Abrasion test of flexible protective materials on hydraulic structures

Xin WANG*, Shao-ze LUO, Guang-sheng LIU, Lu-chen ZHANG, Yong WANG

1. State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Nanjing Hydraulic Research Institute, Nanjing 210029, P. R. China

2. Beijing Oriental Yuhong Waterproof Technology Co., Ltd., Beijing 100123, P. R. China

Abrasion test of flexible protective materials on hydraulic structures

Xin WANG*1, Shao-ze LUO1, Guang-sheng LIU2, Lu-chen ZHANG1, Yong WANG1

1. State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Nanjing Hydraulic Research Institute, Nanjing 210029, P. R. China

2. Beijing Oriental Yuhong Waterproof Technology Co., Ltd., Beijing 100123, P. R. China

In this study, several kinds of flexible protective materials sprayed with polyurea elastomers (hereinafter referred to as polyurea elastomer protective material) were adopted to meet the abrasion resistance requirement of hydraulic structures, and their abrasion resistances against the water flow with suspended load or bed load were studied systematically through tests. Natural basalt stones were adopted as the abrasive for simulation of the abrasion effect of the water flow with bed load, and test results indicate that the basalt stone is suitable for use in the abrasion resistance test of the flexible protective material. The wear process of the polyurea elastomer protective material is stable, and the wear loss is linear with the time of abrasion. If the wear thickness is regarded as the abrasion resistance evaluation factor, the abrasion resistance of the 351 pure polyurea is about twice those of pure polyurea with a high level of hardness and aliphatic polyurea, and over five times that of high-performance abrasion-resistant concrete under the abrasion of the water flow with suspended load. It is also about 50 times that of high-performance abrasion-resistant concrete under the abrasion of the water flow with bed load. Overall, the abrasion resistance of pure polyurea presented a decreasing trend with increasing hardness. Pure polyurea with a Shore hardness of D30 has the best abrasion resistance, which is 60 to 70 times that of high-performance abrasion-resistant concrete under the abrasion of the water flow with bed load, and has been recommended, among the five kinds of pure polyurea materials with different hardness, in anti-abrasion protection of hydraulic structures.

flexible protective material; polyurea elastomer material; abrasion resistance; hardness influence; hydraulic structure

1 Introduction

Erosion and abrasion have been the most common problems in hydraulic structures. Overflow surfaces, flip buckets, spillway tunnels, flushing sluices, and stilling pool base slabs are all easily damaged by abrasion. The Fengman Hydropower Station overflow dam’s ogee section was damaged to a depth of 3 to 4 m because of flow erosion, and the maximum abrasion depth of the apron reached 4.5 m (Dai and Xu 2009). Behind the work gate of theSanmenxia No. 2 bottom hole, a large area of abrasion-induced damage occurred, and the average abrasion depth was 14 cm. In the Yantan Hydropower Station, the bucket concrete surface was widely eroded, and the average thickness of exposed aggregate was 2 to 5 cm (Xia 1988). The stilling basin of the Indian Barkla Dam also had severe sediment erosion records (Vegas Merino et al. 2005). With increasing water conservancy project scale, the flow rate over discharge structures generally exceeds 35 m/s, and in some cases reaches 50 m/s, leading to more serious abrasion.

At present, in order to resist abrasion, high-performance concrete is mainly used. Lots of related research has been conducted, and some new kinds of abrasion-resistant concrete have been produced, with the abrasion resistance improved to some extent. With the progression of the research, polyurea, a new organic polymer abrasion-resistant material, has gradually drawn people’s attention (Henningsen 2002; Chen 2006). As an effective and environmentally friendly material, polyurea was used in construction of spillways and flip buckets at the Xin’anjiang and Fengman hydropower stations (Sun et al. 2006), the concrete volute at the Nierji Hydropower Station (Sun et al. 2009), the de-silting tunnel at the Xiaolangdi Hydropower Station, the plunge pool at the Xiaowan Hydropower Station, the stilling basin at the Guandi Hydropower Station, and the middle hole of the Three Gorges Dam in China. Outside of China, the discharge hole of the Tehri Dam in India is the most typical example of polyurea application in hydropower projects.

Spraying polyurea has been presented in the field of water resources and hydropower engineering for a few years. So far, though, the abrasion resistance of the polyurea material has not been studied systematically. Few indicators related to abrasion resistance can be referenced, and optimization of components of polyurea protective materials has never been conducted to meet the requirement of the abrasion resistance of hydraulic structures. Existing research on abrasion resistance is only based on a few simple tests and roughly qualitative evaluations. Through the high-speed erosion test, Wu (2005) showed that the abrasion resistance of polyurea materials was much higher than that of the C60 silica fume concrete. Zhong et al. (2007) evaluated the abrasion resistance of the polyurethane- or polyurea-coated layer in high-speed sediment jets using the wear-and-tear experimental machine and high-pressure water jet erosion tester. Guo et al. (2011) analyzed the main factors leading to the surface abrasion of the elastomeric coating. It was shown that the impingement of high-speed particles could lead to untimely dissemination of the stress wave in elastic bodies, which caused surface debacle. Chen et al. (2011) found that two-component polyurea material has a high ability to resist water erosion and abrasion. Some foreign research has focused on the mechanical properties of polyurea (Grujicica et al. 2010; Roland and Casalinj 2007; Sarva et al. 2007; Aly and Hussein 2010). However, study of the abrasion resistance has not been found reported aboard. This study aimed to determine the law of abrasion-induced damage and indicators related to the abrasion resistance of polyurea materials based on a series of tests.

2 Materials and samples

Polyurea elastomer protective material has the properties of fast curing, high impermeability, a high degree of toughness, high tensile strength, elongation, chemical resistance, abrasion resistance, impact resistance, and aging resistance, as well as strong bonding with a variety of substrates, and the proportion of its components is arbitrarily adjustable. It has been used in many tasks, especially in waterproofing, wearing resistance, anticorrosion, and decoration.

Beijing Oriental Yuhong Waterproof Technology Co., Ltd. has been committed to development of abrasion-resistant polyurea material for anti-abrasion protection of hydraulic structures, and has produced three formulas for abrasion-resistant polyurea material. The main performance parameters of three kinds of abrasion-resistant polyurea materials used as the surface protective materials in this study are listed in Table 1, and are abbreviated as follows: S for the 351 pure polyurea, G for the pure polyurea with a high degree of hardness, and Z for the aliphatic polyurea.

Table 1 Performance parameters of polyurea protective material

The concrete specimens were made according to the test content and equipment requirements in this study. Abrasion-resistant concrete, CM for short, used in the Longtan Hydropower Station, was used as the concrete substrate of specimens, and the contents of water, cement, fly ash, sand, and stone of the concrete substrate were 146, 414, 73, 584, and 1 134 kg/m3, respectively, with a reducer ratio of 1%. The compressive strength of the concrete substrate at an age of 60 days could reach 64 MPa. The spraying of surface protective materials began when the concrete substrate had been cured for 28 days. The curing process of concrete specimens was the same as that of on-site construction. The spraying thickness of polyurea protective materials was 4 mm, and the test was conducted after the specimens sprayed with protective materials had been cured for seven days.

3 Test methods

Abrasion tests of concrete samples with and without spraying surface protective materials were conducted under the abrasion conditions of the water flow with suspended load and bed load, respectively. Silicon carbide was used as sand in high-speed flow to simulate the abrasion conditions of the water flow with suspended load, and underwater balls and basalt stones were employed to simulate the abrasion conditions of the water flow with bed load. The relative resistance of the surface material corroded by underwater high-speed moving mediawas determined, and the surface abrasion resistance was evaluated. Improved wear testing equipment was used in the abrasion test of the suspended load flow, and the maximum test flow speed reached 60 m/s. Compared with traditional equipment, the new equipment had stronger destructive effects, and the test time was shortened (Gao et al. 2011; Wang et al. 2012). In this test, the sand rate was 7%, the flow speed was 40 m/s, and the test time was determined by the effects of abrasion. The HKS-II anti-abrasion testing machine was used in the abrasion test of the bed load flow. The test was carried out according to the Test Code for Hydro-concrete (SL352-2006), and the specimens were abraded for a period of 72 hours.

The test process was as follows: At first, the specimens were soaked in water for 48 hours to reach full saturation before the test. Then, we took the specimens out of water and wiped off the surface water. After that we weighed the specimens and put them into the test apparatus. After testing, the specimens were removed from the test apparatus, with the surface water wiped off. We weighed the specimens again and calculated the weight loss, wear rate, and abrasion resistance strength. In addition, the morphological changes and damage characteristics of the specimens were observed.

4 Test results and discussions

4.1 Abrasion by suspended load flow

The surface morphology contrast of the specimens with or without a protective coating before and after two hours of abrasion by the suspended load flow is shown in Fig. 1. For the specimen with a protective coating G, the protective coating and concrete substrate remained firmly bonded without bubbling or peeling after two hours of abrasion. Although the original smooth coating surface became corrugated, and rippled abrasion marks appeared, the destruction was slight, and there was little variation of the coating thickness. However, the concrete surface without a protective coating flaked with exposed aggregates, and was more seriously damaged than the protective coating.

Fig. 1 Surface morphology comparison after two hours of abrasion by suspended load flow

The average wear rate and abrasion resistance strength of specimens after two hours of abrasion in water flow with suspended bed are shown in Table 2. The abrasion resistance of the protective coating is more than 10 times higher than that of the concrete surface according to the measured data of mass loss, and the wear thickness of the concrete specimen without the protectivecoating is about five times those of the specimens with the protective coating. The 351 spraying pure polyurea has the best abrasion resistance of the three kinds of protective coatings.

Table 2 Abrasion resistance parameters after two hours of abrasion by suspended load flow

4.2 Abrasion by bed load flow

4.2.1 Abrasion by underwater steel balls

The surface morphology contrast of the specimens before and after 72 hours of abrasion by underwater steel balls is shown in Fig. 2. The diameter of all the circular specimens is 29.5 cm. As shown in Fig. 2, the protective coating was basically intact, in addition to the fact that the surface gloss receded slightly. However, the concrete surface was seriously damaged with exposed coarse aggregates. Table 3 shows large differences between damage of the protective materials and the concrete specimen without a protective coating after the abrasion test by underwater steel balls, but the abrasion resistances of the three kinds of protective materials are almost the same.

Fig. 2 Surface morphology comparison after 72 hours of abrasion by underwater steel balls

Table 3 Abrasion resistance parameters after 72 hours of abrasion by underwater steel balls

The abrasion tests showed that the polyurea elastomer protective coatings were basically intact under the abrasion by underwater steel balls but were slighted damaged under the high-speed suspended load flow. This is mainly due to different wear mechanisms of the two abrasion conditions. The wear mechanism of the high-speed suspended load flow is mainly the impacting and cutting actions of silicon carbide on the specimen surface, while the wearmechanism of underwater steel balls is mainly the rolling, jumping, and friction actions of underwater steel balls (bed load). For the polyurea elastomer coating, the cutting action of the suspended load flow will cause a certain degree of abrasion, but with elastic deformation, it is difficult for the smooth ball to cause the damage by abrasion. Therefore, underwater steel balls are not suitable for use in the abrasion test of polyurea elastomer protective materials, and other appropriate methods should be proposed for reasonable simulation of the abrasion action of the bed load flow.

4.2.2 Abrasion by basalt stones

To compensate for the deficiency of underwater steel balls in the abrasion resistance evaluation of polyurea elastomer protective materials, hard natural basalt stones with different shapes and sharp corners were proposed to replace steel balls as the abrasive. In this study, natural basalt stones of 1 to 2 cm in diameter and 1 kg in weight were chosen at random, as shown in Fig. 3(a), and added into each test apparatus. After a continuous 24 hours of abrasion, the edge of basalt stones became rounded and smooth as pebbles, and the abrasion effect became relatively weak, as shown in Fig. 3(b). Thus, basalt stones should be replaced every 24 hours in the test process.

Fig. 3 Variation of basalt stones before and after 24 hours of abrasion

The abrasion process of the high-level hardness pure polyurea is shown in Fig. 4. Except for the small central region, most of the area of the sample surface was significantly worn. With the abrasion going on, the protective layer gradually became thin. The test indicates that basalt stones are more suitable for simulation of the abrasion effect of the bed load flow than steel balls. The surface morphology contrast of abrasion-resistant concrete and three protective materials after 96 hours of testing is shown in Fig. 5. The concrete surface without a protective coating was seriously damaged, while the three kinds of protective materials were slightly worn.

Fig. 4 Surface morphology variation of concrete specimen with coating G during testing process

Fig. 5 Surface morphology comparison of different materials after 96 hours of abrasion

The mass loss during the abrasion process and the abrasion resistance parameters are listed in Table 4. The mass loss of each material was relatively uniform for every 24 hours, and the non-protective concrete surface was severely damaged. The three kinds of protective coatings have far superior abrasion resistance to concrete, about 40 to 100 times according to the mass loss and about 20 to 50 times according to the wear thickness. The abrasion resistances of the high-level hardness pure polyurea and aliphatic polyurea are almost the same, while that of the 351 pure polyurea is about twice the amount. Therefore, the 351 pure polyurea has superior abrasion resistance.

Table 4 Abrasion resistance parameters during abrasion test with basalt stones

The average wear thickness was calculated according to the mass loss, and the relationship between the average wear thickness and testing time of the three kinds of protective coatings is shown in Fig. 6. It can be seen that the 351 pure polyurea wears slowest and has the optimal abrasion resistance.

Fig. 6 Relationship between average wear thickness and testing time

4.3 Influence of hardness on abrasion resistance

The test of three kinds of polyurea materials indicates that the 351 pure polyurea has better abrasion resistance than the aliphatic polyurea and high-level hardness pure polyurea. In fact the 351 pure polyurea and high-level hardness pure polyurea are only different in hardness. To further examine the relationship between the hardness and abrasion resistance of pure polyurea, five kinds of pure polyurea specimens with different Shore hardness, i.e., D25, D30, D40, D50, and D60 from soft to hard, hereinafter referred to as the Shore D25, Shore D30, Shore D40, Shore D50, and Shore D60 polyurea, were subjected to anti-abrasion performance testing with the basalt abrasive. Three specimens were made for each kind of hardness, and the results were obtained by computing their average values. The abrasion resistance parameters are listed in Table 5.

Table 5 Abrasion resistance parameters for pure polyurea of different hardness

As shown in Table 5, the average mass loss of pure polyurea specimens increased with the abrasion time, demonstrating a linear relationship. The abrasion resistance presented a decreasing trend with the increase of the hardness of pure polyurea. The relationship between the average wear thickness and Shore hardness of pure polyurea is plotted in Fig. 7. The Shore D25 and the Shore D30 polyurea specimens have smaller wear volumes, and their abrasion resistances are almost the same, with that of the Shore D30 polyurea specimen being slightly better.

Fig. 7 Hardness influence on abrasion resistance of pure polyurea

The scanning electron microscope was used to observe the micro-morphology of specimens for study of the wear features of pure polyurea with different levels of hardness. The surface and inner micro-morphologies of pure polyurea before testing are shown in Fig. 8. The coating surface was flat and smooth before abrasion, and there were many inner bubbles with diameters of several tens of micrometers. The surface micro-morphologies of the three kinds of pure polyurea materials with the Shore hardness of D25, D40, and D60, after 96 hours of abrasion, are presented in Fig. 9. It can be clearly found that wear of different degrees occurs on the surface of all the specimens, and that the wear features of pure polyurea of different levels of hardness are different. The softest Shore D25 polyurea showed little wear loss but had the roughest surface. Pits appeared at the site of inner bubbles, and regular fish-shaped wear marks appeared at the other sites after the abrasion test. The surface smoothness of the Shore D40 polyurea was significantly better than that of the Shore D25 polyurea. Some etch pits appeared at the site of inner bubbles. The wear feature presented significant directivity, and part of the bubble surface was still left without complete abrasion. Slight wear marks also existed on the other smooth area and were not as serious as those of the Shore D25 polyurea. The Shore D60 polyurea showed the most wear loss, but the abraded surface was very smooth, and few etch pits or wear marks appeared.

Fig. 8 Surface and inner micro-morphologies of pure polyurea before testing

Fig. 9 Surface micro-morphologies of pure polyurea with different levels of hardness after 96 hours of abrasion

It can be concluded that the hardness has significant influence on the abrasion resistance of pure polyurea. The soft polyurea material has a high degree of toughness and will deform and absorb most of the energy under impacting, cutting, and friction effects of the abrasive.The brittleness of pure polyurea increases with the hardness. Thus, the abrasion resistance decreases, and the wear surface becomes smooth with the increase of hardness. Of the five kinds of pure polyurea materials with different levels of hardness, the Shore D30 polyurea has the best abrasion resistance. If the wear thickness is considered the evaluation index, the abrasion resistance of the Shore D30 polyurea is 60 to 70 times that of abrasion-resistant concrete.

5 Conclusions

The abrasion resistance of a variety of polyurea elastomer protective materials was examined in this study, and the following conclusions can be made:

(1) The traditional underwater ball method is not suitable for the abrasion resistance evaluation of flexible protective materials, while the proposed basalt abrasive method is effective for simulation of the abrasion effect of the bed load flow on the polyurea elastomer protective material.

(2) The wear process of the polyurea elastomer protective material is stable, and the wear loss is linear with the abrasion time. The abrasion resistance of the protective material is far superior to high-performance abrasion-resistant concrete. The abrasion resistance of the 351 pure polyurea is about twice those of pure polyurea with a high level of hardness and aliphatic polyurea.

(3) If the wear thickness is regarded as the abrasion resistance evaluation factor, the abrasion resistance of the 351 pure polyurea is over five times that of high-performance abrasion-resistant concrete under the abrasion by the suspended load flow and about 50 times under the abrasion of the bed load flow.

(4) Overall, the abrasion resistance of pure polyurea shows a decreasing trend and the wear surface varies from rough to smooth with increasing hardness. Of the five kinds of pure polyurea materials with different hardness, the Shore D30 polyurea has the best abrasion resistance, which is 60 to 70 times that of abrasion-resistant concrete. Therefore, it is recommended in anti-abrasion protection of hydraulic structures.

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(Edited by Ye SHI)

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This work was supported by the National Natural Science Foundation of China (Grants No. 51109143 and 51209144), the Natural Science Foundation of Jiangsu Province (Grant No. BK2011109), and the Foundation of Nanjing Hydraulic Research Institute (Grant No. Y113004).

*Corresponding author (e-mail: xwang@nhri.cn)

Received Oct.16, 2012; accepted Sep. 16, 2013