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The slabs constructed on ground must be designed taking into consideration the crack control factors. Reinforcement details for slabs-on-ground is discussed.
The thickness and the design of the slabs-on-ground are based on the cracks that are formed due to the external loading. The slab thickness is calculated based on the assumptions of a slab that is unreinforced and uncracked.
For certain conditions of slabs on ground, the steel reinforcement can be provided as a solution. The features of this method will include:
- The limiting of cracks caused due to shrinkage
- Employ larger joint spacing that is greater than the unreinforced slabs
- Provision for moment capacity and stability for those areas there are chances to form cracks
The incorporation of reinforcement won’t prevent cracking. These elements help in reducing or controlling the crack width that is formed by actually increasing the frequency of cracks that is formed.
A reinforcement design that is properly proportioned and placed can help drastically in limiting the width of the crack and hence the serviceability period of the slab is not affected.
Thickness Design Methods for Slabs-on-Ground Construction
Uncracked strength of the slab is less influenced by using more and more reinforcement in the slab design. The different thickness design methods like Pickett’s Analysis Method (PCA), WRI (Wire Reinforcement Institute) Design Method and COE methods can be applied to the design of slabs on ground with reinforcement.
Reinforcement for Slabs-on ground for Crack Width Control Only
There are two factors based on which the crack width can be controlled by reinforcement. These are the joint spacing and the thickness of the slab. Or we can tell the crack width control is a function of joint space and slab thickness.
A minimum steel ratio of 0.5% of the slab cross section is recommended so that the saw cut of contraction joints can be eliminated.
During the placing of the reinforcement, it is advised to place the reinforcement bars closer to the surface of the slab top surface without compromising the minimum concrete coverage over the reinforcement.
Reinforcement for Slab-on Ground for Moment Capacity
When a slab on ground is designed with reinforcement to work for moment capacity, it becomes equivalent to a uncracked plane concrete section. This slab design will require joint spacing and discontinuous reinforcement at the joints.
At the mid depth, the reinforcement steel quantity is
Here, the cross-sectional area of steel is given by As in in2/ft or mm2/m of the slab designed. The thickness of the slab is given by h.
fc = compression strength of concrete, psi (MPa)
fy = yield strength of reinforcement, psi(MPa)
fs = 75% of fy maximum
The modulus of rupture is given by MOR. This, when used in the design of unreinforced design of concrete, takes psi. This value may range between 7 to psi.
Location of Reinforcement for Crack Control Width in Slab on ground
The reinforcement provided for slab constructed on ground, intended to control the crack width must be placed within the mid depth or above the mid depth of the ground. This must not be placed below the mid depth of the slab.
Mainly the reinforcement placing is specified by keeping it such a way that a cover of 1.5 to 2inch is below the top surface of the concrete.
The reinforcement that is provided for the moment capacity must be placed at the centroid of the tensile area that will be at the uncracked concrete section.
Construction of Slabs on Ground -Design Considerations Based on ACI and ASTM