Tech Stuff: Curing and Hydration

Two half truths don't make a whole

from Ken Hover, Ph.D., FACI, guest writer for Concrete News

The microscopically small bonds that hold portland cement concrete together are formed from the reaction between cement and water. Figure 1 shows grains of Portland cement before water addition, and Figure 2 shows the grains bound by interlocking hydration products about a week after mixing. Water is consumed two ways in this process. First, the hydration products themselves contain water, in the proportion of about 25 pounds of water per 100 pounds of cement. But as you can see in the Figure, the hydration products are highly porous and take on water as they are being formed. It has been known for over 50 years that hydration requires this physically-consumed water as well, for a total water requirement of almost 45 pounds of water per 100 pounds of cement. This means that considerable water is required for portland cement concrete to reach is fullest potential of strength and durability.

Figure 1. Unhydrated particles of portland cement have no ability to bond to one another prior to the addition of water.
Figure 2. Partially hydrated grains of portland cement, with surfaces covered with the products of hydration. The cement grains are bound to one another through the connections among the hydration products much as the connections in Velcro. The void space between the cement particles was originally occupied by the mix water.

Scanning electron microscope photographs: by Eric Soroos, Cornell University.

A brainteaser:
This gets to be a bit of a brainteaser, however, since it is also well known that concrete strength continues to increase and permeability continues to decrease at lower values of water / cement ratio. This is because the same mix water that hydrates the cement also occupies space between the cement particles. The good news is that the more water in the mix, the more water is available to hydrate the cement; but the bad news is that the more water in the mix, the further apart will be the cement particles, and the harder it will be to link them together. On the other hand, at low water/cement ratios the cement particles are very close together, but they cannot hydrate to their fullest potential due to the limited availability of water. This situation is even worse in low w/c mixes when some of this critically needed water, which is already in short supply, is lost to evaporation!

What's the solution?

  1. Restrict mix water content to bring the cement grains close together, and
  2. apply effective curing measures to minimize water loss, and whenever possible water-cure to externally provide the water needed to sustain hydration.
This whole issue has been misunderstood in our industry because of two popular half-truths.

  1. The first is that cement hydration only requires 25 pounds of water per 100 pounds of cement-but this counts only the water that is chemically combined and ignores the other 20 pounds or so that are physically combined. Knowing only half the story often justifies a lax attitude about the need for curing, believing that more than enough water has been provided in the form of mix water. This is related to the second great half-truth,
  2. that the amount of water needed to provide workability is greater than that amount needed to hydrate the cement. This half-truth goes on to define the amount of water in excess of that required for hydration as the so-called "water of convenience." A natural extension of this notion is that the "water of convenience" is also the amount of water that can be lost from the mix due to evaporation before the concrete suffers any loss of strength or durability. Unfortunately, it just ain't necessarily so!
Let's take for example an air entrained concrete mix with a 1-inch nominal coarse aggregate, a 4-inch slump obtained with a low dose of a mid-range water reducer, and a cement content of 6-1/2 sacks of cement per cubic yard. The amount of water required to bring the cement to its full potential will be about 275 lb per yard. The amount of water required for the 4-inch slump will be about 265 lb per yard. What happened to the water of convenience? There isn't any! This mix cannot afford to lose a drop of water due to inadequate curing, and, in fact, would benefit from water curing to make up the difference.

As another example, consider a high-performance, slip-formed concrete paving mix with a 1-1/2 inch aggregate, 6-1/2 sacks of cement, entrained air, water-reducer, and a target slump of 1-1/2 inch. Water required for hydration is again about 275 lb/CY, and water for workability is about 240. Once again, zero water of convenience and a critical need for curing! The pattern here is that high quality mixes are generally characterized by high cement contents and are designed to reduce the amount of water required for workability. In such mixes there is little or no "water of convenience," and the need for effective curing is acute.

At the other end of the concrete quality scale, let's look at a 5-sack mix, 3/4 inch stone, air, and no water reducer. A mix like this could consume about 210 pounds of water per yard in the hydration of the cement, but will require somewhere around 310 lb of water to achieve a 4-inch slump. One hundred pounds/CY of water of convenience! But what quality mix is this? This mix has a w/c of about 0.66, a 28-day compressive strength of 3000 psi or less, high permeability, and a poor forecast for durability. While it might at first appear that we could ignore curing of such a mix, let's look again. If this mix were used for a thin, 4-inch slab on grade, the concrete would only have to lose 0.3 lb of water per square foot until hydration was inhibited within the top one-inch of the slab. Depending on evaporative conditions, the concrete could lose this much water in the first couple of hours after finishing, and then the owner would be in double trouble. The high w/c ratio makes for a large spacing between cement particles, but the loss of water inhibits the formation of the bonds that develop between the grains. What would we expect? High shrinkage and curing, cracking, dusting, low strength and low abrasion resistance. Suddenly the water of convenience doesn't sound very convenient!

About the Author
Ken Hover, Professor of Structural Engineering, Cornell University.

Earned Ph.D. at Cornell University. Faculty member in Cornell's School of Civil & Environmental Engineering for 18 years, teaching courses in reinforced concrete design, concrete materials, and construction management. Research concentrated in concrete durability and the impact of construction operations on concrete quality. Earned the top teaching awards at departmental, college, and university levels. Appointed to rank of Full Professor in 1992. Undergraduate Dean of College of Engineering, 1996-1999.

Received ACI's National Educators Award (the Kelly Award) in 2001. ASCE Best Basic Research Award, 1992.

Current member of ACI 305-Hot Weather, 308-Curing (Past Chairman), 309-Consolidation, 318A-General Concrete and Construction, Elected to ACI Board of Directors, 1999. Chair of 308 Subcommittee for new "Guide to Curing Concrete." Fellow since 1992.

Prepares and presents seminars and short courses nationally and internationally. Developed Federal Highway's short courses on Concrete Materials and on Concrete Mix Design and Proportioning. Rated top technical speaker at World of Concrete every year since 1994.

Licensed Professional Engineer since 1974.

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© 2002 L&M Construction Chemicals, Inc. | ConcreteNews Summer 2002.

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