The Easy Way to Test Hardened Concrete

Concrete is an extremely versatile building material that can be supplied ready-mixed, prepared freshly on site or delivered as precast component parts that can be produced to the exact specification and quantity required. However, the almost infinite range of variables in the properties of a concrete

mix can result in inconsistent tolerances and compromised safety unless thorough and accurate testing is carried out. For this reason, material and component manufacturers, material testing laboratories and civil engineering construction companies are now investing in the latest testing technology, to ensure that concrete consistently demonstrates the requisite qualities for a given application. By making testing more efficient and reliable, this technology is helping to increase productivity, improve safety and reduce costs.

As well as testing fresh concrete to assess the suitability and consistency of a mix, hardened concrete should also be thoroughly tested to ensure that the precast component parts to be used have the desired properties and that concrete already in use has retained its required characteristics. It is often advantageous to specify plant-produced concrete as precast components are manufactured under controlled conditions and offer precisely produced products and high levels of consistency. However, even when this is the case, there remains a requirement for samples to be taken from site, or the production plant, to be tested under controlled conditions.

Concrete density

One of the properties of hardened concrete that should be measured is density, which can affect its durability, strength and permeability. Hardened concrete density is determined either by simple dimensional checks, followed by weighing and calculation, or by weight in air/water buoyancy methods. The density of cubic or cylindrical samples can be quickly and accurately determined using a buoyancy balance consisting of a rigid support frame and a platform-mounted water tank. A mechanical lifting device is used to raise the water tank through the frame height thereby immersing the specimen suspended below the balance. The balance supplied may also be used as a standard weighing device, which provides a versatile and comprehensive weighing system in the laboratory.

Engineers are also able to measure drying, shrinkage and moisture movement in pre-hardened concrete samples. Initial drying shrinkage tests can be taken, measuring the length of a moulded and cured specimen under specified conditions and comparing it to its final constant length when dried. In a similar test, drying shrinkage can be measured, i.e. the difference in length of a matured specimen cut from concrete and saturated, and its final length upon being dried. A third test can be carried out for moisture movement, whereby the difference between the constant length of a specimen when dried and its length when subsequently saturated in water is recorded. These three tests to determine the change in length of a concrete sample brought about by a change in moisture content, enable engineers to make accurate predictions of how a precast section will behave when in contact with moisture.

Perhaps the most important test for hardened concrete is compressive strength testing, which enables engineers to assess the strength of a concrete sample and its performance under actual loading, as opposed to the design loading. Furthermore, any deterioration, from chemical action, weathering, fatigue or excessive loading, can be precisely measured.

The latest generation of compression testing equipment offers extremely reliable results, automating a number of processes for ease of use. Equipment typically combines a high stability load frame, hydraulic ram assembly and high-specification loading platens, with incorporated digital and microprocessor technology to provide highly accurate results. Closed-loop microprocessor control is invaluable in today's compression testing equipment, significantly increasing productivity and offering outstanding levels of accuracy and consistency in testing cycles.

Advanced microprocessor-controlled units

Advanced microprocessor-controlled units can be used to provide fast, accurate and detailed results recording and analysis for high throughput testing, both on site and in the laboratory. These machines are fully automatic and require minimal operator involvement, improving productivity and reducing costs. A wide range of different-sized samples can be accommodated by the equipment, which is designed for different frequencies of use and varying levels of data handling. The units are also compliant with the latest EN specifications and employ specially-designed upper platen/ball seating arrangements to ensure there is no movement of the assembly after initial contact with the sample. Research has shown that any movement of the upper platen, coupled with frame deformation during the loading cycle of standard cube samples, can induce tensile cracks and result in variances of measured strength.

Compressive load is achieved using a hydraulic power unit to provide the required pressure to the ram/cylinder unit of the load frame. Hydraulic systems that incorporate automatic closed-loop controlled loading with a variable output pump benefit from low-temperature operation, as only the required amount of hydraulic fluid is delivered to meet the loading requirements. A further benefit is that service life of components is extended due to reduced operational stress. While much of the operation process is automated, users have access to a simple control panel for calibration, sample selection, test control and options to save, print or download test data to a computer.

Concluding remarks

The latest testing equipment allows hardened concrete, including precast components and extracted samples from completed structures, to he tested more efficiently and accurately than previously possible. Through the use of advanced microprocessor technology, many testing processes can be automated, reducing human error and enabling detailed results to be easily distributed. By investing in testing, risk and uncertainty can be efficiently and cost-effectively minimised, resulting in sound concrete structures.