What's necessary to prepare, construct, and protect masonry in hot weather? The 1999 version of the Masonry Standards Joint Committee's Specification for Masonry Structures dictates, among other things, flushing equipment with cool water and fog spraying.

Following a series of Masonry Construction-sponsored tests, conducted by Atkinson-Noland & Associates in Boulder, Colo., we reached a few conclusions:

  • Hot-weather conditions have a relatively minor effect on the compressive strength of masonry and grout.
  • Fog-spray curing also has a relatively minor effect on the compressive strength of masonry and grout.
  • Flexural strength, on the other hand, can end up much lower for masonry built during hot weather, but spray curing can prevent much of that strength loss.

The Specification sets two temperature- and wind-speed thresholds requiring different preparation, construction, and protection methods. Our test program was based on the lower threshold (100° F with no wind), where the Specification requires the contractor to "fog spray ... masonry until damp, at least three times a day until the masonry is 3 days old."

We set out to:

  • Construct and test masonry (both clay and concrete) at hot (100° F) and at normal (70°) temperatures.
  • Evaluate the effect of different curing conditions on the strength of masonry built in hot weather.
  • Compare the strength results (both compressive and flexural) to the code requirements.

The fog-spray curing only slightly improved the compressive strength of the clay prisms over the hot-cure strengths and decreased the strength of the grouted concrete prisms over the hot-cure strengths.

Even though the hot-cure test results for the clay masonry prisms were the weakest, they were still substantially higher than the specified strength. This suggests that even with the 13% to 15% reduction in hot weather, masonry compressive strength is still in the acceptable range. We also observe that with the Code-required fog spraying, hot-weather compressive strengths were still reduced by 8% to 11%, meaning that this expensive practice yielded very little gain.

We tested two different grout strengths, one of medium strength and one with a rather low strength. When compared with the lab-bag cured specimens, the hot-bag cured grout strengths showed mixed results. For medium-strength grout, the hot-bag cure tests were 31% weaker, whereas for low-strength grout, the tests showed a hot-bag cure that was 15% higher. The results for hot-cure also varied, with medium-strength grout 36% weaker, but low-strength grout 9% stronger, than the lab-cured specimens. In these tests, fog-spray curing significantly improved the compressive strength results for medium-strength grout by increasing the strength to 95% of the lab-bag cure. However, in the low-strength grout tests, fog spraying reduced the strength compared to the hot cure.

In our samples, the hot and dry concrete masonry units had the expected effect on the grout: Since water was quickly removed from the fresh grout, shrinkage cracks were abundant in all specimens. For the medium-strength grout, we had a difficult time determining the timing and amount of reconsolidation and subsequently observed extensive shrinkage cracks in the hardened cores. Because of this, we were more careful in consolidating the grout in the low-strength specimens.

Overall, we believe the dramatic differences in strength are related more to the skill and experience of the mason who reconsolidates the grout and the subsequent formation of shrinkage cracks than to any temperature effect. The benefit of spray curing is minimal, because it does not prevent the formation of shrinkage cracks.

Even though our medium-strength and Axsom's high-strength, hot-cure test results were the lowest among the curing conditions, they were still about equal to or substantially higher than the minimum Code-allowable grout compressive strength, f'g, of 2000 psi. This would suggest that the hot weather effects on grout strength are minimal as long as the grout is well-consolidated.

For the clay masonry prisms, the highest average flexural strength of 144 psi was for the lab-bag cure, and the lowest average strength of 51 psi was for the dry hot-cure. Thus, dry hot weather reduced the flexural strength by a factor of almost three. The fog spraying, however, produced a higher flexural strength of 83 psi.

For the concrete brick masonry prisms, the highest average strength of 117 psi was for the fog-spray prisms. The lab-cure prisms averaged only 75 psi, and the hot-cure prisms averaged 43 psi. Fog spraying increased the average strength over the dry hot cure by a factor of almost three.

Axsom's tests also show that fog spraying significantly increases the flexural strength.

Both our test results and Axsom's match test results from NCMA (Ref. 13). Although NCMA's bond wrench tests were conducted at more normal temperatures (75° ± 15° F), the bond wrench test results for fog-sprayed specimens were about three times higher than bag-cured specimens.

The building codes (Refs. 14 and 15) specify an allowable flexural tensile strength of 40 psi, normal to bed joints between solid units. The average hot-cure test results were just barely over 40 psi. Therefore, for unreinforced masonry, which relies on the masonry for flexural strength, hot-weather construction would not provide sufficient strength capacity without fog spraying.

The bottom line for designers is that although strength may decrease slightly during hot weather construction, reinforced masonry structures should not need any additional hot-weather precautions. On the other hand, unreinforced masonry structures, relying on the Code-allowable flexural tensile strength, must be fog sprayed during hot weather construction to achieve the expected performance.

If, however, the designer chooses to base the structure's design on actual field prism and grout tests (the prism test method rather than the unit strength method), these tests should be conducted in hot weather. If they are not, the strength values chosen by the designer in the project documents should account for hot-weather strength reductions and should specify the required hot-weather curing methods.