APPLICATION OF SELF-COMPACTING CONCRETE IN FRAME- SHEAR WALL STRUCTURE AND ROAD CONSTRUCTION

Transcription

1 APPLICATION OF SELF-COMPACTING CONCRETE IN FRAME- SHEAR WALL STRUCTURE AND ROAD CONSTRUCTION Shi-yong Wu (1) (1) Shanxi University, Taiyuan, Shanxi, China, Abstract More than 20,000 m 3 of self-compacting concrete was used in the construction of Shanxi University Arts Building, Students Dormitories, roads and concrete caissons. Its excellent mechanical properties after hardening fully demonstrate that self-compacting concrete can be applied to various types of constructions. Self-compacting concrete saves labor and time, produces lower noise and fewer disturbances to residents, reduces the labor intensity, improves the construction quality, shortens the construction period and brings in large economic and social benefits. Key words: Viscosity, Quality control, Acceptance regulation 1. INTRODUCTION Self-compacting concrete is also called high-flowability concrete or vibration-free concrete, which is a modern technique that has been developed in the last two decades. Because it enables constructions with high speed and good quality, reduces the labor intensity, and produces lower noise and fewer disturbances to residents, it becomes more and more popular. In China, about 2 billion m 3 of concrete is used every year, 90% of which is of middle or low level. Therefore, research on middle or low level self-compacting concrete is of great significance. In the past 4 years, we have used more than 20,000 m 3 selfcompacting concrete of level C 25 -C 35 in the construction of Shanxi University Arts Building, student dormitories, roads and concrete caissons. The total construction area was 30,000 m 2. By using self-compacting concrete, good economic and social benefits were obtained. 2. RAW MATERIALS AND MIXING PROPORTION 2.1 Cement Two kinds of ASTM І Portland cement denoted as C1 and C2 were used. Their properties are listed in Table 1. Page 1

2 2.2 Sand Selecting Shanxi Douluo sand and Wenshui Sand was taken as fine aggregate, with fineness modulus 2.3 to 2.8, mud content 2.7%, apparent density 2650kg/m 3, stacking density 1500kg/m 3 and boulder clay content 0.4%. 2.3 Stones Selecting Shanxi Zhencheng stone with particle diameter 5 to 25 mm, apparent density 2670kg/m 3, stacking density 1470kg/m 3 and boulder clay content 0.2% was taken as coarse aggregate.. Table 1 Physical and mechanical properties of cement No. C 1 C 2 brand name Handan Taihangshan Handan Taihangshan level Residual ratio by mesh sieve with diametr of 80µm Initial setting time Final setting time Flexural (MPa) Compressive (MPa) 3d 28d 3d 28d P.S P.O Fly ash Rank II fly ash from Taiyuan No.1 Heat and Power Plant was used. It physical properties and chemical composition were given in table 2 and table 3, respectively. 2.5 Plasticizer The plasticizer is made by Shanxi University, with the main component of β- naphthalenesulfonate acld-formaldehydew. It has good plasticizing effect. There is no loss to the slump within 1.5 hours and the enhancement effect is obvious. Table 4 shows mixing proportion parametr of self-compacting concrete. 3. SELF-COMPACTING CONCRETE QUALITY CONTROL AND ITS MECHANICAL PROPERTIES INDEX AT 28D After careful investigation, we chose Shanxi Jinyang Concrete Co. Limited to produce SCC because of its good quality control system and the workers high proficiency level. The production techniques and raw materials of SCC were the same as those of normal concrete, and no special treatment was applied. When the self-compacting concrete was transported to the construction site, convenient and fast testing was applied to check if it was qualified for construction. After several trials, Page 2

3 we finally established the method to use slump barrel together with filler box to measure the properties of SCC at construction sites. Table5, Table6 and Table7 show the testing results of mechanical properties and working parameters of self-compacting concrete in laboratory and in job sites, respectively. Table 2 Physical properties of fly ash Inspection items Units Standard No. 1 Fineness (45µm square mesh sieve residue) Inspection results % Water requirement ratio % Weight loss after burning % Sulphur trioxide % Table 3 Chemical composition of fly ash Chemical composition SiO 2 Al 2 O 3 Fe 2 O 3 CaO MgO SO 3 Content (%) Table 4 Self-compacting concrete mix proportion Concrete per unit volume (kg) Waterpowder content Sand Strength No. level Fly ratio (%) Cement Sand Stone Water Plasticizer ash 1 C Page 3

4 Table 5 Mechanical properties and working parameters of SCC after hardening in laboratory Inspection Inspection item Units Standard No. results 1 Compressive 3 days MPa days MPa days MPa Flexural MPa Gas content % Water resistance MPa Frost resisting property (D50) Loss Mass of Loss of % % Slump degrees mm Spread degrees mm Upending the slump barrel Second 3-6 Table 6 Working parameters at construction sites Concrete parts cast Beam, slab and column Building foundation Slump(mm) Spread degree(mm) Filling Capacity Filler box(%) Road Upending the slump barrel(second) Page 4

5 Table 7 Mechanical properties of SCC at 28d in construction sites Inspection item Units Standard No. 1 Compressi ve 3 days MPa days MPa days MPa d MPa days MPa days MPa Water resistance MPa Gas content % Frost resisting property (D50) Loss of Mass Loss of % % Yield MPa 4.5 Remarks Inspection results Remarks This experiment is performed according to Standard Test Methods for Mechanical Properties of Normal Concrete GB/T and Concrete Plasticizer GB The concrete is collected from the construction site of Shanxi University 0 # Students' Dormitory Note: The above data is measured by Architecture and Construction Quality Control Office of Shanxi Provincial Technical Control Bureau 4. RESULTS AND DISCUSSION Self-compacting concrete has three basic properties: high flowability, high stability and the ability to flow through narrow spacing between reinforcement bars. According to fluid mechanics, concrete is Bin Ham fluid. Its fluid mechanical equation is: dr τ = τ o + η dt τ : Shearing stress caused by own weight τ : Yield shearing stress caused by friction between particles o η : Plastic viscosity, which is the property to prevent relative motions between different layers of liquid. When τ τ o is fixed, the time needed dt for the concrete to undergo a small deformation dr is proportional to the plastic viscosity η. That is, when η is larger, the flowing velocity is smaller, and vice versa. It can be clearly seen that the yield shearing stress τ 0 and plastic (1) Page 5

6 viscosity η are the two dominant factors that determine the properties of the concrete mixture. In construction, such situations can occur: although the slump degrees are the same, about 260 to 270 mm, the spread degrees are quite different one is mm, while the other is mm. This is demonstrated by the fact that one has high flowability while the other flows very slowly. The latter situation is caused by the excessively high viscosity of SCC and the solution to this problem is to add more plasticizer. Sometimes such situation happens: even the sand content is very high, segregation still occurs. This is usually because stone content in the sand is too high or the grading is not good. With the practice of mixing 30% Wenshui sand that has fineness modulus 2.3 and 70% Douluo sand that has fineness modulus 2.8, the problem of segregation is well solved. Fly ash is the best additive to self-compacting concrete. From the analysis of the micro structure of the concrete, we come to the conclusion that: firstly, fly ash has water-reducing effect; secondly, it acts like bearing in cement grouting to produce lubricating and rolling effect, so that it increases the flowability of concrete; thirdly, fly ash also has micro aggregate effect, thus it can replace part of sand to reduce the consumption of sand; finally, its density and reactivity can increase the of the concrete. The keys to improving the quality of self-compacting concrete and reducing unit price lie in two properties of the plasticizer water-reducing rate and the adaptability to various types of cement. Therefore, plasticizer should be the major focus of research for self-compacting concrete. The biggest disadvantage of self-compacting concrete is that it cannot be applied to slopes. When we cast stairs using SCC in constructing Students Dormitories, self-compact concrete overflowed the formwork, so we repositioned the formwork to make it completely sealed. But this method brought in two other problems. First was that the self-compacting concrete did not flow downward because of air resistance. Second was that after we removed parts of the stair formwork, although the concrete flowed, its impact was so big that it damaged the wood formwork, made it separate from the base plate of the stairs, and the concrete overflowed. The biggest advantage of using SCC in road construction is that it is possible to first lay the curb stone, and then directly cast concrete. Comparing to the traditional method of first cast the road and then lay curb stone, the application of SCC needs no formworks, improves the quality of construction, greatly reduces labour intensity and has good visual effects. But the disadvantage is that because of its high viscosity, it is harder to compress the road surface and draw slip-resistant lines comparing to normal concrete. 5. APPLICATIONS OF SELF-COMPACTING CONCRETE 5.1 Shanxi University Arts Building Shanxi University Arts Building is a major project in Shanxi province, which uses frameshear wall structure. The 15 th floor cast by self-compacting concrete is 60.5 meters above the ground and the clear height of the floor is 5 meters. It has concrete beams (simple support, continuous beam), slabs, columns and shear walls of various cross sectional dimensions. At the intersection, there is almost no spacing between the steel reinforcement bars. This project well represents frame structure constructions. Page 6

7 5.2 Students dormitories The three dormitory buildings (0 #, 5 #, 6 # ), which began construction in 2004, each has 6 floors and construction area of 6000 m 2. The total construction area is 18,000 m 2. The foundation is concrete slabs with thickness of 400 mm, the upper and lower reinforcement is φ14@200, the floor transverse reinforcement is φ8@150, the longitudinal reinforcement is φ8@200, the negative moment reinforcement is φ10@150 (longitudinal) and φ8@150 (transverse), the cantilever reinforcement is φ12@150, the entrance hall vertical prop is 12φ20, the hoop reinforcement is φ8@100/200 with cross-section 400x400(mm), and the constructional column is 4φ14, φ6@200 with cross section 240x240(mm). The water tank rooms from the foundation to the top floor of the three dormitory buildings are all built with self-compacting concrete with level C 25 -C 35, and the total consumption of which is 5000 m Roads and concrete caissons Self-compacting concrete roads with the length of 1600m, width of 10m and thickness of 0.28m are built. The level is C 25, and total consumption is 4480m 3. Concrete caissons that are used to drain away the municipal sewage lie below the selfcompacting concrete road foundation. The total length of the concrete caissons is 600m, with inner diameter 1.8m and height 1.5m. The thickness of the top, base and side plates are all 0.25m. For the top plate, the upper and lower reinforcement is φ14@200 and the longitudinal reinforcement is φ10@200; for the base plate, the upper and lower reinforcement is φ12@200 and the longitudinal reinforcement is φ10@200; for the side plate, the inner and outer reinforcement is φ12@200 and the longitudinal reinforcement is φ10@200. Along the longitudinal direction of the four corners, reinforcements of φ10@200 are placed. The concrete caissons are cast twice: firstly, the base plate is cast; then when the base plate became hardened, the top and side plates are cast. The level of the self-compacting concrete is C 25 and the total consumption is 2280m ANALYSIS OF ECONOMIC AND SOCIAL BENEFITS 6.1 Economic benefits Firstly, self-compacting concrete improves the quality of concrete and prevents undervibration or overvibration of concrete due to technical problems in construction. Secondly, since there is no need to vibrate, it guarantees the positions of reinforcements, embedded parts and preformed holes, and reduces the corresponding ening costs. Thirdly, the mixing of large amount of fly ash reduces the consumption of cement and sand. Fourthly, its flowability makes it easier to pump the concrete, extends the service life of the pump and lowers the maintenance cost. Fifthly, it expedites the construction progress, shortens the construction period to almost half of the time compared to that of normal concrete, and requires 2/3 less constructors. Finally, including all the costs in raw materials, water, electricity, labour, machines and expedited construction progress, one m 3 of self-compacting concrete costs 25 RMB less than normal concrete. Page 7

8 6.2 Social benefits. Self-compacting concrete uses large amount of fly ash manufacturing every m 3 of concrete requires more than 100 kg of fly ash. Since no noise is produced in construction, it causes no disturbance to residents. For instance, 0 # Students Dormitory was constructed in the summer and the construction usually began at 5 a.m. everyday. Although the construction site was surrounded by dormitories and apartments, because the self-compacting concrete produced no noise, it didn t disturb the students and residents, thus prevented conflicts with residents caused by the noise of vibration. Hence this technique could be used in continuous constructions in downtown and residential areas, and shorten the construction period. 7. CONCLUSION Through the use of self-compacting concrete in the construction of Shanxi University Arts Building, three Students Dormitories, roads and concrete caissons, it has been exhibited that this technique can be widely used in various kinds of constructions. However, in structures with large slopes, i.e. stairs, it is more difficult to use selfcompacting concrete. Therefore, we still need to find methods that can apply SCC to these kinds of constructions without raising the cost. To conclude, we should complete the design, construction and acceptance system of selfcompacting concrete as soon as possible to help spread the standard application of selfcompacting concrete. REFERENCES [1] Li, Qing-he, 'High and vibration-free self-compacting concrete' (in Chinese), Building Technique Development 24 (6) (1997) [2] Ma, Bao-guo, Zhu, Hong-bo, He Xing-yang, Dong, Rong-zhen and Su, Ying. 'Study on highperformance concrete with middle and lower ', In 'Study on Fire Resistance of New Types of High Performance Concrete and Their Engineering Application' (in Chinese), (Yao, Yan ed., China Building Material Press, 2004) [3] Zhao, Jun, 'The research and application of self-compacting concrete' (in Chinese), Concrete (164) (2003) [4] Wu, Shi-yong, 'The influence of curing environment on the of high-flowing fly ash concrete' (in Chinese), Journal of Shanxi University (Natural Science Edition) 25 (3) (2002) [5] Wu, Shi-yong, 'The mechanism of high-flowing concrete and the application in constructions' (in Chinese), Shanxi Architecture, 28 (1) (2002) Page 8