EXPERIMENTAL STUDY ON MECHANICAL PROPERTIES OF MORTARS AT ULTRA-LOW TEMPERATURE

Transcription

1 EXPERIMENTAL STUDY ON MECHANICAL PROPERTIES OF MORTARS AT ULTRA-LOW TEMPERATURE Zhengwu Jiang (1), Xiongying Li (1) (1) Key Laboratory of Advanced Civil Engineering Materials of Ministry of Education, Tongji University, China Abstract Properties of cement based materials at ultra-low temperature quite differ from those at room temperature or at low temperature. In this paper, the influences of curing time at ultra-low temperature, curing time at room temperature and moisture content on mechanical properties of three types of mortars (ordinary mortar, high Strength Mortar, air entrained high Strength Mortar) at ultra-low temperature(-11) were investigated and compared with those at room temperature. The results show that mortar strength at ultra-low temperature greatly depends on not only curing time at ultra-low temperature, as well as other parameters of mortar such as water to cement ratio, curing time at room temperature, porosity, and moisture content and freezing degree of pore water etc. Flexural strength and compressive strength of mortar at ultra-low temperature grows rapidly with the increase of ultra-low temperature curing time at early stage and stabilizes after 3, 7 days respectively. The ratio of compressive strength to flexural strength of mortar at room temperature is higher than that at ultra-low temperature, indicating the decrease of brittleness of mortar at ultra-low temperature. Moisture content is one of the key factors affecting mortar strength at ultra-low temperature, which improves flexural strength greater than compressive strength. 1. INTRODUCTION Concrete is widely used in many concrete structure cases such as large, high-rise, special buildings under extreme conditions etc. due to its excellent mechanical properties and durability [1-3]. With the development of national liquefied gas energy strategy, concrete is gradually applied in the structures, such as liquefied natural gas storage tanks and cold storage warehouse at ultra-low temperature [2-5]. Generally speaking, ultra-low temperature means that temperature is lower than -5 C or more. However, mortar, concrete and other cement-based materials show quite different mechanical properties and durability at ultra-low temperature from those at low or room temperature. At present, because of deficiency of experimental conditions and methods, little work on concrete properties at ultra-low temperature was carried out. Initial researches demonstrate that concrete will be strengthened at ultra-low temperature, and the higher the water-cement ratio, entrained air content and 723

2 water content, the greater the strength of concrete [5]. However, it is still unclear that the influences law and mechanism of ultra-low temperature on the properties of concrete at ultra-low temperature. It is generally considered that the higher the water content and the lower the ultra-low temperature, the higher the strength at ultra-low temperature [5], which is different from that disputed conclusions as Monfore held that concrete at ultra-low temperature got maximum strength at -1 C [6]. Therefore, it is of great significance to explore mechanical behavior of cement-based materials such as mortar and concrete etc. at ultra-low temperature to guide their engineering applications in extreme special environments. In this paper, the influences of curing time at ultra-low temperature, curing time at room temperature and moisture content on mechanical properties of three types of mortars (Ordinary Mortar, High Strength Mortar, Air Entrained Mortar A) at ultra-low temperature(-11 C) were investigated and compared with those at room temperature. 2. EXPERIMENT 2.1 Materials Ordinary Portland cement with strength grade 42.5 and natural river sand with its modulus 2.49 were used. Polycarboxylate superplasticizer with 3% solid content was adopted. Sodium dodecyl sulfonate was selected as air-entraining agent, with its dosage.2% of cement by weight. 2.2 Experimental Methods Curing and Testing Methods at Ultra-low Temperature Figure 1: Ultra-low temperature curing box Figure 3: Ultra-low temperature incubator In this experiment, a large ultra-low temperature curing box was used for the curing of mortar specimens, whose internal minimum temperature was -15 C. To explore the cooling rate of surface and internal center of specimens in the ultra-low temperature box, mm specimens were placed in the box with the curing temperature set to -11 ± 3 C, Thermocouples were embedded in the center of mortar and attached to the surface of mortar respectively to monitor the temperature change accurately, which were connected to the Data Collector, with 2mm intervals between the surface and the center. As shown in Fig.2 the measured cooling process of center and surface of specimen, the surface temperature falls slightly faster than the center, but both of them was -18 C simultaneously after 2 hours and 13 minutes, and balance to the curing box temperature -11 C after 3 hours and 6 minutes. 724

3 temperature/ center -8 surface time/min temperature/ center surface time/min Figure 2: The cooling curve of specimen in ultra-low temperature curing box Figure 4: The warming curve of specimen in ultra-low temperature incubator Testing methods of flexural and compressive strength of mortar at ultra-low temperature Mortar specimens with the size of mm were used in flexural and compressive strength testing at ultra-low temperature. Specimens, firstly cured at standard conditions or in the water after molding for a certain age, were cured and tested at ultra-low temperature. As shown in Fig.3, a small ultra-low temperature incubator was adopted to guarantee the temperature of specimens at -11 during transportation. Ultra-low temperature incubator, mainly composed of incubator and six small cold storage boxes, was cooled to -11 in the ultra-low temperature box. Four cold storage boxes and six specimens at -11 were placed in the ultra-low temperature incubator in service. As shown in Fig.4 that the warming process of specimens in the ultra-low temperature incubator, the temperature of surface and center reaches -95 and -17 respectively, with corresponding temperature loss rate 13.6% and 2.7% respectively in 6 minutes. Two specimens were used for each flexural and compressive strength testing. It needs only two minutes for each testing. Therefore, ultra-low temperature incubator can ensure the ultra-low temperature of specimens during the whole testing. In Fig. 5, it is shown that specimens warm up faster in the air at 2 than in the ultra-low temperature incubator, indicating good heat insulation of the incubator. temperature/ surface -8 center time /min Figure 5: The warming curve of mortar specimen in the air at room temperature 725

4 2.2.3 Testing methods of specimens with different moisture content The flexural and compressive strength of mortars were tested in three conditions (fully, surface-dry, oven dry) in the experiment. Fully mortars were cured in the water at 2 ± 3, while ones were cured at 2 ± 1 together with humidity over 9%, and oven dry ones were cured and desiccated in the drier at 6 ± 3 for 2days before placed in the ultra-low temperature curing box. To guarantee the oven drying of given specimens, silane agent was used for waterproofing and sealing. 2.3 Test program In this study, as mix proportions of mortars are shown in Table 1, the influences of ultra-low temperature, initial strength of mortar and moisture content on compressive and flexural strength of, and A at ultra-low temperature were investigated, and compared with the strength at room temperature. Initial strength of mortar represents the strength directly tested after room temperature curing, depending on water to cement ratio, room temperature curing time and curing conditions etc. Table 1: The mixing proportions of three kinds of mortars Mortar types cement water sand Superplasticizer Air-entraining agent Ordinary Mortar, High Strength Mortar, Air entrained High Strength Mortar, A RESULTS AND DISCUSSION 3.1 The influences of curing time at ultra-low temperature on mortar strength The influences of ultra-low temperature curing time on strength of, with moisture content 4.7% after curing for 28days at room temperature were given, as shown in Fig flexural strength/mpa f l exur al st r engt h 7 compressi ve strength ultra low temperature curing time/d 1 9 compressi ve strength/mpa Figure 6: The influences of curing time of ultra-low temperature on mechanical properties 726

5 In Fig.6, compressive and flexural strength at ultra-low temperature increases with the increase of curing time at ultra-low temperature. Flexural strength of mortar at ultra-low temperature grows rapidly in the first day and gradually stabilizes after 3 days, while compressive strength increases gradually in first 7 days, and stabilizes afterwards. It is generally considered that strength of mortar at ultra-low temperature depends on not only the initial strength of mortar but also strength of frozen pore water in mortar governed by moisture content and the degree of freezing. The water in the pores with different sizes gradually freezes with the descent of temperature. While temperature reaches -11 or below, the pore diameter with unfrozen water approximates [7], indicating almost all the pore water are frozen into ice. The ice strength increases with the increase of ultra-low temperature curing time [5]. Pore water almost has been frozen into ice with its strength stabilizing after 7days, resulting in little increase of compressive and flexural strength after 7days. The increasing rate of flexural strength of mortar is higher than that of compressive strength. It mainly lies that ice substrate bond is very high [8] and the adhesive strength under tension is much greater than that under shear[9], while compressive strength and flexural strength reflects different destruction mechanisms as shear failure and tension destruction respectively. Because the strength of mortar stabilizes after 7 days curing at ultra-low temperature, all the specimens were tested after 7 days curing at ultra-low temperature after room temperature curing for a certain age in the following program if no special instructions. 3.2 The influences of initial strength of mortar on the strength at ultra-low temperature The influences of curing time at room temperature on the strength at ultra-low temperature As shown in Fig.7 and Table 2, the influences of curing time at room temperature on μ c, μ f of three kinds of mortars were given. Herein, μ c, μ f represents the ratio of the compressive and flexural strength at ultra-low temperature to the strength at room temperature respectively. Table 2 Flexural and compressive strength at room temperature and ultra-low temperature respectively Compressive Curing Flexural strength/mpa strength/mpa time/d A A

6 μc A room temperature curing time /d μf A room temperature curing time/d Figure 7: The influences of room temperature curing time on μ c and μ f of mortars In general, μ c & μ f, between 2 and 4, decrease with the increase of room temperature curing time and stabilize after 14 days as shown in Fig.7. decreases at a higher degree than and A. μ c & μ f develop with room temperature curing time differently. For μ f, with water content 7.1% is the highest, with water content 4.7% is secondary, and A with water content 4.4% is the smallest. For μ c, is higher than the other two mortars which are nearly the same. The strength of mortar at ultra-low temperature is governed by its initial strength at room temperature and strength of frozen pore water. Due to gradual hydration of mortar, the porosity together with moisture content decrease, resulting in the increase of initial strength of mortar and the decrease of frozen ice. Therefore, μ c and μ f decrease. Over 14 days room temperature curing, the initial strength stabilizes and so μ c and μ f trend to 2. μ c, μ f of, due to its higher water to cement ratio and moisture content, is higher than that of &A at early stage and decrease rapidly with the increase of room temperature curing time The influences of mortar types on strength at ultra-low temperature In Fig.8, it is shown that the influences of mortar types on flexural and compressive strength of mortar at ultra-low temperature. Mortars were firstly cured at room temperature and then cured at ultra-low temperature. In general, mortars strength increases gradually with the increase of room temperature curing time. Although differed at early stage, flexural strength of three kinds of mortars trend to 24MPa after 28 days curing at room temperature. Compressive strength of mortars, similar at early stage, reaches 9MPa and 19MPa respectively for and after 28days cuing at room temperature. But strength developing law is similar for and A because of the same water to cement ratio. Therefore, mortar type is one of the key factors affecting the strength of mortars at ultra low temperature. 728

7 nd International Symposium on Service Life Design for Infrastructure flexural strength/mpa A room temperature curing time /d compressive strength /MPa A room temperature curing time /d Figure 8: The influences of mortar types on the flexural and compressive strength of mortars at ultra-low temperature The comparison of the ratio of compressive strength to flexural strength at ultra-low temperature and room temperature respectively In Fig.9, it is demonstrated that the ratio of the compressive strength to flexural strength of three kinds of mortars at room temperature and ultra-low temperature respectively. Because of the stable initial strength of mortar, the ratio stabilizes after 14 days curing at room temperature. It is greater at room temperature than the one at ultra-low temperature, indicating that frozen pore water improves flexural strength greater than compressive strength and the brittleness of mortars decrease at ultra-low temperature. r ength t s 7flexural to strength compressive of ratio the A A room temperature room temperature r oom t emper at ur e ultra low temperature ultra low temperature ultra low temperature the curing time of r oom t emper at ur e/ d Figure 9: The ratio of the compressive strength to flexural strength at room temperature and ultra-low temperature respectively 3.3 The influences of moisture content on strength of mortar at ultra-low temperature 729

8 In Table 3 and Fig.1, it is shown that the influences of moisture content on strength at ultra-low temperature together with μ f and μ c of three kinds of mortars at fully, surface- dry and oven dry conditions. The moisture content of fully, and A is 7.1%, 4.7% and 4.4% respectively, while the corresponding surface-dry ones is 2.3%, 3.4% and 3.2% respectively. Table 3: The influences of moisture content on μ f and μ c Mortar A conditions μ f μ c μ f μ c μ f μ c Saturated Surface-dry Oven dry In Fig.1, for the same type mortar at ultra-low temperature, compressive strength increases with the increase of moisture content while fully one is highest, the surface-dry one is secondary and the oven dry one is the smallest. However, the flexural strength of the same type mortar at three conditions varies at ultra-low temperature, but the same strength is achieved for the ones despite different initial mortar strength. So the influences of moisture content on compressive strength are inferior to that on flexural strength. In Table 3, μ c & μ f of the same type mortar increase with the increase of moisture content. Therefore, mortar is enhanced by moisture at ultra-low temperature, while flexural strength is improved greater than compressive strength. So moisture is the key factor affecting mortar strength at ultra-low temperature. flexural strength/mpa A mortar types surface-dry oven dry compressive strength /MPa A mortar types surface-dry oven dry Figure 1: The influences of moisture content on flexural and compressive strength of mortars at ultra-low temperature 4 CONCLUSIONS 1. The curing and testing methods at ultra-low temperature proposed in this paper is feasible. 2. Flexural and compressive strength at ultra-low temperature grow rapidly with the increase of ultra-low temperature curing time at early stage, stabilizing after 3, 7 days respectively. 73

9 3. μ c and μ f are between 2 and 4. The ratio of compressive strength to flexural strength is greater at room temperature than the one at ultra-low temperature. 4. Strength of mortar at ultra-low temperature depends on not only the curing time at ultra-low temperature but also the water-cement ratio, curing time at room temperature, porosity, moisture content and freezing degree of pore water etc. 5. Moisture content is the key factor affecting the strength of mortar at ultra-low temperature and improves its flexural strength greater than its compressive strength. ACKNOWLEDGEMENT The authors gratefully acknowledge the support provided by Key project for the young fund of Key Laboratory of Advanced Civil Engineering Materials of Ministry of Education, Tongji University. REFERENCES [1] Zhengwu Jiang, Nemkumar Banthia and sarah delbar, Effect of cellulose fiber on properties of self-compacting concrete with high-volume mineral admixtures, Proceedings of the 2 nd International symposium on design, performance and use of self-consolidating concrete, 29.6, [2] Lahlou Dahmani, Amar Khenane, Salah Kaci. Behavior of the reinforced concrete at cryogenic temperatures. Cryogenics [J]. 47 (27): [3] Vandewalle L. Bond between a reinforcement bar and concrete at normal and cryogenic temperatures. J Mater Sci Lett 1989; 8: [4] Zheng Minsheng. Study and application on the high performance concrete in the project of FUJIAN LNG extremely low temperature tank [J]. Fujian Architecture & Construction, 26, 6: [5] Shan Genzhao, Zhao Ke-Zhi. Ultra-low temperature concrete [J]. Low Temperature Architecture Technology, 198, 1:6-63. [6] G.E.Monfore and A.E. Lentz. Physical properties of concrete at very low temperature [J]. J PCA Res Devel Lab, 1962, [7] Deng Shouchang, LI Kejian. PROPERTIES OF ULTRALOW TEMPERATURE CONCRETE [J]. Low Temperature Architecture Technology, 22, 3:5-7. [8] Susanta Chatterji. Aspects of the freezing process in a porous material water system Part 1. Freezing and the properties of water and ice, Cement and Concrete Research [J]. 29 (1999): [9] W.D. Bascom, R.L. Cottington, C.R. Singleterry, Ice adhesion to hydrophilic and hydrophobic surfaces, J Adhesion 1 (1969):