Cracking in Concrete Flatwork: A Practitioner's View
Concrete is one of the oldest building materials in the history of humankind. With over 2500 years of use and case study, you would expect that cracks in concrete should have been eliminated. However, cracking in flatwork is still the most distressing complaint of owners, designers and constructors. While we are not engineers or educated in the formulas and stresses of concrete, we have employed the many different opinions and theories of such people over the past twenty-five years. We would like to offer our practitioner's view on reducing concrete flatwork cracks.
It is important to recognize that cracks in concrete flatwork, in both interior slabs on grade and exterior paving, cannot be eliminated. The one guarantee any concrete contractor will give you is that the concrete will crack. Our efforts need to be directed towards controlling the shrinkage of the flatwork and therefore reducing the occurrence of random cracks. The most common solution to controlling shrinkage has been to add secondary reinforcing in the form of weld-wire fabric. The problem with such reinforcing is that it does not reduce random cracking, but only reduces the crack width when it does occur. And then, such reinforcing only functions if it has been placed at the upper elevation of the slab thickness and remains flat. This will never happen by "pulling " the mesh. For contractors to place secondary reinforcing properly, supports must be used. The weld-wire fabric should be replaced with narrow diameter deformed bar at intervals of at least 18" to allow the craftsmen to walk between the bars. The problem here is you have just increased your flatwork price by at least twenty percent.
Given these considerations, we have been proposing plain (non-reinforced) concrete flatwork. Encouraged by ACI Committee Report 330, "Guide for Design and Construction of Concrete Parking Lots," we began constructing plain concrete flatwork in the early nineties. After successes in parking lots and paving, and supported by ACI 302, "Guide for Concrete Floor and Slab Construction," we began using the same concept on interior slab on grade with even greater success. Unlike secondary reinforcing, we are not trying to reduce crack width, but reducing the area of slab shrinkage by paying greater attention to joint designs and patterns. In lieu of reinforcing, joint intervals do not exceed 30 times the slab thickness. Therefore, if we can reduce the area of shrinkage, we can reduce the likelihood of random cracking. In addition, the cost of the flatwork has been reduced by five to ten percent.
Unfortunately, it is not as simple as "pulling out the mesh." Many other factors such as subgrade load capacity, drag from the subbase material, anticipated load and load transfer requirements, proper joint installation and curing application need to be considered. We would be very interested in you thoughts on this proposal. Please contact us with any comments or requests for additional information.
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