As a manufactured material created by a complex process involving multiple ingredients, some with variable properties, testing concrete is essential to ensure compliance with specification and application requirements. Owners, designers, builders, contractors, and inspectors all have different priorities, so it is vitally important that testing meets the expectations of all stakeholders, particularly in relation to the test standard that is applied.
In addition to changes in concrete materials and in concrete testing standards, test equipment is also continually evolving. From a manufacturer’s perspective, new equipment is constantly being developed to meet the needs of the materials testing engineer. At ELE that means the instrument design teams are looking to enhance safety, usability, reliability, and transparency. Improvements in usability reduce opportunities for error and make training less onerous, and enhancements in reliability and transparency improve confidence in concrete quality and greatly enhance dispute resolution.
Tests on hardened concrete
Compressive strength (Standards: ASTM C31, C39, C192; AASHTO T-22, T-23, T-126; EN 12390-1, 12390-2, 12390-3, 12390-4, 12504-1) — The compressive strength of concrete is usually the most important part of a specification, so the measurement of the compressive strength of the mixture is very important in the structural performance of an engineering project. It is necessary to determine whether a delivered concrete mixture meets the strength specified by the design engineer to guarantee the structure’s ability to perform adequately under the applied static or dynamic loads.
Design engineers use compressive strength to determine the size of the structural members to accommodate the design loads of the structure. Compressive strength tests may be used for quality control, design mixture approval, and acceptability. In-place compressive strength tests are used for verifying strength development and scheduling construction activities.
The compressive strength of concrete is carried out by measuring the breaking load of cylindrical or cube concrete specimens under a constant rate of loading in a compression testing machine. The strength is calculated by dividing the breaking load by the cross-sectional area of the specimen, perpendicular to the loading direction. It is then reported as the compressive strength in units of pressure (i.e., psi, kg/cm2 or MPa).
Test results are derived from the average strength of several specimens casted from the same sample and tested at the same age of curing, generally specified to have full designed capacity at 28 days (28-day strength). To comply with the compressive strength requirements for a project, the average of three consecutive test results must meet or exceed the specified strength and no test must fall below a certain percentage of the specified strength.
The latest compression machines have water and dust-proof, touch-screen color displays with remote connectivity so that operators can run a test from a remote PC. Standard methods are programmed into the instruments and loading is managed automatically, leaving the operator free to conduct other work. All test data is logged internally, providing full traceability.
The necessary equipment includes a compression testing machine in addition to sample preparation equipment, including molds, mixers, and vibration equipment, and sample curing equipment.
Tensile strength (Standards: ASTM C31, C78, C192; AASHTO T-23, T-97, T-126; EN 12390-5, 1339, 1340, 1521) — Although concrete is not normally designed to resist direct tension loads, concrete structures are highly vulnerable to tensile cracking due to various loading effects such as dynamic loading, as well as temperature variation. Tensile strength of concrete is relatively low in comparison — approximately 10 to 15 percent of the compressive strength. However, depending on the desired application for the mix, it may be advantageous to study the bond strength of the concrete components.
There are several tests available to measure the tensile strength of concrete: direct tension force application, flexure, and indirect split cylinder tests. Because of the difficulty in applying a direct tension force that is free of eccentricity and concentrated stresses, uniaxial tension is not a common test, although it is specified in several standards.
The flexural beam test is the most commonly used test and involves application of a bending load to an unreinforced concrete prism or beam using symmetrical two- or three-point loading. As the concrete beam bends under the load, the bottom fibers of the cross-section are placed in tension. The failure load of the beam is then used to calculate the tensile strength or modulus of rupture.
In split cylinder tests, a cylindrical specimen is placed with its long axis horizontal between a compression machine’s platens and a load is incrementally applied to the side of the specimen until it fails by causing the cylinder to split along its axis.
In addition to a compression testing machine, the tensile test also requires sample preparation equipment including molds, mixers, and vibration equipment, and sample curing equipment. For the flexural tests, either a flexural beam frame or flexural platens are installed on the compression machine. For the split cylinder test, a testing jig and wood strips are required to apply the splitting load.
Density (Standards: ASMT C29, C138; AASHTO T-19, T-121; EN 12390-7, 1097-3) — The density of the concrete mix can be adjusted to suit different applications. High-density concrete is often used in casting pretensioned reinforcing elements such as tensioned cables or reinforcement bars. The concrete is poured over the pretensioned cables to provide additional bonding strength between the bars and the concrete. Once the concrete hardens, the bars are released causing compression of the concrete element. This is used on high-strength elements such as bridge girders and pretensioned floor slabs.
Low-density lightweight concrete and air entrained concrete is used in on-grade floor slabs to improve performance in harsh weather conditions. Concrete density varies depending on the density of its ingredients, aggregate, and cement, as well as its air content.
There are several methods for measuring concrete density. The buoyancy balance method is widely used, whereby a known volume of concrete is weighed in both dry and submerged conditions, allowing the calculation of both concrete density and specific gravity. Alternatively, density can be calculated by means of yield buckets in which a fresh concrete sample is placed inside a metal vessel of known volume. The fresh concrete is compacted into the vessel, filled to the top, scraped level and weighed. Density is then determined by a simple weight-over-volume calculation.
Depending on the chosen method, the applicable equipment includes cylinder molding equipment, a buoyancy balance and frame, or unit weight vessels, scale, tamping rod, strike-off plate, and miscellaneous laboratory equipment.
Tests on fresh concrete
Slump (Standards: ASTM C143; AASHTO T-119; EN 12350-2) — The concrete slump test is used to measure the consistency and workability of freshly made concrete as well as the ease with which it flows. High-flow mixtures are used for casting concrete inside heavily reinforced forms to ensure adequate and homogenous mixture is distributed throughout the reinforced concrete element. Therefore, the main purpose of measuring slump is to achieve acceptable workability.
Concrete slump tests are carried out in every batch of freshly made concrete to check for uniform quality of the mix during construction. The test shows the water-cement ratio, with higher water contents showing higher slump values. Slump is an indicator of the compressive strength of hardened concrete; in general, for standard weight concrete, the higher the water content, the lower the strength.
The slump test is simple to perform; it requires low-cost equipment and gives immediate results onsite. It is therefore used widely both at concrete manufacturing plants and construction sites to evaluate the workability of the fresh mix. The test is carried out with a conical-shaped mold called the Slump Cone or Abrams Cone, of standardized dimensions, which is filled with fresh concrete. When the cone is removed, the fresh concrete settles vertically, and the slump value is the measurement of vertical settlement, or slump, from the original height.
The results of the test are interpreted by examining the shape and slump of the mix. The shape shows the mixture has a true slump when the settlement is even and not excessive. Shear slump has a partially collapsed shape on one side and a collapsed slump with excessive settlement indicates that the mixture is too wet.
Equipment requirement includes a metal slump cone, tamping rod, slump cone base, and tape measure.
Air content (Standards: ASTM C231; AASHTO T-152; EN 12350-7) —Air entrainment is often necessary in areas where concrete is exposed to cycles of freezing and thawing. Water expands when frozen 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 using 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 usually 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 compression 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 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 following impact is then measured. The hammer is held perpendicular to the surface being tested and the rebound varies according to the hardness 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 measured 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.
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.
Alfonso J. Rivera, P.E., is with ELE International (www.ele.com), which specializes in the design, manufacture, and supply of high-quality construction materials testing equipment and environmental instrumentation.