From my experience, smacking it with a hammer will only pack it in tighter. The whole problem was the Superplasticizers they were using. They wanted the concrete to flow like it was wet without adding water to it. That is something you do when truck pouring. Pumping is the alternative to using Super P. They should not be used together. Sure enough when they stopped using it, everything worked well.

Superplasticizers are also known as high range water reducers. Basically it’s juice you put in concrete to make it temporarily act as if you added a lot of water but without actually adding any water. It will increase the slump for a short period of time making it easier to place. Since you are not having to add water then you are not decreasing the cement to water ratio thereby reducing the final compressive strength of the concrete. Mechanical means can be used as well suck as using vibrators or pumps.

On the flip side of that coin is the explanation that the more water you add to concrete beyond the desired cement to water ratio the weaker the concrete’s final compressive strength will be. So a good rule of thumb is to maintain the proper compressible strength but have a higher “slump”, use a mix with more cement. Instead of 3000lb mix at a 4 inch slump you use a 3500lb mix at a 6 inch slump. They were using 5000lb mix for an application that could probably have been done with a 4000lb mix at a 6 inch slump. So personally I think trying to maintain a low slump using the water reducer Super P was just a waste. We were not truck pouring so a higher apparent slump was not needed to achieve what the pump does naturally.

The slump of concrete is how far a cone made of concrete “slumps” down from a 12 inch height. The wetter the concrete the easier it flows and therefore the more the cone will fall. A 1 inch fall, or slump, means the mix is tight and probably very close to the maximum cement to water ratio which dictates the final compressive strength. A 12 inch slump means you have a puddle of self leveling concrete that is easy to work with as no raking is needed but it will probably have no strength left of the original design strength.