Standard cubes (M25, 28 days) are tested for crushing failure and standard cylinders are tested for split tensile failure. Ultrasonic through transmission measurements are made at different stress levels by indigenously developed PC based system. The system consists of

• pulser which gives the energising pulse of 600 Vpp in 2 microseconds,
• receiver, gain 40 dB,
• attenuator 20 dB, step variation 2 dB,
• level conditioner to limit the input to the flash ADC to 2 V,
• digitizer with flash ADC speed of 6 MHz and (f) FIFO memories.

Experiments in cubes indicate that microcracks due to compression mode of failure can be predicted by increase in transit time i. e., path length. At about 75% ultimate load, received energy can be considered as better indicator of macrocracking. Split tensile cylinder tests indicate that microcracking in tension mode can be predicted by received energy. Hence, it is possible to evaluate structural integrity and stiffness degradation of concrete by proper inservice inspection/on-line monitoring and subsequent analysis of signal parameters such as changes in transit time and received energy (v²xt).

Through transmission technique can be used as a reliable tool for structural integrity evaluation by proper planning at design stage, inspection at construction stage and inservice inspection. State of concrete even after stress redistribution can also be predicted in case of post accidental conditions. REFERENCES

1. Clause 10502,: Inservice Inspection and Surveillance", Code of practice for nuclear safety related civil engineering structures, AERB Draft code no. SC/CSE/3, published by Atomic Energy Regulatory Board of India, 1992
2. Section 3.2.4, "Inplace methods for determination of strength of concrete", ACI 228. 1R-89, published by American Concrete Institute.