Slab on Grade Foundation

slab on grade

Slab on Grade Foundation

Slab on grade construction is the process of setting a concrete slab on the existing native soil. A layer of engineered fill may be contained in native soil in order to drive the slab to proper elevation. In many cases, the native soil is considered the sub-base. The base course is combined above the sub-base which supplies additional bearing support and a flat surface. Then concrete slab is constructed whose thickness is based on the type of loading and also quality of the native soil. Many times a vapour retarder is kept between the concrete slab and the base course to stop moisture from wicking up across the concrete from the native soil.

In order to decrease the occurrence of cracking, proper construction should be done. We should identify the proper sub-base and proper base and mix design of concrete. We should also provide control joints and a manner of curing.

When the slab dimension is greater than 5m, we should supply proper control joints to decrease the cracking as we know that concrete is a brittle material. Designers and contractors can work with slab on ground construction. The most trouble prone element of concrete work is slab on ground construction, so proper care is essential.Slab on grade foundation is constructed mostly in warmer climates where there is less ground freezing and heat ducting underneath the floor is not required. 

Design of Slab on Grade

Slab on grade is built as a pavement design when moving loads are experienced as this design contains consideration of flexural stresses encountered and repetitive loading. The special consideration must be done when we involve high loads or patterned loads.

We can design slab on grade construction based on a ‘drag formula’ that accommodate the shrinkage of the concrete and control of the granular base material. There is also another method for the construction of slab on grade i.e. by limiting the flexural tensile stresses which is created by loading to a portion of the concrete modulus of rupture for flexure. We can also include granular base materials and sub-base in the design. In order to achieve high quality Slab on grade construction, we can use a multiple layered approach.

Base and Sub-base

To examine the consistency of bearing materials, the sub-base can be proof rolled. We can excavate the soft areas and re-compacted with engineered fill. If the base and sub-base are more uniform than we can construct better slab on grade construction. 

The base must be a uniformly graded quality granular material which can easily be close-packed. It helps to produce a smooth hard surface and a bearing surface for the concrete. It helps to transfer load from the concrete slab to the sub-base which is situated below the base. A uniform surface should be provided that helps to reduce cracking.

Vapour Retarder

We can keep a polyethylene vapour barrier between the slab and base to reduce moisture from entering the slab on grade from the sub-base and granular base material situated below. It is also placed 50 mm of sand material between the polyethylene and the slab in some places in order to minimize curling of the slab. It helps to produce a uniform moisture content all over the concrete slab by allowing water from the concrete mix to flow in the sand layer.

Concrete Required for Slab on Grade

We should use a low slump concrete in order to reduce the shrinkage. A large proportion by weight of flyash should be made in consideration which reduces shrinkage but generates a slower strength gain. The finishing of the concrete surface is affected by large amounts of flyash.

We should consider the strength of the concrete, air content and water: cement ratio when we have to expose the slab to freeze-thaw conditions.

There will be additional considerations to be taken for the design of concrete like strength, water: cement ratio, curing and sealing when we have to expose the slab to deicing chemicals.

There will be additional considerations to be taken for the design of concrete like sulphate resistant cement, strength, water: cement ratio, curing and sealing when we have to expose the slab to sulphates.

There will be additional considerations to be taken for the design of concrete like strength, water: cement ratio, curing and sealing when we have to expose the slab to sulphates and chlorides.

We should minimize the temperature differential between the top of the slab and the bottom of the slab.

We can determine the thickness of slab with the design loading and the quality of the base and sub-base materials below. We should use a minimum 125mm slab thickness. The concrete thickness should be three times the maximum aggregate size.

Reinforcing Steel

The upper one-quarter of the slab thickness has reinforcing steel kept on it. We place the steel in the top to minimize cracking on the top surface. If there is a crack on the top surface, then it is not pleasing as it can be visible. We place steel in a single layer situated near the top of the slab. 0.2% of the concrete area should be provided as area for reinforcing steel. To eliminate visible cracking, we can increase this proportion to 0.5% or 0.6%. There still occur cracks but have reduced crack width.

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