Concrete quality assurance

At a recent meeting of key managers for China’s Three Gorges project, premier Zhu Rongi once again emphasised the high priority given to quality control. ‘Quality is the life of the Three Gorges project,’ he said at the meeting in Beijing on 18 June 2000.

For each area of this complex scheme, personnel are monitoring procedures and working to achieve high quality construction at what is currently the world’s largest civil construction job. The placement of concrete and its quality control are of immense importance — the central structures of any hydroelectric project are primarily concrete but the unprecedented size of the Three Gorges gives an unusual dimension to the work in hand. On site batch plants, conveyor belts and trucks of all sizes are working around the clock to place the 27.9M m3 of concrete that will be used. To appreciate the enormity of the scheme the concrete to be placed is enough to construct a two-lane highway longer than the entire coastline of China.

A total of seven batching and mixing plants have been constructed to supply the concrete for the Three Gorges dam and power houses. The plants are located at four separate elevations, both upstream and downstream from the dam. Two plants are on the downstream right bank, both with rated capacities of approximately 350m3/hr. Five more batch plants are located on the left bank, two upstream and three downstream. The capacities of these plants range from 240-400m3/hr. All seven plants are computer controlled, and have cold air aggregate cooling, watercooling and ice capabilities. Concrete for the navigation locks is supplied by two adjacent concrete plants located on the downstream right side of the locks.

Much of the concrete is placed using six tower belt systems that run from the batch plants to the placement points, but haul trucks are also being used to bring concrete to feeder conveyors or lay-down buckets. In addition to placing the concrete at temperatures consistent with international standards for mass concrete, all mass sections have post-placement cooling pipes that are instrumented to monitor hydration temperatures.

Supervising construction

Construction supervision on a project like the Three Gorges covers a spectrum of activities including materials and concrete production inspection, placement inspection, schedule control, cost control, technical management, concrete materials management and overall co-ordination. The four organisations supervising project construction are Changjiang Water Resources Commission, North-West Investment and Design Institute, Zhongnan Investment and Design Institute and Huadong Investment and Design Institute.

US company Harza Engineering has been providing quality assurance and quality control services for the Three Gorges project since the mid 1980s.

S T Su, Harza’s area manager for China and the Far East, says the company has provided advisory services on several levels and since 1998 has been providing construction quality assurance services for concrete placement on the power houses and spillways.

Harza’s advisory role has provided the opportunity to work with the supervisory organisations and contractors in a variety of capacities. This has meant being involved in the development of quality control and assurance plans for concrete quality, based on international standards, and implementing a system for supervising the enforcement of those standards. Harza personnel have been working in the field with supervisors to develop educational tools for supervising concrete construction and quality, as well as training courses in concrete technology. Consultation for concrete placement technology, temperature control and related technical specifications have been important aspects of the advisory reviews, and resulting technical reports address many of these aspects.

Harza’s technical advisor, Ernie Mitchell, spoke about Harza’s role within the quality control groups at the project. ‘We have been invited to be a small part of the effort at Three Gorges to achieve the best possible quality,’ he said.

‘It is a well-known fact that concrete construction is not an exact science. For that reason, the quality control groups are an essential key to maintaining high performance with the concrete. By working with these groups and sharing project knowledge and international standards, we can better predict material performance and use that knowledge to optimise materials.’

Project characteristics

Completion of the project will provide a reservoir with a flood control storage capacity of 22B m3, and will significantly expand navigational capabilities from Yichang City to the Chongqing Municipality. More than 18,000MW of power will be supplied to east and central China and the Chongqing Municipality. Recreation, fisheries and improved water quality are other benefits of this project.

‘The power from hydroelectric plants encouraged industry and opened lands for agriculture in the western US,’ Mitchell observes. ‘We see even greater potential for the people of China in the project regional area.’

Located near the city of Yichang in the Hubei province the project comprises a spillway dam in the centre of the river channel, with intake and non overflow dam sections on either bank and permanent navigation facilities located to the left of the dam structures. The two integral intake power plants will contain a total of 26x700MW Francis-type turbine generator units. The concrete gravity dam will be 2310m long and 181m high. The spillway section contains 23 low level outlets and 22 surface sluice gates to allow for discharge of sediment and the probable maximum flood.

Construction of the project has been divided into three phases. Phase I of this effort, comprising the preparation work, cofferdams, diversion channel excavation and river diversion, is now complete. Phase II began in 1998. This current phase includes construction of transverse cofferdams to create the construction pit for the spillway dam and the left bank power house. At the same time the permanent navigational facilities are under construction. During phase III, beginning in 2003, the first power plant and shiplock will be put into operation, and the right bank power plant and dam will be constructed. Though construction is scheduled for completion in 2009, the work completed in phase II will allow partial power generation capacity to start up in 2003.

As would be expected, the construction of the principal structures and diversion works for the Three Gorges involves the movement of large quantities of materials. More than 103M m3 of earth and rock are being excavated. The reinforcing steel required for the concrete equals about 463,000t.

Construction of the Three Gorges project is going well and is on schedule. In 1999 it broke the world record for concrete placement at 4.4M m3. If construction crews maintain their current pace through the second half of 2000, the project will break its own record by placing more than 5M m3 of concrete.