A K Sthapak, Mangdechu Hydroelectric Project Authority, Bhutan
Rajbal Singh, Central Soil and Materials Research Station, New Delhi

Six different grades of Self Compacting Concrete (SCC) varying from M15A20 to M35A20 were used in various components of Tala Hydroelectric Project in Bhutan in Himalayas. This paper deals with the mix design, application and performance of a wide range of SCC mixes used in gate grooves in dam, back fill and lining of surge shaft and pressure shafts of Tala Hydroelectric Project. In the present application, two important parameters viz. ease of placement and better filling ability of SCC without vibration weighed in favour of SCC despite its somewhat higher cost as compared to conventional concrete. In case of gate grooves, the space to be filled is vary narrow, in which reinforcement further obstruct the flow of concrete. Therefore, a rich mix with desired attributes was designed. In case of back fill, due to large dimensions, minimum possible cement content was used to avoid thermal problems. The SCC mixes were designed with and without viscosity modifying admixture without adding any fine powder. Under the given placement condition, the SCC was placed successfully without difficulty. The field performance was monitored by testing cubes, cores, beans etc.

Introduction

Tala Hydroelectric Project is a run of the river scheme, located 3 km downstream of the existing 336 MW Chukha Hydroelectric Project on river Wangchu in South Western Bhutan in Eastern Himalayas. The project has constructed a 92m high concrete gravity dam at Wangkha, three desilting chambers each of 250m x 13.90m x 18.5m size for removal of suspended sediments of 0.2mm and above size coming with the river water diverted through the intake structure; a modified horse shoe tunnel of 6.8m diameter and 23km in length, a surge shaft of 15m/12m diameter and 178m height; two pressure shafts of 4m diameter and 1.1km long each and an underground powerhouse (206m x 20.6m x 44.5m) for utilizing a gross fall of 861.5m. A tail race tunnel of 3.1 km length and 7.75m diameter discharges the water back into river Wangchu. The installed capacity of powerhouse is 1020 MW (6 x 170MW).

The project was taken up for execution from the zero level of infrastructure development from 1st October 1997 and its first unit was commissioned on 31st July 2006 and all six units were commissioned by March 2007. The project has been constructed by Tala Hydroelectric Project Authority (THPA), an autonomous body constituted jointly by the Government of India and the Royal Government of Bhutan.

The self compacting concrete (SCC) was used for backfill behind steel liners in pressure shafts, behind steel ribs in surge shaft, lining of surge shaft, and as second stage concrete in dam. The concrete grades of M15A20 and M20A20 were used for backfill in surge shaft and pressure shafts, respectively. M30 and M35 were used in lining of surge shaft. M25 with 10mm and 20mm MSA was used in gate grooves of stop log and radial gates in dam. Conventional pumped concrete could not properly fill all rock crevices and voids were noticed between two consecutive ribs when shutters were removed in surge shaft. Vibrating the concrete was practically difficult. Typically, SCC contains cementitious materials in the range of 400 to 600 kg/m3 including ultrafine inert materials (EFNARC, 2002). However, in the present application, an attempt has been made to produce SCC at a cement content of 310 to 360 kg/m3 with 20 to 25 kg/m3 micro silica content without any fine filler material by using viscosity modifying agent and a well suited proportioning of aggregates.

The mix design, application, and performance of various grades of SCC along with the advantages have been discussed in this paper.

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