The factors have been found to influence the test results to a great extent are: 1. Effect of Size and Shape of Specimen 2. Effect of Conditions of Casting 3. Effect of Moisture Content of Specimen 4. Effect of Temperature of Specimen at Test 5. Effect of Bearing Conditions 6. Effect of Rate of Loading.
Factor # 1. Effect of Size and Shape of Specimen:
As the concrete is composed of elements of variable strength, it is reasonable to assume that larger the volume of concrete subjected to stress the more likely it is to contain an element of an extreme low strength. Hence the measured strength of a specimen decreases with increase in its size.
From experimental results, cylinders of height equal to twice the diameter and cubes are taken as standard specimens. Such cylinders are neither so short that small variation in length affects the strength considerably nor so long that column action will affect its strength. The common size of cylinder is 15 x 30 cms and that of cube 15 cms. The standard specimens for flexural strength are rectangular beams with ratio of breadth to depth as 1.5. The standard size is 15 x 15 x 70 cms. If the normal maximum size of aggregate does not exceed 20 mm then 15 x 15 cms. x 50 cms beam may be used.
When a concrete is tested in compression by means of cylinders of the like shape, but of different sizes, it has been found that larger the specimen, lower the strength. In Table 14.7 the strength of various sizes of cylinders relative to 15 x 30 cms cylinder expressed as percentage of 15 x 30 cms cylinder is shown. As the size of cylinder is increased above 45 x 90 cms, the variation in relative compressive strength is small.
Experimental results also have shown that the decrease in strength with the increase in size of the specimen is less pronounced in lean mixes than in rich mixes. For example the strength of 45 cms and 60 cms diameter cylinders relative to 15 diameter cms is 85% for rich mixes and 93% for lean mixes Fig. 14.9. Fig. 14.10 shows the relation between mean strength of cylinder and cubes of different sizes.
Specimen Size and Aggregate Size:
The least dimension of test specimen should be appreciably larger than the largest size of the aggregate particles used in concrete. Generally the ratio of least dimension of test specimen and largest size of the aggregate should be between 3 and 4.
Effect of Height to Diameter Ratio:
Experiments conducted by U.S.B.R. have shown that the ratio of height to diameter of the specimen affects the compressive strength of concrete to a great extent.
When cores or cylinders of non-standard shape are tested, the equivalent strength of standard shape (h/d = 2) can be computed approximately by multiplying the observed compressive strength by the corresponding factor given in the following Table 14.8:
The correction factors as recommended by ASTM standards C 42-77 and BS 1881 part-4, 1970 are shown in Table 14.9 below:
As per I.S. 516-1959, this relation is shown in Fig. 14.11. The strength of 10 cm cube is 10% higher than 15 cms cube.
On the basis of their experimental results Murdock and Kesler have found that the correction depends on the strength of concrete. High strength concrete is less affected by variations in the proportions and shape of the specimens. It is of interest for low strength concrete only.
Factor # 2. Effect of Casting Conditions:
The method of moulding a specimen has been found to have an important effect on the indicated strength. The compaction of concrete influences the strength of the concrete each one percent less compaction results in the reduction of 5% strength that of fully compacted concrete. Use of card board cylinder mould resulted in 3 to 9% less strength than steel moulds due to relatively rough walls of card board moulds.
Factor # 3. Effect of Moisture Content on Specimen:
Compression specimens tested in the air dry condition have been found to exhibit 20 to 40% higher strength than those of corresponding concrete tested in a saturated condition.
This may be probably due to the following reasons:
(a) Greater density of dry paste.
(b) Development of initial tensile stresses in the paste due to localized restraint of paste shrinkage by pieces of aggregate.
(c) Development of hydrostatic pressure in saturated paste.
Thus a compressive specimen at the time of test should be saturated as saturation condition is uniform and can be achieved easily, whereas dry condition is variable and indefinite. The cores taken from a structure are socked for 40 to 48 hours immediately prior to the compression test. However test specimens should be surface dry saturated.
With regard to the moisture content of flexure specimens at time of test, conflicting results have been reported by different researchers, due to non-uniformly drying of the specimens as it is very difficult to get uniform drying. Thus the modulus of rupture of concrete specimen which has been allowed to dry is lower than that of a similar saturated specimen. This difference is due to the tensile stresses induced by restraint and non-uniform shrinkage prior to the application of the load. The magnitude of the loss of strength depends on the rate at which moisture evaporates from the surface of the specimen.
Further the relative humidity also has a marked influence on the strength of concrete. It has been observed that when no moisture gradients are present, strength compressive or tensile decreases significantly with an increase in relative humidity, but extreme drying causes a drop in strength. If the relative humidity increases from 20% to 80% the flexure strength drops from 60 kg/cm to about 48 kg/cm2 i.e. about 20% loss.
Factor # 4. Effect of Temperature of Specimen at Test:
The temperature of specimen at the time of test has a marked influence on the indicated strength of concrete. Temperature of specimen is quite distinct from curing temperature.
For compression some researcher at University of California have indicated that compressive strength at 25°F is 40% higher than that of the same concrete at 70°F and at 130°F is 15% lower than that of the corresponding specimen tested at 70°F. Flexure test of mortar at University of Texas has indicated that modulus of rupture at 40°F is 12% higher and at 100°F is 20% lower than that at 70°F. Thus on an average, a variation of 1°F to 4°F in testing temperature results in a difference of 1% in strength.
Factor # 5. Effect of Bearing Conditions:
It is important that concrete specimen should be loaded uniformly over the entire area, otherwise localised concentration of load will result failure by splitting the specimen rather than in compression or flexure.
Thus in order to develop the full strength of the specimen following bearing conditions must exist:
1. The specimen should be accurately centred in the testing machine and the axis of the specimen should be vertical.
2. The bearing surfaces should be perpendicular to the axis of the specimen.
3. A spherical seated bearing block should be used and it should be accurately centered on the specimen with the centre of curvature of its spherical surface in the plane of contact with the specimen.
4. The bearing surface should be plane. The variation in the end surface of a cylinder from true plane even by 0.25 mm has resulted reduction in strength by 35%. Hence bearing faces of testing specimen should be plane within 0.05 mm if they are not, then they should be capped. The bearing faces of testing machine should be plane within 0.025 mm of true plane. It has been observed that convex end surfaces cause greater reduction than concave faces. The reduction is high in high strength concrete.
5. If a capping material is used, it should have strength and elastic properties as near as possible to those of the specimen.
Factor # 6. Effect of Rate of Loading:
The rate of loading also affects the indicated strength of concrete. It has been observed that more rapid the static loading of concrete, higher the observed strength. As per I.S. 516, the standard rate of loading should be 140 kg/cm2 per minute. If the rate of loading increased to 4200 kg/cm2 per minute, then the observed strength will be about 12% higher than that obtained from the corresponding specimen at standard rate of loading.
On the contrary, if the rate of loading is reduced to 4.2 kg/cm2 per minute, then the observed strength will be 12% lower than that obtained at standard loading. Further it has been found that concrete can withstand indefinitely stresses upto about 70% of the strength determined under standard loading. The increase in strength with more rapid loading is proportionately greater for leaner mixes. In an experiment on increasing the rate of loading to 50 times, the increase in indicated strength with given aggregate/ cement ratio was found as given below in Table 14.10.