ABRASION-EROSION RESISTANCE OF HYDRAULIC CONCRETE UNDER HIGH-VELOCITY SANDBLASTING

Transcription

1 ABRASION-EROSION RESISTANCE OF HYDRAULIC CONCRETE UNDER HIGH-VELOCITY SANDBLASTING Xinhua Cai (1,2), Zhen He (2), Yaojiu Rong (1) and Wang Lei (2) (1) Headquarters of Armed Police Hydropower Troops, Beijing , China (2) State Key Laboratory of Water Resource & Hydropower Engineering Science, Wuhan University, Wuhan , China Abstract The abrasion-erosion resistance of hydraulic concrete incorporate different mineral admixtures under high-velocity sandblasting (the velocity exceeds 40m/s) had been investigated by means of a self-designed sandblasting apparatus. The effects of class F fly ash, silica fume, and ground granulated blast furnace slag (GGBFS) on the abrasion-erosion resistance of concrete had been explored by tests. Furthermore, the effects of impinging angle on the abrasion-erosion resistance of concrete had also been considered and the abrasionerosion failure mechanism had been analyzed through model analysis. The results revealed that certain of amount of silica fume and GGBFS combined with fly ash could effectively enhance the abrasion-erosion resistance of concrete, and then single using the class F fly ash had positive effect on the abrasion-erosion resistance of concrete while the content of fly ash was increasingly large. The concrete was subjected to wearing when the impinging angle was smaller, and to impacting when the blast angle become larger. 1. INTRODUCTION Hydraulic concrete, used for the constructions of dams, canals, drainage tunnels, etc., is affected by abrasion-erosion due to the environmental water and flowing suspensions. The damage of concrete surfaces under water flow, caused by abrasive action of waterborne solid particles, is one of the major issues when designing the operation of hydraulic structures. The issue is especially severe in spillways and outlets of dams on rivers with significant torrential character. The most significant damage problem is due to the attack of high-velocity flowing water over the hydraulic structures surfaces. It is estimated that about 70% of all damage to hydraulic constructions is attributed to the abrasion-erosion by high-velocity water flow carried on solid particles [1]. Solving the abrasion-erosion resistance of hydraulic concrete involves two issues that should be understood, that is, the materials and the failure mechanism. There are many methods for testing the abrasion-erosion resistance of concrete [2,3], but the failure mechanism

2 of concrete subjected to high-velocity water flow carried solid particles has not been made clear. Many researches have focused on finding the treasures to improve the abrasion-erosion resistance of hydraulic concrete. Several strategies are known to improve the resistance of hydraulic concrete against abrasion or erosion, namely the addition of polymers to the matrix, mixing silica fume into the matrix, the addition of plasticizers to the mix, or the inclusion of steel fibers into the concrete mixture [4-8]. Up to the present, the kinetic abrasion-erosion by high-velocity water flow has not been established due to the complexity of abrasion-erosion failure mechanism. At the same time, many important projects, like Xiluodu, Xiangjiaba, Jingping, etc., are facing the abrasionerosion problem brought by the high-velocity water flow carried solid particles. So this work is attempting to investigate the abrasion-erosion failure mechanism of concrete subjected to high-velocity water flow carried solid particles, and seek to the approaches for improving abrasion-erosion resistance of concrete. It will be the objective of this paper to investigate the abrasion-erosion resistance of hydraulic concrete with different mineral admixtures at various flow velocities and for different angles of attack. 2. EXPERIMENTAL MATERIALS AND TEST METHODS 2.1 Raw materials and mix proportions The raw materials used in this work are as follows: (1) The cement is Portland cement PII 42.5 being allowed to code GB (China). (2) The fly ash is Grade I being allowed to code GB/T (China). (3) The Ground Granulated Blast Furnace Slag (GGBFS) is Grade S95 being allowed to GB/T (China).(4) The coarse aggregate is gravel with size scale 5~25mm, and the ratio of gravel for 5~10mm to 10~20mm is 2:8. (5) The fine aggregate is river sand with modulus of fineness 2.7. (6) The superplastisizer is polycarboxylic acid ZWL-A-IV with water-reducing ratio 31%. Table 1 The basic mix proportion Raw materials Cement Sand Gravel Water Superplastisizer Mass (kg/m 3 ) Table 2 Detail parameters of all mix proportions Serial number Cement Fly ash GGBFS Silica fume JS-1 100% / / / JS-3 60% 40% / / JS-5 60% / 40% / JS-8 60% 20% 20% / JS-9 60% 10% 30% / JS-13 80% 15% / 5% JS-14 60% 20% 20% / The specimens used in this study are cubic specimens of 150mm side length. The compressive strengths are 65~75MPa according to their different mineral admixtures. The basic mix proportion is listed in table 1. Keeping the water to cementitious materials (W/CM) constant, different mineral admixtures with different content were added into concrete for replaced the cement in order to investigate the influences of these mineral

3 admixtures on the abrasion-erosion resistance of hydraulic concrete. Table 2 shows the details of all mix proportions covered this work. 2.2 Test methods and precedure A self-designed sandblasting apparatus (showed in figure 1) was applied to investigate the failure mechanism of concrete subjected to different velocities and attack angles. The sandblasting apparatus was composed of air compressor, cold dryer, test chamber, dust removing plant, pressure gauge, air inlet pipe, water inlet pipe, abradant hopper and automatic mechanism for shifting the specimens from side to side. The test bed (on the automatic mechanism) could give different angles of attack (from 0 to 90 ), and the pressure gauge could give different air pressure to obtain the required velocity, v v air compressor 2. cold dryer 3. test chamber 4. dust removing plant 5. pressure gauge 6. air inlet pipe 7. water inlet pipe 8. abradant hopper 9. specimens 10. automatic mechanism from side to side Figure 1 Schematic map of the sandblasting apparatus In this work, we carried out three experiments for three issues as follows: (1) One is the effects of impact velocity on erosion resistance of concrete. In this issue, the air pressure was applied to represent the impact velocity. Here, the air pressures were 0.3MPa, MPa, 0.5MPa, MPa and 0.7MPa. (2) The other is the effects of impinging angles on abrasionerosion resistance of concrete. Here, we chose five impinging angles for consideration, that is, and 90. (3) The third one is the effects of mineral admixtures on abrasion-erosion resistance of concrete. Fly ash, ground granulated blast furnace slag (GGBFS), and silica fume were chosen for replacing the cement with different content. The abrasive particles used here is quartz sand, with 450μm±30μm mean particle diameter. 3. Test results and discussions 3.1 Effects of impinging velocities The result obtained from the experiment was mass loss of the specimen, and it was not suitable for evaluating the abrasion-erosion resistance. So the calculated mass loss rate and the abrasion resisting strength were used for evaluating the abrasion-erosion resistance of

4 concrete. The mass loss rate and the abrasion resisting strength were calculated by equation (1) and (2) respectively. Here, L(α) is the mass loss rate, g/kg, which denotes the mass loss of the specimen per kilogram abratant used, G 1 and G 2 is the mass of the specimen before and after testing, g, m is the mass of abratant used for every time, kg, f a is the abrasion resisting strength, h/cm, T is the testing time, h, ρ c is the density of concrete, g/cm 3, A is the area of specimen subjected to abrasion, cm 2. L(α)=(G 1 -G 2 )/m (1) f a =Tρ c A/(G 1 -G 2 ) (2) The mass loss rate and abrasion resisting strength of concrete under different air pressures were shown in figure 2. It s seen that the mass loss rate was in proportional to the air pressure, and the abrasion resisting strength was inversely proportional to the air pressure. These results were in accord with the Refs [11]. Mass loss rate /g/kg mass loss rate abrasion resistance strength air pressure/mpa Figure 2 Relationship of air pressure and mass loss rate/ wear resistance strength (JS-1, impinging angle is 90, 5min) According to abrasion mechanism of metals and ceramics, the velocity of abratant particles has remarkable influence on the abrasive resistance of materials, because there are closely relation between the abrasive mass loss and the kinetic energy of abrasive particles. A lot of abrasion-erosion experiments carried out by different abrasive particles with different size revealed that the relationship of abrasive particles mass loss rate and the abrasive particles velocity accord to expression (3), Loss=Kv n (3) Here, v denotes the velocity of the abrasive particles, and n is a parameter related to materials, e.g. n equals to 2.3~2.4 with regard to plastic materials, but 2.2~6.5 to brittle materials. In this work, the accurate velocity of the abrasive particles has not been measured, but it can be calculated by hydromechanics theory approximately Abrasion resistance strength /h/cm

5 3.2 Effects of impinging angles The relationship of impinging angle and mass loss rate is shown in figure 3. It s seen that the mass loss rate grew with the impinging angle increasing. It grew sharply from 30 ~60, but tended to grow gently from 60 ~90. This results is similar with brittle materials, like glass. So it s recommended that the selection of impinging angle of concrete should be perpendicular to the incidence direction (the impinging angle is 90 ) in favor of testing efficiency and convenience. Furthermore, it can be observed that the failure mode of concrete subjected to highvelocity water flow is close to wearing when the impinging angle is small (such as 15 ), but to impacting when the impinging angle is large(such as 90 ). 0.7 mass loss rate Mass loss rate/g/kg Impinging angle/degree Figure 3 Relationship of impinging angle and abrasion resistance of concrete (JS-1, MPa, 5min) 3.3 Effects of mineral admixtures Figure 3 shows the effects of different mineral admixtures on the compressive strength and abrasion-erosion resistance of concrete. Compressive strength/mpa compressive strength abrasion resisting strength Abrasion resisting strength/h/cm 0 JS-1 JS-3 JS-5 JS-8 Figure 4 Relationship of mineral admixtures and abrasion resistance of concrete (MPa, 5min, impinging angle was 90 ) It s seen that the compressive strengths of all series (from JS-1 to JS-14) have no decided changes, which scale is 65~75MPa. But it is worth notice that the abrasion resisting strengths JS-9 JS-13 JS

6 of these concretes were very different. Two series of concretes, which the cement being replaced by 20% fly ash and 20% GGBFS (JS-8), by 15% fly ash and 5% silica fume (JS-13) indicated remarkably higher abrasion resisting strength. Single replacing cement by fly ash (JS-3) or GGBFS (JS-5) didn t reveal preferable abrasion resistance. The combination using of fly ash and GGBFS or silica fume might be better for improving the interface between aggregate and paste, or enhance the abrasion-erosion resistance of hard cement paste. 4. CONCLUSIONS The flow velocity has significant effect on the abrasion-erosion resistance of hydraulic concrete. The mass loss rate of concrete has an approximate two power relation of water flow velocity. The material loss rate is in connection with the impinging angle. For concrete, the material loss rate increases with the impinging angle being larger, and the maximum material loss rate is occurred at the angle of 90. Furthermore, the material loss rate increases sharply when the angle extends from 15 ~60, but gently between 60 ~90. The additions of mineral admixtures have different effects on the abrasion resistance of concrete. Combination using of fly ash and GGBFS or silica fume could greatly improve the abrasion resistance of concrete, but single using of fly ash or GGBFS has hardly any action on its improving. ACKNOWLEDGEMENTS The authors are grateful for the financial support from the Major State Basic Research Development Program (No. 2009CB623200) and National Natural Science Foundation of China (No ). REFERENCES [1] He, Z., Hu, S.G., Liang, W.Q., and Li, B.X., 'Research of concrete abrasion-erosion in hydraulic structures', Journal of the Chinese Ceramic Society, 28 (12) (2000) [2] ASTM C , Standard Test Method for Abrasion Resistance of Concrete by Sandblasting. [3] ASTM C1138. Standard Test Method for Abrasion Resistance of Concrete (Underwater Method). [4] Andrej, K., Matjaz, M., Jakob, S. and Igor, P., 'Abrasion Resistance of Concrete in Hydraulic Structures', ACI Materials Journal, 106 (4) (2010) [5] Liu, Y.W., Yen, T., Hsu, T.H., 'Abrasion erosion of concrete by water-borne sand', Cement and Concrete Research, 36 (2006) [6] Yen, T., Hsu, T.H., Liu, Y.W., Chen, S.H., 'Influence of class F fly ash on the abrasion-erosion resistance of high-strength concrete'. Construction and Building Materials, 21 (2007) [7] Hu, X.G., Momberm A.W. and Yin Y., 'Erosive Wear of Hydraulic Concrete with low steel fiber content', ASCE Journal of Hydraulic Engineering, 132 (2006) [8] Momber, A.W. 'Short-time cavitation erosion of concrete', Wear, 241 (2000) [9] Hu, X.G., Momber, A.W., Yin, Y., Wang, H. and Cui, D.M., 'High-speed hydrodynamic wear of steel-fiber reinforced hydraulic concrete', Wear, 257 (2004) [10] Momber, A.W., 'An SEM-study of high-speed hydrodynamic erosion of cementitious composites', Composites: Part B, 34 (2003) [11] Hocheng, H. and Weng, C.H., 'Hydraulic erosion of concrete by a submerged jet', ASM International, 11 (2002)