Getting the best from Materials Tests
By Michael and Wilma "Willy" Morrison

If you work in the construction industry, you're going to deal with testing or test results at some point in your career. Contractors may find that project payouts are linked to test results. Producers use tests to sell their products. Designers decide to specify products based on test results. Litigation, of course, often hinges on test results.

With so many aspects of our industry riding on test results, the quality of testing services is extremely important. This article will help you get the most from your material testing program by selecting the appropriate test for the problem at hand, taking samples correctly, working with capable laboratories, and analyzing your results properly.

Selecting the proper test
Selecting the proper test is the first important step. Sometimes one test will give you all the answers you need, but usually there are several tests you can use to evaluate a situation.

Consider these questions to help determine the right test:

  1. Does the problem concern concrete, repair mortars or grouts, overlays, sealers, coatings, curing compounds, or epoxy systems?
  2. Is the question to be answered related to strength, scaling, cracking, freezing, or chemical exposure?
  3. Did the product set up too fast or not at all?
  4. Is it blistering, peeling, delaminating, or discolored?
  5. Was a product applied too early or too late?
  6. What surface preparation was done and how?

You need a clear idea of the problems involved before deciding what tests can help identify or verify their causes. If you're not sure about the problem, you might contact your supplier or contractor to see if they can help. They may send out a technical service representative to help you. You can also contact a reputable testing laboratory and ask for assistance. An experienced staff member of a firm specializing in construction materials testing should ask the right questions and help you set a plan of action.

For instance, if a large exterior concrete slab exhibits sporadic scaling across its surface, testing the concrete's compressive strength is not necessarily the first action to be taken. Yet many people who contact us with this problem ask for this test first. While the compressive strength of a core may be tested for verification, we would recommend petrographic examination of the core first. Compressive strength testing of cores is relatively cheap compared to the petrographic examination, but you could easily spend $600 to $700 only to find out your concrete strength is fine and is not causing the scaling. Then you'd need to extract more samples, try alternative tests, and spend more money.

Petrographic examination can determine a number of things, such as:

  • Is chert present in the aggregate?
  • Does the aggregate appear not to be freeze-thaw durable?
  • Is the concrete air-entrained?
  • Does the air void system look uniform?
  • Is the water/cement ratio higher at the surface of the concrete than in the body of the concrete?
  • Was the concrete finished too early?

A petrographic exam will answer many more questions and give you much more value for the dollars you spend-and you can still verify the strength if it seems necessary.

Sometimes the problem is less obvious than it appears. Consider the peeling and discoloration of surface treatments. Suppose a product you applied to a large warehouse floor is peeling off the floor in some areas and not in others. Your clients claim you sold them an inferior product. They want you to remove it all and replace it with something better. Or worse, they've hired your competitor to remove your product and put down his own. In this case, some of the first things to consider are surface preparation and bond strength. You can hire a lab to come onsite, perform bond pull-off tests, and remove sections to verify proper surface prep and profiling. It's also important to verify the presence of a moisture vapor barrier beneath the slab. You can do this with a full-depth core. If no vapor barrier is present, the moisture vapor rate exceeds 3 to 5 lbs/1000 ft2, and your product is not breathable, this most likely is the problem. Now instead of being the cause of the problem, you can give your clients the information to solve their problem. You might even become part of the team to help fix it.

In another example, the air content in your concrete is slightly below the specified limit. You also know that a little more water than you'd have preferred was used in the mix. The job is in a northern, cold-weather state, and you're concerned about how the concrete will perform in the future. What should you do to alleviate your concerns? Some would take a core and check the strength. However, when the air content is low and the w/cm ratio is high, the real concern probably is the durability of the concrete. You should consider performing freeze-thaw durability testing (ASTM C 666) as well.

Cracking is another problem with many different possible causes. Is it plastic, thermal or drying shrinkage? Is it D-cracking or the effect of alkali-silica reactivity (ASR)? Is the issue joint spacing or lack of reinforcement? Is it improper subgrade preparation or was the concrete overloaded?

Of course, testing is used not only to provide answers to problems, but also to evaluate products. Federal, state, and local agencies usually require independent testing to evaluate product properties relative to established standards. Manufacturers can use test results to satisfy these governmental entities, and also as a marketing tool. Producers need test data to show consumers, contractors, and specifiers the benefits of using their products, especially when they're competing with relatively similar products.

Materials problems often can be viewed as a mystery, and testing can provide the clues needed to solve the crime.

Sampling for Testing
Proper sampling is just as important as selecting the proper test. In fact, most standard tests are governed by standard sampling methods meant to ensure that results from test to test will be comparable.

If you're taking samples for chloride determinations, review AASHTO T 260 for sampling methods. When sampling aggregates, refer to ASTM D 75; when sampling cement, use ASTM C 173; when sampling fresh concrete, follow ASTM C 172; and for sampling hardened concrete, ASTM C 823. ACI publication 201.1R-92 provides a guide for making a condition survey of concrete in service.

Here again, you should consult with your test lab if you're not sure what type of sample to take. They should be able to help you obtain the correct sample type to run a valid test.

ASTM C 42 describes proper methods for obtaining cores and sawed beams of concrete for testing. Taking cores for compressive-strength testing requires the core rig to be secured with an anchor system so as to produce a smooth and perpendicular specimen. Testing a core that has ridges on it or is not perpendicular will result in lower measured strengths. A core's length-to-diameter ratio and its orientation in the structure are also critical. Make every effort possible to avoid reinforcing steel when obtaining cores: using a cover meter to locate rebar before coring will help. If a core has steel in it, the core must be cut to remove the steel, and if the resulting core is below the 2:1 L/D ratio, correction factors must be applied to the compressive strength. Proper core conditioning prior to testing and proper lab procedures also are critical.

When obtaining samples to evaluate a surface-scaling problem, don't take cores or samples only from the areas where the scaling is very severe and the aggregate is visible. If you do, you're not giving the test lab much to work with. The source of the problem may lie in the surface that has scaled away. Therefore, samples for petrographic examination should be taken near the edge of the scaled area, so that the top of the core has half of its original surface and half that has scaled away. Better yet, take one core from the perimeter of the scaled area with the surface intact and one from within the scaled area for comparison. If you're considering ASTM C 672 deicer scaling resistance in your test program, be aware that all samples must have their surface intact prior to testing.

Choosing the testing laboratory
You might think that getting a laboratory to follow proper test procedures would be the easiest part of this process, but it is not. First, make sure the lab you hire is familiar with the particular tests you are requesting. Whenever possible, it's wise to find a laboratory that not only offers testing but also has experience with materials consultation. And note that you may need to go beyond your local area to find one. A good place to start to look for a lab is on the AASHTO Web site (www.aashto.org). Laboratories listed on this have been inspected by the Cement and Concrete Reference Laboratory at the National Institute of Standards and Technology. They have satisfactorily met standards for the tests that are listed. Also, the listed labs must have demonstrated proper quality control measures in the areas of calibration, testing procedures, and staff experience.

The ASTM web site (www.astm.org) also has a large listing of laboratories not only for construction materials but also for many other materials (rubber, plastics, stone veneer, metal, steel, etc.). ASTM does not accredit these laboratories. Rather, labs pay to be listed. Still, we've found ASTM's listing useful when referring inquiries outside our area of expertise.

Interpreting test results
Now that you have the results from your testing program, what do they all mean? A capable test lab should be able to help put the results in perspective. The lab should explain how the results relate to the problem under investigation, whether they can be used to verify material properties, or serve the purpose of quality control.

Results do not always turn out as expected or how the client wants them to. Occasionally, they don't tell us anything except that more testing is needed to solve the problem. Most of the time, however, if you gather enough information from the beginning to select the correct tests, you should be able to put the pieces of the puzzle together.

More often than you might think, putting a quality test lab on your team can help you save money and satisfy your customers.

About the Authors:

You can contact the Morrisons at CTL:
847-965-7500 or e-mail
Willy: wmorrison@ctlgroup.com Mike: mmorrison@ctlgroup.com
Willy Morrison is a Sr. Materials Technologist, Supervisor of Physical Testing Services and Quality Assurance Manager at Construction Technology Laboratories, Inc. (CTL). She has been with CTL for 19 years and has extensive experience with evaluation and testing of construction products and materials such as cement, cementitious materials, aggregates, admixtures, concrete, grouts, repair and epoxy mortars, stone veneers, and cast stone. Willy is an active member in ASTM C09 and serves on a variety of committees. She is a member of ACI and is currently the ACI-Illinois Chapter President.

Mike Morrison is Sr. Materials Technologist and Group Manager of Materials Testing & Analysis at Construction Technology Laboratories, Inc. (CTL). He has been with CTL for 20 years and has a extensive experience in the area of acceptance testing and evaluation of construction materials including cement, cementitious materials, aggregates, admixtures, concrete, grouts, sealers, coating and curing compounds. He is an active member of ACI and ASTM.


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© 2003 L&M Construction Chemicals, Inc. | ConcreteNews Summer 2003.

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