In the article “Using High-Volume Fly Ash,” by Richard Szecsy (January 2006), the author focuses on the positive contributions of HVFA as a green material. I agree with this assessment but also wish to raise the issue of when an HVFA application would not be appropriate—such as in conjunction with concrete densification and polishing of the flooring. Concrete densification relies on the chemical interaction of a densifier (for example, RetroPlate is a modified sodium silicate) with the calcium hydroxide (free lime) component of cement. This interaction creates the crystalline growth that we commonly refer to as densification of the slab. Through testing we have established that excessive substitution of fly ash (greater than 20% of the cement weight) will reduce the improvement in abrasion and impact resistance that our customers can expect from our process. In addition, mix designs with excessive fly ash are not able to be as tightly finished, and subsequently do not appear to polish as well. If concrete containing fly ash is to be treated with the RetroPlate System, it is preferable to use Class C fly ash as it possesses cementitious properties of its own. Nevertheless, since most batch plants rarely distinguish between the classes of fly ash, our recommendation is to keep the substitution rate of fly ash below 20%.
— Peter Wagner
Kudos to Tom Klemens for tackling (braving) such a difficult topic (“How to Moisture Test Concrete Floors,” February 2006). His opinion and conclusion are shared by my office; different methods of moisture testing and other tests are necessary to get a better picture of what is going on when concrete scheduled for flooring installation has failed or not performed properly for whatever reason. We use calcium chloride tests to determine moisture volume/content, but we like humidity probes to determine where the moisture originates. Humidity probes cannot determine volume since they measure relative humidity rather than absolute humidity (relative humidity changes approximately 5% for each 2° F change in temperature even when the volume of water hasn't changed). Determining the moisture volume is important to establish if the moisture originates from underneath, from environmental influences in the room, or from some other source. Correlated with the results from CaCl tests, you can determine the concrete porosity. Moisture volume entering the floor assembly can be tested by the CaCl method, but CaCl only tells you what is in the top ½ to ¾ inch of concrete. This feature of comparing the two different methods gives a much clearer picture of what is going on. In the article, although distinctions were made between the different humidity tests, I was surprised the distinctions between the calcium chloride tests were not made. Most of the calcium chloride test kits are usable only once, easy to damage, and, if an error is made, must be disposed of without obtaining any meaningful results. Our dome is reusable and doesn't require sealing to establish an airtight contact with the concrete surface. Our test kit is also virtually indestructible, much faster to place and pick up than the disposable kits, and much less expensive since the same dome can be used indefinitely.
— Bob Higgins
In Problem Clinic, February 2006, you were asked how to place concrete in the winter the same as in July. Your answer was that you can't, but maybe now the answer is you can. I have been in the concrete business for 19 years. Over this time, cold weather concrete problems have cost me several hundred thousand dollars. I have used every method and device known to the concrete world for thawing ground and curing concrete in the cold weather. None of them is practical or cost effective, and some just don't work. About a year and a half ago, out of pure frustration, I developed a heated concrete curing blanket and a heated ground thawing blanket that we call the Power Blanket. In the past few months we've sold more than 700 blankets to the concrete and construction industry. Many contractors and producers now have the ability to do cold weather work without all the old hassles. The Super Duty Power Blanket will thaw up to 8 inches of ground or 1 foot of snow, overnight. It is now possible to pour concrete in cold weather on the ground or on steel decks that have been preheated. Curing concrete has now become affordable and realistic in cold weather. Current test results report that on nights the air temperature fell to -10° F the temperature under the Power Blanket on newly poured concrete maintained an even 65° to 70° F. The Power Blanket is designed to provide years of service. It operates on 120V power; a 20 amp circuit can cover more than 600 square feet. With the Power Blanket you will achieve the desired strength in as little as two days while maintaining optimum moisture and reducing and eliminating cracking.
— David Naylor