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Specification and Design of Commercial Concrete Slabs on Grade

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Specification and Design of Commercial Concrete Slabs on Grade 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 <insert name of company and report reference> and a copy of the report forms part of the contract documents. The target “K” value for the subgrade of this project is K = <insert value after discussing with geotechnical Engineer>. The slab design is to be based upon this value when preparing tender prices. Once the Contractor has been appointed, the K value is to be site checked in accordance with the following clauses. The slab design shall be modified if the K value varies significantly from that expected. The Contractor shall advise the Engineer within 5 days of the site testing and prior to placing any subbase. • 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.
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