ASSESSMENT OF MATERIAL PROPERTIES OF CONCRETE GRAVITY DAMS – AN INDIAN CASE STUDY
2. Field Studies on Concrete Gravity Dam
Non-destructive tests were conducted in the galleries (both inside and outside) and the accessible portions downstream and upstream of the dam. The tests mainly included an Ultrasonic Pulse Velocity (UPV) test, Half Cell Potential test, in situ moisture content using a humidity meter, resistivity test and air permeability test. The UPV test measurements produced using the surface probing method (as per IS: 13311 Part-I-1992) with pulse velocity varied between 3.56km/sec and 4.03km/sec. When these values were compared with the velocity criteria of IS: 13311 (Part 1)-1992, the overall quality of the concrete was assessed to be “good” in general.
The half-cell potential measurements were taken at nine different locations, and when these results were compared with the corrosion criteria as per ASTM C-876 it is seen that there is no corrosion of the reinforcing bars (i.e. 90% chance of no corrosion). The chemical analysis of the concrete core samples was carried out, and from these results it was seen that the pH values were on the higher side, indicating no loss of the passive layer around the reinforcement which protects the steel from distress. The moisture content in situ readings were taken at eight different locations along the downstream side, upstream side and four galleries at different elevations. The moisture content varied from 4.64% to 5.20%. The moisture content on the upstream side varied from 4.95% to 5.20%, and on the downstream side it was around 4.64%. The test results indicated that moisture content in the upstream side was marginally higher than the downstream side. The coefficient of air permeability (kT × 10-16) for the upstream right bank, upstream left bank, gallery at elevation 1073’ upstream and gallery at elevation 1608’ downstream varied from 0.016 to 0.046, which indicated that the quality of near-surface concrete falls in the ‘good’ category. The coefficient of air permeability (kT × 10-16) for the downstream right bank, downstream left bank, gallery at elevation 1268’ upstream and gallery at elevation 1548’ downstream varied from 0.269 to 0.590, which indicated that the quality of near-surface concrete falls in the ‘normal’ category.
The values of electrical resistivity ranged from 77.00kΩcm to 95.37kΩcm. The test
results indicated that moisture content in the upstream side was marginally higher than the downstream side, and the same was confirmed by the humidity meter test results. Ambient
temperature, CO2 level and humidity were measured randomly at the downstream side, upstream side and four galleries at different elevations. The ambient temperature varied
from 23ºC to 28ºC, and CO2 level varied from 274ppm to 497ppm. Humidity level varied from 27.8% to 72%.
The variation between the ambient temperature on the downstream side and upstream side concerning the gallery at four different elevations was about 2ºC to 3ºC. This data was reported based on the field investigation carried out by NCB for a particular period, and the data
may vary seasonally, and CO2 level may vary over the period of the year. No carbonation was observed in any of the concrete samples when concrete was exposed and sprayed with a 1% phenolphthalein solution in ethanol at all the locations.
Vol XXXII Issue 1 DAM ENGINEERING 7
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