Slump Loss and Mix Water Content
Q.: We need to submit a concrete mix to be placed by pump. Slump and air are specified at the point of placement. Can we increase the initial mix water content to compensate for water lost in pumping and also satisfy the limit on water-cement ratio at the point of placement?
A.: You've identified a common misconception about concrete pumping. Concrete does change consistency somewhat during pumping, but this is not due to a loss of water. Specified limits on water-cement ratio apply to the concrete before discharge into the pump. Under most conditions slump loss should not be too great.
Your question suggests that you may not be comfortable with concrete pumping. Before proceeding, we recommend that you review "Placing Concrete by Pumping Methods," ACI 304.2R, American Concrete Institute. Chapter 4 discusses proportioning pumpable concrete. You also should study field procedures before the work. Plan ahead to secure joints in the pump line and replace worn gaskets. On some jobs the engineer may agree to added water reducers or superplasticizers when problems arise because of slump loss due to hydration, temperature, or time. It's best to work this out at the pre-pour conference.