HIGH QUALITY RECYCLED AGGREGATE CONCRETE (HIRAC) PROCESSED BY DECOMPRESSION AND RAPID RELEASE

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1 HIGH QUALITY RECYCLED AGGREGATE CONCRETE (HIRAC) PROCESSED BY DECOMPRESSION AND RAPID RELEASE Yoshimoto Kimura 1), Keiichi Imamoto 2), Masaru Nagayama 1) and Hiroshi Tamura 1) 1) General Building Research Corporation of Japan 2) Ashikaga Institute of Technology, Japan Abstract Quality of concrete with recycled is generally lower than that of virgin. The main reason is that recycled with its higher water absorption property has porous mortar matrix around the virgin and hence develops an inferior bond. In order to improve the quality of recycled concrete, an innovative method is proposed in this paper. High quality recycled concrete (HiRAC) can be obtained through a decompression and rapid release (DC-RR) procedure applied after normal mixing of concrete with recycled. Through this procedure, the quality of transition zone between and cement matrix can be dramatically improved. This paper describes an experimental study on the effectiveness of DC-RR procedure on some of the mechanical and physical properties of recycled coarse concrete. 1. Introduction One of the problems involved in applying recycled to concrete is that its water absorption is higher than that of natural, which results in inferior quality concrete. Previous methods of enhancing the quality of recycled concrete are either improvement of the quality of the recycled itself (1) or use of high range water reducing agent and other additives to improve the performance of the cement paste matrix (2). However, the former method needs expensive equipment and the latter might result in high cost of concrete. This study proposes a decompression and rapid release (DC-RR) procedure applied after normal mixing of concrete which aims to obtain high quality recycled concrete (HiRAC). The DC-RR procedure is very simple and does not need any special equipment. This paper reports on experimental studies undertaken to verify its effectiveness. 2.Decompression and rapid release (DC-RR) procedure Murata et al. reported on the production of concrete made with blast furnace slag in a low-pressure mixer (3). In this process, the pressure in the mixer is reduced during mixing

2 76mmHg 13mmHg mixing mixing stopped decompression completing of DC-RR rapid release interface repressurization force induced by rapid release Fig.1: Diagrammatic representation of the changing pressure exerted on the concrete and schematic drawing of changes at the paste- interface (decompression), then, when the pressure reaches a certain level, it is returned rapidly to atmospheric level (rapid release) in order to bond the and the paste. The DC-RR procedure used in this study is similar to the method developed by Murata et al but differs in that mixing is not performed during the decompression and rapid release processes. The omission of mixing in the DC-RR procedure makes it possible to use air tight tank such as concrete hopper. Fig.1 shows a diagrammatic representation of the changing pressure exerted on the concrete and a schematic drawing of changes at the paste- interface. The decompression level is 13mmHg for 76mmHg of atmospheric pressure (i.e. the pressure is reduced by 63mmHg). Generally, it takes about 2 minutes to complete a DC-RR procedure. 3. Change of transition zone between and paste matrix by DC-RR procedure In order to investigate the effects of the DC-RR procedure on the transition zone between and paste matrix, two specimens were prepared in which a single natural coarse is placed in each of the two vessels, as shown in the upper part of Fig.2. After pouring cement paste of.5 W/C into each of the vessel, one was processed by the DC-RR procedure and the other was not. After 14 days, the samples were cut open vertically, and the exposed surfaces were polished and observed with an electron microscope as shown in the lower part of Fig.2. The photographs show partial images of the transition zone between and paste matrix observed across the entire. Whereas the image of the non DC-RR sample shows a separation of about 1µm, that of the DC-RR processed sample shows an almost complete contact. The reason for the separation in the former case might be shrinkage of the paste and external force during the cutting and polishing. It can be seen that the weakness of transition zone is remedied by the DC-RR procedure. However, if the composition of the samples differs from that of normal manufactured concrete, further investigation of the transition zone might be needed.

3 Non decompression 76mmHg cement paste 76mmHg 13mmHg paste paste Ca(OH) 2 Fig.2: Images of background reflection electron microscope of polished surface specimens 4.Applicability of DC-RR procedure with recycled coarse concrete 4.1 Outline of experiment (1) Materials The source of recycled used in this study was a plain concrete block with a W/C ratio of.6, and compressive strength of 32MPa at 28 days in air-dried curing. Properties of materials and mixture proportions are tabulated in Tables 1 and 2, respectively. In this study, hard sandstone was used as natural coarse. Table 1: Materials Material Properties Cement Normal Portland cement, Density 3.16g/cm 3 Natural coarse Hard sandstone, Density 2.64g/cm 3, Absorption.64% Recycled coarse Density 2.43g/cm 3, Absorption 6.42% Fine River sand, Density 2.6g/cm 3, Absorption 1.98% Water Tap water Water reducing agent Lignin sulfonate acid polyol compound Table 2: Mix proportion Aggregate type Natural coarse Recycled coarse Slump (cm) Air content (%) W/C (%) Water content (kg/m 3 ) s/a (%)

4 Mix for 3sec. 6sec 3sec 2 min. Mixing stopped Cement Fine Water Coarse agg. DC-RR Normal mixing DC-RR procedure Item Test method Slump, air JIS A 111,1128 Compressive strength JIS A 118 Specimen size:φ1 2cm shrinkage days at drying:7days measurement:embedded gauge Specimen size:φ1 2cm Creep days at loading:28days measurement:embedded gauge Freezing and thawing JIS A 624 Carbonation Draft of JIS, Specimen size:φ1 2cm Fig.3: Concrete manufacturing procedure and test items and its methods (2) Concrete manufactures and test items A concrete manufacturing procedure, test items and test methods are shown in Fig. 3. Mixing of the concrete was performed using a mixer of.1m 3 in capacity and each mixing batch was.6m 3. Then the DC-RR procedure was performed by using another mixer tank. 4.2 Test results and discussion (1) Slump and air content of fresh concrete Table3 shows the test results for fresh concrete. As apparent from the table, both concretes made with natural coarse and with recycled coarse have decreased by 2.cm in slump, but 2.% and 1.% in air content, respectively. The decrease in slump might be due to air release effect in concrete by decompression. (2) Compressive strength of concrete The developments of compressive strength of concrete are shown in Fig.4. As shown in the figure, with regard to the normal mixing method, the compressive strength of concrete with recycled coarse was about 1% lower than that with natural coarse at 28 days under normal curing. In contrast, the DC-RR procedure has helped to improve the strength of concrete with recycled coarse by about 2%, which is of almost equal strength to that of natural coarse. In addition, it also improves the compressive strength of natural coarse concrete. In this respect, the DC-RR procedure seems to be effective in improving the mechanical performance of recycled coarse concrete. Table 3: Test results for fresh concrete Aggregate type Manufacturing Slump Air content Concrete temp. method (cm) (%) ( ) Natural coarse Normal mixing DC-RR Recycled coarse Normal mixing DC-RR

5 Compressive strength (MPa) Natural agg. Nomal mixing Natural agg. DC-RR Recycled agg. Nomal mixing Recycled agg. DC-RR Age (days) Fig.4: Developments of compressive strength (3) Shrinkage and creep (3)-1 Shrinkage Fig.5 shows changes in concrete shrinkage strains. The test specimens were cured for 28 days in 2 C water and then left in drying condition of 2 C and 6% R.H. The top and bottom end surfaces of the specimens were covered with aluminum foil in order to prevent moisture evaporation. Shrinkage strain was measured at the center of the specimen with embedded gauge. The shrincage strain value adopted in this study was the mean value of two specimens. As shown in the figure, the shrinkage strain of concrete with recycled coarse was about 2% greater than that with natural coarse after 1-day drying. It seems that the DC-RR procedure was not effective in reducing the shrinkage of concrete with recycled coarse. (3)-2 Creep Fig.6 shows changes in concrete creep strains. The test specimens were cured on the same condition as the specimens for shrinkage measurement, and after 28 days of curing, loaded in the drying condition of 2 C and 6% R.H.. The loading stress was 3% of compressive strength of concrete after 28 days. Total strain (creep plus shrinkage strain) was measured at the center of the specimen with embedded gauge. The creep strain was obtained by subtracting shrinkage strain from the total strain. The creep strain per unit stress adopted in this study was the mean value of two specimens. Shrinkage strain (µ strain) Natural agg. Nomal mixing Natural agg. DC-RR Recycled agg. Nomal mixing Recycled agg. DC-RR Elapsed time (days) Creep strain (µ strain/mpa) Natural agg. Nomal mixing Natural agg. DC-RR Recycled agg. Nomal mixing Recycled agg. DC-RR Elapsed time (days) Fig.5: Changes in shrinkage strain Fig.6: Changes in creep strain

6 As shown in the figure, with regard to normal mixing method, the creep strain per unit stress of concrete with recycled coarse was about 2% greater than that with natural coarse after 1-day loading. It can be seen that the DC-RR procedure reduces the creep strain of concrete with recycled coarse by about 2%, and that the procedure causes a similar creep deformation as that of natural coarse. Furthermore, the creep strain of natural coarse concrete is also reduced by about 25% with the DC-RR procedure. The DC-RR procedure seems to be effective in reducing the creep strains of recycled coarse concrete. (4) Freezing-thawing resistance Fig.7 shows the changes of durability factors of concretes. The durability factor was obtained by calculating the ratio of dynamic elastic modulus at each cycle to the primary modulus. Test specimens were cured for 14 days in 2 C water and subjected to freezing-thawing cycle. The value of durability factor adopted in this study was the mean value of two specimens. As shown in the figure, the changes of durability factors in concretes with recycled coarse were similar to those with natural. As described previously, the DC-RR procedure reduces the air content of concrete. Hence, the durability factors of concretes tend to be slightly decreased. However, this decrease might not be harmful to concrete. It seems that the DC-RR procedure slightly decreases the durability factors of concrete with recycled coarse, but this decrease is not harmful. (5) Carbonation Cylindrical specimens of 1 cm in diameter and 2 cm in height were used in this test. Test specimens were cured for 56 days; 28 days in 2 C are and 28 days in 2 C and 6% R.H. air. After the curing, the specimens were left in 5% CO 2, 2 C and 6% R.H. condition. Table 4 shows the measurement of carbonation depth of concrete after 13 weeks. The specimens were split lengthwise and the average of carbonation depths measured at 22 points represented with is shown in Fig.8. As shown in the figure, with regard to the normal mixing method, the carbonation depth of concrete with recycled coarse was about 1% greater than that with natural coarse. The DC-RR procedure reduces the carbonation depth of concrete with recycled coarse by about 3%. The carbonation resistance of the recycled coarse concrete in this procedure is greater than that in the case of natural coarse concrete produced by the normal mixing method. Furthermore, the carbonation depth of natural coarse concrete is also reduced by about 3% by the DC-RR procedure Durability factor Natural agg. Nomal mixing Natural agg. DC-RR Recycled agg. Nomal mixing Recycled agg. DC-RR Cycles Fig.7: Changes in durability factor

7 5cm Table 4 : Measurement of carbonation depth Aggregate type Producing Carbonation depth method at 13 weeks Natural coarse Normal mixing 13.1 mm (1.9 mm) DC-RR 1. mm (1.3 mm) Recycled coarse Normal mixing 14.3 mm (2.3 mm) DC-RR 9.4 mm (1.9 mm) Brackets in the table indicate standard deviation of measured values Fig.8: Measurement point The DC-RR procedure seems to be very effective in reducing the carbonation depth of recycled coarse concrete. 5. Examination of the application to various recycled s The application of the DC-RR procedure was examined on various recycled s whose water absorptions were different. Fig.9 shows the relation between water absorption of the recycled coarse and the rate of increase in the compressive strength. Here, the rate means the proportion of the increase in the strength of concrete in the DC-RR procedure compared with that of natural mixing concrete. However, the compressive strength changes according to the amount of air. Thus, it is assumed that when the amount of air increases by 1%, the strength will decrease by 5%. The Figure shows that the rate of the increase in the compressive strength becomes higher as the water absorption of the coarse increase. In other words, DC-RR procedure is effective in the case of concrete with low-quality recycled coarse. However, it should be noted that the effect of the DC-RR procedure is hardly seen when high-quality recycled coarse is mixed with low-quality one (See the triangular shape in fig.9). It seems that further studies are necessary concerning the results that the strength in this experiment was not so much improved as that in the experiment shown in fig.4, and that no effect was seen with a natural. Rate of the increase in the compressive strength (%) Natural Mixed Natural with recycled Water absorption of coarse (%) Fig.9: Relation of the water absorption of the coarse and rate of the increase in the compressive strength

8 6. Conclusion and overview of the future Test results in this study are summarized in the following Table 5. The DC-RR procedure proposed in this study has the possibility to improve the quality of concrete with recycled coarse and expected to be one of the effective methods for actualizing high quality recycled concrete (HiRAC). The DC-RR procedure is still developing, and further investigation might be needed. In Japan, a research group for HiRAC was organized and various research and investigations have been performed. Hopefully, we can carry on further research. Table 5: Effects of DC-RR procedure on the characteristics of concrete with recycled coarse Item Effect Compressive strength About 2% increase Shrinkage No marked change Creep About 2% decrease Freezing and thawing resistance No marked change Carbonation About 3% decrease References [1] Ishikura T, et. al.: Development related to high-quality recycled manufacturing technology [II] (1~5), Architectural Institute of Japan, Material and Construction, pp. 697~76, (in Japanese) [2] Namba T, Abe M, et al.: Research into quality improvement of recycled concrete. Concrete Engineering, No.2.pp (in Japanese) [3] Murata J, et al.: Research into the properties of concrete made with blast furnace slag in a low-pressure mixer. Concrete Engineering Scientific Paper Collection, vol. 2, no. 1, pp. 45~55, 1991 (in Japanese)