An improved process for the preparation of concrete wherein the improvement comprises incorporating into the concrete-forming mixture from about 0.1 parts to about 10 parts by weight based on solids content thereof, per 100 parts by weight of concrete-forming mixture, of an aqueous solution which comprises the product obtained by admixing, at a temperature in the range of about 25.degree. C. to about 100.degree. C., (i) a polyethylene glycol having a molecular weight from 600 to 3000 with (ii) from 2 to 5 equivalents, per equivalent of said glycol, of a mixture of polymethylene polyphenyl polyisocyanates containing from 25 to 90 percent by weight of methylenebis(phenyl isocyanate) the remainder of said mixture being polymethylene polyphenyl polyisocyanates of functionality higher than 2.0, and, during the time when said admixture is completely soluble in water, dissolving said product in water in a proportion of from 1 to 20 parts by weight of said product per 100 parts by weight of water to form a clear aqueous solution.