Building Materials, Third Edition

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The principle object of proportioning concrete is to achieve requisite strength with desired
workability for which due attention should be paid to the selection of cement and aggregates
according to the specifications. Concrete mix design is dealt in detail in chapter 11. Some of the
considerations to be taken note of are:

1. The mix must be workable so that it can be placed and finished without extra effort.


2. Low cement consumption consistent with the attainment of desired properties.


3. High cement content improves strength, impermeability, density and workability.


4. With cement content, ingredients and workability remaining constant, the strength and
   impermeability of concrete increase with the density of mix.

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The standard method of evaluating the quality of concrete in buildings or structures is to test
specimens cast simultaneously for compressive, flexural and tensile strengths. The main
disadvantages of such methods are the delay in obtaining results and the fact that the test
specimens may not be truly representative of the concrete used in the structures. Factors
affecting the strength of concrete are so many that it is almost impossible to get samples
identical in quality with the entire mass of concrete and so the test samples should not be
regarded as a true representation of the quality of concrete in-situ. There is no strength test,
which provides the requisite information on concrete in-situ without damaging the concrete.
These and other drawbacks of destructive test methods have led to the development of non-
destructive methods of testing. Non-destructive methods are quick and can be performed both
in laboratory and in-situ with convenience.
Although there can be no direct measurement of the strength properties of structural concrete
for the simple reason that strength determination involves destructive tests, several non-
destructive methods of assessment have been developed. These depend on the fact that certain
physical properties of concrete can be related to strength and can be measured by non-destructive
methods. Such properties include hardness, resistance to penetration by projectiles, rebound
capacity and ability to transmit ultrasonic pulses and X- and –rays. These non-destructive
methods may be categorized as penetration tests, rebound tests, pull-out techniques, dynamic
tests, radioactive tests and maturity concept. Non-destructive test (NDT) can be performed on
fresh (green) as well as hardened concrete with equal ease and are described as follows.

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There are two tests, the maturity test and the ultrasonic test.

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Xis based on the principle that concretes having equal maturities will have equal
compressive strengths. The maturity of the in-situ concrete at the early ages can be determined
with the aid of an instrument known as maturity meter. This is used to determine the earliest
safe time for removal of formwork. The results are authentic provided the concretes have initial
temperature between 15-26°C and there is no loss of moisture during the period of curing.

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X Ultrasonic pulses are released from one of the transducers placed in contact
with one of the surface of the freshly placed concrete. The pulse is converted into an electric