Question: For high-volume fly ash concrete, a low water-cementitious materials ratio is needed. We are contemplating using a polycarboxylate to improve its workability. I know polycarboxylates are expensive but their performance in terms of slump retention, water reduction, and early strength gain is supposed to be second to none. Is it worth the performance enhancement and peace of mind to go with the more expensive polycarboxylate admixture as opposed to using a naphthalene-, lignin-, or melamine-based admixture?
Answer: Absolutely. Polycarboxylates are an excellent product. Not only do they have the positive attributes you mentioned but they also do not have some of the disadvantages of other superplasticizers. For example, they do not retard the concrete like some other admixtures do at high dosages. Also, they can be batched at the plant, eliminating the need for workers to climb truck ladders onsite.
Beware—some polycarboxylate superplasticizers require regular circulation or agitation to stop a component that inhibits air entrainment from separating out. If this occurs, you can get a high air content and a corresponding drop in strength. Otherwise, great stuff!
Also, you should be using a lower dosage rate with a polycarboxylate than you would with a more traditional water reducer. Look at the entire mix rather than just try to substitute the polycarboxylate for whatever high-range water reducer you are currently using. There are differences, but it should take you no more than a day and about six to eight 3-yard trial batches to figure out what works best. Your admixture representative should be able to supply you with helpful information.