Keeping it cool at Three Gorges

AS A multi-purpose dam, the Three Gorges Project (TGP) on the Yangtze river in China will have benefits in flood control, power generation and navigation. Since work began in 1993, the combined effort of the contractors has allowed the project to progress smoothly. It has been built within the master schedule, to quality requirements, within budget. What is more, there have been some significant breakthroughs on technical issues and new construction records have been set.

There were a number of options available for construction methods and a choice of major equipment, prompting a scientific assessment to be conducted. The method chosen was to use gantry cranes, tower cranes and cable cranes supplemented by a tower belt. Five concrete batching systems were employed, with a design capacity of 2500m3/h. Six tower belts were used for the dam and powerhouse. They offered continuous pouring, high efficiency and industrialised concrete production, with an average productivity about 100m3/h and 200m3/h at peak and an average monthly production of 30,000-40,000m3. The four-graded concrete was placed by the tower belt and it was used to pour various kinds of concrete on one block.

The materials excavated from the foundation pit were fully utilised and, in addition, a new technology was developed that combined sand-making (using a Barmac 9000) and a mixing mill.

In 2000, the company claimed records for concrete poured on a yearly (5.48Mm3), monthly (553,500m3) and daily (22,000m3) basis. In total, 14.09Mm3 was placed in 1999-2001 and 28Mm3 in total.

A complete set of quality assurance and modernised construction management systems were set up to manage prompt concrete pouring based on TGP practices. The pouring block was specially designed and a complete set of associated construction technologies were formulated.

An integrated computer monitoring and control system for concrete production, transport and pouring was developed, allowing real-time monitoring and control, dynamic adjustment and optimal regulation of the whole process of concrete construction. Because of the complexity of the concrete placement, the construction options were assessed and programmed scientifically, instead of through the traditional 'judgement by experience'. To develop this process a system was developed to simulate concrete placement and this improved efficiency in practice .

Following a long series of tests, an artificial aggregate of non-alkali active granite was selected as the best material to meet the concrete durability requirements of TGP. The alkali content was strictly controlled: it was less than 0.5% in the cement clinker, and less than 2.5kg/m3 in total in the cement and in the Class I fly ash mixed into the concrete. The fly ash has a high proportion of fine particles so it can be used as a functional material to improve the workability of the concrete, reduce water consumption, and slow the alkali active reaction to save consumption of slurry.

This helped prevent cracking due to temperature and reduced dry contraction. In addition, a high-quality and high-efficiency water reduction agent was mixed with Class I fly ash, allowing water consumption for the grade four concrete (made of artificial granite aggregate) to be reduced from 110kg/m3 to 85kg/m3. Increasing the mix amount of flyash by reducing the water-gel ratio improved the durability of the concrete.

Cooling systems

Strict control of the temperature is particularly important at Three Gorges, partly because of the large size of the cylindrical blocks and the temperature difference of foundation and also because frequent sudden drops in the temperature of the dam area make crack prevention on the surface of concrete quite difficult.

To manage this, TGP implemented an integrated temperature control scheme for the whole process, developed after a wide analysis of the temperature control measures used for projects at home and abroad. A complete set of temperature control measures have been taken: selection of quality raw materials; optimisation of the concrete mix; control of exit temperature and pouring temperature of the concrete; water cooling; surface heat preservation; and flow-water curing.

Theoretical calculation methods had to be used, as there was a lack of reference information corresponding to the rate of temperature rise of concrete during transportation by tower belt. This is the first time that high-speed prompt concrete pouring by tower belt in the constraint zone of the dam in the high temperature season, had been attempted at home or abroad. What is more, there was no construction experience or calculation methods that could be used as a reference to determine the rate of temperature rise of the concrete during transportation. To this end, new calculation models were established.

The simulation feedback analysis system for temperature control of dams was specially developed. A complete set of advanced calculation methods were developed to carry out analysis on temperature control. This provided the simulation, forecast and feedback analysis, during the construction and operation periods, of the 63 dam sections, 200 monoliths and tens of thousands of pouring blocks of various types.

The low-temperature concrete production system at TGP is the largest in the world and the strictest in terms of temperature control. Concrete produced in the summer must be at 7? at the point of exit, with a design production capacity of 1720m3/h and a peak pouring intensity of 440,000m3/month in summer.

In the light of the special characteristics of the TGP and its requirements for pre-cooling, a new technique - secondary air-cooled aggregate - was used after a series of tests and studies.

The 7degrees low-temperature concrete was produced by using the aggregate regulation bin provided for secondary screening on the ground as a cooling bin, providing primary air-cooling of the coarse aggregate. The material bin of the mixing plant was used for secondary air-cooling and sheet ice or low-temperature water was added to the concrete mix. The innovation was in the secondary air-cooling aggregate: the usual water cooling of the aggregate was replaced by ground-air cooling, which, in conjunction with the highly efficient cooling fan and its corresponding distributing units, formed an enclosed cool-air circulation system so that the aggregate was continuously cooled.

In the summers of 1999 to 2001 the measured exit temperature of the concrete was 1.6-12degrees. The average temperature was 6.8degrees, with around 85% at less than 7degrees.