Q. I would like to specify an architectural concrete topping of about 2 inches on a 30-year-old concrete bleacher/gallery structure. I have been told I should use a bonding agent, a water reducer to prevent cracking and a higher than normal compressive strength.
What would be an appropriate strength to specify? Should I also specify slump? What about the water-cement ratio?
Cracking was a problem with a previous repair attempt. Is there a good way to avoid that this time? It's interior work where the temperature will be controlled. Is there need for control joints? The existing concrete does not have any.
A. The compressive strength you need depends the loads you are going to have on the concrete. Remember, you have an existing slab to work with as far as strength and support is concerned. As a guide, though, the compressive strength should be at least 3000 psi.
The slump test can be an indicator of workability but is most effective in checking the uniformity of successive loads. Talk with your concrete supplier and go for a slump that allows the concrete to be placed and finished well while keeping the water-cement ratio at or below 0.45. Keeping the water down will help reduce shrinkage and shrinkage cracking. (See also “Much Ado About Shrinkage” March, p. 42.)
Even so, shrinkage is going to occur. One rule of thumb is to multiply the slab thickness, in inches, times 2.5 to find the joint spacing, in feet. For example, a 2-inch slab would have joints every 5 feet. Make sure all of the cracks in the existing slab are repaired. Otherwise they may reflect through the new concrete.
Finally, consider using synthetic fibers in the concrete. They give the concrete early tensile strength to withstand the stress the new concrete will go through as it changes from a plastic to a hardened state.