Something uncommon

By: Rachel J. Detwiler and Ella Shkolnik, Contributing Authors

You are the concrete supplier on a large paving project. The concrete exhibits considerable delays in setting, poor strength development and ultimately severe cracking of the pavement. Why is this happening?

You are a tech services representative for a cement company. You receive reports from the field of intermittent problems with early stiffening when your cement is used with a Class C fly ash and a water reducer. Is your cement to blame? What should you recommend to your customers?

Your paving operations were going along fine, and then you had to switch cements because of a shortage. Now you can’t place the concrete before it starts to stiffen. Is there something wrong with the new cement?

You specify materials carefully and make sure they all conform to the relevant AASHTO standards, yet somehow a particular combination results in setting problems—too slow or too fast, consistently or intermittently. Maybe you’ve used this cement successfully for the past three years, and now that you’ve started using Class C fly ash, you’re having setting problems. Is it because of the fly ash?

Maybe not. You may be experiencing incompatibility. Incompatibility is not a new problem, but we seem to be seeing more of it now than in the past.

Although we don’t admit it, we tend to treat concrete as an “abuser-friendly” material. We know what ACI and other authorities recommend as good practice, but competitive bidding and tight schedules can make compliance difficult. There are new admixtures on the market, and we use more of them than before—in combinations of two, three or more. We also make greater use of supplementary cementitious materials: fly ash, slag and sometimes silica fume. These materials are all beneficial when used appropriately, but the more ingredients in the concrete, the greater the likelihood of an unexpected interaction between them.

Sulfate solutions

In essence, concrete sets because of the reaction of the C3A (tricalcium aluminate) in the cement with water. Gypsum and other sulfate-bearing components are added to the cement during grinding to control the reaction and prevent flash set. Setting, in effect, is a race between the sulfates and the aluminates (C3A in the cement and sometimes in Class C fly ash). If there is enough sulfate in solution at the right time, the hydration of the C3A is controlled; if not, the concrete experiences flash set.

What affects the performance of the sulfates? In various investigations of setting problems in the field, we have seen the following:

Not enough sulfates in the cement, sometimes because of limitations imposed by the state DOT;
The wrong kind of sulfates. In the cement mill, gypsum can dry to form hemihydrate or anhydrite. Some artificial gypsums (byproducts of various industrial processes) can break down during storage or shipment. Some sulfate-bearing compounds dissolve quickly, some are slower to dissolve and some hardly dissolve at all. The various types of sulfate compounds can be identified and quantified in the laboratory by differential scanning calorimetry;
Lignosulfonates in the water reducer limit the solubility of sulfate and calcium ions. Check the manufacturer’s literature to see whether the water reducer contains lignosulfonates; and
Hot weather makes the sulfates less soluble.

What affects the behavior of the aluminates? Our investigations have found various possibilities:

Highly reactive C3A in the cement;
Type I cement finer than 400 m2/kg. If the cement is very fine, it’s harder to control the setting unless there is enough sulfate. Small variations in fineness can result in big changes in behavior;
Class C fly ash with a high Al2O3 content. Check the chemical analysis; the higher the Al2O3 content, and the more fly ash you use, the likelier you’ll experience premature setting. X-ray fluorescence will quantify the Al2O3 present; x-ray diffraction will indicate whether it is in the form of C3A;
Alkalis. Cement and fly ash can both contain alkalis. Alkali aluminates are especially quick to react, so they increase the rate of hydration;
Free lime (CaO) in fly ash. Free lime combines with Al2O3 and sulfate to form ettringite, contributing to flash set. Some free lime is necessary, though, because it aids in the hydration of C3A, which is responsible for the strength of the concrete. In a system with a high C3A content, the free lime is consumed quickly. The removal of calcium from the pore solution may in turn retard the strength gain of the concrete. Free lime can be detected by x-ray diffraction;
TEA (triethanolamine) in water reducer. Check the manufacturer’s literature to see whether it contains TEA, which makes the C3A react faster. We found one admixture that contained both lignosulfonates and TEA, that is, it both limited the solubility of the sulfates and made the aluminates react faster; and
Hot weather. As with most chemical reactions, C3A hydration is faster at high temperatures.

Buy materials, pay attention

In practice, you buy cement, supplementary cementitious materials and admixtures without paying much attention to alkalis, sulfates or aluminates. What might cause the problems you see in the field?

Class C fly ash: Some Class C fly ashes are excellent materials, and some cause a lot of problems because they contain too much alkali, free lime or aluminates. Unfortunately, they all meet the requirements of AASHTO M 295;
Alkalis in cement and fly ash: High alkali contents in cement or fly ash will cause the concrete to set faster. That may be good if you are working in cool weather or you are using slag in your concrete;
Very fine cement: If the fineness of a Type I cement (provided on the mill certification) is above 400 m2/kg, setting may prove difficult to control. The limits on fineness set by AASHTO M 85 are designed to prevent this problem;
Water reducer: If it contains TEA or lignosulfonates, you could have premature setting. Some water reducers contain both;
Sulfates: There should be enough of the right kinds of sulfates in your cement. If you are using a Class C fly ash that has a high alumina content, you are both increasing the C3A content and reducing the sulfate content of your concrete;
Admixture overdoses: Overdoses of water reducers—even high-range water reducers—may retard setting. Consult the manufacturer’s literature for the recommended dosage. This problem can be identified by conduction calorimetry; and
Hot weather: At high temperatures, the aluminates are more reactive and the sulfates less available. Also, you need higher dosages of air-entraining admixtures to get the desired air content.

Try it out first

Incompatibility is not caused by a single material but by a combination of materials. The single best way to prevent incompatibility problems is to make trial batches of the concrete. Because hot weather is a common factor contributing to setting problems, make some of your trial batches at 80 or 90°F if you expect to be placing concrete in the summer. Check the setting time and air content of the fresh concrete, loss of slump and air content with time and the performance of the hardened concrete. You might want to experiment to see what happens when you exceed the manufacturer’s recommended dosages of your admixtures so you know what to expect—and what to avoid—in the field.

In the field, follow recommended good practice. Consult the admixture manufacturer’s literature for proper dosages and addition sequences. Do not directly combine admixtures. Once you’ve qualified a mix design with a given set of ingredients, don’t substitute any other ingredients without testing.

Didn’t set right?

OK, you meant to do all that, but now you’re under the gun because you’re having setting problems in the field. What can you do now? Here are some things to try:

Use a different water reducer. Try to find one without TEA or lignosulfonates;
Delay the addition of the admixture. Sometimes just delaying the addition of the admixture by about 30 seconds can make a difference;
In hot weather, follow hot weather concreting practice. Review the recommendations of ACI 305R and put as many of them into practice as necessary; and
Reduce the dosage of Class C fly ash. Some contractors have been able to solve their setting problems by reducing the dosage from 20% to 15% fly ash. Do not reduce the dosages of any other supplementary cementitious material without the approval of the engineer of record, because these materials are generally specified for other purposes such as control of alkali-silica reaction.

Helping the detectives

Sometimes it isn’t enough to make the problem go away. You have to find out why it occurred. Maybe you play detective yourself; maybe you hire a consultant to find out what happened and why. Either way, there are things you can do on the job that will make it much easier to figure it out after the fact. The first is to keep good records: weather conditions; materials used; mix designs; delays on site; where in the structure or pavement each batch of concrete was placed; times of batching and placement, addition of water; and any test results obtained. The second is to retain a sample from each material shipment. A 5-gal pail of each aggregate and cementitious material and a glass pint jar of each liquid admixture, labeled with the manufacturer’s information and the date the shipment was received, can be invaluable after the fact if anything goes wrong. Sometimes the difference between a problem concrete and a good concrete is very subtle, and comparisons between ingredients that worked and ones that didn’t—even, say, different shipments of the same cement from the same plant—will help you isolate the problem. Also, if you want to see whether you got the cement content, water-cement ratio or other mix proportions you specified, having the individual ingredients available will make it possible to get good test results.

About The Author: Detwiler is principal engineer at Construction Technology Laboratories Inc., Skokie, Ill. Shkolnik is a senior materials technologist at Construction Technology Laboratories Inc.