STUDY ON THE MECHANICAL PROPERTIES OF RECYCLED CONCRETE HOLLOW BLOCKS BASED ON ORTHOGONAL EXPERIMENT

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1 STUDY ON THE MECHANICAL PROPERTIES OF RECYCLED CONCRETE HOLLOW BLOCKS BASED ON ORTHOGONAL EXPERIMENT Guoliang BAI (1), Fengjian ZHANG (1) (3), Yuyi AN (1), Hui XIAO (2), and Zonggang QUAN (2) (1) Xi an University of Architecture and Technology,Xi an , China; (2) Xi'an Research and Design Institute of wall & Roof Materials, Xi an , China; (3) Henan University of Urban Construction, Pingdingshan , China Abstract The mixture ratio is very important to the basic mechanical properties of recycled concrete block, the basic mechanical properties include compressive strength and bending strength, which is affected by the four main factors: water usage per unit volume, water-cement ratio, recycled coarse, and recycled fine. In this paper, based on the orthogonal test, the influence rules of the four factors to the mechanical properties has been found, and at the same time the effect of sand percentage to the mechanical properties has also been studied. The analysis of the test results shows that the recycled fine is the main influence factor, the recycled coarse and water-cement ratio are the important factors, and water usage per unit volume is the minor factor. According to the study, if the mixture ratio is reasonably selected, the recycled concrete block would have better mechanical properties and can be used for load-bearing wall materials. Keywords:Orthogonal test; Recycled ; Recycled concrete block 1. INTRODUCTION In recent years, with the rapid development of Chinese urban and rural construction, and with the implementation of new planning system in many cities, large amounts of demolished buildings have produced a lot of construction waste. At the same time, natural disasters, particularly earthquakes also produce large amounts of building waste. If the construction waste can t be disposed effectively, it not only takes up a lot of landfill space, but also pollutes the environment [1, 2]. It s an effective treatment to produce recycled concrete blocks using recycled coarse and fine from crushed construction waste. Because the density of recycled is smaller than the natural, so recycled concrete blocks have the characteristics of lightweight, thermal insulation, and satisfying 645

2 energy-saving requirements, and the technique is worthy of application and popularization. Compared with natural, the recycled are very different in many performances, and it is the key to this application adopting a reasonable proportion to produce recycled concrete block with the better mechanical properties. In this paper, through orthogonal experiment, the mechanical properties of recycled concrete block, i.e. compressive and flexural properties, are studied, and the rules of four main factors affecting the recycled blocks have been obtained (the four main factors : replacement ratio of fine, replacement ratio of coarse, the water-cement ratio, water usage per unit volume). And on this basis, the sand percentage affecting on the mechanical properties of recycled concrete block has also been studied. At the last, several proposals of producing recycled concrete block have been suggested. 2. TEST MATERIALS Water used for the test experiments is from the Xi'an city running tap water, cement is PC32.5 Portland cement, natural fine are from the Weihe river sand, and natural coarse are from the Weihe River pebbles. Recycled fine and coarse are from construction waste, it is from demolished components of a factory in Xi'an. Waste concrete had been screened after crushed by jaw crusher, and three size ranges of recycled can be obtained: greater than 10mm, between 5mm and 10mm, less than 5mm. In this experiment recycled fine of less than 5mm and recycled coarse of 5~10mm are adopted. The properties of are shown in Table 1. Compared to natural, the performances of recycled are very different, as can be seen from Table 1. The water absorption of recycled are much larger than natural, and the apparent density and bulk density is smaller than the natural, which is because there is a lot of mortar blocks in recycled, and there are a large number of cracks in the recycled [3,4]. Table 1: The properties of Type of Natural fine Natural coarse Recycled fine Recycled fine 3. TEST PLAN Particle size (mm) Apparent Density (kg/m 3 ) Bulk density (kg/m 3 ) Water Absorption (%) < ~ < ~ Orthogonal experiment design of mix ratio and production of specimens In this test, recycled concrete blocks with a single row holes were used, it s specification is 390mm 190mm 190mm, and thickness of rib and outside wall are 30mm, as shown in Figure 1. In the orthogonal test of mixture ratio, the four influential factors in mechanical 646

3 properties of recycled concrete block are selected. They are namely the replacement ratio of recycled fine, replacement ratio of recycled coarse, water-cement ratio, water usage per unit volume. For each factor, three levels are selected and analyzed in the test (see table 2). Orthogonal test adopted orthogonal design scheme with L9(34) (table 3) [5]. In orthogonal test the quantity of water is the net usage of water, additional water would be added by calculating when mixing concrete [6]. The sand percentage of each mixture ratio is 40%. Table 2: The levels of orthogonal test Levels Water usage (kg/m 3 ) Water-cement ratio Replacement of recycled coarse (%) Replacement of recycled fine (%) Recycled concrete blocks were produced in Xi'an blue-green-clear Science and Technology Development Co., and concrete block-making machine was made in Xi'an Orient Group, the number of specimens for each group was 5, the total number of specimens was 45. Because the amount of concrete for each group was not much, the artificial mixing was adopted. First, specified amount of cement, fine and coarse were mixed evenly, then water were added, and the mixture was stirred until the concrete was entirely homogeneous. Concrete mix was plunged into the hopper of block-making machine, and then shocked, extruded and demoulded. Recycled concrete block produced were cured through the way of natural conservation, with the sealed plastic film to prevent water evaporation. The compression test has been carried on after blocks have been cured for 28 days. 3.2 Mechanical Properties Compressive property test was carried on RFP-03 intelligent dynamometer machine of Wuxi Xinluda Equipment Co. (fig.1). The compressive strength of each mix ratio is denoted by the average compressive strength of the 5 specimens. Test results are shown in table 3. Table 3: The orthogonal design scheme and results of orthogonal test Groups A Compressive Bending (kg/m 3 B C(%) D(%) ) strength (MPa) Strength(MPa) (A water usage per unit volume; B water-cement ratio; C replacement ratio of recycled coarse ; D replacement ratio of recycled fine ) 647

4 When the specimen was loaded firstly, there were no significant changes in recycled concrete block, with the load increasing, individual minor cracks appeared on the surface of block. With continuously loading, cracks developed further, some of which connected with each other and sounds of crackle could be heard clearly. As the load was approaching the ultimate value, the block was cracked and divided into two inverted cone, then the deformation increased dramatically, while the pressure increased slowly. Eventually the block was completely destroyed. In the test, on the fracture surface of recycled concrete block, the recycled coarse are most cut directly, which shows the extremely weak resistance of recycled. The fracture surfaces of natural coarse are similar to ordinary concrete, on some sections natural are broken, and on other sections bond surface between the coarse aggregate and mortar are disconnected. Bending test was carried on the same dynamometer instruments as compression test (fig. 2), Result is shown in Table 3. Fig. 1: Compression test Fig. 2: Bending test When the specimens were loaded firstly, the data of the machine was not very stable. With the load increasing, the data increased evenly, and the surface of the specimen hadn t showed any change. Then the data began to decrease, and the minute cracks on the surface emerged. When the load continued to increase, specimen suddenly fractured in two parts, which made a large sound, at the same time data decreased rapidly. This shows flexural damage of recycled concrete block is brittle failure without sign of damage before failure. Compared with the compressive strength test, failure cross-section of recycled concrete block in flexural test is near the middle of the specimen, and the fracture is flush. There is no broken concrete fragment in the damage process. Most recycled and some of natural are directly cut off, and some of natural break from the joint surface between cement mortar and. 4. RESULT ANALYSIS OF ORTHOGONAL TEST AND OPTIMIZATION TEST 4.1 Range analysis of orthogonal test The range analysis of orthogonal was shown in table

5 Table 4: The range analysis of orthogonal test Analysis of Compressive strength Analysis of Bending strength A B C D K K K k k k range K K K k k k range Through the range analysis of results on compressive strength and bending strength, it can be seen that for the four factors, range of D is the biggest, and it s the main factor influencing the mechanical properties. The second is the range of C and B, C and B are the important factors. The range of A is the minimum, and it is the secondary factor. Therefore, according to range of each factor, the order should be D, C, B, A. A3B1C1D1 is the optimization combination in the test of mechanical properties. 4.2 Optimization of different sand ratio test According to the analysis, in this orthogonal experiment, A3B1C1D1 combination is the optimal combination of recycled concrete blocks. The combination hasn t appeared in the experiment. In the 9 groups of orthogonal experiment, percentage of sand are all selected as 40%. In order to examine how the optimal combination and different percentage of sand influence on recycled mechanical properties of concrete block, based on the orthogonal experiment, 4 Groups of A3B1C1D1 optimal combination test with different percentage of sand was designed. The concrete mix-ratio and the test results are shown in Table 5. According to Table 5, the curve of sand percentage affecting on the compressive and flexural strength of recycled concrete blocks is drawn, which is shown in Figure 3. Table 5: the optimal test and the result of test group sand percentage (%) Unit water per volume (kg/m 3 ) Mixture ratio Compressive strength(mpa) Bending strength(mpa) A3,B1, C1,D combination

6 6.0 Stress(MPa) Compressive strength Bending strength % 40% 50% 60% 70% Sand percentage 80% Fig. 3: The curve of sand percentage and the compressive and flexural strength As can be seen from the fig. 3, with the rate of sand increasing, compressive strength and flexural strength of recycled concrete block first increased and then decreased. When the rate of sand is between 30% and 50%, the compressive and bending strength of recycled concrete block increase linearly; when the rate of sand is between 50% and 60%, they decrease linearly; and when sand percentage is 50%, compressive and bending strength all attach the maximum. According to the analysis on optimization test results under different percentage of sand, the optimal percentage of sand is selected as 50%, and the highest compressive strength of recycled concrete block can catch up to 6.2MPa by testing. So they can be used as load-bearing blocks. 4.3 Analysis on factors affecting compressive strength of recycled concrete block (1) Recycled fine are most influence on the mechanical properties of recycled concrete blocks. In the process of crushing construction waste into recycled, will generate a lot of powder, and in the production process there are some mud mixed in. During being screened, these fine particles are all contained in the fine. If these particles exist in large quantities, the bond of cement paste and sand would be weakened, and the strength of recycled concrete block would decline. (2) Recycled coarse are an important factor to the mechanical properties of recycled concrete block. Recycled coarse contain some of the mortar blocks during the production process, and some of mortar will bond on the surface of recycled. During the production process, number of micro-cracks will form in recycled under the action of mechanical forces; these would make the performance of recycled coarse lower than natural. With the replacements of recycled coarse increasing, the mechanical properties of recycled concrete block decreases. (3) Water-cement ratio and water usage also affect the mechanical properties of recycled concrete block. Because recycled concrete block used is dry-hard concrete, and water absorption of recycled aggregate is large. These two factors particularly affect on the consistency of concrete. The consistency directly affect the forming of block, if it s too large, block would easily deform after demoulding, and if it s too small, forming of block would be difficulty. While block being demoulded, the continue to absorb the moisture of block, resulting in the decline of hydration water for cement. If curing of the block is not kept well, hydration of cement would be insufficient, leading to the lower strength of the blocks. (4) Percentage of sand also affect the mechanical properties of recycled concrete block as a factor, and changes in the percentage of sand directly affect gradation of aggregate, reasonable 650

7 gradation access to higher compressive strength. When the mix ratio of recycled concrete with percentage of sand between 30% and 50% become more and more reasonable, the strength of recycled concrete blocks gradually increases. 50% should be the best percentage of sand. 5. CONCLUSIONS (1) The orthogonal test results show that: the content of recycled fine is the main factor affecting the compressive strength of concrete; content of recycled coarse aggregate and water-cement ratio are important factors; and water consumption is a secondary factor. (2) Through orthogonal test and the optimal combination test with different percentage of sand, it s determined that water is 150kg/m 3 ; water-cement ratio is chosen as 0.4; replacement ratio of coarse is 40%; replacement ratio of fine aggregate is 40%. In the optimal combination of recycled concrete blocks, percentage of sand is 50%. (3)Under a certain mixture ratio, the strength of recycled is less than the strength of cement paste, and the destruction section of recycled concrete block fracture mostly through the. (4) In the test, the highest compressive strength of recycled concrete block is up to 6.2MPa, and it has reached the required strength of MU5.0, which shows that if the appropriate mix ratio is selected, recycled concrete block with single-row holes can be used for load-bearing wall materials. ACKNOWLEDGEMENTS This work was supported by the National Technology Support Foundation of China, (No. 2008BAK48B05), The Key Project of Natural Science Basic Research Plan in Shanxi Province (2009JZ012) and The Technical Project of Industrial Application in Xi an (CXY09025(5)). The National Key Laboratory of Structural Engineering at Xi an Univ. of Arch. & Tech. (Xi an, P.R. China) is thanked for supporting Technology and Equipment. REFERENCES [1] Ma Rong, The Sense of Reproduce Concrete in the Ecological Construction, Concrete, Vol 10, pp.21-23, [2] Wang Zhiwei, Industrialization chain and economy analysis of recycling waste concrete, Vol 4, pp , [3] Liu Shuhua, The study main properties of recycled, Advanced material industry, Vol 8, pp.63-67, [4] Guo Yuanchen, Basic Properties of Recycled Aggregate and Engineering Verification of Recycled Road Materials, Journal of Qingdao Technological University, Vol 30,pp.60-64, [5] Chen Wei, Experimental Design and Analysis, Tsinghua University Press, ISBN: /o.461, June [6] Xiao Jianzhuang, Recycled concrete, China Building Industry Press, ISBN: , June