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Allen Face has been writing articles for the industries informational resource Concrete Construction since 1982. Please visit their website to browse through his articles as well as a trove of other useful information.

For your convenience, we have provided a summary and links to several of his articles here for you as well.


That Pesky Moisture Gradient, Part 3

Published Feb 10, 2011

The material ultimately created by the cement’s hydration—the gray stuff we know as hardened cement paste—is called a colloidal gel—a curious sort of “permanently damp solid” wherein very tiny bits of liquid (the molecules of water and dissolved ions of calcium and hydroxide) are evenly dispersed throughout a porous random nanostructure of insoluble hygroscopic solid material (the precipitated crystalline hydrates).

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That Pesky Moisture Gradient, Part 4

Published March 8, 2011

Portland cement paste shrinks primarily in response to the removal of free water from its capillary and gel pores. Immediately after mixing, while the paste is a murky fluid, its volume will be reduced as gravity forces the solid cement particles to settle. In this initial stage, the lost paste volume equals the volume of water bled to the surface. Once the paste’s solid structure begins to form, its reduction in volume changes to a decreasing fraction of the water lost. Logically, the mechanisms that give rise to the lion’s share of the paste’s shrinkage throughout the hardening process must involve the replacement of the original liquid water with a gaseous mixture of air and steam (i.e. water vapor).

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That Pesky Moisture Gradient, Part 5

Published March 28, 2011

The surplus paste water is the most mobile and least dense of the four principal concrete components, and the only one capable of changing phase from liquid to gas. Immediately following placement, this surplus water starts to percolate upward, and the slab begins a process of sedimentation wherein the solid particles of cement, sand, and stone sink to the bottom while the lighter liquid water works its way, or bleeds, to the top.

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Throwing Water

Published June 1, 2011

The following rule corrects a common misconception regarding the floating and troweling of concrete.

Rule No. 13a: Until the final burnishing stages, a slab becomes increasingly trowelable because its surface is drying—not because it’s hydrating.

This rule is manifestly true. If we had to wait on substantial formation of the C-S-H skeleton before machining, then the progressive densification of the top layer accomplished during finishing operations would be destructive. Vacuum dewatering’s ability—even in cold weather—to shorten the waiting period before first floating is further confirmation of Rule 13a, as is the accelerative effect of removing the paste after floating by bump cutting.

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Steel and Cracks, Part 1

Published June 30, 2011

Few subjects are mired in more confusion than the function of steel reinforcement—especially with regard to its effects on cracking. To see what does happen when a steel rod is solidly embedded in concrete that is stretched, imagine a 15-foot-long 18-inch-wide block of 6-inch slab that can be uniformly tensioned lengthwise. Let the block be crack free and made of mature 3500-psi, 150-pcf concrete. How much tension can be applied to the block before it pulls apart, and how much will the block have stretched at that moment?

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