Residual Compressive Strength of Recycled Brick Aggregate Concrete at High Temperatures

Transcription

1 Residual Compressive Strength of Recycled Brick Aggregate Concrete at High Temperatures Kasi Rekha 1, Dr. M. Potharaju 2 1 Assistant Professor, 2 Professor, Department of Civil Engineering, GITAM University, Visakhapatnam, Andhra Pradesh, India. Abstract--Utilization of construction debris as recycled aggregates in the production of concrete is gaining momentum nowadays due to the availability of large quantity of demolition waste. The recycled brick aggregate (RBA) concrete is made used for the production of low grade recycled aggregate suitable for concrete production. The risk of concrete structures exposed to fire is also on the increase due to the increased fire accidents and blasts. This paper presents the results of an experimental investigation on the effects of high temperatures on the properties of a standard RBA concrete mix made with 25% of crushed clay bricks as the coarse aggregate. The concrete cubes were casted with crushed clay brick and granite aggregate. The specimens of both RBA and Granite aggregate (GA) concrete were subjected to temperatures ranging from 100 o C to at an interval of 100 o C for duration of Three hours. The compressive strength of both the concretes before 1000 o C and after exposure to high temperatures was compared to assess the relative performance. The results showed that RBA concrete preformed better than GA concrete at high temperatures. Keywords-- Recycled Brick Aggregate, Granite Aggregate, Concrete, High Temperatures, Colour Change, Crack Pattern, Weight Loss, Compressive Strength. I. INTRODUCTION Numerous civilizations have reused building materials of earlier civilizations of their own destroyed architecture (either through war or natural causes) to construct new buildings. The remains of ruined Romanesque churches supplied the stone for various farm houses. A huge quantity of demolition waste, which constitutes a major portion of the total solid waste stream, is generated every day. The demolition waste includes demolished concrete, bricks, masonry, plaster, etc. The nature of waste is generated due to demolition of the damaged old structures like buildings, pavements, bridges etc. Natural disasters like earthquake, cyclone and floods etc., also leads to the increase in demolition wastes. The enormous quantities of demolition waste has created a great challenge in its disposal and its impact on the environment. Some of the important elements needs to be considered for protecting the environment are the reduction of the consumption of energy and consumption of demolition waste. Nowadays, this issue has also drawn considerable attention under sustainable development. The utilization of recycled aggregate is particularly very promising as 75 per cent of volume of concrete is occupied by the aggregates alone. Recycled Aggregate was initially used as landfill because of limited recycling facilities and for economical efficiency. However, this situation has changed after extensive research work and significant advancement in concrete technology. At present, they have been widely used for non-structural concrete applications such as masonry partition wall blocks, flooring, foundation mats, coarse materials of road base, paving blocks. As for building applications, masonry partition wall blocks, flooring prepared with recycled aggregates may be affected by fire. The behavior of concrete in fire depends on its mix proportion and constituents and is determined by complex physiochemical transformations during heating. Reduction in compressive strength of concrete was observed when the temperature is raised. This reduction is due to the physical or chemical breakdown of aggregates when heated to high temperatures normally above 600 o C. However concretes with siliceous aggregates perform better at elevated temperatures by exhibiting lesser loss in strength. As brick contains 55% of silica in its chemical composition, the recycled brick aggregate concrete gives better performance than the normal aggregate at elevated temperatures. II. LITERATURE SURVEY A substantial number of researchers have reviewed the properties of recycled aggregate concrete. Khalaf F M [1] et al, investigated the properties of crushed clay brick concrete subjected to elevated temperatures. The results were compared with natural granite aggregate concrete. 159

2 The physical properties of brick like uniaxial compressive strength, aggregate impact value, aggregate relative density, brick and aggregate water absorption, aggregate porosity, concrete density were determined before using them in concrete. The authors also concluded that the fire resistance of crushed clay brick aggregate concrete is as good as that of natural aggregate concrete. The bond between the brick aggregate and he cement paste were also found good after breaking the specimen to crush. Xiao Z [2] et al, studied the properties of partition wall concrete blocks made with recycled clay brick aggregate. The recycled clay bricks were collected from construction and demolition waste streams and replaced the coarse and fine aggregate in percentages of 25, 50,75 and 100 by volumes. The compressive and flexural strength of concrete were determined after exposing to temperatures of 300, 500 and 800 o C. The authors concluded that the concretes retained about 48 to 91 % of their original compressive strength and 8to 41% of their initial flexural strength. Yang, J [3] et al, studied the performance of concrete by replacing the natural coarse aggregate with recycled concrete aggregate(rca) and Crushed clay brick(ccb). The authors concluded that the content of CCB will increase the permeability. But upto 20% inclusion of CCB in RCA will still produce concrete with very good protective quality. Arundeb Gupta [4] et al, studied the mechanical and micro structural properties of uncoated and Geopolymer / Cement coated recycled aggregate concrete (RAC) after exposed to elevated temperature. The test specimen were exposed to temperatures of 400, 600,800 o C for a period of 6 hours. The authors concluded that at elevated temperatures the recycled aggregate concrete coated with Geopolymer showed higher compressive strength than uncoated and cement coated recycled aggregate concretes. Xiao JZ [5] et al, investigated the residual compressive and residual flexural strengths of recycled concrete with 0,30,50,70,100% replacements of recycled coarse aggregate subjected to elevated temperatures.the authors concluded that the compressive strength and flexural strength of recycled aggregate concrete (RAC) at elevated temperatures is better compared to Natural Aggregate Concrete(NAC). No explosive spalling was observed at elevated temperatures in case of RAC. Sedat Karaman et al [6] studied the firing time effect on compressive strength, water absorption, weight loss, clay mineralogy etc. It is observed that as the temperature increases the strength of the brick is also increasing to some extent. 160 As the temperature increases the weight loss is also increased as a result of loss of organic matter in clay. III. MATERIALS USED A. Cement: The cement used was Portland Pozzolona cement with 28 days compressive strength of 62.4 N/mm 2 which satisfies the limits given in IS [7]. The same cement was used to study the performance of both RBA and GA concretes. B. Fine Aggregate: Locally available natural sand was used as fine aggregate. The sieve analysis carried out in accordance with IS 2386 (Part 1)-1963 [8]. C. Granite and Brick Aggregate: Natural crushed 20 mm single sized granite aggregate was used in the investigation so that comparisons could be made with the crushed clay brick aggregate. The collected recycled brick are then crushed down to the 20 mm and 10mm aggregate manually. The RBA is then coated with cement slurry (1:4 ratio) to reduce its water absorption before using them in concrete. The GA and RBA are used in SSD condition. Fig.1 shows the different aggregates used in mix design. Fig. 1: Aggregate used in concrete Representative samples from each type of aggregates were collected and tested for its specific gravity and grading in accordance with the IS 2386 (Part 3) [9]. The sieve analysis was carried out on both recycled brick and granite aggregates in accordance with IS 2386 (Part 1) :1963 [8]. The results of these properties were given in Table 1.

3 Table 1 Comparison of the Properties of Natural and Recycled Brick Aggregate Property Specific Gravity Fineness Modulus Granite Aggregate Recycled Brick Aggregate IV. EXPERIMENTAL INVESTIGATION An attempt is made in this paper to study the affect of high temperatures on a standard type of concrete suitable for low-level civil engineering applications. Concrete mix was designed for the granite aggregate in accordance with IS [11]. In order to investigate the effects of high temperatures on the compressive strength of RBA concrete, it was first necessary to produce a standard concrete mix with a characteristic mean strength of f ck =20 N/mm 2 and a target mean strength f ck =26.6 N/mm 2 for granite aggregate, to act as a control (GA concrete) mix (Table 2). The next stage of the testing program was to produce an RBA concrete mix with 25% of crushed brick aggregate as a coarse aggregate. The concrete mixes were made in accordance with IS [12]. Each batch was of sufficient volume to produce six 150 mm cubes for crushing at 28 days. The only difference in the mixing process of GA concrete and RBA concrete was that prior to mixing, the brick aggregates are coated with cement slurry (1:4 ratio). The compressive strength of concrete made with both the aggregates were determined in accordance with IS [12]. A set of six cubes for each mix were crushed at room temperature to act as a control mix. Grade of Mix Type of Aggregate Charcteristic Mean Strength (N/mm 2 ) Table 2 Mix Proportions of fresh Concrete M20 Granite Recycled Brick Target Mean Strength (N/mm 2 ) 26.6 Cement/Sand/ Aggregate Ratio 1: 1.25: 2.80 Water / Cement Ratio V. HIGH TEMPERATURE TEST A total of sixty 150mm size cubes for each type of concrete were casted to expose them to temperatures ranging from C to 1000 o C. Six cubes for each temperature were considered from each concrete to test for compressive strength. The test cubes were placed in a Bogie Hearth Furnace (Fig.2) and subjected to designate temperatures of 100 o C to 1000 o C at an interval of 100 o C for a period of 3h. The cubes were tested hot within 15 min after removal from the furnace for compressive strength in unstressed condition. Fig. 2 : Bogie Hearth Furnace for controlling Temperatures VI. RESULTS AND DISCUSSIONS The average strength results are presented in Table 3 for both the GA and RBA concrete mixes. The recycled brick aggregate produced standard concrete of an acceptable values, with the average strengths of GA concrete (25.25 N/mm 2 ) and RBA concrete (25.15 N/mm 2 ) more or less same. A. Visual Observations of Specimens After Exposure to High Temperatures Both the RBA and GA concrete cube specimens were heated in accordance with the curve of ISO: [13]. After the temperature reached the designated temperature, the furnace was kept constant for 3 h (soaking period). The specimens were then taken out from the furnace for visual observation of colour change and surface cracks. 161

4 a) Colour Change Different colours were observed on both RBA and GA concretes after exposure to high temperatures. It was found that both the concretes exhibited the same colour variation when the specimens were heated to different temperatures. The variation in colour change is shown in Fig.3. The variation in colour with a rising temperature can be classified in to three main ranges: normal grey when the temperature was below 400 C, pale grey when the temperature ranged between 400 and 700 C, and buff colour when the temperature exceeded 700 C. 400 o C 800 o C From Fig.4, it is observed that the micro cracks in RBA concrete are much less in comparision with GA concrete after heating to 800 o C, which explains the higher strength of RBA concrete than GA concrete C C GAC RBAC GAC RBAC C GAC RBAC GAC RBAC 1000 o C GAC RBAC Fig 3: Colour Change of GA Concrete and RBA Concrete at Different Temperatures b) Crack Pattern Figure 4 shows the surface cracks on the concrete samples of RBA and GA concretes after heating. At 600 C both the concretes demonstrated visible surface cracks on both the concretes. However, there were no cracks in both the concretes with a temperature < 600 C. At 800 C, the number of surface cracks increased in GA concrete compared to RBA concrete. These cracks crossed the crosssection of the specimen of GA concrete when the temperature is raised beyond 800 o C affecting the structure of the concrete to a considerable extent. Whereas at this temperature the RBA concrete did not exhibit pronounced damage of the structure of the concrete. GAC RBAC Fig.4. Creack Pattren of both GA and RBA concrete at high temperatures B. Loss of Weight The weights of the specimens before and after exposure to fire were measured to assess the percentage loss of weight in both the concretes. Figure 5 shows the variation of percentage loss of weight with temperature in both the concretes. The percentage loss of weight increased with the increase in the temperature in both concretes. The loss of weight in GA concrete might be attributed to the release of water leading to the formation of air voids. The structural integrity of the specimens deteriorated at very high temperatures as confirmed by the increase in weight reduction and cracks with increased temperature. Though the RBA concrete exhibited the increased loss of weight with temperature as that of GA concrete, the percentage loss of weight in RBA concrete is pronounced compared to GA concrete. This higher weight loss in RBA concrete may be attributed to the combination of loss of organic matter in clay and bound water. 162

5 Fig 5: Variation of Percentage loss of weight of GA and RBA concretes C. Compressive Strength Fig. 6 shows the percentage residual compressive strengths of both GA and RBA concretes at elevated temperatures. At 100 and 200 o C both GA and RBA concretes exhibited higher percentage residual strengths than that at room temperature. Table 3 Compressive Strengths Of NAC And RBAC At Different Temperatures The increase in compressive strengths of both the concretes can be due to the effect of accelerated hydration. From Fig. 6, a sudden fall in compressive strength was observed between 200 and 300 o C in both the concretes. At 300 o C both the concretes retained only a 60% of its room temperature strength. Beyond 300 o C a gradual decrease in residual compressive strength was recorded. However the RBA concrete was still able to retain more residual compressive strength than GA concrete. At 400 o C the residual strength of RBA and GA concretes are 55% and 46% respectively showing a strength gain of 10% in RBA concrete. Upto 600 o C RBA concrete exhibited more or less similar gain in the strength where as at 700 o C RBA concrete exhibited a strength gain of 14% than GA concrete. Between 700 o C to 1000 o C the gain in strength of RBA concrete ranges between 14 to 17% compared to GA concrete. At 1000 o C, both the concretes showed severe deterioration due to the decomposition of CSH gel. RBA concrete lost 70% of its strength after exposure to 1000 o C in contrast to 86% after exposure to 1000 o C temperature for GA concrete. Sl.No 27 o C 100 o C 200 o C 300 o C 400 o C 500 o C 600 o C 700 o C 800 o C 900 o C 1000 o C NAC RBAC The higher residual strength of RBA concrete at these temperatures can be attributed to the.stiffness of the recycled clay brick and additional hydration of un-hydrated cement paste D. Bond After crushing, the concrete cubes made with GA and RBA aggregate were examined closely before and after heating. On inspection from Fig. 7 it was found that many of the RBA particles were broken right through while still being adhering to the cement paste even after fire. This proves that a good bond exists between RBA particles and the cement paste even though some of the particles have a smooth surface. The reason for the good bond of RBA may be due to the larger surface area of RBA than GA. Fig 6: Variation of Residual Compressive Strength of GA and RBA concretes 163

6 Fig. 7 : Bond between RBA and GA after exposure to 1000 o C VII. CONCLUSIONS Standard concrete can be produced using crushed clay brick upto 25% replacement as the coarse aggregate. It is important that the brick aggregate is in a SSD moisture condition and should be coated with cement slurry before use in concrete. The results showed that RBA concrete tested can be used in producing PCC for low-level civil engineering applications. From the view point of surface colour, different colours were observed on the heated RBA concrete after high temperatures. However, it was found that at high temperatures no colour variation was observed in RBA concrete when compared to GA concrete. The variation in colour with a rising temperature can be classified in to three main ranges: normal grey when the temperature was below 400 C, pale grey when the temperature ranged between 400 and 700 C, and buff colour when the temperature exceeded 700 C. The recycled brick aggregate concrete demonstrated more loss of weight than granite aggregate concrete due to the loss of organic matter in clay at high temperatures. It was visually observed that fine cracks appeared on the surfaces of the specimens of both RBA and GA concretes at a temperature of 600 C. However, there were no cracks in both the concretes at a temperature less than 600 C. More number of cracks were formed in the surface of GA concrete up to a temperature of 800 o C where as only a few cracks were observed on the surface of RBA concrete at this temperature. These cracks crossed the cross-section of the specimen of GA concrete when the temperature is raised beyond 800 o C. However these cracks have not penetrated in to the cross section in case of RBA concrete. A good bond exists between RBA particles and the cement paste even after exposure to high temperatures as that of GA particles. The residual compressive strength of both the RBA and GA concretes have exhibited rising to falling trend when the temperature is about 200 C. Beyond 200 C both the concretes exhibited falling trend. The RBA concrete show better compressive strength than GA concrete at all temperatures due to the nature of the clay brick making the RBA concrete stronger and more stiff. The RBA concrete could able to retain about to percent of its original compressive strength where as the retention in GA concrete is only about to percent between 400 o C to 1000 o C. The fire resistance of recycled brick aggregate concrete is better than the fire resistance of granite aggregate concrete. REFERENCES [1] Fouad M. Khalaf and Alan S. DeVenny, Performance of Brick Aggregate Concrete at High Temperatures, Journal of Materials in Civil engineering, Vol. 16, No. 6, December 2004, pp [2] Zhao Xiao, Tung-Chai Ling, Chi-Sun Poon, Shi-Cong Kou, Qingyuan Wang, Runqiu Huang, Properties of partition wall blocks prepared with high percentages of recycled clay brick after exposure to elevated temperatures, Construction and Building Materials Vol 49, 2013, pp [3] Yang, J, Qiang, D and Bao, Y, Concrete with recycled concrete aggregate and crushed clay brick, Construction and building material(2011), Issue 25, pp [4] Arundeb Gupta Somnath Ghosh & Saroj Mandal, Coated Recycled Aggregate Concrete Exposed To Elevated Temperature, Global Journal of Researches in Engineering Civil And Structural Engineering, Volume 12, Issue 3, 2012, pp [5] Jianzhuang Xiao, Yuhui Fan, M.M Tawana, Residual compressive and flexural strength of a recycled aggregate concrete following elevated temperatures, Technical report Structural Concrete 14 (2013), No. 2, pp [6] Sedat Karaman et al Firing temperature and firing time influence on mechanical and physical properties of clay bricks Journal of Scientific & Industrial Research Vol. 65, February 2006, pp [7] IS , Specification for Portland Pozzolona Cement Parts 1 & 2, Bureau of Indian Standards, New Delhi. [8] IS 2386 (Part 1)-1963, Specifications for Methods of Test for Aggregates For Concrete, Bureau of Indian Standards, New Delhi. [9] IS 2386 (Part 3)-1963, Specifications for Methods of Test for Aggregates For Concrete, Bureau of Indian Standards, New Delhi. [10] IS: , Recommend Guidelines for Concrete Mix Design, Bureau of Indian Standards, New Delhi. [11] IS , Method of Test for Strength of Concrete, Bureau of Indian Standards, New Delhi. [12] ISO: , Fire Resistant Tests - Elements of Building, International Standards Organization, Geneva, Switzerland. 164