Over the past 50 years, CC has answered thousands of questions in its Problem Clinic. In the very first issue, there was a department named “Questions and Answers.” In May 1961 the name was changed to Problem Clinic with the editor (Bill Avery) noting that “We hope more readers, stimulated by the brief discussions presented here, will send in questions which can be dealt with in future columns.” Problem Clinic remains today as one of CC's most popular departments.
The following is a collection of some of the best advice in Problem Clinic—plus some interesting pieces from the past. The advice given even in the 1950s remains valid today. All of these questions and answers, and thousands more, can be found on our Web site.
Q. What is meant by false set?
A. False set is a premature stiffening that differs from flash set in that the cement paste does not liberate heat at a rapid rate and in that very mild mixing will restore plasticity. False set rarely shows up in ready-mixed concrete because of the working effect of the mixer.
Q. We've noticed that concrete from onsite mixers often stiffens within minutes of pouring, then loosens up later. What causes this?
A. You're seeing an example of false set—a pronounced loss of plasticity without much heat evolution shortly after mixing. It's mainly a nuisance rather than a major problem because plasticity can be restored by remixing or vibrating the concrete. The concrete will then set normally. False setting is more likely when onsite mixers are used. That's because ready-mixed concrete is usually being mixed or agitated during the period when false setting occurs. But sometimes, when a set-retarding admixture is used with a cement that has false setting properties, false set occurs after ready mixed concrete has been discharged. Several possible mechanisms of false setting are described by Mindess and Young in their book Concrete, published by Prentice-Hall. One explanation is that false set is caused by dehydration of gypsum added to cement during manufacture to control setting of tricalcium aluminate. At high temperatures during the cement grinding process, water is driven from the gypsum, converting some of it to plaster (calcium sulfate hemihydrate). When water is added during mixing of concrete, the plaster rehydrates back to gypsum, forming a matrix that stiffens the mix. Because there's so little gypsum in the cement, the effect doesn't last long and further mixing destroys the matrix.
Cold weather concreting
Q. How soon after concrete has been poured can it be frozen without sustaining damage?
A. In “Recommended Practice for Winter Concreting,” ACI Committee 604 [now ACI 306] noted that the principal factor is the rate at which the concrete gains strength. Experience indicates that concrete may not be seriously damaged by one or two cycles of freezing if it has attained a strength of more than 500 psi. Limited tests indicate that concrete placed at a temperature of 75° must be subjected to severe freezing within about 6 hours for any serious damage to be sustained. If it is placed at 40° F, even a mild freezing at 25° may result in a 50% strength loss.
Q. Is it true that concrete will no longer freeze after it reaches 500 psi and, if so, what is the best method for field-testing the concrete to determine when it reaches 500 psi?
A. It is not true that concrete will not freeze after it reaches a strength of 500 psi. However, it is a rule of thumb that concrete will not be destroyed by a few cycles of freezing and thawing after it has attained about 500-psi strength. In other words, 500 psi is the minimum strength required to resist the first exposure to freezing. In most cases it is not really practical to determine that concrete has attained this minimum “safe” strength. Most engineers would be more inclined to estimate the rate of strength gain on the basis of the known low-temperature behavior of the concrete mix being used. Then if there is any likelihood whatever that the concrete might not attain 500 psi before it freezes, they would make provision for heating or insulating it enough to prevent freezing. Actually, the recommended method is to not rely at all on either tests or estimates but to follow the practices for protecting concrete against freezing in accordance with ACI 306 “Recommended Practice for Cold Weather Concreting.” That standard provides tables which show the amount of protection required for various concrete sections, types of cement and degree of load as well as the length of time the protection should be supplied.
Q. How long must concrete be protected from freezing temperatures, with and without accelerators or admixtures?
A. Fresh concrete should be protected from freezing for the first 24 hours after placement, by which time it should have a compressive strength of 500 psi. After that, the amount and duration of protection will depend on the desired rate of strength development.
Generally, the greater the cement content and the section thickness, the greater the heat of hydration and thus more rapid strength gain of the concrete. You can use the maturity method from ASTM C 1074 if you want to correlate the temperature-time factor of the in-place concrete to its strength.
Other references that may be of interest are ACI 306 “Cold Weather Concreting,” and the first section in ACI 201 “Guide to Durable Concrete.” Beyond that, the 14th edition of Design and Control of Concrete Mixtures gives a maximum saturation of capillary pores at 91.7% filled with water as the point at which concrete with or without air will not be affected by freezing and thawing cycles. The 9% expansion of water as it freezes has the room for expansion it needs to avoid disruptive internal pressures.
Another good resource on this subject is T.C. Powers' Prevention of Frost Damage to Green Concrete, which is available as a free PDF file at www.cement.org/bookstore (search for “frost damage”). That is the reference for the 500 psi minimum for a single frost cycle in ACI 306. Remember that the re-saturation of pores requires very little water from an external source to render a non-air entrained concrete vulnerable to damage.
Compressive and flexural strength
Q. In normal concrete work, the maximum compressive strength that can possibly be obtained is generally reckoned to be about 7500 psi for a 28-day cylinder. It would be interesting to know if there is a conceivable maximum limit using materials and techniques available.
A. The limit at the moment seems to be about 16,000 psi, although this can be obtained only under laboratory conditions. Using crushed rock or granite aggregates in an aggregate/cement ratio of 3:1 and pressure compaction, 6-inch cubes with a 14,000 psi strength at between 28 and 36 days can be obtained without any great difficulty. It would appear that the achievement of maximum density depends on the development of some radically new type of vibrator, perhaps ultrasonic.
[Mike Pistilli, Prairie Materials, tells us that today ready-mixed concrete can be routinely produced at 18,000 psi and laboratory concrete as high as 30,000 psi.]
Q. Our ready-mixed concrete company has received a request for concrete with a flexural strength of 670 psi. We've never had this kind of order before. Can you tell us what equivalent compressive strength such a concrete represents?
A. This sounds like an order for paving concrete, which is commonly specified in terms of flexural strength. Flexural strength can be related to compressive strength of concrete by the following formula:
This says that the flexural strength is roughly equal to 9 times the square root of the compressive strength. On this basis, a flexural strength of 670 psi would be equivalent to 5540 psi compressive strength. The formula is not exact because it is influenced by the type of aggregate.
Q. Before placing the concrete for the last pier of a drilled-pier foundation job, the foreman decided to add water to the ready mix truck. The inspector didn't like the looks of the watered-down concrete and took test cylinders that represented that one pier. The specifications call for a 28-day strength of 3000 psi. After the lab broke the seven-day cylinders, the cylinder from the pier with added water broke at 1980 psi. The other seven-day cylinders were as high as 2620 psi. The engineer is concerned that the concrete will not meet the specified strength. I realize that adding the water was the wrong thing to do, but I don't want to remove the pier if it is of adequate strength. Will it reach the specified 3000 psi?
A. As this case shows, it is often useful to extrapolate 28-day strengths from seven-day strengths. Of course, the amount of strength gain varies between the seven-day and the 28-day tests. Cement type and curing conditions are two factors that affect the amount of strength gain to be expected. Concrete, by Mindness and Young, gives a general rule: The ratio of 28-day to seven-day strength lies between 1.3 and 1.7 and generally is less than 1.5, or the seven-day strength is normally between 60% and 75% of the 28-day strength and usually above 65%. The cylinder that broke at 1980 psi is 66% of the specified 3000 psi. According to Mindness and Young's rule, it should meet the specified strength at 28 days. Most likely, the mix wasn't designed for 3000 psi but for a higher compressive strength to account for variability. By adding the additional mix water you raised the water-cement ratio which, in turn, reduced the strength. The piers placed before the water was added will probably have strengths higher than the specified 3000 psi. The pier in question, however, will most likely meet the specified strength. If after 28 days the cylinders still do not meet specified strength, take cores to verify the strength before implementing a costly pier removal.
Q. Is there any truth to reports that concrete buildings are radioactive?
A. Yes, but it doesn't seem to justify any anxiety. All structures are somewhat radioactive, but masonry materials, including concrete and brick, show greater radioacitivity than wood. The phenomenon has nothing to do, of course, with the atomic age. We've been using radioactive materials for shelter ever since the first man holed up in a cave. In modern structures, minute amounts of radium and thorium in the building materials appear to be the main source of radioactivity.
Q. How much sugar is required to keep concrete indefinitely in a plastic condition?
A. The amount of sugar that should be used to keep concrete from fully hardening ranges from 1.0 to 1.5 percent by weight of cement. It is important to note, however, that the effect of sugar is not to keep the concrete permanently plastic but to keep its strength at a low enough level so that it can be easily broken up.
Q. I built a patio in my backyard last July and did the concrete work myself. I used a six-bag mix with 5% entrained air and the concrete wasn't too soupy—not more than a 5-inch slump. It was a very hot day but my neighbor and I gave the surface a steel trowel finish before it had set up too much. I got some burlap bags from the local feed store and covered the surface immediately after finishing, then wet it down well and covered the whole thing with polyethylene sheets. I kept the burlap wet for a week. When I took it off there were several places on the surface where it looked as though the concrete still hadn't set. The coarse aggregate was showing in these spots and yet most of the patio looked fine. What could have caused this?
A. One possible cause for this problem is that some of the burlap bags you used were contaminated with an organic substance such as sugar that retards concrete setting. Because you gave the patio a steel trowel finish and may have finished it too early, it's also possible that blistering was the problem. Blisters occur when entrapped air or bleed water gets trapped in the concrete. However, since you said that it looked as though the concrete hadn't set in the affected areas, our first guess is probably the more likely answer.
Q. Occasionally one sees hints that plastics may eventually oust portland cement concrete in construction. Is this really feasible?
A. At the moment plastics, and chiefly the thermosetting (permanently hardening under the action of heat) resins, serve only as valuable aids in concrete construction, for example as adhesives or in toppings. Waterstops of polyvinylchloride (PVC) are an example of the auxiliary function of the thermoplastics (those showing an unlimited ability to be remoulded under the action of heat). As far as can be seen, and unless there are radical changes in the existing methods of producing plastics, it would be unlikely and there is no danger of plastics ever ousting present-day concrete before supplies of raw materials for portland cement production are exhausted.
Q. Has the entrainment of air in concrete any advantages besides increasing its resistance to freezing and thawing?
A. Yes, air entrained concrete has less tendency to bleed, it is considerably more plastic than ordinary concrete, and it generally shows less segregation.
Q. Is there any way to reduce the air content of a mix which is considerably higher than specified or intended?
A. Over-entrainment of air can be caused by inadvertently adding double dosages of admixtures or by using more than one admixture, both of which entrain some air. Some aggregates have been known to release air into the mix. The most widely used air-detrainer is tributyl phosphate. Dibutyl phosphate, water-insoluble alcohols, water-insoluble esters of carbonic and boric acids as well as silicones may also do the job. Obviously they must be used judiciously for the concrete to end up with the right amount of air.
Q. I have a concrete wall that builds up heavy efflorescence in the spring. There doesn't seem to be any other source of efflorescence except the wall itself, but I have cleaned it two years in a row and the white deposit still keeps coming back. How can I stop it?
A. Spring rains are undoubtedly soaking your wall and picking up a load of lime or other soluble material from within the wall. The water slowly migrates back to the outer surface, where it deposits the crystals as it evaporates. This process probably will continue as long as there is soluble material left in the wall. One way to overcome the problem is to repeat the process purposely, but to arrange to have the efflorescence deposited outside the wall, not on its surface. To do this you saturate the wall from the outside with water and immediately plaster on a thick layer of papiermache made of chewed-up newspaper, wet enough to stick to the wall. When the wall has dried the efflorescence will be in the papiermache, which can be peeled off. It may be necessary to use two or three repetitions. This technique was suggested some years ago by the observation that farmers never had any efflorescence on barns at the place where the manure pile lay against the wall. One farmer even plastered the manure on, like the papiermache, and got rid of all the efflorescence.
Q. Lately we have had contractors placing too large an area of concrete floor slabs, so that finishers are not able to properly finish it before it rains or darkness falls. Is there any rule of thumb about the number of finishers required to finish a given area of concrete?
A. We have not been able to find any. The amount of time that must be spent waiting for a slab of concrete to be finishable varies widely with the concrete mix, the slab thickness, the ambient temperature, and other variables. Furthermore, the amount of work that must be done will depend on the total number of steel trowelings required. In some parts of the country it seems to be acknowledged that 500 square feet of floor per man is a big day's work, although in New York City 1000 square feet with a fairly good trowel finish is not unusual.
[In 2006, a finishing crew with a laser screed and riding trowels can easily finish more than 3000 square feet/man/day.]
Q. What can cause a wide variation in mixing water needed to get a uniform slump for ready mixed concrete? For a 4-inch slump, I normally need to batch about 150 gallons of water in a 5-yard load. Sometimes, though, it takes as much as 170 gallons to get the same slump. Why?
A. Several factors can cause slump variations. If moisture content varies in the aggregate stockpile, switching from a wet to a dry portion of the pile can increase water demand. If admixtures are being used, a dispenser malfunction can result in not enough water-reducing or air-entraining admixture being added. If aggregate segregation has occurred in the stockpiles, a large amount of fines might be put in one batch. Edward Wegner, formerly a technical services representative for a cement company, described his solution to a similar problem. He visited a plant plagued by erratic variations in water demand. Sand from a 40-foot-high stockpile was charged directly into a scale hopper with a front end loader. It was a dry, windy day and Wegner noted that the wind was depositing a drift of very fine sand near the toe of the pile. Instead of reblending this material, the operator periodically swung over, picked up the drift, and charged it directly into the hopper. The excess of fine sand dried up the mix and increased water demand. After the loader operator started blending drift sand back into the stockpile, there were only minor variations in water demand.