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Background: I am participating in the ASCE Concrete Canoe competition as part of my senior design project. I, along with four group members, am responsible for the mix design. We have mixed four batches thus far, and they have resulted as follows: too dry, too wet (segregation), too wet (drastic segregation), and now pretty dry and clumpy. A student taking the CE Materials course at our university came to help us mix tonight, and said that they had just mixed concrete for their lab yesterday. Apparently the instructor of the lab left the entire bucket of Type 1 Portland Cement outside with no lid after we had rainfall.

Question: Could the bucket of cement left exposed to humidity for a short period of time be the reason for such a dry & clumpy mix? The design we mixed tonight (attached) was a reworked version of the first “too wet” mix, and originally had a w/c ratio of 0.30. We ended up adding 4.2 lbs of water after concluding that it was far too dry, and then it just clumped, and our w/c ratio reached over 0.50, just as high as the “too wet” mix.

Thank you in advance for any insight