QUESTION: One of our biggest problems is getting consistent quality of concrete — concrete that has the same quality and the same slump batch after batch. How do we improve that?
ANSWER: Say you order concrete with a 5-inch slump. As a contractor, what do you feel is a consistent slump? Plus or minus ½ inch? Your producer can give you that consistency but he will have to slow down to do it. So, the first thing is understanding and communicating what you and the producer think is consistent. For a ready-mix producer — what is your idea of consistent slump? More like plus or minus 1 inch. So the first thing is to have a preplacement meeting where you agree on these things. Also, agree on where you are going to measure the slump. It could be coming out of the plant at plus or minus 1 inch but that may not be the case at the jobsite. Remember that concrete is a time-dependent material, just like a banana that you peel. As soon as it’s open it starts to go. It’s a fresh material, that’s why we use it, but as soon as it’s mixed it starts getting hard. So depending on when you take your sample you can — you will — get very different results. And if it starts setting up and you want to add water to bring the slump back up, remember that adding a gallon of water to a yard of concrete is going to delay set by about 15 minutes. And then not only do you have to wait longer for the initial set but you also have more bleed water that you’re waiting to evaporate. And then you may have trucks lined up behind that one and you’re waiting for the slab section to set up. This goes back to the prepour planning meeting. You need to evaluate realistically how fast you are going to pour and how many trucks the producer needs to dedicate to the job to make sure everything is consistent.
This question and answer was generated during the Breakfast with the Experts at the 2015 World of Concrete, sponsored by BASF. If you have problems on your jobs that you would like to pose to the experts, plan now to attend one or more of the 2016 breakfasts. Another question: "Repairing a Structural Beam."