How to Control Cracking by Using Reinforced Concrete Slab?
You will find that most slabs-on-ground are unreinforced or even nominally reinforced for crack-width control. The steel reinforcement can control the widths of random cracks, when placed in the upper or top portion that might occur due to the shrinkage of concrete and factors such as temperature restraints, subbase settlement, applied loads or other issues.
This type of reinforcement is what is commonly known as shrinkage and temperature reinforcement.
There is a difference between shrinkage and temperature reinforcement and structural reinforcement. In case of structural reinforcement, it is usually placed in the bottom portion of the slab thickness so as to increase the load capacity of the slab.
Both top and bottom layers of reinforcement are present in most structural slabs-on-ground to be able to control crack-widths and increasing load capacities. Since the two layers of reinforcement can be costly as well as can create issues in constructability, therefore, structural slabs-on-ground are not as commonly used as nonstructural slabs.
Even though there are a number of options in reinforcement for nonstructural slabs-on-ground, in this article we will only focus on steel reinforcing bars and welded wire reinforcement which is used for crack width control.
The basics to learn
Cracking cannot be prevented by steel reinforcing bars or welded wire reinforcement. Till the point that the concrete cracks the reinforcement remains in a dormant state. It is only after the cracking starts that it becomes active and then can control crack widths by restricting crack growth.
Reinforcement is required if slabs are placed on high quality subbases with a uniform support and if it also consists of low shrinkage concrete with joints that are properly installed with spacings of 15 feet or even less. There will be a chance for a few random or out-of-joint cracking to occur. In case of the occurrence of random cracks it should remain quite tight because of the limited joint spacing and low concrete shrinkage which then limits future serviceability or maintenance issues.
In case where slabs are placed on problematic subbases and also has risks of non-uniform support or consist of moderate to high shrinkage concrete or joint spacings which can even exceed 15 feet, that is when reinforcement is necessary so that the cracks width can be limited if it starts to occur.
With the crack widths growth and approach to about 35 mils (0.035 inches) there will be a deterioration in efficiency of load transfer as the aggregate interlock will diminish. It may also lead to a difference in vertical movements all across cracks or there can be the occurrence of slab "rocking".
It is due to this that the crack edges become exposed and there will be a chance for edge spalling to take place, the chances of this happening are higher in case the slab is exposed to wheeled traffic and especially hard-wheeled lift trucks. When spalling begins, crack widths at the surface will become broader and there will be an increase in cracks with the deterioration of slab.
Shrinkage and temperature reinforcement arenecessary when the joints in contraction are either unacceptable or not installed. This design approach is at times even referred to as continuously reinforced or joint-less slabs and it allows a number of closely spaced (3 to 6 feet), fine cracks to occur all across the slab.
If there is an unrestricted crack width growth then it will lead to spalled edges along with out-of-joint cracks when it is exposed to wheeled traffic, all the more in case of hard-wheeled lift trucks.
Crack control options
Here we have stated two options for controlling cracks in slabs-on-ground which are generally used: 1) installation of contraction joints in the location of cracking to control(this will not control crack widths) or 2) Installation of reinforcement to control crack widths (this does not control crack location).
In the first case, joint spacing and concrete shrinkage control the slab where it is cracked and also the widths of contraction joints or cracks in the joints. With the increase in joint spacings and concrete shrinkage there is an increase in joint widths. Much like cracks, if joint widths approach about 35 mils, there can a reduction in the efficiency of the aggregate interlock to be able to transfer loads and prevent differential vertical movements across joints. It is for this very reason that a number of designers use load-transfer devices such as steel dowels, plates or continuous reinforcement through contraction joints to confirm positive load transfer and also to limit differential vertical movements throughout joints.
Now in case of the second option, the slabs can crack randomly but there is a control in the crack widths with steel reinforcing bars or welded wire reinforcement. Usually contraction joints are not installed in this case. What happens in this case is that the cracksoccur randomly and results in the formation of numerous, tightly held together cracks. Due to its appearance, it is important to discuss this crack control option with the owner.
Cutting reinforcement at joints
When you are using both crack control options in the same slab it is important to be careful. Joints can become extremely stiff if too much reinforcement passes through contraction joints, this can lead to inability to crack and open as designed.
If due to reinforcement the contraction joints do not activate (i.e., crack and open) then out-of-joint or random cracking usually occurs. In case both the options are used then it is important to restrict the amount of reinforcement which will pass through joints to confirm proper activation.
It depends on the designers, while some designer?sadvice to cut all the reinforcement at contraction joints whereas the others might specifically ask to cut every other bar or wire. With the help of cutting every other bar or wire, the remaining reinforcement will help provide load-transfer and also decrease differential panel movements at the same time it will not limit joints from activating.
In case the temperature and shrinkage reinforcement at joints are not indicated in specifications and construction drawings then the contractors must submit a request for information. A number of times contractors are unreasonably blamed for out-of-joint cracking which is actually a design issue.
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