STRUCTURES + BUILDINGS
Testing concrete for quality and reliability
Test equipment evolves to support changes in concrete materials and testing standards. By Alfonso J. Rivera, P.E.
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 manu- facturer’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 oppor- tunities for error and make training less onerous, and enhancements in reliability and transparency improve confidence in concrete quality and 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 ap- plied 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 con- struction activities. greatly enhance dispute resolution. Tests on hardened concrete The compressive strength of concrete is carried out by measuring the breaking load of cylindrical or cube concrete specimens under a con- stant 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).
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.
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 per- centage 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
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csengineermag.com
april 2018
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