The effect of steel platens of testing machine in uniaxial loading In compression test, tangential forces being developed between the end surfaces of the concrete specimen and the adjacent steel platens of the testing machine. The steel platen will restrain the lateral expansion of the concrete in the parts of the specimen near its ends: the degree of restraint exercised depends on the friction actually developed. When the friction is eliminated, e .g by applying a layer of graphite or paraffin wax to the bearing surfaces, the specimen exhibits a large lateral expansion and eventually splits along its full length. With friction acting, i.e. under normal conditions of test, an element within the specimen is subjected to a shearing stress as well as to compression. The magnitude of the shearing stress decreases and the lateral expansion increases, with an increase in distance from the platen. As a result of the restraint, in a specimen tested to failure there is a relatively undamaged cone or pyramid of height approximately equal to 0.5d√3

Tensile strength a. Direct tension test: Although concrete is not normally designed to resist direct tension, the knowledge of tensile strength is of value in estimating the load under which cracking will develop. The absence of cracking is of considerable importance in maintaining the continuity of a concrete structure and in many cases in the prevention of corrosion of reinforcement. There are two types of test for strength in tension: direct tension test, and splitting tension test. a. Direct tension test: Dog bone sample

b. Splitting tension test:

Arrangement for the modulus of rupture test

There exists also a test for flexural strength under centre-point loading, prescribed in ASTM C 293-94. In this test, failure occurs when the tensile strength of concrete in the extreme fibre immediately under the load point is exhausted. On the other hand, under third-point loading, one-third of the length of the extreme fibre in the beam is subjected to the maximum stress, so that the critical crack may develop at any section in one-third of the beam length. Because the probability of a weak element (of any specified strength) being subjected to the critical stress is considerably greater under two-point loading than when a central load acts, the centre-point loading test gives a higher value of the modulus of rupture, but also amore variable one. In consequence the centre-point loading test is very rarely used.

Factors affecting strength of concrete: 1.Water/cement ratio: In engineering practice, the strength of concrete at a given age and cured in water at a prescribed temperature is assumed to depend primarily on two factors only : the water/cement ratio and the degree of compaction. When concrete is full compacted, its strength is taken to be inversely proportional to the water/cement ratio. It may be recalled that the water/cement ratio determines the porosity of the hardened cement paste at any stage of hydration. Thus the water/cement ratio and the degree of compaction both affect the volume of voids in concrete.

Gel/space ratio: It is alternative parameter to porosity. This ratio is defined as the ratio of the volume of the hydrated cement paste to the sum of the volumes of the hydrated cement and of the capillary pores. The gel/space ratio can be used to estimate the minimum water/cement ratio required for the cement gel just to occupy the available space.