Q: This picture shows a foamy area that showed up on a 35-foot column after the forms were stripped. On close inspection you can see exposed aggregate, both fine and coarse, in the defective area. In some places you can even see exposed rebar. We had similar troubles on other tall columns. Some 15-foot columns were not quite as bad. The mix contained superplasticizer and the slump after adding this admixture was 9 inches. Is it possible to prevent this trouble when superplasticizer is used?
A: This kind of trouble is fairly common in mixes that have low sand contents. Low-sand mixes are often used for concrete in large members and they are standard in pavements and dams. Superplasticizer should not be used without making trial mixes that include the superplasticizer with the job materials unless you have prior experience with the same mix. Trial mixes will show whether the proportion of fine to coarse aggregate needs to be increased, and if so, how much. Segregation, poor workability and sand streaking are common troubles when the aggregate proportions have not been properly adjusted for use with the superplasticizer. If you do have trouble, first look at the quantity of the superplasticizer chosen, and then the aggregate proportions. One or both may be at fault.