SEMINAR SERIES 2016
Specification and design of commercial concrete slabs on grade
Presented by The New Zealand Concrete Society
Seminar Notes (TR62) Presented by The New Zealand Concrete Society
This Seminar is Made Possible By:
www.bosfa.com www.contech.co.nz
www.conslab.co.nz
Presenters:
Andrew Dallas Conslab Ltd
Rhys Rogers BBR Contech
Alan Ross BOSFA
Concrete slabs on grade constitute a significant portion of total build costs for commercial buildings, and the quality of the floor can have major implications on the productivity and efficiency of the finished building. There is an ever present demand to reduce project costs by minimising construction programmes and slab thicknesses, even as expectations for floor finishes and physical loadings increase. Consequently, optimising the design, layout and thickness of commercial concrete slabs on grade is essential. The design of commercial slabs on grade in New Zealand is usually conducted by specialist slab designers and constructors through a design and build process. The competitive design and build tender process is structured to obtain the lowest possible cost, and the construction industry tends to view concrete slabs as simple to design and construct, which often results in a lower priority being assigned to quality. Yet when concrete slabs on grade go wrong the costs are disproportionately large. Consequently, it is essential that those tasked with obtaining slabs on grade convey the correct design information and objectives to slab designers at the appropriate stages of the project to ensure that uncertainties and risks are managed, and a cost effective and fit for purpose slab is achieved. Understanding concrete floors and the risks and opportunities they provide should be fundamental to anyone involved in the construction of commercial and industrial buildings. Appropriate specification clauses are critical to reduce the risks associated with the construction of concrete floor slabs for the designer, contractor, suppliers and the end user. Under specification can lead to a slab which is not fit for purpose, while over specification can tie the hands of the slab designer resulting in unnecessary costs, and poorly considered clauses can lead to undesirable side effects. It is therefore important for all parties to understand what the risks are with specification clauses which appear regularly today. The Seminar will cover: An overview will be provided of the common types of commercial slab on grade in NZ, specifically fibre slabs, combi-slabs, post-tensioned slabs, and conventionally reinforced slabs. Design concepts, detailing and typical layouts will be discussed for each slab type. A worked example will be used to demonstrate the development of appropriate slab layouts for various areas of a typical commercial warehouse, and relevant considerations of specification and design will be discussed. The progression of a typical commercial building project will be outlined and the necessary information and decision making process required at each stage explained. Common floor finish and flatness specifications and their corresponding advantages and disadvantages will be covered. Considerations factoring into the plan layout of slabs will be discussed, along with detailing requirements for edges, doorways and trafficked joints and for adjoining foundation and building structural elements.
Contents
1. Cross section of a floor slab 1.1. Subgrade 1.2. Subbase 1.3. Slab 1.4. Surface
2. Floor slab types 2.1. Fibre slabs 2.2. Post-tensioned slabs 2.3. Conventionally reinforced slabs
3. Typical building layout considerations 3.1. Fibre slabs 3.2. Post-tensioned slabs
4. Design decisions
5. Progression of a construction project
Session 1
Cross Section of a Floor Slab
1. Cross section of a slab on grade
Andrew Dallas Technical Manager and Director Conslab Ltd.
Wearing Surface Concrete Slab DPM Blinding layer Subbase Subgrade
2
1 Sub-Grade • Should provide uniform support • so hard or soft spots removed • Consideration of settlement – Due to differences in the subgrade – Restraint from piles – Could check for the stresses induced by rotations but generally if you have differential settlement you will have cracking.
Makes little change to the slab thickness Significant cause of slab failures
Designed using the Modulus of Subgrade Reaction “k”
Confirmation of Properties • NZ: CBR or Benkleman beam • TR34: Plate bearing test (750mm) • For heavily loaded slabs we recommend having a geotechnical engineer recommend the k value
2 Source: Conslab from a report by OPUS
Source: Extract from TR34 4th Ed
3 Specification Clause
• the sub-grade or sub-base is tested using a Benkelman beam at 5m centres followed by a proof roll to identify weak spots. 90% of readings to be within the deflection limit and no reading greater than the maximum deflection limit.
CBR% 90% Deflection Limit mm Maximum Reading 3 4.1 5.0 4 3.1 4.0 5 2.5 3.5 10 1.2 2.0 15 0.9 2.0
Source: Conslab Contract Documents
– TESTING OF THE SUBGRADE • A geotechnical investigation has been carried out by
• Subgrades in Cut • The Contractor shall cut the site to approximately 150mm above Subgrade level and at this stage the Contractor shall arrange for the Geotechnical Engineer to carry out Benkelman Beam tests or other approved testing to determine the actual K value of the Subgrade and provide the results to the Engineer for approval. Following approval the final trimming to subgrade level shall be completed.
• Subgrade in Fill • Where the site is to be filled then the compacted engineered fill material will be placed up to the design subgrade level. At this stage the Contractor shall arrange for the Geotech Engineer to carry out Beam testsor other approved testing to determine the K value of the subgrade. Provide the results to the Engineer for approval. Following approval the final trimming to Subgrade level shall be completed.
Specification clauses courtesy of Conslab, MSC Consulting Engineers and Jawa Consulting Engineers
4 Wearing Surface Concrete Slab DPM Blinding layer Subbase Subgrade
9
Sub-base • Provides a working platform for construction • Provides a level formation • Transmits load to subgrade • Granular material , min thickness 150mm • Not used for enhancement of modulus of subgrade reaction • Proof roll to ensure no obvious soft spots
5 Wearing Surface Concrete Slab DPM Blinding layer Subbase Subgrade
12
6 • Blinding layer • Closes off any rough spots in the subbase • Allows for finer tolerance on the subbase level • Depth of Sand 0-5mm max • Depth of PAP7 or GAP7 up to 20mm
Specification Clause
– SUB-BASE PREPARATION • The basecourse shall be compacted in accordance with the <“Earthworks” or “Siteworks”> Section of the General Specification. • The basecourse layers will be suitable hardfill graded and compacted to provide a closed even surface not prone to rutting. • The top surface of the basecourse will be placed to a levels tolerance of +0mm or - 10mm. A certificate and levels plan is to be provided by a Registered Surveyor engaged by the Contractor certifying that these tolerances have been achieved. A copy of these reports is to be provided to the Engineer for review.
7 Wearing Surface Concrete Slab DPM Blinding layer Subbase Subgrade
15
DPC is there to stop moisture rising through the floor Two layers for PT and jointless fibre The second layer is simply to reduce friction
8 Specification Clause
– Damp Proof Membrane • Under all ground floor slabs provide an effective DPM shown to have a water flow resistance of not less than 90 meganewton seconds per gram. • Side laps and end laps shall be taped and sealed in accordance with manufacturers instructions
Wearing Surface Concrete Slab DPM Blinding layer Subbase Subgrade
18
9 Concrete Mixes • The key is consistency! • Slump and set times • Simplicity generally leads to success • Avoid superplasticisers • Minimum cement for abrasion • Don’t get hung up on minimum slumps • Consider how to get the concrete placed • No air entrainer.
19
10 Need to consider the practical aspects:
• How are you going to get the concrete into the slab? • Leave panels off • What is the access like for the finishing equipment? • Can you hang the finishing equipment off columns? Access for the workers?
Specification Guide (not clauses) – The key is consistency – must allow controlled water addition to trucks below the target slump – Specify the strength required, any minimum cement content. – Any other durability requirement. Generally nothing – Specify the testing required. – Do not specify no pump mixes allowed – Do not specify air entrainment (unless outside a freezer roller door) or superplasticisers – There are no such things a high grade or special grade plants. “Concrete must come from a plant which hs been audited as meeting the requirements of NZS3104”
11 Wearing Surface Concrete Slab DPM Blinding layer Subbase Subgrade
23
Abrasion • Finishing, curing, strength in that order • Around 300- 320kg cement • toppings /dry shake
• Most commercial/industrial is 35-40 MPa 40 MPa will have adequate cement
12 13 Specification Clause Surface Finish
– Surface finish and surface tolerances need to be separated
Abrasion Requirement The concrete is to have a minimum cement content of 325kg. The surface is to be repeat power trowelled to a dense burnished finish and then water cured for 7 days.
(Information Note: Do not specify to AR1 or other finishes as per NZS3101 as there is very limited capacity to measure this.)
Concrete Finishes The class of finish to concrete surfaces shall comply with NZS3114. Floor tolerances are specified under tolerances. Any defects to the concrete shall be made good at the Contractors expense and in a manner agreed with the Engineer.
14 15 Specification Clause - CURING • The slab shall be water cured for a minimum of 7 days. The contractor shall provide a method statement as to how this will be achieved. Alternatively the Engineer may approve curing using a membrane forming compound that complies with the requirements of NZS3109:1997. • Should a membrane curing system be approved then it shall be compatible with future painted line markings or any other surface coating required for this floor.
16 Session 2
Floor Slab Types
2. Floor slab types
Specification and design of commercial concrete slabs on grade
2.1 Fibre reinforced slabs
Alan Ross Sean Page-Wood CPeng, IntPE General Manager or Business Development Manager BOSFA BOSFA
Specification and design of commercial concrete slabs on grade
1 PRESENTATION OVERVIEW • Slab usage and requirements • Different types of loads • Ultimate Limit State design • Fibre types • Joints vs. controlled cracking, Slab types and Detailing – Part 2
Specification and design of commercial concrete slabs on grade
Normal Busy Warehouse
Specification and design of commercial concrete slabs on grade
2 Bulk Storage Warehouse
Specification and design of commercial concrete slabs on grade
Specification and design of commercial concrete slabs on grade
3 Maintenance / industrial
Specification and design of commercial concrete slabs on grade
Automated warehouse
Specification and design of commercial concrete slabs on grade
4 Retail
Accelerated Bridge ConstructionSpecification in and design of Seismiccommercial Areas: concrete slabs on grade design detail of
Container Pavements
Specification and design of commercial concrete slabs on grade
5 PRESENTATION OVERVIEW • Slab usage and requirements • Different types of loads • Ultimate Limit State design • Fibre types • Joints vs. controlled cracking, Slab types and Detailing – Part 2
Specification and design of commercial concrete slabs on grade
Loads - Containers ◦ Are not UDL’s but point loads ◦ How close can they stack? ◦ How many high ◦ 20’ or 40’ ◦ Maximum weights?
Specification and design of commercial concrete slabs on grade
6 Loads - Containers Handling Equipment
Vehicles
Straddle Carrier Reach Stacker
Specification and design of commercial concrete slabs on grade
Loads
Specification and design of commercial concrete slabs on grade
7 Warehouse - Nasty Fork Lifts
Type of Wheels? Tyre Pressure? Number of tyres? Repetitive zones? Axle loads?
Specification and design of commercial concrete slabs on grade
Warehouse - Distributed Loads
Typically 30 to 40 kPa Can Govern with pattern Value used for marketing by real estate agents
100 % 50% UDLUDL 0%
Specification and design of commercial concrete slabs on grade
8 Warehouse - Point Loads (Racking)
• Typically 5 to 9 tonnes • Recommend 7 Tonnes min. • The double load in the middle is nasty • Spacing is important • Normally governs
Specification and design of commercial concrete slabs on grade
Automated warehouse
Specification and design of commercial concrete slabs on grade
9 Point Loads - Racking
Leg Spacing? Size?
Specification and design of commercial concrete slabs on grade
Loads
An accurate loading specification is critical: Construction loads Racking layout Forklift types Containers Trucking Other…
Accelerated Bridge ConstructionSpecification in and design of Seismiccommercial Areas: concrete slabs on grade design detail of
10 Examine all scenarios
Centre Dowelled Edge Double Loading Corner Loading Edge Loading Loading
Specification and design of commercial concrete slabs on grade
Specify the loads clearly
Specification and design of commercial concrete slabs on grade
11 PRESENTATION OVERVIEW • Slab usage and requirements • Different types of loads • Ultimate Limit State design • Fibre types • Joints vs. controlled cracking, Slab types and Detailing – Part 2
Specification and design of commercial concrete slabs on grade
Steel Fibre Reinforced Concrete (SFRC)
Fibres – like all reinforcement – become effective AFTER cracking develops
Specification and design of commercial concrete slabs on grade
12 SFRC properties
Increase toughness – post crack / residual strength
Test set-up Plain concrete Steel Fibre concrete beam-test
no ductility high ductility
Specification and design of commercial concrete slabs on grade
SFRC properties • Residual flexural strength • Re3 value = residual flexural strength / concrete flexural strength Test set-up Plain concrete Steel Fibre concrete beam-test
no ductility high ductility
Specification and design of commercial concrete slabs on grade
13 ULS design – Plain Concrete No Cracks No Cracks
Cracking okay
Specification and design of commercial concrete slabs on grade
ULS design - SFRC No Cracks No Cracks Add capacity to the slab with SFRC Cracking okay
Specification and design of commercial concrete slabs on grade
14 ULS pavement design guides
TR34, CUR (Holland), others
Specification and design of commercial concrete slabs on grade
Specification and design of commercial concrete slabs on grade
15 Loads and material factors
Ultimate limit state (ULS) Load factors dependent upon type of load ◦ Permanent actions e.g. racking 1.2 ◦ Variable actions 1.5 ◦ Dynamic effects 1.4+ Material reduction factor ◦ Generally divide characteristic strength by 1.5 Serviceability limit state (SLS) is avoidance of cracking Safety factor on actions becomes 1.0
No specific SLS design carried out for most SOG. Checks can be for temp gradient, shrinkage stress and diff deflection…..but all overridden by detailing and experience to avoid service cracking
Specification and design of commercial concrete slabs on grade
SLS design - Jointed concrete slabs
• Traditional pavements are based on experience to avoid cracks: • limited joint distance • saw-cut or saw-cut free floors in indoor environment • saw-cut floors in outdoor environment • no connections to fixed points • rectangular field shape • length/width ratio ≤ 1:1.5 • minimum thicknesses • etc.
Specification and design of commercial concrete slabs on grade
16 PRESENTATION OVERVIEW • Slab usage and requirements • Different types of loads • Ultimate Limit State design • Fibre types • Joints vs. controlled cracking, Slab types and Detailing – Part 2
Specification and design of commercial concrete slabs on grade
Fibre types Quality ◦ Fibre properties that influence performance ◦ CE documentation ◦ Specifications…
Specification and design of commercial concrete slabs on grade
17 Fibre properties that influence performance high length-diameter ratio A (L/D ratio) A system of glued fibre bundles enables fibres with a high L/D ratio to be mixed easily and uniformly throughout the Hooked ends concrete
High tensile strength steel
Controlled pull-out (due to deformation of the hook)
Specification and design of commercial concrete slabs on grade
DoP reference
Declared values
Minimum fibre dosage to achieve nominated post crack strength
Specification and design of commercial concrete slabs on grade
18 Steel Fibre specification
Steel fibres manufactured in accordance with EN14889-1 CE label and Declaration of Performance provided to the project engineer Design provided by flooring designer or, specify a specific fibre type and dosage
Specification and design of commercial concrete slabs on grade
Thank you and Questions.
Specification and design of commercial concrete slabs on grade
19
2.2 Post-tensioned slabs on grade
Rhys Rogers Structural Engineer – R&D and Design BBR Contech
Specification and design of commercial concrete slabs on grade
Post-tensioned slab on grade
• Definition, applications & advantages • Typical layouts • Elements of a PT slab • PT slab design for mobilisation • PT slab design for loadings (thickness)
Specification and design of commercial concrete slabs on grade
1 Post-tensioned slab on grade
Specification and design of commercial concrete slabs on grade
Post-tensioned slab on grade
Specification and design of commercial concrete slabs on grade
2 Post-tensioned slab on grade
• PT stress puts concrete in compression
– PT stress prevents shrinkage cracking by allowing slab to shrink as one unit
– Enhances load bearing capacity
– Causes any cracking that does occur to tend to remain narrow or to close up
Specification and design of commercial concrete slabs on grade
Post-tensioned slab on grade
• PT provides resilience and robustness – Allows slab to recover from minor overloads – Causes any cracking to tend to remain narrow or close up • Can accommodate moderate levels of differential settlement, and deformation
• Minimises interference with pouring and finishing – Gives placers best opportunity to achieve finish & flatness requirements
Specification and design of commercial concrete slabs on grade
3 Why select a PT slab?
• Increased joint spacing – Construction joints only, no sawcuts • Minimises shrinkage and flexural cracks • Lower life-cycle cost – Fewer joints to maintain – Better durability • Enhanced serviceability – Minimum down-time for joint repair/maintenance ACI 360R-10 Guide to design of slabs-on-ground
Specification and design of commercial concrete slabs on grade
Why select a PT slab?
• Better preservation of floor flatness by minimising joints • Decreased slab thickness • Increased load strength • Resilience and recovery capability from overloading
ACI 360R-10 Guide to design of slabs-on-ground
Specification and design of commercial concrete slabs on grade
4 Post-tensioned slab on grade
• Definition, applications & advantages • Typical layouts • Elements of a PT slab • PT slab design for mobilisation • PT slab design for loadings (thickness)
Specification and design of commercial concrete slabs on grade
Typical layout of PT slab • Objective is to minimise joints – Fewer joints results in better operational productivity and lower life cycle maintenance • Primary limitation on joint spacing is max. pour size – single pour usually <≈ 2500-3200m2
• No sawcuts or joints required within each pour
Specification and design of commercial concrete slabs on grade
5 Typical layout of PT slab • Multiple pours can be post-tensioned together • Day joint between 2 slabs (allows up to 6000m2) • Coupled joints can join more than 2 slabs • Creates tight/closed joints – Good durability, minimal impact on forklifts – No joint armouring required • Allows very large slabs with no opening joints – Shrinkage movements restricted to slab edges
Specification and design of commercial concrete slabs on grade
Multiple pours post-tensioned as one slab (Day joint)
Day 1 Day 2 Pour Pour PT tendons PT tendons stressed after second pour
Specification and design of commercial concrete slabs on grade
6 Multiple pours coupled together
Pour 3 Pour 1
Stress 1 Stress 3 and 4 Pour 4 Pour 2
Stress 2
Specification and design of commercial concrete slabs on grade
Tight construction joints • No armouring • Durable joint • Minimal impact on forklifts operations • Clients should expect some minor cracking around joints, these don’t usually affect slab performance
Specification and design of commercial concrete slabs on grade
7 Post-tensioned slab on grade
• Definition, applications & advantages • Typical layouts • Elements of a PT slab • PT slab design for mobilisation • PT slab design for loadings (thickness)
Specification and design of commercial concrete slabs on grade
Elements of a PT slab Edge Reinforcement
Casting
Strand Onions
Strand
Grout Vents
Spiral reinforcement
Ducting
Duct Chairs
Barrels & Wedges
Specification and design of commercial concrete slabs on grade
8 Elements of a PT slab
Specification and design of commercial concrete slabs on grade
Elements of a PT slab
Specification and design of commercial concrete slabs on grade
9 Elements of a PT slab
Specification and design of commercial concrete slabs on grade
Stressing access: Infill strips
• Nominally 1m wide infill strip provides access for stressing • Most flexible way to provide access • Usually tied to slab with rebar so opening joint is at wall • Adds another tight joint, but usually hidden by racking
Specification and design of commercial concrete slabs on grade
10 Stressing access: Pockets
• 300x110mm pockets cast into wall panels • PT Slab extends to wall • Must isolate slab from panels and columns • PT slab design required before PC panel production begins
Specification and design of commercial concrete slabs on grade
Stressing access: Pockets
Specification and design of commercial concrete slabs on grade
11 PT slab design concepts
• Design for mobilisation (concrete shrinkage) – PT design – Detailing – Early shrinkage & Relative shrinkage
• Design for loading – Design methodology – Types of loading – Sensitivity to different types of load
Specification and design of commercial concrete slabs on grade
Post-tensioned slab on grade
• Definition, applications & advantages • Typical layouts • Elements of a PT slab • PT slab design for mobilisation • PT slab design for loadings (thickness)
Specification and design of commercial concrete slabs on grade
12 PT design for mobilisation • Avoidance of Shrinkage cracks – The PT is designed to be sufficient to overcome subgrade friction and allow the entire slab to shrink as one unit
– Restraint of this movement will result in cracking (detailing critical)
– Concentrates shrinkage gaps at slab edges (can be large for long slabs)
Specification and design of commercial concrete slabs on grade
PT design for mobilisation
• Shrinkage starts when concrete begins to harden • Nothing to prevent cracking before PT is applied • Rate of shrinkage drops off quickly with age
– Our observations indicate shrinkage movement occuring up to 2 years age
– Minimal shrinkage occurs after 2 years
– Restraint over first six months most likely to cause problems
Specification and design of commercial concrete slabs on grade
13 PT design for mobilisation
• At any given section:
FPT >= Ffriction • Friction force prop. to weight of slab and
applicable loadings End Stress Dead End Dead • PT force varies along length of tendon
Specification and design of commercial concrete slabs on grade
PT design for mobilisation
• Mobilisation equation at given section
ntendons * Ftendon >=
Aslab to be mobilised * μsubgrade *
(UDLslab weight + UDLapplicable loads)
Specification and design of commercial concrete slabs on grade
14 PT design for early shrinkage
• Initial PT force must be applied ASAP – Must prevent shrinkage cracking from starting (<=24 hrs after pour)
• Allowable PT force is limited by early concrete strength – cylinders required for initial stress and final stress
• Initial PT force must be sufficient to overcome subgrade friction and allow slab to shrink as one unit
• Only need to mobilise slab weight (no loads)
Specification and design of commercial concrete slabs on grade
PT design for early shrinkage
• At application of initial PT force – No load on slab, friction due to slab weight only – Friction coefficient is large • Friction coefficient tends to reduce as shrinkage movement occurs, worst case is for initial movement – Initial load varies with slab size and layout, usually about 1/3 to ½ of final PT force
Specification and design of commercial concrete slabs on grade
15 PT design for mobilisation (medium to long term) • More variables to consider – Greater PT stress available to mobilise – Must mobilise slab & realistic proportion of loading
• Long term, slab movement must not be restrained to allow thermal movements
Specification and design of commercial concrete slabs on grade
PT design for mobilisation (medium to long term)
• Med-long term mobilisation is usually not critical
• Should consider when the following coincide: – Long slabs or multiple coupled slabs (>≈120m) – Early application of heavy UDLs (<≈ 6 to 12 months) – Limited load cycling is expected
Specification and design of commercial concrete slabs on grade
16 PT design for mobilisation (medium to long term) • When we do consider med/long term mobilisation, we need to get an accurate picture of loading – Full UDL unlikely to apply to full floor area • Unloaded aisles and forklift manoeuvring areas • What proportion of full UDL is realistically expected in loaded areas? – True UDL can be as low as 50% of design UDL
• Will the load be cycled? – Shrinkage movement can occur piecemeal when loads are low
Specification and design of commercial concrete slabs on grade
PT design for mobilisation (medium to long term) • Design options – Increase PT stress • More PT gives a more resilient slab over all • Greater PT can allow thinner slab – Reduces friction force and increases PT force
– Divide the slab with armoured opening joints • Limits slab length and required mobilisation force • Also reduces magnitude of shrinkage movement – Easier to detail for gaps at slab edges and doors
Specification and design of commercial concrete slabs on grade
17 Differential shrinkage
Specification and design of commercial concrete slabs on grade
Differential Shrinkage
• Slab 1 is poured & begins to shrink
• Slab 2 is poured & wants to shrink faster/further
• Slab 2 is restrained by contact with slab 1
Specification and design of commercial concrete slabs on grade
18 Differential Shrinkage
• Total shrinkage for each slab will be the same
• Say 20% of shrinkage occurs before slab 2 is poured
• The 20% differential is permanent at the joint
• 0.2*750με = 150με = 0.15mm/m = sum of crack widths /m
Specification and design of commercial concrete slabs on grade
Layout and sequence
1. Layup slabs 1A & 1B 3. Pour 1B (day after) 5. Layup slab 2 4. Initial stress 1A & 1B 6. Final stress 1A & 1B 8. Initial stress 2 9. Final stress 2 7. Pour slab 2 2. Pour slab1A (2 weeks after 1B)
Specification and design of commercial concrete slabs on grade
19 Differential shrinkage
Slab 1A Slab 2A
Slab 1B shrinkage Slab 2B 2 week differential differential week 2 4 week differential 4 week differential shrinkage shrinkage Slab 3A Slab 4A
Slab 3B shrinkage Slab 4B 2 week differential differential week 2
Specification and design of commercial concrete slabs on grade
Differential Shrinkage
• Expect some cracking on coupled joints • Aim is to end up with many narrow cracks – Good detailing, construction programming, concrete, curing, PT processes required
• Mitigate risk of undesirable cracking by: – Limit time between adjacent pours – Use concrete with low early age shrinkage properties – Apply PT stress before shrinkage begins so that joint remains tight – Detailing • PT couplers must transfer PT stress all the way to edge of first slab • Rebar in relevant locations to maintain small crack widths
Specification and design of commercial concrete slabs on grade
20 Post-tensioned slab on grade
• Definition, applications & advantages • Typical layouts • Elements of a PT slab • PT slab design for mobilisation • PT slab design for loadings (thickness)
Specification and design of commercial concrete slabs on grade
PT slab design for loading
• Loading design concepts
• How to consider common loading types – Rack loading – Axle loads • Key design information / Sensitivity
Specification and design of commercial concrete slabs on grade
21 PT slab design concepts
• Design objective: – Slab remains crack free during normal operation (governs design)
• Slab is allowed to crack under minor overloads – SLS EQ or SLS overload cases – PT maintains small crack widths which close up when load is removed
Specification and design of commercial concrete slabs on grade
PT slab design concepts
• Not designed as a “crack free slab” – Client expectation is often greater than reality
• Need to manage this to avoid disappointment • PT slabs designed for water tightness have 2-3 times more PT stress than average PT slab on grade • Minor cracking at interfaces due to mobilisation and differential shrinkage can’t be prevented by more PT
Specification and design of commercial concrete slabs on grade
22 PT slab design concepts
• NZS1170 limit states don’t directly apply – ULS • On the ground so slab can’t collapse • Must maintain integrity to prevent racking collapse – Resilient nature of PT slabs maintains bearing ability – Uplift forces restricted by yielding baseplates – SLS • Cracking is permitted under NZS1170 SLS cases • PT Slab designed not to crack under normal operation • Small crack widths under SLS EQ/overload
Specification and design of commercial concrete slabs on grade
PT slab design concepts
“Avoid the use of multiple factors of safety. Post-tensioning provides reserve capacities, and factors of safety greater than those for nonprestressed slabs should not be used. Cracking under the concentrated load is permissible for post-tensioned slabs, and it can be taken into account by using structural design requirements of ACI 318.”
ACI 360R-10 Guide to design of slabs-on-ground
Specification and design of commercial concrete slabs on grade
23 Loading design methods • Westergaard method • Other design approaches – Graphical methods (based on Westergaard) • PCA (Portland Cement Association) • WRI (wire reinforcement institute) • COE (Corps of Engineers) – Finite Element Modelling – can be sensitive to interpretation/experience (rubbish in rubbish out!) – Proprietary slab design software
Specification and design of commercial concrete slabs on grade
Westergaard slab design • Westergaard model is most common – Most design standards suggest this approach – CCANZ design guidance is available – Relatively simple to automate in a spreadsheet
– Based on un-reinforced slab design • PT stress is added to the tensile strength of concrete • Design variables: Slab thickness & PT stress
Specification and design of commercial concrete slabs on grade
24 Westergaard slab design
Flexural stress from loading <= Flexural tensile strength of concrete
• Accounts for fatigue and time to loading with factors on flexural tensile strength
Specification and design of commercial concrete slabs on grade
Westergaard slab design
• Considers subgrade as a winkler spring bed – Modulus of subgrade reaction = k • Often given as N/mm2/mm or MN/m3 (misleading) • Measure of pressure per unit displacement (kPa/mm)
• Usually have to convert from CBR which can be inaccurate • Ideally Geotech engineers should do conversion – Provide k value for design, – Benkelman beam test targets for construction
Specification and design of commercial concrete slabs on grade
25 Westergaard design for loads
• UDL loads not usually critical – Evenly loaded floor has no flexural stress in slab – Aisle loading rarely governs
• Westergaard formulae deal with point loads – Equations for interior, edges and corners – Stresses from multiple loads must be combined
Specification and design of commercial concrete slabs on grade
Westergaard PT slab design
• Stress due to interior load P
l is dependant on slab depth, typically 0.5 to 1m
Specification and design of commercial concrete slabs on grade
26 Westergaard PT slab design
• Stress due to single point load depends on: • P = Load magnitude • h = Slab depth (cubed, big influence) • k = Modulus of subgrade reaction • μ = Poisson ratio of slab (0.15 static, 0.2 dynamic) • b = loaded radius
Loaded radius is calculated based on post area for rack loads. Baseplates are too thin to spread load
Specification and design of commercial concrete slabs on grade
Westergaard PT slab design
• Combination of loads – When two loads are close together (<2h) • combine loads (2*P) • Increase loaded radius (r) – When two loads are further apart (>2h) • combine stresses in x & y directions at critical point using graph
Specification and design of commercial concrete slabs on grade
27 Loading types
• Rack loading • Forklifts (MHE)
• Other types of loading – Container loads / Swinglifters – Truck loading
Specification and design of commercial concrete slabs on grade
Back to back rack loading 300mm B2B rack
Large B2B spacing Aisle Feet considered separately (1xP) Less severe load case
200mm B2B rack
Small B2B spacing Aisle Feet combined (2xP) More severe load case
Edge rack
Specification and design of commercial concrete slabs on grade
28 Back to back rack loading
Aisle
Sensitive to: 1. Post load Aisle 2. B2B spacing 3. Post size 4. Short leg spacing
Specification and design of commercial concrete slabs on grade
Double deep racking
Aisle
Double deep B2B racking
Aisle
More Critical than B2B racking
Specification and design of commercial concrete slabs on grade
29 Shuttle Racking
• Approximately square grid, 1.1-1.6m spacing • Generally less critical than B2B racking • No B2B posts
Specification and design of commercial concrete slabs on grade
Rack loadings
• Slab design & thickness is sensitive to: – Post load – Racking type (B2B, double deep, shuttle) – Back to back rack spacing (clear spacing) – Short direction post spacing (c/c or outside dim?) – Post size (not baseplate size)
– Modulus of subgrade reaction (k)
Specification and design of commercial concrete slabs on grade
30 Forklift Loadings
Specification and design of commercial concrete slabs on grade
Forklift loadings
• Machine + load weight balanced on front axle • Reasonable factor for impact and uncertainty
Specification and design of commercial concrete slabs on grade
31 Forklift loading
• Loaded radius based on tyre pressure – Pneumatic tyres (700-1000 kPa) – Solid “Cushion” tyres (1200 to 1600kPa) – Solid rubber tyres (1700-1800kPa) – Solid polyurethane wheels (>6900kPa)
Specification and design of commercial concrete slabs on grade
Forklift loading
• Slab design and thickness is sensitive to: – Axle load / rated capacity of forklift – Axle width – Single or double wheeled axle – Tyre type / pressure
– Modulus of subgrade reaction (k)
Specification and design of commercial concrete slabs on grade
32 2.3 Traditional mesh slabs
Andrew Dallas Technical Manager and Director Conslab Ltd.
Traditional Mesh Slabs • Traditional mesh slabs are essentially unreinforced when it comes to structural design. • Top Mesh is in for shrinkage control purposes • Or you can undertake a mesh design with mesh in the bottom of the slab which does add to the structural strength See TR34
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