Non destructive testing is important for Quality Assurance of hardened concrete and the evaluation of existing concrete structure with regard to their strength & durability.
Types of Non Destructive Testing
Following are the commonly done Non Destructive Tests for concrete:
This test is done to assess the quality of concrete. The method consists of measuring the time of travel of an ultrasonic pulse passing through the concrete being tested. Comparatively higher velocity is obtained when concrete quality is good in terms of density, uniformity, homogeneity etc.
Rebound Hammer test is a Non-destructive testing method of concrete which provide a convenient and rapid indication of the compressive strength of the concrete. The rebound hammer is also called as Schmidt hammer. It consist of a spring controlled mass that slides on a plunger within a tubular housing. When the plunger of rebound hammer is pressed against the surface of concrete, a spring controlled mass with a constant energy is made to hit concrete surface to rebound back. The extent of rebound, which is a measure of surface hardness, is measured on a graduated scale. This measured value is designated as Rebound Number (rebound index). A concrete with low strength and low stiffness will absorb more energy to yield in a lower rebound value.
Carbonation occurs in concrete because the calcium bearing phases present are attacked by carbon dioxide of the air and converted to calcium carbonate. Depth of Carbonation is assessed using a solution of phenolphthalein indicator that appears pink in contact with alkaline concrete with pH values in excess of 9 and colourless at lower levels of pH.
This test provides an indicator of the level of corrosion or the corrosive potential within the concrete.The test is used extensively for condition surveys of suspect RCC structures to identify areas with corrosion activity for further analysis to establish the cause of corrosion and estimate residual service life.
The concrete core is tested for compressive strength of hardened concrete in the structure. Concrete core is drilled from the RCC member and testing is done to find the compressive strength.
Chemical analysis of concrete is done to determine the pH value, chlorides, sulphates etc to understand the causes of failure of concrete. Whenever there is chloride in concrete, the risk of corrosion increases with warm, moist conditions. The chloride content should not be more than 0.6 Kg/Cu.M of concrete. Sulphates present in the concrete can cause expansion and disruption of concrete. The totalwater-soluble sulphatecontentof the concrete mixexpressed as SOl’ should notexceed 4 percent by massof the cement in the mix.