Post Tensioned Concrete Pavements - .meters of post-tensioned slabs and the majority of the major

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  • Post TensionedConcrete Pavements


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    THE COMPANY PT-Pave is a dynamic venture between Amsteele Systems and Chris Howes Construction, aimed at providing a complete package/one-stop-shop for post-tensioned concrete pavements, also known as post-tensioned slab on grade. Amsteele Systemss beginnings date back to the late 1960s, with CCL (SA) being the forerunner for Amalgamated Prestressing, who merged with Steeledale Systems in 2000 to form Amsteele Systems. Millions of square meters of post-tensioned slabs and the majority of the major highway bridges in South Africa, and beyond, have been prestressed using systems licensed to Amsteele Systems and its predecessors. Amsteele Systems have built up a reputation for excellence in the post tensioning industry and are market leaders in this field in Southern Africa. Founded in 1993, Chris Howes Construction (CHC) has become known for delivering products and service of the highest quality. Identifying the niche of concrete flooring within the construction industry CHC-SA has evolved as national leaders in this field since 1993, delivering flooring solutions that are recognised to be of the best quality on an international standard. Utilising state-of-the-art equipment, maintaining constant supervision and on hand labour, CHC-SA has achieved preferred supplier status among organisations like MASS MART (Makro, Game & Builders Warehouse) and the JD Group.

    TAILORED SOLUTIONS Post tensioning of concrete pavements, for both internal and external use, have provided many developers and clients across the globe with a cost effective concrete pavement solution for more than 40 years. Benefits realised with post tensioned concrete pavements include:

    Large joint free slab areas Reduced long term maintenance due to the reduction of joints Reduced construction costs Reduced subgrade preparation Highly effective on poor ground conditions Accelerated construction Increased load capacity Reduced permeability

    Some of the more common applications for post-tensioned concrete pavements include:

    Distribution facilities (internal and external pavements) Warehouses (internal and external pavements) Factories (internal and external pavements) Bulk storage facilities (internal and external pavements) Container storage and handling facilities Aircraft hangers Airport apron slabs Indoor and outdoor sports courts Freezer stores Raft slabs Floor slabs for water retaining structures

    Residential PT raft slab with un-bonded PT





    PT Pave provides a complete package to clients, from

    schematic design, to detail design as well as site construction of

    internal and external post-tensioned concrete pavements

    All designs and site works are

    covered by standard construction industry warranties, such as PI

    and defects liabilities

    External PT concrete pavement under construction with bonded PT tendons


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    INDUSTRIAL CONCRETE PAVEMENT REQUIREMENTS An industrial concrete pavement should be free of excessive cracking, suitably flat and durable. To achieve this requires interaction between the slab designer, end user/owner, geotechnical specialist and contractor PT Pave considers it important to consider the following factors:

    Environmental conditions Site geotechnical conditions Imposed loads on the pavement, including traffic loads Slab thickness the reinforcement requirements Joint positioning Slab construction methodology

    THE SYSTEM Post tensioning in concrete pavements has two functions, firstly to counteract short term shrinkage cracking from about 18 24 hours onwards, then to pre-compress the concrete to counteract cracking due to ongoing long term shrinkage, flexural tensile stresses from service loads and temperature gradients in the concrete. The post-tensioning used can be a bonded or un-bonded flat slab system with the basic methodology as follows:

    Slabs are cast with high tensile pre-stressing strands running through the slab from edge to edge, at regular spacings,

    Tendons are un-profiled (flat) in one layer, with equal or close to equal quantities in orthogonal directions,

    Tendons are anchored at the slab edges and stressed after concrete placement, thereby placing a permanent two-way compression force on the slab,

    If a bonded PT system is used, this force is then locked in by grouting the ducts with a high strength grout which also acts as corrosion protection,

    Tendons can be stressed continuous between slab panels by using coupling or intermediate stressing anchors, which allow the total elimination of movement joints between panels as adjacent panels are pulled together through the post-tensioning process,

    This can result in 1000s of square metres floor slabs without movement joints. Careful planning of the floor construction methodology is required, with the final slab edges requiring special consideration.

    ELIMINATE JOINTS Factors affecting industrial and warehousing facilities are:

    Temperature variations High imposed loads - racking & traffic from forklifts and trucks, High abrasion due to the above

    Consequences of joint and floor failures in industrial concrete pavements are:

    Greater requirement for costly joint maintenance Slabs require repair due to joint failures Increased maintenance cost to equipment (forklifts and vehicles) Loss of productivity in the facility

    Post-tensioned concrete pavements are used in facilities where the main objective is to eliminate joints that are the major weakness in any concrete pavement. The post-tensioning system allows a significant reduction in the number of joints while keeping the structure within allowable tensile stresses, which leads to lower maintenance costs over the design life of the slab.


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    THE BEST SOLUTION Concrete has limited capacity to resist tensile stresses. For conventional reinforced concrete slabs or slabs with steel fibre reinforcement, the thickness and primary reinforcement are increased so that the stresses do not exceed the concrete tensile strength and satisfy the design code requirements. Post-tensioning allows balancing of the tensile stresses in the concrete, leading to thinner slabs without the need to significantly increase the amount of reinforcement. The major advantages of using a post-tensioned slab on grade solution compared to other ground slab systems is as follows: Effective load support It is a properly engineered floor designed for any load system Active crack control - Post-tensioned slabs reduce the risk of cracking far more effectively than any other method due to the active compression force exerted on the slab Fewer or no joints - Use of a post-tensioned slab on grade solution greatly reduces the joints required in a floor, which results in reduced maintenance Curling - The risk of slab curling is greatly reduced with fewer joints and greater joint spacing. This produces a smoother ride and less maintenance for forklifts Poor ground - Greatly increased resistance to problematic soils. Reduced sub-grade preparation and a thinner slab with fewer or no joints save on time and cost during construction. Post-tensioning achieves the highest efficiency, based on:

    The external effects that apply to the structure (including temperature, temperature differential and loadings)

    The internal effects such as concrete shrinkage The size of the structure The quality of the supporting ground

    DESIGN CONSIDERATIONS Aspects considered during the design process include:

    External design loads, All external design loads are catered for in the design, such as racking, wheels, containers, bulk materials, load cycles etc.

    Internal and external thermal effects Daily ambient temperatures lead to temperature gradient stresses through the slab depth, which need to be accounted for in the design. Typical gradients of 0.02C/mm and 0.04C/mm are used at times for internal and external slabs respectively, which can cause bottom fibre tensile stresses that are additional to the stresses caused by external design loads


    PT places a two way compression force on slab

    Internal PT pavement with a bonded PT system

    prior to concrete placement


    Slabs are analyzed and designed using Finite Element modeling of the slab and subgrade


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    Subgrade friction Elastic and shrinkage movements give rise to frictional restraint stresses between the slab and the sub-grade. The typical design friction coefficient for concrete laid on a plastic membrane over clean sand bedding is between 0.5 and 0.6 Sub-base parameters A typical PT-Pave design will include a detailed analysis of the slab supported by the underlying sub-base. Modeling of the sub-base will typically be based on geotechnical data such as California Bearing Ration (CBR) and the modulus of subgrade reaction etc.

    CONSTRUCTION CONSIDERATIONS The main aspects to consider are Pour size The main constraint is the capability of concreting and finishing of the concrete in a day. Pour sizes of between 1500m2 and 2000m2 should typically be considered and planned, although pour sizes in excess of 5000m2 have been done Pour sequence The sequence of slab pours and their stressing requirements require careful planning and should be optimized for the best program outcome Curing and protection Due to the larger pours, PT pavements are susceptible to initial shrinkage effects, and it is therefore important to cure and protect the slab from extreme condi