Chapter 6 Formwork

Chapter 6 Formwork 6.1 Introduction As a rule, formwork constitutes one third of the costs of reinforced concrete, but can be more ex- pensive 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 in- fluence 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 expen- sive 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. 419 • 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 . 420 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 421 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. 422 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 423 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. 424 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.

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