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 ten- sion is not a common test, although it is specified in several standards. The flexural beam test is the most commonly used test and involves ap- plication 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 condi- tions. Concrete density varies depending on the density of its ingredi- ents, aggregate, and cement, as well as its air content. There are several methods for measuring concrete density. The buoy- ancy balance method is widely used, whereby a known volume of con- crete 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
An Ultrasonic Concrete Tester is a non-destructive testing device used to measuret concrete uniformity and determine the presence or absence of voids, cracks, and other imperfections.
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 heav- ily 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 re- moved, 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-
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