Inspite of large amount of research papers and experimentation, both in the field and in the laboratory on the durability aspects of concrete, little attention is paid to the fresh concrete in underground-water environment in particularly to the concrete shafts of cast is situ piles either bored or driven. The published material on the effect of aggressive ground water on the rheological properties of concrete is scarce.
The instability of freshly laid cast in situ piles as consequences of certain elements like chlorides and sulphate percentage is studied in depth. The most of the published material relates to hardened concrete. Moreover the harmful composition of water is not completely revealed by the existing specification of water testing procedure. Many a times the damage is not due to chloride or sulphate principally but some other phenomenon which clears the ground for sulphate/chloride attack leading to the disintegration of piles.
It in this background the modified design procedure for ground water particularly from suitability to freshly mixed concrete based on the experimental results and observed case histories is proposed in the present paper.
Present Testing Procedure
The testing procedure of water plays a vital role in ascertaining the aggressive nature of ground water or otherwise. The engineer or designer can ensure a durable pile shaft if the water testing procedure gives him full information about the water quality. At present testing procedure of ground water includes tests like pH, chloride, sulphate and organic contents. Such test procedure serves the purpose in case of many situations. But it has been observed in number of cases that inspite of the quality of water being reported as non aggressive in nature (in accordance with prevailing water testing procedure) the constructed piles when exposed were found to be defective, deteriorated and in degraded condition
In last two decades it was observed at number of sites that bored cast in situ piles on exposing were observed to be deteriorated. The studies under-taken to analyze revealed that proper investigation, including testing of soil and water samples confirming to the relevant Indian Standard specifications (I.S. 456) was carried out. Only after proper analysis of data of soil investigation, designers proposed the bored cast in situ piles and yet the piles on exposure were found to be deteriorated as a consequence of adverse influence of aggressive elements in ground water which are not revealed by testing.
An accidental testing revealed that there are some other parameters unknown to foundation engineer and not covered by present standard procedure for testing of water.
This observation necessitated a deep probe in the matter so as to know the various causes which can lead to the deterioration of pile shafts. A detailed experimental scheme was planned. Based on the results a modified procedure for water of testing for under ground concrete construction is proposed.
Theoretical Background Of Harmful Properties Not Covered By Present Testing Methods
Though the deterioration of piles observed in last two decades in an around Mumbai shocking to the foundation engineers, the literature on quality of water for drinking purpose outlines at length the chemical phenomenon involved due to unsaturated calcium carbonate in water which has lead to such situation. This concept addressed as calcium carbonate saturation index is assessed by conducting Ryzner Index test as outlined in IS.
Similarly oxygen demand in ground water, either because of bacteria or because of chemical demand of oxygen by the inorganic matter can causes the deterioration of the fresh concrete. Both these characteristics of ground water play a very significant role in deterioration of piles particularly when concrete
It is quite common in metropolitan cities that the leakage of sewage pipelines leads to contamination of soil and water around it. Such water is very harmful to concrete. It is in that perspective BOD and COD test as outlined in respective IS code need to be conducted.
It is therefore essential that quality of water with respective to Ryzner, BOD and COD must be ascertained.
Additional tests and their permissible limits
The table below gives the permissible range of Ryzner Index, BOD, and COD for three types of cement which may be referred in addition to pH, chloride and organic matter tests.For further details:
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