A Study of Properties of Mortar Mixed with Hydrophobic Agents: Water Absorption, Strength and Drying Shrinkage Y. Luan 1 , H. Furuta 1 , S. Asamoto 1 and T. Yoneda 2 1 Department of Civil and Environmental Engineering Saitama University, Saitama JAPAN 2 Technical Institute Maeda Corporation, Tokyo JAPAN E-mail: [email protected]Abstract: Physical and chemical deteriorations of concrete structures are often closely related to the penetration of water. In this study, a new mortar which has the property of internal hydrophobicity was made experimentally. Using hydrophobic agents, two hydrophobizing methods were carried out. One was to add the hydrophobic agent directly during the mixing. The other was that the hydrophobic agent was sprayed onto sand before mixing. Hydrophobicity was evaluated by measuring contact angle and water absorption ratio. It was found that compared to the normal specimens, the contact angle of the internal hydrophobized ones increased greatly, and water absorption ratio decreased. Moreover, water tightness of cracked mortar was investigated. The result showed that even if crack occurred, the hydrophobized mortar still had relatively good water tightness. Therefore, the internal hydrophobic mortar is promising to be used to increase water tightness of concrete structures. The compressive strength and drying shrinkage of the specimens was also investigated in this study. Keywords: Internal hydrophobic; mortar; sand spraying; contact angle: water absorption; crack 1. INTRODUCTION Physical and chemical deterioration of reinforced concrete structures is often closely related to the ingress of water. Deleterious substances such as chloride and sulfate ions diffuse into concrete with water, causing corrosion of steel bars and sulfate erosion, respectively. Water itself, as an agent, can also cause chemical or physical attack to concrete. For example, in the case of alkali silica reaction, gel products swell when absorbing water, and cause expansion pressure to lead to map cracking. In freezing and thawing process, water is directly involved to cause internal stress arising from the expansion when freezed. In the viewpoint of durability, improvement of water tightness of concrete is an important measure for alleviating deterioration and extending the service life of structures. Surface coating with water repellent agent is usually used to protect concrete from water ingress, but several problems exist. It is reported that under continuous solar radiation, the ultraviolet component can gradually undermine the coating layer, decreasing the water tightness (Kubo et al. 2001; Watanabe et al. 2011). The coating process costs labor and time, and also it is difficult to ensure the unevenness of the coating layer. Among those problems, the most fatal one is that, when even one crack occurs at the surface, water can easily penetrate through the cracks into the interior. Consequently, the overall tightness is lost, even if the coating layer is intact. In order to overcome the weaknesses, in particular that caused by crack, a concept of internal hydrophobizing treatment has been proposed and paid attentions (Yano et al. 2002, Moriconi & Tittarelli 2003). Compared with the coating method, which endows only surface hydrophobicity, the internal hydrophobizing treatment enables the interior of concrete repellent to water. Even if crack occurs, there will be little decrease of the overall water tightness. Based on this concept, the authors made such mortars by mixing hydrophobic agents in cement paste or mortar, and verified their water repellency. In particular, in the case of crack occurrence, the internal hydrophobic mortar showed good performance to resist water ingress (Furuta et al. 2015). In this study, using two different hydrophobic agents as admixture, the authors further investigated water repellency properties quantitatively, especially under cracked conditions. Moreover, strength and drying shrinkage of the hydrophobized mortar were also investigated experimentally. 213
Transcript
A Study of Properties of Mortar Mixed with Hydrophobic Agents: Water Absorption, Strength and Drying Shrinkage
Y. Luan1, H. Furuta1, S. Asamoto1 and T. Yoneda2
1Department of Civil and Environmental Engineering Saitama University, Saitama
Abstract: Physical and chemical deteriorations of concrete structures are often closely related to the penetration of water. In this study, a new mortar which has the property of internal hydrophobicity was made experimentally. Using hydrophobic agents, two hydrophobizing methods were carried out. One was to add the hydrophobic agent directly during the mixing. The other was that the hydrophobic agent was sprayed onto sand before mixing. Hydrophobicity was evaluated by measuring contact angle and water absorption ratio. It was found that compared to the normal specimens, the contact angle of the internal hydrophobized ones increased greatly, and water absorption ratio decreased. Moreover, water tightness of cracked mortar was investigated. The result showed that even if crack occurred, the hydrophobized mortar still had relatively good water tightness. Therefore, the internal hydrophobic mortar is promising to be used to increase water tightness of concrete structures. The compressive strength and drying shrinkage of the specimens was also investigated in this study.
Physical and chemical deterioration of reinforced concrete structures is often closely related to the ingress of water. Deleterious substances such as chloride and sulfate ions diffuse into concrete with water, causing corrosion of steel bars and sulfate erosion, respectively. Water itself, as an agent, can also cause chemical or physical attack to concrete. For example, in the case of alkali silica reaction, gel products swell when absorbing water, and cause expansion pressure to lead to map cracking. In freezing and thawing process, water is directly involved to cause internal stress arising from the expansion when freezed. In the viewpoint of durability, improvement of water tightness of concrete is an important measure for alleviating deterioration and extending the service life of structures.
Surface coating with water repellent agent is usually used to protect concrete from water ingress, but several problems exist. It is reported that under continuous solar radiation, the ultraviolet component can gradually undermine the coating layer, decreasing the water tightness (Kubo et al. 2001; Watanabe et al. 2011). The coating process costs labor and time, and also it is difficult to ensure the unevenness of the coating layer. Among those problems, the most fatal one is that, when even one crack occurs at the surface, water can easily penetrate through the cracks into the interior. Consequently, the overall tightness is lost, even if the coating layer is intact.
In order to overcome the weaknesses, in particular that caused by crack, a concept of internal hydrophobizing treatment has been proposed and paid attentions (Yano et al. 2002, Moriconi & Tittarelli 2003). Compared with the coating method, which endows only surface hydrophobicity, the internal hydrophobizing treatment enables the interior of concrete repellent to water. Even if crack occurs, there will be little decrease of the overall water tightness. Based on this concept, the authors made such mortars by mixing hydrophobic agents in cement paste or mortar, and verified their water repellency. In particular, in the case of crack occurrence, the internal hydrophobic mortar showed good performance to resist water ingress (Furuta et al. 2015). In this study, using two different hydrophobic agents as admixture, the authors further investigated water repellency properties quantitatively, especially under cracked conditions. Moreover, strength and drying shrinkage of the hydrophobized mortar were also investigated experimentally.
In the experiment, for the sake of accompanying water repellency in the hydrophilic hydration product of cement, two types of silicone-based hydrophobic agents were used as admixture, respectively. They were both commercial products, and herein were abbreviated as admixture A and D. Referring to the previous experiments by the authors (Furuta et al. 2015), two hydrophobizing methods were used as follows.
Direct Mixing method
The hydrophobic agent was directly mixed with water, cement and sand to make mortar.
Sand spraying method
The hydrophobic agent was first sprayed onto the surface of dry sand, and stirred repeatedly to ensure evenness. With this approach, the sand was hydrophobized. Then, the sand was dried for 24 hours, and mixed with cement and water to make mortar.
The experiment groups in this study are shown in Table 1. Water-to-cement ratios of all the groups were 0.5. The reference group was normal mortars without any hydrophobizing treatment. For the groups of the direct mixing, the addition ratio of the agent to the cement, by weight, was 3.0%. For the groups of sand spraying, 50 millilitres of agent was sprayed per kilogram of sand.
Specimens with three types of shape and size were prepared. One was square prisms with the size of 20 × 20 × 40 mm. The second was also square prisms, 40 × 40 × 160 mm. The third was cylinders, with the diameter 50 mm and height 100 mm. All of the specimens were cured with sealing until the age of 7 days. For some of the cylindrical specimens, crack was induced to investigate its water tightness under cracked condition. Tensile splitting load was applied. For those specimens, hard steel wires bent in the ends were preliminarily embedded in the mould before mixing (Figure 1). Before loading, 15 mm in both side were cut off. During the loading, when the load exceeded the splitting strength, a penetrated crack in the center occurred. The load was held on, and due to the confinement of steel wires, the crack enlarged gradually rather than sudden splitting. When the crack width approached the desired value, which was 0.5 mm in this study, the load was removed.
2.2 Measurement of contact angle and water absorption ratio
The hydrophobicity of the specimens was evaluated experimentally by measuring contact angle and conducting water absorption test. The contact angle is an index evaluating water repellency. In the experiment, cylindrical specimens were used. After curing, the specimens were dried at 20 ˚C and 60% for 4 days, and then the upper surface was polished. A drop of water was dripped at the surface. Using digital microscope, the tangent line of the drop at the liquid-solid surface was drawn, and the contact angle was measured (Figure 2). Generally the higher contact angle indicates higher water repellency.
In the water absorption test, square prism specimens with the size of 20 × 20 × 40 mm were used. Since water absorption ratio was influenced by the initial water content of specimens before absorption, two types of specimens with high and low water contents, were prepared. Initially, some specimens were dried at 20 ˚C, and the others were dried at 40 ˚C. Higher temperature accelerates water evaporation, so it could be regarded that the specimens dried at 40 ˚C had lower water content than those at 20 ˚C. After drying, those specimens were immersed into water. Weight gain during the absorption was measured. Absorption ratio was calculated by dividing the weight gain by the original weight of the specimen. Generally the lower the absorption ratio is, the more hydrophobic the specimen can be regarded.
2.3 Investigation of water tightness under cracked condition
In this study, the wetting-drying condition was applied to specimens to test its water tightness. Cylindrical specimens, with the size of φ50 x 100 mm, were prepared. As described, for some specimens, a splitting crack around 0.5 mm wide in the center, was induced using tensile splitting load. After that, the side and bottom surfaces were sealed using epoxy resin. The top surface was opened, and a 10 mm depth sink was set. The sink was kept full of water during the wetting (Figure 3) to control water head and provide water penetration by gravity. The cycle was 5-hour wetting and 19-hour drying. The temperature was 20 ˚C, and relative humidity for drying was 60%. Weight gain and loss were measured in the process, and the absorption ratio was calculated.
Figure 3 Specimen during wetting
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The groups in Table 1 were prepared. Except for that, two other groups using surface coating method were also prepared. Agents A and D were sprayed and coated at the absorption surface of the cylinders, respectively, to represent the general case of surface coating. For all the groups, there were two types of specimens, cracked and uncracked ones.
2.4 Measurement of strength and drying shrinkage
To consider the influence to strength, compression strength test of the mortar was carried out. Cylindrical specimens with the size of φ50 x 100 mm were prepared and test at the age of 7 day. In addition, drying shrinkage was also measured. Square prism specimens with the size of 40 × 40 × 160 mm, after curing, were dried at 20 ˚C and 60%. Two metal chips were attached at the surface using glue before drying, and their distance was measured to calculate the drying shrinkage strain. At the same time, water loss was also measured.
3. RESULTS AND DISCUSSIONS
3.1 Contact Angle and Water Absorption Ratio
Figure 4 shows the result of the contact angle measurement. According to the results, the average contact angle of the mortar without hydrophobizing, was 3.4˚. For the groups using agent A, the contact angles for the mixing method and the spraying method were 9.5˚ and 31.1˚, respectively. Those values for groups D were much larger, which were 91.1˚ and 98.5˚. Water tended to be spherical shape at the surface of mortar in those cases. Therefore, it can be concluded that silicone agent A had some effect for the hydrophobizing, whereas silicone agent D was much more effective.
Figure 4 Result of contact angle
The results of the absorption ratio with the time are shown in Figure 5. For those specimens dried at 40 ˚C, because of lower initial water content before absorption, their absorption ratios were higher than those dried at 20 ˚C. Furthermore, it can be found that, for all of the specimens using hydrophobic agents, the absorption ratios were lower than the reference ones. For the groups using agent D, they had lower absorption ratios than those using agent A, especially when initially dried at 40 ˚C. For the groups using agent A, there was just little effect if the specimens were initially dried at 40 ˚C. In generally, the results of absorption ratio were consistent with the measured contact angle. It can be concluded that hydrophobic agent D had better effect than agent A. In addition, the method of preliminary sand spraying seemed better than that of direct mixing.
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Figure 5 Result of water absorption ratio
3.2 Water Repellency under Cracked Condition
Figure 6 shows the absorption ratios of cylindrical specimens under drying-wetting cycles. The results of uncracked specimens are shown in Figure 6 (a), whereas those of the cracked ones are shown in Figure 6 (b). In case of the uncracked condition, except A-coating, for all of other specimens with hydrophobizing treatment, surface coating, direct mixing, or sand spraying, the absorption ratios were lower than that of the reference one. Thus, it is concluded that both the surface and internal hydrophobizing treatments were effective when the specimens were sound. On the other hand, in the case of the cracked specimens, the absorption ratios of both the surface coating and the reference ones increased significantly, whereas those of the internal hydrophobized ones were kept low like that of the uncracked ones. The surface coating ones had a slightly higher absorption ratio than the reference one, which may be attributed to the slight difference of the crack width. Undoubtedly, the surface coating ones lost their water repellency when crack occurred, but the internal hydrophobized ones had good water repellency all the same.
(a) Cylinders without crack (b) Cylinders with crack
Figure 6 Results of water absorption ratio under drying-wetting cycles.
3.3 Strength and Drying Shrinkage
Figure 7 shows the result of compressive strength at 7 day. It can be found that the strength of internal hydrophobized mortar tended to be much smaller than the reference one. The strength of reference mortar was 49.3 MPa. The strength of A-3 was 27.5 MPa, while that of A-S was 31.5 MPa. The strength of D-3 was 36.8, while that of D-S was 25.8 MPa. On the other hand, the measured air contents are shown in Table 2. It can be seen that air content increased, especially when the mortar was made with direct mixing method. The air content of reference mortar was 3.7%, whereas those of A-S and D-S were 4.8% and 5.2%, respectively. For A-3 and D-3, the values were 8.3% and 8.2%. It
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can be deduced that one of the reason for strength decreasing may be the increased air content. Besides, it can be found that, A-3 and D-3 increased much more than A-S and D-S in the air content, while they had similar level with A-S and D-S in the strength, or even higher. For the case of sand spraying, the surface of sand became hydrophobic, so the interfacial transition zone (ITZ) between cement paste and sand was further weakened. Except for the above reasons, hydrophobic agent may inhibit hydration process of cement, and thus decrease the strength. This needs to be investigated in future study.
Figure 7 Result of compressive strength
Table 2 Result of air content
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The results of water loss ratio and drying shrinkage are shown in Figure 8 and Figure 9. For water loss ratios, except A-3, all of the other hydrophobized mortars had lower values than the reference one. D-S and D-3 had lower water loss ratios than their counterparts, A-S and A-3, respectively. As to the manufacturing method, the sand sprayed ones had lower values than those made by direct mixing. A possible reason, for the lower water loss of hydrophobized ones, may be due to the inhibiting effect for water evaporation from micro-pores of mortars. Another reason, on the other hand, may be that water moved out of mortar during the sealing stage, and this portion was not caught in the measurement starting from drying point. In the experiment, when removing sealing film, water was found between the surface of mortar and film. Further investigation for water loss, including the sealing stage, should be done in the future.
As to drying shrinkage, the reference one was found to have the largest shrinkage, which was above 500 micros at 7 day of drying. The hydrophobized ones all had smaller shrinkage. Especially D-3 and D-S had much lower values, which were just one third of the value of the reference one. This trend was consistent with the past research (Yano et al. 2002). The relationship between water loss and drying shrinkage is shown in Figure 10. It can be seen that when having the same water loss ratio, the hydrophobized mortars all had lower drying shrinkage than the reference one. As contact angle increases, water tends to be spherical drops rather than spreads in the interface of water and solid. From microscale aspect, it can be imagined that water in micro-pores also tends to be spherical shape. The result is that the contact area between water and pore walls decreases. Generally, the
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driving force of shrinkage is caused by negatively pressure of water in micro-pores due to meniscus. As the contact area decreases, the driving force of shrinkage decreases, and so does drying shrinkage.
Figure 8 Result of water loss ratio Figure 9 Result of drying shrinkage
Figure 10 Relationship between water loss and drying shrinkage
4. CONCLUSIONS
In this study, the authors attempted to make internal hydrophobic mortar experimentally. Two different hydrophobic chemical agents were tried. Two manufacturing methods, which were directly mixing and preliminary spraying on sand, were conduct. To evaluate the hydrophobizing effect, contact angle and water absorption ratio of the specimens were measured. It is found that out of the two agents, a silicone chemical D was more effective, especially when it was sprayed on the sand and then mixed into mortar. The contact angle was increased and water absorption ratio was decreased greatly. Subsequently, to verify the internal hydrophobicity, wetting-drying test was conduct for uncracked and cracked mortars. It is experimentally confirmed that the specimens with surface coating, when crack occurred, lost their water repellency, whereas the internal hydrophobized specimens remained water repellent state. Moreover, the hydrophobizing increased air content and enlarged the interfacial transition zone between sand and hardened cement paste, causing great decrease of the strength. It was reflected in the compressive strength test. On the other hand, drying shrinkage of the hydrophobized mortars were lower than those of the reference ones.
ACKNOWLEDGMENTS
This work was supported by JSPS KAKENHI Grant Number 26820175. The authors would like to express the gratitude to their support.
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REFERENCES
Furuta, H., Luan, Y., Asamoto, S. & Yoneda, T., 2015, 'Development of an internal hydrophobic cementitious material', Proceedings of 4th International Symposium on Engineering, Energy and Environment Engineering for Sustainable Society: a Move toward a Better World, Chonburi, Thailand, Nov. 8-10, 2015, pp. 357-362.
Kubo, Y., Tamai, Y., Kurihara, S. & Miyagawa, T., 2001, 'Durability of Hydrophobic Effect of Concrete Impregnated by Silane, Proceedings of the Japan Concrete Institute, Sapporo, Japan, Jul. 4-6, 2001, pp.421-426. (In Japanese)
Moriconi, G. & Tittarelli, F., 2003, 'Use of Hydrophobic Agents as Concrete Admixtures', ACI special publication, Vol. 212, pp.279-288.
Watanabe, S., Sagawa, Y., Tanikura, I. & Nojima, A., 2011, 'A Study of Surface Properties of Concrete Coated with Different Ratios of Silane-based Agents', Proceedings of the Japan Concrete Institute, Osaka, Japan, Jul. 12-14, 2011, pp.1619-1624. (In Japanese)
Yano, U., Kikuchi, M. & Koyama, A., 2002, 'Experimental Study of Properties of Mortar Mixed with Silicon-based hydrophobic agent', Research Report of Kanto Chapter, Architectural Institute of Japan, pp. 61-64. (In Japanese)
