Test method explained
Concrete Testing
Aitken Rowe is NATA accredited for concrete testing at all three laboratories — Wagga Wagga, Griffith and Albury/Wodonga — with the capacity to test hundreds of specimens. Six tests matter on a typical job, and only one of them is strength.
More than a 28-day number
Concrete has been used for centuries, but modern concrete is a specialised mix of sand, gravel, cement and additives that produce specific strengths and properties. Engineers specify a mix to reach a minimum compressive strength at 28 days — but the design also accounts for site location and climate, flexural strength, air voids and slump.
The strength test is the obvious one. The others are what confirm the supplied concrete actually suits its intended use.
Two things are certain with concrete
There is an old joke on site that only two things are certain with concrete: it will get hard, and it will crack. The second one is why drying shrinkage testing exists — cracking is managed, not eliminated, and the test proves the mix stays inside the limits the designer allowed for.
Compressive strength
Fresh concrete is compacted into specialised moulds, 200mm high and 100mm in diameter. Once it has hardened enough, the cylinder is removed from the mould and placed in a temperature-controlled water bath to finish curing under controlled conditions.
After the specified number of days the ends of the cylinder are capped — with either a sulphuric compound or calibrated rubber discs — and loaded at a set pace rate until the specimen fractures. The failure load divided by the surface area gives a pressure in MPa. The strength at 28 days is the nominal strength: 32MPa concrete must reach 32MPa at 28 days. This is run on most commercial and residential projects.
The same test applies to grout, cast as cubes rather than cylinders. We can supply moulds for self-sampling or attend site and sample ourselves. For concrete that has already cured, we retrieve cylinders of various diameters using our own coring equipment.
Schmidt rebound hammer
The most precise way to determine strength is a 28-day compressive test on cylinders sampled during the pour. When that did not happen, there are non-destructive ways to estimate it. The most common is a rebound hammer: a spring-loaded mass of known energy is pushed into the concrete surface and bounces back, and the amount of rebound is converted into an estimated compressive strength.
We use a digital rebound hammer that can be used horizontally or vertically. To confirm the strength of an existing slab or large footing we test in a grid across it and then take a minimum of three core samples, correlating the cores against the hammer results for a more accurate answer.
Slump
Workability is one of the key parts of a concrete design. A wetter, high-flowing mix is useful where concrete must be pumped to height or needs to self-level. Low workability concrete suits mass structures such as dams, where too much moisture generates heat that can damage the structure.
The test uses a steel cone on a flat, smooth surface, filled with compacted concrete in three layers. Once filled and levelled the cone is lifted clear and inverted beside the slumped concrete, and the difference in height is measured as the slump.
Flexural strength
Flexural strength is the indirect tensile strength of the concrete. A beam is cast in a rectangular mould and loaded in a specialised roller frame — two bottom support rollers and two upper rollers pressing down — until the beam breaks. This test is typically specified on commercial-scale projects.
Drying shrinkage
Cracking is minimised with mix additives, strict moisture control and properly positioned control joints — pre-planned cracks. Drying shrinkage testing confirms the mix stays within the limits the designer specified.
A specimen is cast in a rectangular mould with stainless steel studs set in place. Once hardened it is demoulded and placed in a controlled environment where its length is monitored. Monitoring can run up to three months, though most specifications require a 21 and 56 day result.
Air content
Specialised mixes for certain climates require strict control of air voids. Too little air reduces freeze-thaw resistance; too much considerably reduces compressive strength.
The method compacts concrete into a special bowl, attaches an upper air chamber, replaces the surrounding air with water and fine-tunes with valves. A hand pump introduces pressure into the upper chamber, which is released into the lower bowl, and the gauge displays the apparent air content. We run it alongside the routine tests on the same visit.
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