2.70 112.4 Cassette 0,70 x 2.70 107.5 Cassette 0.60 x 2.70 97.8 Cassette 0.55 x 2.70 93.1 Cassette 0.45 x 2.70 83.4 In side corner 0.35 x 0.35 x 2.70 102.7 Large cassette* 2.40 x 3.00 344.1

* to this large cassette, the same width is available as for the other cassette. Table 6.06 Manto cassettes

The shoring-up of large panel formwork is performed by using inclined struts/braces (also known as push-pull-props), as shown in Figure 6.22. The innermost tube can be roughly adjusted in relation to the outermost tube. During mounting, the flat-plates can be positioned in plumb (verti- cally) through the fine adjustment of an encapsulated thread. The inclined struts are also used for the shoring of concrete elements in element construction projects. They are also available in fixed lengths, i.e., without telescopic extension means.

The bearing capacity curves for inclined struts are valid for the effects of stress (pressure); in cases of tension, the values for the minimum lengths in stress situations can be used, as in this situation, the danger of deflection is not the decisive issue, but the fixing-bolts above and below is the issue.

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6.5.3 Formwork for columns

Column formwork can normally be rectangular, square, or round in shape. Because of the relatively small amounts of concrete in columns, their formwork is normally filled in one go. The maximal concreting pressure is, therefore, the hydrostatic pressure.

Figure 6.23 and 6.26 shows different ways of laying-out the formwork for columns. In addition to the traditional formwork for columns shown, reusable steel formwork and disposable surface- treated cardboard formwork is available today for building columns.

Adapted side-flanges are used for the sides of traditional column formwork, which are erected within a bottom framework and shored with struts. The reinforcement is placed in position beforehand in the larger columns. The wooden braces that hold the column formwork flanges together can be extended a board thickness beyond the flange edge in order to support the neigh- bouring side flange. In order to stiffen the column formwork, locking boards, double boards (walers) with Tie Bars, or column vice-frames of steel, which are tightened around the formwork like a belt with wedges, are used. These steel vice-frames are positioned over the wooden braces of the column formwork’s side flanges. Table 6.07 shows examples of such vice-frames.

Type V consists of four flat steel plates with punched-out elongated holes and bent ends. The system is completed with 4 steel wedges on chains. Type H consists of two identical angled steel profile frames with locking-devices and wedges.

It is necessary to use timber battens for lining the formwork in the production of traditional round columns. It is the column’s radius that is decisive for which width of batten that has to be used and, furthermore, with which bevel slant they are to be planed. A far cheaper alternative, but also one which gives a slightly different appearance, is to use the aforementioned cardboard forms, which are undergoing a process of development such that there are a huge number of surface structures available. In addition, other than round sections are now available: square, hexagonal, octagonal and rectangular.

Figure 6.28 shows shoring of the timber batten formwork with steel plate belts. The steel plate’s cross sectional area must be big enough to resist the concrete pressure P. If D is the column diameter and L the distance between plate belts, the force F in the steel plate is given by:

F = ½ P·D·L Formula 6.14

If many similar round columns are to be produced, it is viable to use prefabricated forms manu- factured in steel. The forms are assembled in halves with bolts going through protruding flanges. A steel form can be used approximately 200 times.

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Weight Length kg m ”Raking” shore – a shore that is leaning at an angle

Figure 6.22: Inclined strut shoring (“Raking” shore)

In order to prevent twisting of the column formwork during casting of concrete, two inclined struts, which must stand perpendicular to each other, must be used.

Figure 6.29 shows a more complicated column formwork, which is assembled from 4 pieces. The flanges are supported by wooden girders that a held together with U-profiles and 80 kN Doka- anchors.

Figure 6.23: Column formwork with locking boards and with Tie Bars

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Steel wedge

Figure 6.24: Cross section through Tie Bars Figure 6.25: Cross section through column vice-frame

Figure 6.26: Column formwork, square with column vice-frames and rounded locking-boards

Figure 6.27: Round special columns in “cardboard tube forms”

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Table 6.07: Column frames

Figure 6.28: Column form with steel plate belt (collar band)

Figure 6.29: Special column formwork

6.5.4 Formwork for slabs

Traditional slab formwork. Figure 6.30 shows and example of traditional formwork for a slab construction. The props are vertical 100 · 100 mm studs. Often, this dimension can be reduced from a load bearing perspective, but it is retained because there is something to nail into. When erec- ting the formwork directly on the ground, an underlay of planks, soleplates or sleepers is required. In order to ensure an accurate floor height, the props (studs) are placed on leveled wedges. Alternatively, and as shown on Figure 6.30, formwork bearers can be nailed to the sides of the props, which are then supported with fishplates. In order to keep the props in the right place, 25 · 100 mm shoring boards are nailed on the sides of the props. For free-standing formwork, inclined shoring is mounted onto the props’ sides to create spatial stability. The shoring shortens the free column length and thus increases the load bearing capacity of the props (studs). Joists are usually mounted onto the top of the bearers. These are normally 32 mm boards on edge, or

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50 · 100 mm half-timber. As the concrete construction normally sags after the formwork is struck, the formwork bearers are normally given a rise in the middle corresponding to 1/400 of the span. Wooden boards or plywood panels are used as the base for the slab’s soffit in the formwork. Consideration should be taken to possible deformation caused by moisture so that these base-boards are not laid too close together.

Steel pipe props The traditional slab formwork has been improved in several respects, as will be shown in the following. The vertical timber props have been replaced by steel pipe props, as shown in Figure 6.31. They consist of an outer and inner pipe and can be drawn out in the approximate length required, after which a bolt is stuck through a hole. A rough adjustment can thus be made for each 100 mm. Fine adjustment is made by twisting a threaded sleeve, which has a practical handle placed at a suitable working-height. Foot and header plates have nail-holes, but can also be furnished with a fork that fits 50 · 100 mm or 100 · 100 mm timber. Furthermore, special brackets are available that can lock the shoring to the supports with wedges.

Here, the bracket is set on top of the pipe prop and a board is inserted in-between the pipe prop and the wedge, after which the wedge is hammered into place.

By shoring the transition between the outer and inner pipe prop in both directions, the free height is reduced and the load can be increased as shown in a couple of examples in Figure 6.31.

It should always be evaluated whether existing construction members can be used as supports for slab formwork; Figure 6.32 shows some examples of how this can be done. Supports can be brickwork walls, concrete walls (where the top row of coupling-holes can be exploited) or prefabricated beams with mounting holes. This, of course, requires planning in good time in order to achieve economic savings that are latent in the aforementioned process.

For more difficult concrete constructions, as for example bridges, formwork towers or triangular- supports, as shown in Figure 6.33, are used. The towers can comprise of welded frames, which are mounted in such a way that the towers form a square base of 1.0 · 1.0 m. The towers can be built at an arbitrary height. Under normal circumstances, mutual tower shoring is not necessary up to 8 meters’ height, but after that height the towers need shoring per each 3 metres. They are equipped with fixing brackets in all joints and are, therefore, able to sustain tension so that they can be transported by crane. The four tower legs can be loaded differently. However, the weakest loaded leg must have load of 20 % of the maximal load.

A work team of 3 men can mount 5 towers an hour at a height equivalent to 6 m. The super- structure comprises timber or steel profiles, after which joists and casting underlay is laid out. The loads shown in Figure 6.33 are the calculator permissible loads.

Apart for sustaining large concentrated loads for formwork, lattice supports are used as temporary supports for constructions as, for example, concrete slabs that are crane-loaded.

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Shutter

Formwork bearer

”Flying” shores (horisontal) ”Dead” shores (props) (vertical) Figure 6.30 Slab formwork. Formwork joists - sat on top of bearers

Support bracket

Clearance Permissible load in kN height in m Type FA Type FB Type F II Type F IV Type FV 1.50-2.70 2.00-3.30 2.25-3.75 3.25-4.75 3.90-5.50 Without With Without With

support support support support 1.50 30.0 2.50 21.0 23.0 20.5 2.70 14.2 21.8 19.5 3.00 20.0 18.0 3.30 14.5 15.5 22.0 22.0 3.50 13.3 20.0 22.0 3.70 12.6 17.0 22.0 4.00 12.5 22.0 16.0 22.0 4.50 7.8 21.4 12.0 20.5 4.70 7.0 20.0 10.4 17.5 5.00 8.0 13.5 5.50 6.0 10.0 Weight in kg 11.8 14.0 18.0 23.4 23.4 26.0 26.0 Price in kr. 310 350 425 483 483 525 525 hire/day 0.63 0.74 0.89 1.00 1.00 1.11 1.11

Figure 6.31: Steel tube studs (“Dead” shores) with bearing brackets (support fork).

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Rebate in wall

Telescopic traverse lattice girder

Brick wall

Concrete wall

Figure 6.32: Supporting the existing construction.

System truss-girders can substitute formwork bearers and joists. They consist of two different types as shown in Figure 6.34, a steel profile girder and a lattice girder. When mounting, the steel profile girders are slid into the lattice girder until the desired span is achieved; the heavy-duty screw is tightened and the girder is ready for use.

ID 15 ID 20 (with wind load) without wind load 50kN per support leg. 14 m: 73 kN 336 kg 15,482 DKK 10 m: 139 kN 256 kg 11,525 DKK with wind load 5.0 m: 192 kN 150 kg 7,892 DKK 29 -43 kN per support leg. H45 (with wind load) 1.5 m 113 kg 4,760 DKK 5.0 m 166 kg 7,300 DKK 15 m: 100kN 610 kg 30,411 DKK 10.0 m 407 kg 12,070 DKK 10 m: 233 kN 466 kg 25,050 DKK 15.0 m 557 kg 17,200 DKK 5 m: 366 kN 316 kg 18,784 DKK

a) Formwork tower b) Triangular lattice support column

Figure 6.33: Formwork tower and triangular lattice support column.

Each type is produced in different lengths. The girder is furnished with a gusset plate to rest on a support. Often, the system-girders are placed so close together that joists (floor battens) can be avoided, i.e., that shuttering boards can be laid directly upon the girders. When striking the formwork, the adjustment screws are loosened and the formwork girders are lowered slightly in the middle of the decking, then the girders can be pushed together and removed.

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Traverse lattice girders The traverse lattice girders consist of two contiguous parts, which slide in and out of each other, while the girder parts can be pulled completely apart from each other and combined. Thus, steel profile girders can be pushed into the lattice girders from both ends and one can, by using three elements - two lattice girders and a steel profile girder, or one lattice girder and two steel profile girders - obtain a series of possibilities for build-combinations. The girders are normally used only with simple-supported ends, since they cannot absorb negative moments. However, the “Junior” girders with spans over 6.5 m must be intermediately supported.

Aluminium girders The aluminium girder is an extruded 167 mm high profile, which is lined with a strip of wood for nailing. This profile is sold in lengths from 1.5 m to 6 m with increments of 0.5 m. It is not inten- ded for assembling together, but for mounting with an overlap, i.e., staggered. It is worth noting that one can mount the girders cantilevered for 30% of their span.

Lattice girder

Profile girder

Figure 6.34: System girders

Corridor traverse Maximal moment 2.1 kNm Weight Hire Price Max. reaction 10 kN kg kr/day kr Type I 0.7 – 1.25 m 5.5 0.62 192 Type IIA 1.25-2.10 m 12.0 0.80 300 Telescopic traverse Maximal moment 6 kNm Weight Hire Price Max. reaction 10 kN kg kr/day kr Type 1 1.80-3.10 m 24.1 1.13 620 CADET-GIRDER Maximal moment 9 kNm Weight Hire Price Max. reaction 9 kN kg kr/day kr a) Inner girder 1.85 m 14.0 0.54 269 b) Inner girder 2.60 m 29.8 0.72 363 c) Outer girder 1.85 m 11.4 0.56 291 d) Outer girder 2.60 m 16.2 0.82 369 Combinations: Type 1 (a+c) Span 1.85 – 3.00 m Type 2 (b+d) Span 2.60 – 4.50 m JUNIOR-GIRDERS Maximal moment 12.5 kNm Weight Hire Price Max. reaction 13 kN kg kr/day kr a) Inner girder 2.88 m 21.0 0.89 430 b) Outer girder 2.35 m 19.0 1.00 312 c) Outer girder 2.98 m 23.0 1.25 425 Combinations: Type 1 (a+b) Span 2.95 – 4.68 Type 2 (a+c) Span 3.05 – 5.31 Type 3 (b+a+b) Span 4.78 – 6.48 Type 4 (b+a+c) Span 5.39 – 7.11 Type 5 (c+a+c) Span 6.03 – 7.75 Table 6.08 System girders

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TECHNICAL DATA F = 1.68 · 103 mm² Wx = 76.8 · 103 mm² 4 Ix = 6.96 · 106 mm E = 70,000 N/mm² Sr = 126 N/mm² (bending tension) Mr = 9.75 kNm Qr = 25 kN Ar = 40 kN ix = 64.4 mm Weight = 5.8 kp/m Price = 265 kr/m Hire = 0.55 kr/m

Figure 6.35: Aluminium girders

Figure 6.36: Wooden girders and steel props with supports

Wooden girders The wooden girder shown in Figure 6.36 is often used as formwork bearers and joists in fast mounting systems together with steel pipe props that, for example, in every second row are furnished with supports so that shoring can be omitted.

Lattice girders Gigantic lattice girders of steel with spans up to 34 metres can be established for larger construction and civil engineering works. They can be assembled from up to 2 m high individual elements that, with a span of 12 metres, can sustain a load of 54 kN/m and, with a span of 30 metres, a load of 13 kN/m. In the latter case, the dead load of the girder is 3 tons and it comprises 5 individual gigantic girders. It can also be assembled with smaller parts to form arches for building bridges; and here they are equipped with horizontal tension members.

Slab cassette formwork Figure 6.37 illustrates one of the types of slab cassette formwork on the market. The basic cassette is 900 · 1800 mm, executed in aluminium framework and 10 mm edge protected veneer panels, and they weigh only 20.5 kg per element (900 · 1800 mm).

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Price Height in metres for slab thickness in mm Price (price level 2004) Support 100 150 200 250 300 Hire Purchase Cassettes Hire Purchase Kr/d Kr/d kr A260Z 1.96  2.95 2.95 2.95 2.80 2.60 1.02 470 18090 3.85 1625 18075 3.65 1540 A350Z 2.38  3.70 3.70 3.50 3.20 3.00 1.22 572 18060 3.30 1391 18045 2.94 1239 A410Z 2.76  4.45 4.45 4.45 4.20 3.90 1.32 615 18030 2.45 1034

Figure 6.37: Slab cassette system Furthermore, they are available as walls and a number of adaptable cassettes down to 300 mm in width. The last adaptation can, as shown in the figure, be made by using special adjustment girders.

Beam/girder brackets Figure 6.38 shows an example of the use of a beam bracket. This can be adjusted to different dimensions from 150 mm to 700 mm in height and up to 670 mm in width. It is equipped with a built-in wooden strip that makes it possible to directly nail the formwork boards to the vertical side pieces. The beam bracket can both be used for ordinary beams, edge beams and leaps in the soffit of slabs. The weight is 13.5 kg, and the permissible concrete pressure is 20 kN/m².

Figure 6.38: Beam bracket

Form tables Just as it is possible to incorporate formwork for walls into large panel wall formwork, it is also possible to assemble slab formwork into larger fixed units – called form tables, as shown in Figure 6.39. Form tables can be built-up completely in timber or in a combination of timber and steel profiles, or patented sub-constructions from suppliers can be used. The forms, which are transported by crane, must be made in such a way that they can be sunk a little ways (under possible beam height in the slab soffit) during the striking of the formwork. The formwork is mounted on wheels or special rolling equipment in order to be pushed out to the façade where the crane can reach it. The form tables can be furnished with beam-edges for longitudinal beams. These are usually executed with bevel edges in order to ease striking their formwork. 455

Figure 6.39: Form table

Crossing beams cannot, however, be beveled because of difficulties of striking the form with so- called room-corners. The use of form tables is naturally dependent on the number of repetitions in their use, and they are thus especially suitable for multi-storey buildings with identical slabs; the sizes of the tables are only dependent on the load bearing capacity of the crane in use.

6.5.5 Special wall formwork

Circular formwork Circular formwork, e.g., for the construction of round reinforced concrete tanks, can be built-up as shown in Figure 6.40. Templates are used for the setting-up of the internal walls, one at the top and one at the bottom. The template can comprise pieces of timber cut-out in the form of 50 mm boards. If the tank diameter, D, is greater than 9 m, it is possible to elastically bend 25 mm thick casting boards and fix them to vertical formwork soldiers, as shown in Figure 6.40a.

Plywood can be formed as mentioned above. Flatly-placed boards can act as tension members, and Tie Bars must therefore be placed at each of the formwork soldiers.

Figure 6.40b shows a construction method for circular tanks with a diameter, D, less than 9 m. Here, vertical boards that are supported by shoring are used. A 150 mm board can be used to cut-out both the inner and outer shoring. This is again supported by double vertical coupling boards. The extensive cutting-out work for the forming of the shoring will lead to the exploitation of the fact that the concrete pressure drops as you go up, so that the distance between shoring can be increased here.

It must be remembered that, as for the corresponding forces in steel plate tension belts (Fig.6.28) in round column formwork, circular wall formwork have directional tensions that need to be absorbed by the horizontal parts of the formwork and transferred to the joints between these.

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Template Template and shoring

a) Diameter greater than 9 m. b) Diameter less than 9 m.

Figure 6.40: Circular formwork

Ring tension = · D · concrete pressure Formula 6.15

It is also possible to use system girders to produce the round formwork illustrated in Figure 6.41. Trapezoid steel girders are mounted onto a casting-plate, which can be made of boards, veneer, or thin steel plate. Wire-tighteners are mounted and the curve can be formed manually.

It is possible to use straight cassettes for round formwork if the radius is sufficiently large enough; in this way, a multi-sided object is formed with a little more concrete consumption. Raster-formwork can be used from 9 m diameter with 600 mm elements and 50 mm adaptable pieces on the outer sides.

Bolt holes per 290 mm 21 mm plywood

Maximal casting pressure 60 kN/m²

Figure 6.41: Round form produced by using steel trapezoid girders.

Sliding form systems Sliding formwork systems are used, e.g., for silos, chimneys, water towers, bridge piers, etc. A sliding form is understood as being a low formwork system, as shown in Figure 6.42, which is continually moved upwards in step with reinforcement and concreting work. The form is 1-1.2 m high and is held together by moment-resistant yokes. The formwork is lifted upwards by hydraulic jacks, which are located at each yoke and fixed to a climbing-pole of steel. The climbing-poles are joined successively during the process of the casting work and can be enveloped by a glide-pipe so that they can be recovered. The form sides can be made of plywood or steel plate. They are mounted with 3-5 mm freeway below to ease the climbing process.

The yokes are made of steel and are placed per 1.5-2.0 m. The work platform is at the height of the form’s top surface and is supported for spans less then 3 m directly on the form top itself. Material transport is achieved, as a rule, by using a tower crane, whilst manpower transport is achieved by using a lift. The work platform must be equipped with fire fighting equipment. For very high buildings, warning lights must be provided to warn aircraft. Ribbed bar or tentor rebar is used as reinforcement in lengths up to maximum 4-5 m that must be staggered in their joints –

457 i.e., joining of reinforcement must not be in the same horizontal level, but must be displaced with regard to each other. Concreting is done with ordinary concrete with a maximal stone size of 32 mm.

The upward sliding is continuous over a 24-hour period and happens with a speed of 2.5-3.5 m per day. This is equivalent to approximately 0.15 m per hour, which means that the concrete is only 6 hours old when the formwork is struck.

Control of whether the formwork is in plumb and is not twisting must be made at regular inter- vals. A hanging scaffolding is used to inspect the newly cast concrete surface. In order to avoid the formation of cracks because of too rapid a drying-out process, a curing-membrane is applied to the surface to prevent evaporation. The self-climbing formwork is economic at heights of 15 metres or more. Wall thicknesses should be at least 150 mm, otherwise there is a danger of lifting-cracks occurring, which can happen in hot weather when the climb upwards is slow. The friction between the concrete and the formwork can be greater than the counter weight from the concrete’s dead load and adhesion to the reinforcement. As self-climbing formwork is a sort of extrusion process, protrusions in walls cannot be tolerated. Rebates or recesses for later accommodation of beams can be established relatively easily. It is also possible to vary the cross- sectional area by making pauses in the casting and rebuilding the formwork.

Climbing form The climbing formwork system has the same area of use as the self-climbing formwork system. However, it is used, to make formwork for one wall up to a great height, but is also useful where the construction is not round, but rectangular as with, for example, elevator shafts. In climbing formwork, one wall height (e.g. 3 m) is set-up at a time from a hanging scaffold. The scaffold is moved in sections by a crane and is hung on bearing-brackets, which are bolted to inserts in the previously concreted wall.

Apart for a work-platform for setting-up the scaffolding, a smaller platform is hung for post-repairs of the wall that has just been struck for formwork.

Figure 6.43 shows an example of a climbing formwork system at a building gable where only the outer formwork climbs. Climbing formwork can, as mentioned previously, also be used without standing on the decking, but with climbing formwork on both sides.

The work-platform can also be made broader so that the shoring of the formwork can be established on wheels and rails.

“Filigran” walls “Filigran” (name of Danish product) walls are also a form of special wall formwork. They comprise two 5 cm thick concrete plates where all the reinforcement necessary for the finished concrete wall is already cast into the construction from the factory. These two concrete plates are held together by the cast-in reinforcement bars (called filigran girders), whose task is maintain the desired thickness of the wall (forming a hollow), as well as absorbing the casting-pressure when the hollow filigran wall is cast with concrete on site. The advantage of this construction is that, in contrast to ordinary element constructions, it maintains the monolithic water- and sound- tightness within the construction.

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Climbing pole

Oil supply Hydraulic jack

Yoke

Work-platform --- Work-platform

Formwork

Hung scaffolding Concrete

Figure 6.42: Gliding formwork

Figure 6.43: Climbing formwork

Concrete, cast on site “Filigran” girder

Slab Slab height

Joint Possible joint reinforcement

Figure 6.44: “Filigran” element

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6.5.6 Special formwork for slabs

Filigran concrete slab elements Filigran concrete slab elements comprise, as shown in Figure 6.44, of 50 mm prefabricated concrete element soffit plates with the reinforcement necessary for the finished slab’s function above, and a filigran girder to absorb the loads resulting from transport and displacement forces between the element and the surface concrete that will be subsequently cast on site. The advantage of using filigran slabs is that the stability is the same as for concrete cast in-situ, and that slab formwork, and thus time, is saved. The elements must, however, be supported across the filigran girders with steel props and formwork bearers per 2.25 m for 100 mm slab thicknesses, and per 2 m for 200 mm slab thicknesses. The elements weigh only 100 kg/m² and do not, therefore, normally require extended crane capacity. Furthermore, reinforcement work on site is minimized and finishing works are avoided on the soffit surface, which is ready for painting.

Shell construction formwork Shell construction formwork is shown in Figure 6.45. Formwork for a traditional cylindrical roof construction is shown here. If the span is L and the maximal rise-height is H, then the radius R is given by:

If this angle is greater than 25-35°, dependent on the concrete’s slump, a top side shuttering that has great similarities with wall formwork, has to be used as through Tie Bars are used. As a rule, the top side shuttering must be build-up gradually, piece by piece as the casting is executed, as it is otherwise be in the way of the casting and vibrating process.

The casting underlay, which can be made of plywood, is normally supported by joists, which again rest on formwork bearers that are specially cut to fulfil the curve form. The formwork bearers can lie directly on the dead shores, but are often laid (as shown) on beams.

When cutting the formwork bearers, consideration must be taken to their dimensioning. If the construction’s span L = 9 m and its rise-height H = 1.5 m, for a 1.5 m long bearer you have:

i.e., the cross-sectional height must be reduced with 38 mm at the ends.

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The timber stud construction will often be substituted today by steel props; in order to avoid moment-effects on the legs, they are mounted at the same height. Steel profiles and adjusted trusses that have an over-side corresponding to the shape of the construction are laid at the top and form the transition to joists and the caster underlay. For, e.g., long halls and tunnels, it is advantageous to mount the formwork on a mobile undercarriage so that a driving-formwork results. When the formwork is struck, part of the sloped shoring is removed and the formwork is thus lowered so as to release the concrete, after which the form and its scaffolding is driven to the next position.

Finally, it is possible today to get forms that are specially built to accommodate precisely these barrel-vault shapes in the form of tunnel formwork.

Tunnel formwork Tunnel formwork comprises an angular formwork where the walls and slab formwork are set-up simultaneously and concreted in one and the same process. Each form is produced from 4 mm steel plate, which is shored with welded-on steel profiles. Half of the slab-form is hinged to the side of a carriage in such a way that inclined spindles can be used to adjust the slab to the desired rise-height, and lower it again during the striking of the formwork. In order to achieve a faster striking of the formwork and avoid long-term subsidence, the slab is shored-up with supports as soon as the angle-form is removed and before the next form is taken down.

Tolerances Tolerance demands are very variable in Danish tender documents. One often sees special demands regarding the slab formwork having to have the necessary rise-height of 1:400 of the span (but minimum 15 mm) in order to eliminate expected sagging and settlement after the form has been struck.

Figure 6.45: Formwork for vaulted slab

If one has not got a complete overview of all the possible deformation in time, a large span can give great problems with regard to setting-up the wall elements on the next storey.

The precision with which a concrete construction has to be executed will depend on which type of project is in question. Extended demands to precision will correspondingly increase costs. ACI

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(The American Concrete Institute) recommends the following tolerances for concrete in building constructions:

1. Deviations from plumb-line for walls and columns: 2 mm/m (max, 25 mm). Especially important construction members: 1 mm/m. 2. Deviation from horizontal: 2 mm/m Undesirable deepening in flat surfaces: 10 mm For special construction members: 1 mm/m 3. Placement with relation to module lines and mutual distances of individual constructions: At 6 m: 12 mm At 12 m: 25 mm 4. Variations in size and placement of openings: 6 mm. 5. Variations in cross-section of columns and beams and thickness of slabs and walls: -6 mm -/+ 12 mm 6. Variation in foundation widths: -12 mm -/+ 50 mm Variation of foundation thicknesses -5 % 7. Eccentric loading of foundations: 2 % of foundation width in the direction of eccentricity, but max. 50 mm 8. Staircases, risers: 1.6 mm Staircases, goings: 3 mm.

6.6 Construction joints and inserts

6.6.1 Floor formwork

The castings of floor slabs usually happens in sections, and along the boundaries of these sections framework called screed guides are usually laid-down. These have three purposes: • To give the casting-height for the finished slab, i.e., to be the rail on which the smoothing of the concrete surface happens with a light vibrating screed or other straight-edge • To determine the positioning of the construction joint in the floor surface • And finally, to be the delimitation border for the concrete.

Figure 6.46 shows some examples. It is important that the screed guides do not sink, when the weight of the light vibrating screed is applied to it, during vibration and smoothing of the concrete surface. Figure 6.46a is a construction with spacing-timber on which a profile steel rail is attached and a wooden strip, which will form the construction joint in the side of the concrete slab. If the floor is not reinforced, the timber can be a piece of through timber, otherwise there must be spacing pieces which are spaced with a mutual distance of 1-1.5 m when using a 70 · 150 mm steel profile. To support the spacing pieces, small heaps of concrete are laid down with cast-in pieces of timber so that the guiderail can be fastened properly. Figure 6.46b shows a cross-section of a 4 m long guiderail, which is constructed in steel for use in non-reinforced floor slabs. The anchorage of the guiderail is done with 4 soil spikes, which are hammered through round 22 mm holes in the guiderail’s steel bottom plate. Finally, it is possible to acquire screed guides in concrete or steel, as shown in Figure 6.46c (steel not shown). These screed guides are cast into the floor and become a permanent part of the finished floor construction. They can be provided with holes for the reinforcement’s buffer-bars.

If the construction joint is not to function as the underlay for smoothing the concreted floor, it is possible to be content with hammering some soil spikes made of rebars and attach a plastic net.

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Light vibrating screed

Figure 6.46: Screed guides.

6.6.2 Construction joints

Visible construction joints in walls and columns are normally situated/cast 20 mm up into the subsequent slab or beam, and construction joints are hidden in the finished construction. If this is not possible, a profiled (timber) strip member, as shown in Figure 6.47a, must be placed in the construction. When the formwork is struck, and when the casting process is continued again, the profile draws a straight line in the construction joint, which will otherwise be crooked if the profile strip were not placed there. In slabs, the construction joint can be placed as mentioned under the floor. Construction joints in walls can be executed as shown in Figure 6.47b.

6.6.3 Joints

The aforementioned construction joint forms a butt joint: concrete against concrete. They are not watertight, and if this is required, a sealing joint strip must be inserted. Sealing joints are also used as movement or expansion joints, where, furthermore, a joint-plate (e.g., polystyrene plate) separates the two concrete slabs slightly in order facilitate the absorption of movement. To complete this process, the empty joint is filled-in from the top surface with approximately 20 mm of sealant material.

Figure 6.48 shows some examples of the execution of joint strips and water stops in formwork. In the corners, the joint strips are welded together by heating them with a welding device. As conditions on site are often difficult and awkward, it is often easier to order prefabricated joint strip pieces. Many types of joint strip are available on the market today. Normally, the width of a wing of the joint strip should be equal to half the wall thickness, i.e., the whole width corresponds to the thickness of the wall.

In recent years, a new expanding joint strip made of Bentonite clay, which has the characteristic that it expands extensively when it comes into contact with water, has been available on the market. Furthermore, Bentonite has the characteristic that it seeks out water and tightens holes and crevices. In order to hinder the Bentonite in expanding at the wrong time, it is available in a “stocking” made of corn-starch, which means that the joint strip can both be mounted in fresh concrete and in a moist environment. The organic stocking is broken-down by the aggressive concrete. The joint can be mounted both in vertical and horizontal construction joints. The joint strip is pressed into the fresh concrete or fixed with nails per 300 mm. When connecting the strips lengthwise, they are connected as butt joints: end to end. The joint strip is mounted on the water-side of the construction with a covering concrete layer of 50 mm in order to avoid flaking during expansion.

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It is often a problem in larger in-situ cast walls, such as basement walls, that shrinkage occurs in an uncontrollable way with resulting cracks and costs with regard to repairs. It is, therefore, recommended that crack-stopping joints/movement joints are placed as shown in Figure 6.49 per 6 m. A PVC-plate is mounted in the formwork, which is removed after the formwork has been struck, and a joint in cement mortar is established. In this way you can control where to place your cracks.

Casting level Visible wall ------surface Settling level ------

Figure 6.47: Construction joint in walls

Internal joint strips External joint strip Figure 6.48: Casting in joint strips

6.6.4 Casting-in components

It is very seldom that wooden nail-block inserts are precast into concrete to facilitate nailing and screwing these days, but inserts for shoring and supporting cranes and site lifts are appropriate. An insert is threaded so that a bolt can be screwed into it later. The cone-shaped insert shown in Figure 6.51 is used in connection with wooden formwork to prevent the insert from being pressed out into the form during tightening and results in a protruding insert.

Board nails are used for anchorage of insulation boards, which have to be cast together with the concrete; the board nails are pressed or hammered before concreting through the insulation board so that the point of the nail with its counter-barbs are cast into the concrete. Board nails are available in hard PVC and can be nailed to the formwork through nail holes so that the insulation boards can subsequently be pressed in over the nails.

Triangular wooden strips or plastic strips are not permanently cast-in, but are mounted to the formwork to give them bevel edges.

Boxing-out for rebates, which are removed again when the formwork has been struck, can be put together in timber (see Figure 6.50), or plastic pipes, carton or plastic foam can be used as a substitute for timber.

Plastikol joining material, or similar

Wooden strip Filled with cement mortar

Figure 6.49: Crack-stopping joints 464

Figure 6.50: Boxing-out for window rebate, here without a sill

Blinding-layer

Nail block insert Fixing block

Board nail Beveling-strip with nail flange Insert.

Figure 6.51: Members for casting-in

Fixing blocks of fibre concrete with nail-holes and steel nails are used for casting-in. They are fixed into the concrete to be used as screed guides for vertical wall or foundation formwork.

Bushing pipes/separating pipes with plugs and water-bars are used for coupling-rod holes that are to be made watertight. The water-bar or drain-cap, which they are called when made of plastic, are fixed outside the bushing pipe and are cast-in with it, and have the purpose of pre- venting water flowing through the wall.

The plugs, which can be through for fibre concrete pipes for total-closing (i.e., when one insulates against sound) are stuck into the bushing pipes after the formwork is struck and fixed with glue there.

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6.7 Stripping formwork

The risk of fracturing a construction during stripping the formwork should normally not be greater than that which applies to structures already in use. Usually, more stringent restrictions apply to stripping the formwork of horizontal load-bearing structures than for vertical walls and pillars. This is because deformations are often less important for vertical structures. The vertical structures have only a small fraction of their maximum load at stripping, while beams’ dead load may constitute a large proportion of the maximum load.

6.7.1 Basic terminology The relative curing speed, H(θ), is today set as the rate at which cement and water react at 20°C. The following experience-based formulae are available, where θ is the concrete´s temperature:

Since one often examines concrete´s properties from the reference value of 20°C, the term maturity M is introduced, which indicates a specific concrete´s curing state compared to the equivalent age at 20°C. One gets:

Formula 6.23 where the concrete´s mean temperature, θi, is determined for a specific time interval, Δti, and thereby, the hydrating rate R (θi) is determined. A maturity of 30 hrs. corresponds to the concrete having obtained a curing degree corresponding to it being 20°C warm for 30 hours.

The products Δti and H (θi) indicate the concrete´s maturity growth in relation to a curing process at 20°C.

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The concrete´s heat development depends, among others, on the cement type and cement volume. At full hydration, the total heat output is of the order of 400-500 kJ per kg cement. The actual heat development cycle has a graph shape as shown in Figure 6.52.

The specified curve progression can be empirically approximated by the following analytical expression:

Q = the total heat development (kJ/kg) Q = heat development at maturity M (kJ/kg)

M = the concrete´s maturity (hrs.)

Te = characteristic time constant (hrs.)

 = curvature parameters (without parameters)

Figure 6.52: The curve showing the heat development progression.

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Laboratory tests provide the mean values indicated in Table 6.09:

Cement a Quick hardening (curing) 432 12.2 0.9 Normal hardening 375 13.7 0.9

Slow hardening 320 15.2 0.9

Table 6.09: Heat development values

Low alkaline sulphate resistant cement can be considered slow hardening, standard cement is considered normal hardening, and super rapid cement is quick hardening. Rapid cement falls between normal hardening and quick hardening cement.

Similarly, and in principle, the development of strength has the same curve shape over time as that which is applicable to heat development, but without having parameter- community with heat development.

Strength development can, therefore, analytically be expressed by:

Formula 6.25

σ∞ = potential final strength MPa for M

σ = the strength MPa

M = concrete´s maturity, hrs.

τ = characteristic time constant, hrs.

α = curvature parameter

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Table 6.10 indicates strength development parameters for ordinary cured Portland cement. Other cement sorts follow roughly the same strength development, as the difference lies more in how quickly they harden and gain maturity. The named parameters cover the range from to

σ 28 MN/m2 60 55 50 45 40 35 30 25 20 15

α 0.46 0.44 0.42 0.41 0.40 0.38 0.37 0.35 0.34 0.33

τ 38 48 68 100 118 185 224 380 560 820

σ∞ 76 73 72 69 67 63 58 53 49 38

Table 6.10: Strength parameters

If Formula 6.25 were to be used in larger areas, one must expect different curvature parameters in the early phase and in the later progression.

In order to determine the time of stripping the formwork, there are a number of criteria that must be met, which include: safety against frost, collapse, deflection, mechanical damage, loss of moisture, colour variation, thermal cracks, and wind loads. The current available knowledge about the individual criteria will be examined below.

6.7.2 Freezing in connection with pouring of concrete A distinction is made between frost-proof concrete, which refers to the danger of freezing of water in the concrete at the early stages of the curing process, and frost-resistant concrete, which is related to long-term durability to the influence of frost. The latter is often improved by adding air-entrainers additives to the concrete.

Frost-proofing is about the free water in concrete not being exposed to temperatures below 0°C before the concrete has attained sufficient strength. When water freezes to ice, its volume is enlarged by approx. 9%. During freezing of water in concrete that has begun to cure, this volume expansion results in the still unfrozen water being squeezed away from the place where the formation of ice occurs. This causes a hydraulic pressure to occur that can rupture the cement paste, unless it has obtained sufficient strength to withstand this pressure.

Normally, it is said that concrete with a w/c-ratio less than 0.55 is secure against frost when it has a maturity of 15 hours, i.e., corresponding to it having been in a 20°C environment for a whole day. If the w/c-ratio is greater than 0.55, a slightly longer curing time is required.

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Figure 6.53 shows a more nuanced picture of the situation where the curve, on the basis of a series of experiments, provides a reasonable assessment of frost security in relation to the water/cement ratio.

6.7.3 Strength In the majority of cases, the same calculation methods can be used for the determination of safety against fracture during the stripping of the formwork as for the finished works. The dimensional load for decking is thus made-up of the dead load of the concrete and laid-up materials, and random loads – while simultaneously using the standard´s partial coefficients.

In beams, slabs or similar constructions with lax reinforcement, it is normally not the compressive strength that is decisive. The concrete´s tensile strength depends rather on factors such as shifting (displacement), cracking and the reinforcement´s anchorage.

Figure 6.53: Security against frost

For prestressed (pre-tensioned) concrete, the concrete´s compressive strength will often be decisive. When stripping the formwork, the same conditions essentially apply except that the strength development with time may be different for compressive and tensile strength. Usually, the tensile strength is somewhat faster developed than the compressive strength. At the same time, the tensile strength is not proportional to the compressive strength. These two factors counteract each other, so it is reasonable to assume that at an early stage that one can count on the tensile strength, which is related to compressive strength at the current age in the same manner as that prescribed at 28 days of age.

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If the structure´s load-bearing system at the time of stripping the formwork is the same as during the service of the structure, the required strength at dismantling is said to be proportional to the load. One gets:

In some cases, neither the concrete´s compressive strength nor its tensile strength is essential for the finished structure. A separate calculation can then give stripping times that are shorter than those given by Formula 6.26.

The method cannot be used if the structural system at stripping the formwork is different from that which applies to the finished construction. This applies, for example, to a continuous beam over two bays, where only one bay is cast in the first stage. Similarly, the method is not applicable if the load at stripping differs from that in the finished construction, or if there is no proportionality between the load and the required strength. The latter case applies for example to prestressed concrete.

6.7.4 Deflection If decking is loaded early, e.g., after seven days, its deflection would become greater than if it were to be loaded later, e.g., after 14 days. This is due in part to the concrete´s (tensile) modulus of elasticity increases with time in step with strength development. The greatest impact is, however, creeping. Loading after 7 days gives 30-40% greater creeping deformation than loading after 14 days.

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Figure 6.54: Deformation due to creeping

The conditions are illustrated in Figure 6.54, where stripping is done at two different periods, and . The momentary deformation, , is somewhat less than due to the coefficient of elasticity´s change.

Creeping for the early stripping is, in contrast, considerably larger. It may also happen that there are deformations after a certain point in time, which is essential, e.g., the point in time when a non-load bearing wall is erected on a cast concrete beam. Decisive for whether the wall is damaged by the beam´s deflection is the beam´s deformation after the wall has been completed. If it is assumed, in Figure 6.54, that the wall is erected at time t, it is seen that the early stripping of the formwork is best. Studies (Sadgrove) has examined deflection´s importance at early stripping of formwork under two circumstances:

1. < 6

2. > 6

In the first case, of the moment load is applied at the moment of stripping, and full moment load is applied after 28 days. The total deflection was thus increased by 18%.

In the second case, of the moment load is applied at the moment of stripping, increased to maximum moment corresponding to a deflection of 1/250 · L after approx. 7 days.

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The result was that if the beam was loaded when it had reached 13, 27, 40 and 67% of his strength, the total deflection was increased with, respectively, 25, 25, 9 and 3%. The following can be, therefore, concluded: a. As the dead load is rarely below 40% of the total load, and if the carrying capacity is sufficient at the moment of stripping, the deflection will presumably not exceed a 10% increase at early stripping over later stripping. b. Most beams and slabs are under reinforced and become, therefore, rigid faster than the calculations will show from one of the balanced cross sections. c. The concrete strength, and hence the stiffness, is usually greater than assumed, since the characteristic value rarely occurs. The failure criterion given in Formula 6.24 can also be used for deflection.

6.7.5 Mechanical adhesion If adhesion between the formwork and the concrete is greater than cohesion in the concrete, damage will occur during stripping of the formwork. The damage is independent of concrete proportioning. Experiments show that after 12 hours of curing at 20°C, there should be no risk of damage of this kind. Otherwise, a good starting point for stripping the formwork is an in situ strength of 2 N/mm2. There is provided that oiled moulds and formwork are used.

One strikes the formwork because it is usually too expensive to let shuttering be in place in order to protect a wall from further damage during construction.

Stripping of round-shaped total formwork can happen by lifting the whole mould straight up without disassembly. This can take place only when the slump has become 0, which it does after 3½ to 4 hours of maturity. If one gives the formwork/mould a bit of bevel/chamfer, the friction forces can be measured at 3.4 kN/m2 at the earliest possible stripping. For completely vertical sides, the same value can be achieved by lubricating very smooth form sides with a retarder.

6.7.6 Determination of the time for stripping the formwork

Unless otherwise stated in the project specifications, constructions that are effected by bending tension are usually struck after the concrete over the whole surface has attained a compressive strength of 10 MPa, and non-bending/tension effected constructions can usually be struck after the concrete over the whole surface has a compressive strength of at least 5 MPa.

When formwork is stripped, the protective effect of the mould around the concrete is removed.

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The time for stripping the formwork must be aligned with the required finishing of concrete, see 6.7.7.

Concrete´s age1) measured in In maturity hours2) for earliest removal of formwork/cover

Environmental Class: P M A E v/c > 0,55 15 - - -

0,55 ≥ v/c >0,45 15 36 - -

0,45 ≥ v/c >0,40 12 24 120 -

0,40 ≥ v/c 12 24 96 120

1) If setting starts later than 5 hours after mixing, the above maturity hours are extended correspondingly.

2) Documentation of the concrete´s maturity happens by measuring the concrete´s surface layer at a depth of 10 mm.

Table 6.11: Minimum duration for protection against drying-out.

6.7.7 Duration of protection against drying-out Formwork retains moisture. When the formwork is struck, other provisions must be made to retain moisture. The required protection against drying-out can be established by: • Allowing the formwork to be in place and covering the free concrete surfaces. • covering with vapour-proof sheets • maintaining a sufficiently high relative humidity in the environment • constant watering of the surfaces. However, alternating watering and drying-out of surfaces must not occur • use of sealant materials if these substances have an efficiency of 75%, as determined by Test Method TI-B33, unless otherwise required by the project specifications.

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