Slab on Grade. Introduction CHAPTER 10

Transcription

1 CHAPTER 10 Slab on Grade Introduction A concrete slab on grade is a level surface of concrete that lies directly on the ground. Slabs on grade are used for roads, sidewalks, patios, airport runways and basements or ground floors of buildings. Building Construction Lectures-2 nd Stage Page 92

2 Components of slab on grade 1. subbase as the drainage layer 2. vapor retarder 3. layer of fined stone 4. weld wire fabric 5. concrete Subbase as the drainage layer To prepare for the placement of a slab on grade, the unstable topsoil is scraped away to expose the subsoil beneath. If the exposed subsoil is too soft, it is compacted or replaced with more stable material. Next a layer of approximately 20 mm diameter crushed stone at least 100 mm deep, also referred to as a capillary break, is compacted over the subsoil as a drainage layer to keep water away from the underside of the slab. Vapour retarder For interior floor slabs on grade, a moisture barrier also called vapour retarder, usually consisting of a heavy plastic sheet, is laid over the crushed stone to prevent water vapour from rising through the slab from the ground beneath. A layer of fine crushed stone ( mm) thick may be placed over the vapour retarder to Building Construction Lectures-2 nd Stage Page 93

3 protect it from damage. This layer absorbs excess water from the concrete, helping to prevent curling (warping) of the slab that can occur during curing when the top of the slab loses moisture more rapidly than the bottom. Steel reinforcement and its location Welded wire fabric with wire spacing of 150 mm in each direction is laid over the moisture barrier or crushed stone. The grid of wires of bars helps protect the slab against cracking that might be caused by concrete shrinkage, temperature stresses, concentrated loads, frost heaving or settlement of the ground beneath. Note that reinforcement does not prevent the slab from cracking, but it reduces the width of each individual crack, keeping the overall crack width unaffected. Note also that reinforcement does not increase the load-carrying capacity of the slab, which is primarily a function of slab thickness. Because the purpose of reinforcement in slab is to control shrinkage cracks, it should be located as close to the top of the slab as possible, keeping in view the requirements for cover and surface finish. Building Construction Lectures-2 nd Stage Page 94

4 Concrete thickness and strength The thickness of concrete slab is a function of the load it must carry, with a typical minimum of 100 mm. A concrete strength of (20-27MPa) for residential and for commercial and light-industrial slabs is generally recommended. A higher strength is needed only for more heavily loaded slabs or where greater wear resistance is needed. Pouring and finishing the slabs on grade Building Construction Lectures-2 nd Stage Page 95

5 Pouring (casting) of slab commences with placing concrete in the formwork. The concrete is spread by workers using shovels or rakes until the form is full. As the concrete is placed, the concrete masons reach into the wet concrete with metal hooks and raise the welded wire fabric to approximately the top of the slab. The first operation in finishing the slab is to strike off or screed the concrete by drawing a stiff plank of wood or metal across the top edges of the formwork to achieve a level concrete surface. Immediately after striking off the concrete, the slab receives its initial floating. This step is usually performed by hand, using flat-surfaced tools, typically (1.2 to 3 m) in length, called bull floats or darbies. These are drawn across the concrete to flatten and consolidate its surface. After this initial floating, the top of slab is level but still rather rough. If a smooth surface is desired, additional finishing operations proceed after a period of time during which the concrete begin to stiffen and the watery sheen, called bleed water. Shake on hardeners are sometimes sprinkled over the surface of a slab between the screeding and floating operations. These dry powders react with concrete to form a very hard, durable surface for such heavy wear applications as warehouses and factories. Building Construction Lectures-2 nd Stage Page 96

6 When the finishing operations have been completed, the slab should be cured under damp conditions for at least a week; otherwise, its surface may crack or become dusty from premature drying. Damp curing may be accomplished by covering the slab an absorbent material such as sawdust, earth, sand, or burlap and maintaining the material in a damp condition of time. Or an impervious sheet of plastic or waterproof paper may be placed over the slab soon after troweling to prevent the escape of moisture from the concrete. No concrete floor is perfectly flat. The flatness of concrete slab is specified as the maximum gap size, typically in the range of (3-10 mm), permitted under a (3 m) straightedge placed anywhere on the floor. Controlling cracking in concrete slabs on grade Because concrete slabs on grade are relatively thin in relation to their horizontal dimensions and usually are relatively lightly reinforced, they are practically prone to cracking. The stresses that cause cracking may originated from shrinkage that is a normal part of the concrete curing process, from thermal expansion and contraction of the slab, or from differential movement between the slab and abutting building elements. If such cracks are allowed to occur randomly, they can be unsightly and can compromise the functionality of the slab. Most commonly, cracking in slab on grade is managed by introducing an organized system of joints into the slab that allow stresses to be relieved without compromising the appearance or performance of the slab. A concrete slab-onground requires the following types of joints: Control joints Isolation joints Construction joints Building Construction Lectures-2 nd Stage Page 97