Testing of Concrete - Why is it Important?

Certain tests should be undertaken during construction to ensure the quality of building products. This is particularly applicable to the pouring of concrete, as errors made at this stage of the build can lead to costly and potentially dangerous defects becoming apparent in the future.

The four most common mistakes made when pouring a concrete slab in residential construction include:

• Incorrect use or lack of reinforcement
• Incorrect excavation of foundations to a depth or width that suits the soil type that the concrete is poured upon.
• Poor workmanship during the placement of  concrete or insufficient vibration to ensure a good quality of finish
• Incorrect MPa or poor quality concrete used

Testing of concrete should be undertaken to help the building supervisor or engineer determine that the concrete is of a satisfactory standard to be used, and is fit for purpose for its intended application. There are several ways in which testing can be carried out:

• By slump testing
• By compression testing

Slump testing
The slump test is a measure of the consistency or mobility of concrete and is the simplest way of ensuring that the concrete on the site is not varying. This test should be carried out at regular intervals as a measurement of consistency. In situations where the slump varies between testing, it is an indication that the content of the mix has changed, which is often attributed to the moisture content of the mix. The mix can then be corrected to ensure a consistent mix.

Materials required for conducting a slump test include a standard slump cone, a bullet pointed steel rod or tamping rod, an impervious base plate, a rule, and a straight-edge.

The following steps are taken to carry out slump testing of concrete:

1. Moisten the inside of the slump cone and place it large end down on a clean level surface. Hold it firmly in place with a foot on each footrest.
2. Fill the cone, in three approximately equal layers, with concrete from the sample. Each layer should be tamped don exactly 25 times with the tamping rod, which must be allowed to penetrate each layer.
3. The strokes must be uniformly distributed over the whole surface of the layer and not worked up and down continuously in one place.
4. After the top layer has been compacted, the surface of the concrete is struck off level with the top of the cone and any surplus concrete removed from around the base.
5. The cone should be lifted, carefully but firmly, straight up so the concrete is allowed to subside. Lift the cone smoothly and quickly but do not jerk, twist or take off at an angle or a false result may be obtained.
6. To measure the slump, invert the cone and place it alongside the slumped concrete. Lay the tamping rod on top of the cone and measure the amount of slump, measuring to the highest point of the concrete. The slump is recorded to the nearest 10mm.

As a general guide the slump of the concrete should not exceed 75mm. The following shapes may also assist in determining the consistency of the concrete mix:

• True slump: This is a good shape. Concrete settles but stays consistent.
• Zero slump: If the concrete does not slump then the mix is deemed potentially ‘non-plastic’. Additional moisture may be required.
• Collapsed slump: If the concrete slump collapses then the concrete mix is considered potentially non-cohesive. The concrete mix may be too wet in this instance.
• Shear slump: The slump is believed to have no displaced centre, that is, the concrete lacks cohesion. The concrete may undergo segregation and bleeding as a result of shear slump and thus is undesirable for concrete.

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Compression testing
The strength of concrete is determined by making specimens, curing them, and then crushing them to ascertain their strength. The preparation of specimens is crucial in achieving an accurate result as a badly prepared specimen will almost always return a low result.

Compressive test specimens are normally cylinders 150mm in diameter by 300mm high. Materials required to conduct this test are a cylindrical mould (should be metal and rigid enough to retain shape during preparation of specimen), tamping rod, rule, base plate, and mineral oil.

The method of testing is as follows:

1. Before filling with concrete, the mould should be clean and coated inside with a very light film of mineral oil.
2. Place the mould on a level surface and fill with concrete from the sample in three equal layers. Rod each layer 25 times with a bullet pointed rod 600mm long and 16mm in diameter, allowing each stroke to penetrate the previous layer. In this case it is necessary that the concrete be fully compacted and it may be necessary to rod each layer more than 25 times. The rodding must be distributed over the whole surface of each layer and not merely in one place. The concrete in the mould may be compacted by vibration if suitable vibrators are available.
3. After the specimen has been moulded, it should be stored in a place where it will be undisturbed for 18-24 hours, kept moist and at a temperature of between 21 degrees Celsius and 24 degrees Celsius. After 24 hours the specimen should be removed from the mould and again stored under moist conditions and at the correct temperature. This is called curing.
4. For transport to the laboratory, the specimens should be packed in moist sand or hessian so that they will remain moist and be undamaged during transit.

Incorrect placement or use of concrete may lead to cracking or even structural failure. If cracking is found at your property, Cap-It-All Building Inspections will not only identify the cracked areas but also inform you as to the predicted reason for the cracking, and assist you should you require a complaint to be lodged via the building commission.

Our building inspectors have a sound knowledge of Perth’s soils and are aware of the areas where foundation movement is more prevalent. Where cracking is found to be a result of incorrect use of concrete, ensure you choose a building inspector who can assist you accordingly.

Buying Property Perth will continue to release informative blogs on a weekly basis. Visit Cap-It-All Building Inspections website for more information on concrete testing and other important information to consider when buying property Perth.