This is a section of sidewalk in front of a driveway. The scaling shown could have many different causes. Petrographic analysis is the only way to know what really happened, but one possibility is that the contractor placed concrete with too much water causing the surface to become porous (most water goes to the surface).
Credit: Joe Nasvik
What should you do when a load of concrete arrives on your jobsite and it is too wet? One easy answer is to say, “When in doubt, throw it out.” But this kind of a decision is worth some thought. If you reject a load, it will take some time before another one arrives to your jobsite and in the meantime you are paying for nonproductive labor. If you are on a commercial or public project where samples are tested for strength and other characteristics by a testing company employed by the owner, the choice might be very easy—others will reject the load. But if you are on a residential project where no one will know, what do you do? And if you are a decorative concrete contractor, how will concrete with excessive slump affect your work?
When there is too much water in the concrete, there is greater shrinkage with the possibility for more cracks and reduced compressive strength. As a general rule, every additional inch of slump decreases strength by approximately 500 psi. So for example, if you ordered 5-inch slump concrete and received 7½ inches, a mix designed to be 4000 psi would end up being 2500 psi. This represents a serious loss in strength, especially if you were placing exterior concrete in a freeze/thaw climate where the standard requirement is 4000 psi for proper durability.
As you increase the amount of water in a mix, you also increase the porosity of the hardened concrete. Many more canals are formed as water migrates to the top of a slab. The porous surface of the slab reflects light differently and if you are placing integrally colored concrete or applying a chemically stained finish, the results will be different than expected. Increased porosity also results in more efflorescence, causing the color of the surface to be less intense or whitish.
Excessive slump also can result from accidentally adding too much water-reducing admixtures or super-plasticizers to a mix. This can result in aggregate segregation and a serious loss of strength. There is no question about rejecting this concrete. But if the overdosed admixture isn't high enough to cause segregation, the concrete will eventually set and achieve a higher than expected compressive strength.
Weighing options and making decisions
Your first impulse is probably to place the concrete anyway. This option gets the best productivity from labor and doesn't strain relations with your ready-mix producer. The risk, however, is that work can both look and be compromised, and the resulting lower durability might force you to remove and replace at some point in the future.
Another solution is to send the load back to the plant and add more portland cement to “dry it up.” Sometimes more aggregate is added as well. But there is unpredictable chemistry between concrete that is starting to hydrate and newly added material. One outcome is that the concrete passes through the initial set phase very quickly, reducing the time you have to complete your work. If you are stamping patterns in concrete, you may not be able to finish your decorative applications.
If one load of concrete is too wet in a sequence of loads, you should reject that load because the results will be significantly different from the others.
If there is more than one load involved in a placement, another option is to spread the load low, placing concrete with the proper slump on top of it. This can work if the concrete still has structural integrity.
Your decision about whether to accept or reject concrete also will depend on just how wet the mix is. Many specifications permit slump variances of 1 inch. If the slump exceeds 1½ inches from what was ordered, reject it.