The setting and hardening process of concrete can be considered as the most critical time period during the life of a concrete structure. To assure high quality and avoid problems in performance throughout the life of the material, it is essential to have reliable information about the early age properties of the concrete. This paper presents a new method to monitor the hydration process of cementitious materials at early ages. The technique is based on the measurement of the reflection coefficient (or reflection loss) of high frequency shear waves at an interface between a steel plate and e.g. concrete. Several experimental studies that examine the ability of the wave reflection method to qualitatively and quantitatively describe the setting and hardening process of cement paste, mortar and concrete are presented. The results show that parameters such as setting time, compressive strength, elastic and visco-elastic moduli and degree of hydration of cementitious materials can reliably monitored with the wave reflection method. The relationship between the measured reflection loss and the compressive strength was successfully used in a first field trial of the method to determine the inplace strength of precast concrete elements.

An x-ray astronomy experiment consisting of three collimated proportional counters and an X-ray Sky Monitor (XSM) was flown aboard the Indian Satellite IRS-P3 launched on March 21, 1996 from SHAR range in India. The Satellite is in a circular orbit of 830 km altitude with an orbital inclination of 98° and has three axis stabilized pointing capability. Each pointed-mode Proportional Counter (PPC) is a multilayer, multianode unit filled with P-10 gas (90% Ar + 10% CH4) at 800 torr and having an aluminized mylar window of 25 micron thickness. The three PPCs are identical and have a field of view of 2°×2° defined by silver coated aluminium honeycomb collimators. The total effective area of the three PPCs is about 1200 cm2. The PPCs are sensitive in 2-20 keV band. The XSM consists of a pin-hole of 1 cm2 area placed 16 cm above the anode plane of a 32 cm×32 cm position sensitive proportional counter sensitive in 3-8 keV interval. The position of the x-ray events is determined by charge division technique using nichrome wires as anodes. The principal objective of this experiment is to carry out timing studies of x-ray pulsars, x-ray binaries and other rapidly varying x-ray sources. The XSM will be used to detect transient x-ray sources and monitor intensity of bright x-ray binaries. Observations of black-hole binary Cyg X-1 and few other binary sources were carried out in early May and July-August 1996 period. Details of the x-ray detector characteristics are presented and preliminary results from the observations are discussed.