What is good concrete?
For years I’ve tried to answer the question, “What is good concrete?” I’ve written articles on the subject (always a learning opportunity for me), and had come to think I understood what it is and what it isn’t—I probably understand more about what it isn’t.
So when we decided to do a small laboratory research project to see how moisture either moved through concrete or was retained by it, I had already worked out in my mind what the outcome would be. The denser low water-cement 0.35 sample (good concrete) would retain enough water to cure properly in the first 28 days and the sample with the 0.55 would permit moisture to pass through it more easily and wouldn’t attain proper strength. But all four of the mixes we tested maintained moisture levels above 80% relative humidity (RH)—the minimum level for hydration to proceed—during the curing period. They were all good concrete from that standpoint.
We also measured carbonation of the surface of each sample, thinking this would be a way to separate good concrete from bad and we did find differences in the thickness of carbonation between samples. But one of the givens about concrete is that it carbonates, so this doesn’t make it good or bad. In fact, carbonation offers some benefits along with its negatives. The surface carbonation of our samples wasn’t enough to say that one mix might perform better over time than another, though I still have my suspicions.
I hope you’ll enjoy reading the article about the study in the September issue of Concrete Construction, I’m sure you’ll have your own opinions to counter ours about what constitutes good concrete.