Q: Should grout contain any special admixtures when it is used in concrete masonry made with integral water repellent?

A: Typical ASTM C476 grout is mixed with considerable water to facilitate placement. Normally, it is assumed that since the concrete masonry units are highly absorptive, water will be drawn out of the grout during placement. This, in turn, will reduce the water to cement ratio, resulting in grout with excellent bond and compressive strength.

However, concrete masonry units with integral water repellents will absorb considerably less water than units without integral water repellents. Therefore, it is logical to assume that there may be a reduction in grout compressive strength and grout bond strength with units that have very low water absorption.

Grace Construction Products, a manufacturer of integral water repellents, has performed tests in this area. In the results summarized in their Technical Bulletin 17, Grace found no significant difference in the compressive strength of the grout, the compressive strength of the masonry prisms, or the bond strength between the grout and concrete masonry units containing their integral water repellent (DRY-BLOCK) and units without any integral water repellent. The grout was tested in accordance with ASTM C1019, the prisms in accordance with ASTM C1314, and the shear strength in accordance with California State Chapter 2405(c)3.C, Test for Grout Shear Bond Strength.

In its sample specification, Grace recommends performing project-specific tests to verify that the grout shear bond strength of units with the integral admixture is not more that 5% lower than control sample without admixtures.