Innovative materials

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1 Innovative materials

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3 Tailor Made Concrete Structures Walraven & Stoelhorst (eds) 2008 Taylor & Francis Group, London, ISBN The research on early age thermal cracking control of C50 HPC in main tower of Sutong Bridge G.Z. Zhang, L.Q. Tu & S.K. Li CCCG WuHan Harbour Engineering Design & research Co., Ltd, Hubei, China ABSTRACT: In recent years, thermal crack at early age of concrete has been one of the restricted factors affecting the durability of concrete structure and becomes a hidden peril of the structures. Considering the characteristics of Sutong Bridge, such as high main tower, large mass of concrete, high strength grade of concrete, thermal crack at early stage of concrete was studied applying temperature-stress testing machine. When designing mix proportion of concrete, some methods were used, such as mixing a certain amount of fly ash, reducing dosage of cementitious materials, decreasing sand-aggregation ratio and mixing polypropylene fiber in the root of main tower. Early-age curing of concrete was also strengthened. The results indicated that good performance of C50 high performance concrete was got, including the workability, mechanical properties, the anti-cracking and volume stability. And there was no early age thermal crack in the project. 1 INTRODUCTION MainTower of Sutong Bridge adopts invertedy-shaped structure with a total height of meters, which includes the upper tower, the medium tower, the lower tower and beams. And the strength grade of concrete is C50. The root of the tower and the linking solid segment of the medium and the upper of the Tower use C50 mass concrete. In order to ensure the quality of concrete construction of Sutong Bridge, avoid thermal cracks and ensure the service life and safe operation of the Bridge, the main tower of concrete should use high performance concrete. The strength grade of concrete in Main Tower of Sutong Bridge is C50. On one hand, as the section size is large and the hydration heat of concrete is high, so thermal crack is easy to occur. On the other hand, the tower column uses pumping concrete. Furthermore, in order to ensure the workability of the concrete, reduce friction between the pump and concrete and prevent concrete from segregation and blocking the pump, more cementitious material is needed, which is bound to bring up a higher temperature rise of hydration heat. So the concrete is easy to crack. Therefore, how to properly deal with the contradiction between hydration heat and pumpability is the key of concrete mix proportion. So, the tower concrete should use high-performance concrete with low water-binder ratio and high crack-resisting property, and the technical requirements are as follows: (1) Strength grade of concrete: C50; 28 days compressive strength 57 MPa; (2) Setting time of concrete: In order to reduce the temperature peak of hydration heat of concrete in summer construction, the initial setting time should be more than 25 hours; (3) Slump loss of concrete: The slump after 2 hours should be more than 180 millimeters, and the slump flow after 2 hours should be more than 450 mm; (4) To prevent thermal cracks, the internal maximum temperature of concrete should be less than CONCRETE MIX-PROPORTIONING DESIGN 2.1 Materials Normal Portland (P O) 42.5 cement of NanTong Huaxin cement factory was used, whose specific surface area was 340 m 2 /kg. Fine aggregate was from Jiangxi Ganjiang sand, whose fineness modulus was about Coarse aggregate was crushed rock from Zhenjiang Maodi gravel factory whose size grade was 5 25 mm. The other materials including, fly ash, high performance water reducing agent, polypropylene fiber were also commercially available. Grade I 1043

4 Table 1. Properties indexes of concrete. Parts of the Tower Properties index The root (<10 m) m m Mix proportion Cement/kg/m Fly ash/kg/m Fine aggregate/kg/m Coarse aggregate/kg/m Water/kg/m Water-reducing agent/kg/m Fiber/kg/m / / Properties of Slump/mm fresh concrete Slump loss (after 1 h)/mm Slump flow/mm Slump flow loss (after 1 h)/mm Initial setting time/min Final setting time/min Water bleeding/ml / / 10 Workability good good good Bulk density/kg/m Mechanical 7days strength/mpa properties 28 days strength/mpa index Elastic modulus/mpa fly ash was used, from Jianbi Power plant in Zhenjiang. The retardation type water reducing agent was HP400 from Shanghai Huadeng Co., and fibers used were polypropylene fiber net from the United States, whose length was 19 mm. 2.2 Concrete mix-proportioning design The mix proportion design of cable tower included three parts, and they were solid segment concrete of the 10-meter-high root of tower, high-lift pumping high performance concrete of meter-high tower, high-lift pumping high performance concrete of meter-high tower. The specific mix proportion and related indexes were shown in Table 1. 3 CRACK CONTROL MEASURES FOR CONCRETE 3.1 Mixing fly ash to reduce hydration heat C50 mass concrete with 20% content of fly ash was used to deal with the contradiction between hydration heat and pumpability. Moreover, the results indicated that the temperature rise of hydration heat could satisfy the requirement of construction after mixing fly ash. 3.2 Embedding cooling water pipe Solid segment concrete emplaced a total of 10-layer cooling water thin-walled steel pipes, whose diameter was 32 millimeters and wall thickness was 3.25 millimeters. The horizontal interval of steel pipe was 0.8 m, and the vertical interval was 0.9 m and 1.0 m respectively. It was 1.0 meter away from the side of concrete. Water cooling began when concrete poured to the elevation of cooling water pipe of each layer, and it stopped immediately after the temperature peak of concrete occurred. The water flow should be more than 25 L/min in order to create turbulence. 3.3 Strengthening early-age curing After demoulding, wetted the surface, coated polyethylene plastic film immediately, and then covered the concrete with thermal insulation materials, such as geotextiles. On one hand, keeping the concrete surface wet could supply necessary water for continuous hydration, so that the tiny cracks could be healed; On the other hand, covering thermal insulation materials significantly reduced the temperature difference between inside and outside surface. 4 CONCLUSIONS (1) C50 high-performance concrete used had a good workability, such as small slump loss, no segregation, no bleeding, good pumpability and vibrationcompacting property. And the setting time of concrete could meet the requirements of construction. Furthermore, it had a good volume stability, whose dry shrinkage rate was less than that of the ordinary concrete; 1044

5 (2) The experiments in site showed that the high performance concrete with fiber could be pumped to the height of 306 meters whose properties were excellent. And the concrete pump pressure was about 80% of the maximum pump pressure, which did less harm to the pumping equipment. (3) Three aspects, including materials, construction and curing of concrete, were took into account to control the thermal cracks of mass concrete effectively, which provided a strong guarantee to enhance the appearance quality and durability of concrete structure. The project indicated that the effect of temperature control was good, and there were no harmful thermal cracks. 1045

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