The underside of a failing bridge.
Lucideon The underside of a failing bridge.

Concrete can fail in numerous ways and for many reasons. Fortunately, there's more than one way to learn what happened. This understanding allows the most appropriate action to be undertaken to repair, replace, and, where appropriate, apportion responsibility.

Always conduct a visual inspection before deciding on sampling rates, sampling techniques, and testing requirements. An experienced forensic testing consultant can often understand the causes of a failure from visually examining the deficiency. Floor cracking, for example, is often due to poor design and incorrectly positioned movement joints, which can be easily seen.

Flaking corrosion of exposed reinforcing bars indicates carbonation of the concrete, whereas a more penetrative localized corrosion indicates chloride attack. Small samples of concrete can be removed from areas of failure using hand tools. These samples can be tested on-site to establish the depth of carbonation or in the laboratory to detect the presence of chloride ions.

Non-Destructive Examination

A range of non-destructive techniques are also available, each of which has its ideal application and limitations.

Position, size, and depth of reinforcing bars can be mapped using a Ferroscan, also called a cover meter; this data can be used to confirm that the design is fit for the purpose. The meters are simple to operate but can be unreliable if the correct parameters aren't used. Obtaining clear readings can also be difficult, particularly if there is more than one layer of bars and it is placed at a depth greater than 8 inches.

For floor slabs, ground penetrating radar (GPR) can be used to scan large areas to reveal the presence of voids, rebar, and any cracking. As with cover meters, this tool is best used with some previous knowledge of the construction make-up and requires an experienced operator to interpret readings. Both techniques provide much more reliable results if small areas are opened to verify bar diameters, position, and depth of cover in order to calibrate the equipment.

Half-cell potential can be used to map corrosion in reinforcing steel over larger areas. This requires breaking open the surface of the concrete to expose areas of rebar, but the damage is minimal.

When assessing large areas, a visual inspection is often carried out to determine the sites to be sampled; the samples are then generally taken from poorer quality sections. A rebound hammer can be used to investigate the uniformity of the concrete and to delineate the areas of poorer quality. Testing is conducted using a 3-foot-by-3-foot grid pattern, and results can be estimated quickly and easily in-situ. Results are more accurate when the hammer is calibrated against cores sampled on-site and testing is carried out in the laboratory. This technique is ideal for large areas because each test can be done in 10 minutes to 15 minutes without causing damage.

When concrete is degrading, the first characteristic to investigate is compressive strength. Removing core samples from the site is the most reliable method to understand concrete strength. Samples should ideally not contain rebar and must be taken such that when the core is prepared for testing, it has a diameter:height ratio of 1:1. The core should also be prepared correctly, that is, capped at each end before destructive compression testing. Material from the crushed core can also be used to establish mix proportions, presence of sulphates, and aggregate gradations.

Cores are useful not only for strength testing; they can also be thinly sectioned and used for petrographic analysis. This is an excellent technique for forensic analysis and provides details on parameters including mix content, presence of voids, chemical attack, air entrainment, analysis of cracks, and alkali-silica reactivity.

These are the most common testing techniques available. Having experienced technicians conduct the testing will ensure correct sampling and analysis and provide the most time- and cost-effective outcome.

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