Conewago had a couple hundred cubic yards of concrete on order for gawd awful early in the morning. They asked for a close spacing to get it done quick. They were using 4500lb mix which sets up pretty quickly if your not careful. They neglected to communicate with dispatch to let them know to hold for a while. They had a couple silos that would take a lot longer to do. But instead the said nothing. We ended up with a bunch of trucks sitting on the job for much too long. When the crew poured the concrete out they quickly found themselves fighting to keep ahead of the quickly hardening mix.
Most people think of concrete as “mud”. Mix dirt and water and when it dries you get brick. If it gets too dry when working with it just add water to keep it where you want. This is NOT the case.
Concrete can actually set up under water assuming you keep the water from carrying it off. Basic concrete is a mix of aggregates (stone and sand mostly), water, and hydraulic cement. There are special recipes for more specialized concrete but that is for another story. Hydraulic cement is activated when mixed with water. The cement then begins “glueing” the stone and sand together to form a single solid mass. You can not unactivate the cement or undo the bonding (without ruining the concrete). You can add water to increase the slump or maintain the slump as the bonding process continues but this will permanently damage the cement’s ability to bond the particles together. It’s a case of use it or lose it. As time goes on the concrete will either get harder and stronger or if you add water it will delay getting harder but lose strength until there is nothing left after the process runs it’s course.
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.