Q.: We have a project to replace an old concrete deck. The existing pier caps are heavily covered with efflorescence. Your "Guide to Repair of Concrete," March 1977 issue, gives good descriptions (pages 124 and 131) of how to remove efflorescence. But you do not mention how to test to determine whether or not the concrete is still in good condition.

A.: The application of muriatic acid to remove efflorescence from a bridge pier cap is not likely to cause significant loss in structural strength unless there has been some unusual condition or excessive application of acid. Under ordinary conditions the acid washing should affect only about the top l/8 inch of surface. If there are cracks in the pier cap the acid could penetrate more deeply and attack reinforcing steel, but even so the amount of acid ordinarily used would not be likely to cause significant damage provided the cracks were then repaired to prevent subsequent intrusion of water.

If you have reason to want to check the strength and integrity of the pier cap you could perhaps get the best results by the "Standard Test Method for Pulse Velocity Through Concrete," ASTM C 597. The test should be performed by someone who is thoroughly familiar with its use and interpretation. It will give information on the quality and uniformity of the concrete, which can then be compared with that of nearby concrete of the same mix design and age which is known to be good. The test will also detect internal cracks or other discontinuities.

The relative strength can alternatively be checked by "Tentative Test Method for Rebound Number of Hardened Concrete," ASTM C 805, or "Tentative Test Method for Penetration Resistance of Hardened Concrete," ASTM C 803. As in the pulse velocity method, these results also would have to be compared with those for nearby concrete.

A destructive method of test is to take drilled cores from areas suspected of being weak or damaged. This is done according to "Standard Method of Obtaining and Testing Drilled Cores and Sawed Beams of Concrete," ASTM C 42. The cores can be tested directly for compressive strength and examined visually for cracks or other obvious troubles, and, if desired, can be examined petrographically to check for any other kind of deterioration.