There are several ways to evaluate the in-place strength of concrete, such as the maturity test, Windsor probe test, rebound hammer, and the pullout test. Learn more about each.
In previous columns, discussion focused on testing hardened concrete specimens—field-cured versus standard-cured specimens. But, what is the real strength of the concrete in the structure? The best method we have today to determine this is the maturity method (ASTM C1074).
The relationship of strength gain to temperature has been written about since the 1940s. Concrete gains strength faster in warmer weather than in cold weather. By placing a sensor in the fresh concrete and taking temperature readings at predetermined intervals, a maturity meter combines the effects of time and temperature to develop a “maturity number.” Having already developed a maturity curve, a maturity number versus compressive strength for a specific concrete mix design makes it possible to estimate concrete strength at that time and in that location of a structure.
There are a number of benefits to using the maturity method.
- It provides a better representation of in-situ strength gain than in the laboratory or with field-cured specimens. In 1988, the Federal Highway Administration determined that even field-cured specimens do not accurately reflect the true rate of hydration experienced by the concrete in a structure.
- It enables in-situ strength measurements that can be checked at any time. When cylinders are used, they can be tested only once—a problem if strength is below that required for shore or form removal, especially if additional specimens aren’t available.
- It provides better timing for strength-dependent construction activities. Because the strength can be checked at any time, improved timing results in maximum time savings without sacrificing safety or quality. Plus, no time is wasted for samples to be delivered to the lab, or for the lab to call with results.
- It enables in-situ strength measurement at “lowest-strength” locations. Given the fact that concrete subjected to higher temperatures gains strength faster than concrete at lower temperatures, concrete in structures gains strength at different rates in different locations depending on the different temperature conditions within the structure. For instance, thinner sections tend to generate and retain less internal heat than sections containing more mass or less surface area. Similarly, portions of a structure gain strength at different rates due to the effects of shading or of direct sunlight. The maturity method for measuring in-situ concrete strength gain enables you to take measurements at locations where strength gain is likely to be slowest, providing additional insurance that no subsequent work begins until adequate strength has been gained within the entire structure.
- It enables in-situ strength measurement at “critical strength” locations. In addition, the capability of measuring strength via maturity allows the engineer to specifically target strength measurements in those locations where critical stresses are expected for the anticipated loading conditions during subsequent construction activities.
Determining how much maturity testing costs also relates to time. The parking structure expansion performed several years ago at the General Mitchell International Airport in Milwaukee is an example. Concrete in the ramps and decks was tested to determine when post tensioning (PT) work could be performed. The contractor used field-cured cylinders to determine when to stress the tendons, but project managers were not happy with the three days it took to get the minimum strength required by the structural engineer. So the structural engineer approved maturity testing, enabling the contractor to tension the strands in two days, saving an entire day for each of about 50 individual PT placements. So, besides the accuracy of in-place concrete strength measurements, maturity testing also saves time and money.
Windsor probe testing
This method of testing the strength of concrete is performed by penetrating the surface of the concrete with a hardened steel probe with a blunt conical tip. The probe is fired into the concrete with a gun using a powder-filled cartridge. The depth of penetration is measured and the strength of the concrete is taken from a table provided by the manufacturer. However, as ASTM C803 states, a relationship has to be “experimentally established between the penetration resistance and concrete strength using similar concrete materials and mixture proportions as in the structure.” The strength of the paste might not change much, but aggregate strength certainly can change from region to region. Because the probes might penetrate aggregate particles, it really is important to determine the strength versus penetration curve for your area. The manufacturer provides a chart of Moh’s hardness for aggregate versus penetration depth in order to obtain the concrete strength, but this can be subjective, and is not generally good enough for accurate results.
The method for determining the rebound number of hardened concrete is given in ASTM C805. The uses of the rebound hammer are given in Section 5.1 of C805, which states that “this test method is applicable to assess the in-place uniformity of the concrete, to delineate regions in a structure of poorer quality or deteriorated concrete, and to estimate strength.” In practice, we have never seen anyone follow the test method correctly, because Section 5.2 states “Relationships between rebound number and strength that are provided by instrument manufacturers shall be used only to provide indications of relative concrete strength at different locations in the structure.” To use this test method for estimating strength, it is necessary to establish a relationship and rebound number for a given concrete mixture and given apparatus. To establish the relationship you must correlate rebound numbers measured on the structure with the strengths of cores taken from corresponding locations. At least two replicate cores must be taken from at least six locations with different rebound numbers. But often times inspectors take rebound readings from a number of locations without meeting the ASTM requirements. Readings from the same location often can’t be replicated either. For this reason we think the test is almost useless, due to the wide range of results. We always try to talk our clients into using almost any other test method.A variation of the rebound hammer test is the newly developed device made by the Nitto Construction Co., Hokkaido, Japan (see www.concretetester.com). The instrument tests the strength of new or mature concrete with greater precision and speed than conventional rebound hammers, without the cumbersome, time-consuming calibration issues rebound hammers typically have. It takes only a few seconds of setup time to calibrate the instrument. When the operator hits the test section of concrete with the hammer part of the device, it records and analyzes both impact and post impact data, processing the information much more quickly than other rebound hammers.When a worker applies gentle impact force at the point of the hammer on the test section, the instrument reads concrete strength with unprecedented precision. It also can detect previously unreadable defects and be used to detect areas of delaminated concrete surfaces.
The pullout test (ASTM C900) is a slightly destructive test, but the area of the pullout is relatively small and can be patched. A round, metal insert head and connecting shaft is buried in fresh concrete, the top of the shaft being at the top of slab height. The shaft has a smaller diameter than the insert head. When the load on the pullout shaft is increased to failure, a conical piece of concrete will be taken out. The pullout strength can be related to compressive strength to determine whether post-tensioning may proceed, forms and shores may be removed, or winter protection and curing may be terminated. Post-installed anchors also can be used. However, in this test, the range of individual test results can vary by 30% or more.
This article is an overview of the approved test methods for estimating the strength of concrete in a structure. Each of the test methods discussed has many more components than can be mentioned here. We recommend that competent firms and individuals do the tests, and where required in the test method, the tester must hold proper and current certifications.