In order to establish a definite relationship an experimental testing with the help of different machines available is undertaken under controlled conditions. The various factors affecting strength of concrete.

The concrete is heterogeneous material, hence strength of concrete is affected by properties of ingredients, mix proportions of ingredients, extent of mixing, compaction etc. The variation in levels of these factors can result in variation in test results along-with variations attributable to equipment character. In order to establish a fairly accurate regression between ultrasonic velocity and strength of concrete or statistical experiment was designed. The experiment involved testing of concrete cubes casted out of known significant factors and their levels. The factors chosen were age concrete, density of concrete and ultrasonic velocity. The concrete so made was testing by different set of equipment. After testing 200 specimen a non-linear equation is developed to relate ultrasonic velocity and strength. The paper presents an account of the experiments and regression equations arrived at.

The equations obtained between strength and velocity for Equipment A&B are as follows: For Equipment A

C = 79.846 + 4.103 x 10^-9 D³ + 0.00217 t³ + 4.842 x 10^-6 U²

For Equiment B

C = -104.569 - 9.023T + 3.279 x 10^-9D³ + 0.001176 t³ + 2.209 x 10^-5 U²

LEGEND:

C = Compressive strength in kg/sq. cm

D = Density of concrete in kg/cu. m

t = Age of concrete in days

U = Ultrasonic velocity in M/sec

T = Type of curing for curing case = 1, for no curing case C= 0

The relationship so derived reveals equipment character also plays important role and test data of one cannot be interpreted considering experience of the other. Hence it is necessary to define various characters of equipment to arrive at universal relationship. It is also necessary to consider other factors and their levels to have regression of high degree confidence level. Unless this is done the testing has to be always related to bench mark values.