under-vibration of the mix, settlement of aggregate, and others. The number of concrete specimens being compression tested is often too small to be considered more than random testing. It is also important in many cases to determine the compressive strength of aged concrete elements for structure retrofitting, modeling, and analysis. Therefore, non-destructive testing (NDT) performs a vitally important role in the assessment of finished concrete structures. The Concrete Test Hammer, also known as the Rebound Hammer or Schmidt Hammer, is one of the most widely used NDT methods for determination of in-situ concrete strength. The test hammer uses a spring-actuated mass that is released to impact the surface of a concrete sample with a defined amount of energy. The rebound distance fol- lowing impact is then measured. The hammer is held perpendicular to the surface being tested and the rebound varies according to the hard- ness of the sample point. This rebound measurement is then converted into compressive strength by means of a conversion chart. Different conversion charts have been generated to compensate for instrument orientation. These charts were developed by carrying out rebound tests on concrete samples before being crushed under compression. Equipment requirement includes a concrete test hammer and a testing anvil for calibration. Ultrasonic pulse velocity (Standards: ASTM C597; EN 12504-4) — The ultrasonic pulse velocity test method is another NDT method for assessing the quality of finished concrete. The basic principle of this method is that the velocity of an ultrasonic pulse through concrete is related to its density and elastic properties, so the time of travel is mea- sured for an ultrasonic pulse passing through the concrete being tested. Relatively high velocity is observed when concrete quality is good in terms of density, uniformity, homogeneity, etc. Care is necessary when testing, but an experienced operator may obtain a considerable amount of information about a concrete member. The advantage of this method is that the pulse passes through the complete thickness of the concrete so that significant surface and subsurface defects can be detected. There are several applications for the Ultrasonic Concrete Tester. These include measurement of concrete uniformity; determination of the presence or absence of voids, cracks, and other imperfections; deterioration of the concrete that might have occurred due to age or through the action of fire, frost, or chemical attack; the measurement of layer thickness and elastic modulus; and the determination of concrete strength. Summary Concrete testing does more than simply demonstrate compliance with specifications; it provides a check on the variabilities that exist in concrete mixtures and in the processes that concrete undergoes before it becomes a hardened finished product. When conducted properly, testing also provides accurate, reliable, transparent information that greatly improves dispute resolution.
The slump test requires low-cost equipment and gives immediate results onsite.
7) —Air entrainment is often necessary in areas where concrete is exposed to cycles of freezing and thawing. Water expands when fro- zen and this can create internal forces that may exceed the bonded or tensile strength of the concrete element, resulting in cracking. Air in the form of very small bubbles provides void spaces within concrete that act as a reservoir where water can deposit and expand, relieving the internal pressure in the freezing cycle and providing protection to the concrete. Air is homogenously distributed in the concrete mix us- ing mixing blades and additives are used during the mixing process to stabilize the bubbles of entrained air so that they remain once the concrete has hardened. The test for measuring air content in normal weight concrete is usu- ally performed using the pressure method because it is relatively fast. Concrete is placed inside a container of known volume and flushed out at the top. The method is based on Boyle’s law, which states that the volume of air in the voids is proportional to the applied pressure. Pressure is applied to the sealed test container by connecting a separate air chamber equipped with a pump. With the valve closed, the chamber is pressurized to a calibrated operating pressure and the pressure gauge is tared. When the valve is opened, the air in the concrete expands into the test chamber and a gauge provides the reading in units of air content. The necessary equipment includes a Type B pressure air entrainment meter, strike-off bar, and a rubber mallet for aiding concrete settlement in the vessel. Non-destructive concrete testing Concrete test hammer (Standards: ASTM C805; EN 12504-2) —The standard method for determining the compressive strength of concrete (as outlined above) is by crushing cured concrete specimens in a com- pression machine. However, a variety of factors can affect the quality of hardened finished concrete. These include differences in concrete batches, variations in the placement of concrete in forms, over- or
ALFONSO J. RIVERA, P.E. , is with ELE International (www.ele.com), which specializes in the design, manufacture, and supply of high-quality construc- tion materials testing equipment and environmental instrumentation.
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april 2018
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