QUALITIY OF RECYCLED CONCRETE MADE BY A TRUCK MIXER

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Jared Herbert Short
6 years ago
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1 QUALITIY OF RECYCLED CONCRETE MADE BY A TRUCK MIXER Toyoharu Nawa 1), Kenta Tsuruya 1), Hiroshi Hashida 2) 1) Hokkaido University, Graduate School of Engineering, Japan 2) Shimizu Corporation, Institute of Technology, Japan Abstract In this study, a closed recycling system for demolished concrete in urban area is proposed as one of advanced recycling system of concrete resources. The proposed system consists of heat and rubbing plant to produce recycled aggregates and truck-mixing concrete plant for manufacturing recycled concrete. As a site occupied by a truck mixing plant is smaller than that of a stationary mixing plant, it seems that the truck mixing plant is more suitable to a closed recycling system for concrete in urban areas. In order to verify feasibility of the proposed new closed recycling system including the truck-mixing plant, we examined the properties of the concrete made by a truck mixer and compared them with the properties of recycled concrete made by a stationary mixer. High quality recycled aggregates, which were produced by heating and rubbing plant, were used as fine and coarse aggregates. The results of this study show that the recycled concrete made by a truck mixer has only a little difference in performances as compared with that made by a stationary mixer. Therefore, truck mixing is useful in achieving a closed recycling system for concrete in urban areas. 1. Introduction In the last few decades a lot of researches and development have been conducted on the reuse of concrete lumps from demolished structures in Japan. The recycling ratio of demolished concrete reached 96% in 2000, but most of demolished concrete is being reused as sub-base materials for road pavements. It is not the most suitable way of recycling, because the production of new concrete still requires a huge amount of raw materials. Taking into account the environment impact and transportation problems, it is advisable that recycled aggregates made from demolished concrete at a dismantling site can be used in producing new concrete for the same site and/or neighborhood construction site. This recycling system is referred to as a closed recycling system for concrete. The closed recycling system is also being developed in Japan and its effectiveness is confirmed. However there have been several remaining problems. All of current ready mixed concrete plants are equipped with a stationary mixer and this requests an extensive site, accordingly it is inconvenient and uneconomical to take down

2 the concrete-manufacturing plant and move it to another place when dismantling of concrete structure has been finished. Producing ability of the plant is excessive. In addition, the plant does not reach its capacity because the amount of demolished concrete is relatively small in each site. Therefore, it might be necessary to develop a small size mobile production plant of concrete. We focus on a truck mixing plant without a stationary mixer. The truck mixing is not being, recently used in Japan, but is popular in North America and European countries. Thus the objective of this study is to examine the quality of recycled concrete produced by a truck mixer, as compared with that of made by a stationary mixer, in order to verify the feasibility of truck mixing. 2. Outline of Closed Recycle System for Concrete 2.1 Producing equipment and system configuration The concept of new closed recycling system for demolishing concrete lumps from structures is illustrated in Figure 1. The heat and rubbing plant to produce recycled aggregates and truckmixing concrete plant for manufacturing recycled concrete are currently being developed and integrated [1]. The demolished concrete lumps were thermally treated, resulting in the disintegration of cement paste matrix in concrete. After that, it was mechanically treated and separated by the sieves, and consequently, the recycled raw materials such as coarse and fine aggregates, and fine powder were collected. Concrete using high quality recycled coarse and fine aggregates can be manufactured in a dismantling site within an urban area by a small size mobile concrete truck mixer. Furthermore, fine powder can be reused as the raw materials for producing new high quality cement because they include little amount of pulverized siliceous sand. Urban area Structure Demolish Concrete Lump Heating and rubbing plant 2.2 Heating and rubbing plant In our proposed system, demolished concrete lumps are heated at about 300 and rubbed, then almost all of the cement paste attached to the surface of the aggregates is separated. The recycled coarse and fine aggregates are visually Neighbouring site Reconstruction Recycled aggregates Truck mixing plant Powder Recycled cement Fig.1: Schematic diagrams of a closed recycling system of concrete in urban area

clean as shown in Figure 2, and their qualities are almost the same as virgin ones. Thus, these aggregates can be reused as the raw materials for the production of a new concrete.

3 Truck mixing plant This plant consists of aggregate storage, cementitious materials storage, admixture storage, batching equipments, a dust collection system, a feed system, and a truck mixer.

3 clean as shown in Figure 2, and their qualities are almost the same as virgin ones. Thus, these aggregates can be reused as the raw materials for the production of a new concrete. Figure 3 shows a schematic diagram of heating and rubbing plant. 2.3 Truck mixing plant This plant consists of aggregate storage, cementitious materials storage, admixture storage, batching equipments, a dust collection system, a feed system, and a truck mixer. This plant is simpler and smaller than a central mixing plant. This plant can also be decomposed and become movable. It seems that the truck mixer plant is suitable to a closed recycling system of concrete in urban areas because urban areas hardly have a large open space. A truck mixer has a mixing drum with helical paddles inside it. Furthermore, in order to prevent the formation of hardened concrete some steel plates and bars were also attached at the prominent places inside the drum, as shown in Figure 4. Fig.2: Appearances of recycled coarse and fine aggregates Demolished Concrete Lump Hopper Heating Tower Mill Mill Collector Sand Sieve Fig.3: Schematic diagram of heating and rubbing plant Fig.4: Schematic diagram of drum for a truck mixer Powder Gravel 3. Materials Ordinary Portland cement was used. Using the concrete lumps that were demolished from a storehouse built in 1970, recycled coarse and fine aggregates were manufactured by a heating

4 and rubbing equipment. The average values of quality of these aggregates are shown in Table 1. It is evident that their quality was as good as normal aggregates and satisfied the standard value of quality for crushed stone and natural aggregates conforming to JIS A 5308 and JASS 5 specifications. In order to grasp the quality fluctuation of recycled aggregate, the variations in quality for recycled aggregates were examined on the density, water absorption, quantity of impurities, and particle size distribution. Figure 5 shows the variations in density in absolute dry condition for recycled coarse aggregate. It is confirmed that high quality recycled aggregates can be produced using the heating and rubbing plant. 4. Mix Proportions and Manufacture of Concrete Mix proportion conditions are as follows; the slump at 30 minutes after mixing was 18 ± 2.5 cm for the water-cement ratios of 55 and 45%, and 21 ± 1.5 cm for a water-cement ratio of 35%. Air contents in all concrete were set up within 4 ± 1 %. The water-cement ratios of concrete were 55, 43, and 35%. For chemical admixture, AE water reducing agent was used in the case of concretes with water-cement ratio of 55 and 45%, Table 1: Properties of recycled aggregates Item Fine agg. Coarse agg. Maximum size (mm) 5 20 Density in saturated surface-dry condition (g/cm 3 ) Density in absolute dry condition Water absorption (g/cm 3 ) (%) Mass of unit volume (kg/l) Solid volume percent (%) Absolute dried density (g/cm 3 ) Table 2: Mix proportion of concrete W/C s/a Unit content(kg/m 3 ) Admixture (%) (%) W C S G (C %) * *High range water reducer whereas the polycarboxylate superplasticizer was used when the water-cement ratio was 35%. Mix proportions of concrete are shown in Table 2. All concrete ingredients were charged into a drum of truck mixer in following order; half of coarse aggregate, half of water, fine aggregate, cement, the remaining half of coarse aggregate and the remaining half of water. The drum was continued to revolve while charging the concrete ingredients. The maximum of a batch was restricted to about 1.5m 3 because when the excessive amounts of concrete ingredients were charged into the drum, concrete could not be well mixed. After all the materials were charged, mixing process continued for 5 minutes at a speed of 10 revolutions per minute, whereas that of stationary mixer was 1 minute Target Control June 6 月 July 7 月 Aug. 8 月 Sept. 9 月 Oct. 10 月 Nov. 11 月 Dec. 12 月 Month of production Month of production Fig.5: Variation in absolute dried density of the recycled coarse aggregate under production

5 5. Experimental Procedures Slump and air content were measured from a few minutes to 60 minutes after mixing. 100x200 mm cylinders were cast to determine the compressive strength. 100x100x400 mm prisms were cast for to determine the drying shrinkage, the carbonation, and the resistance to freezing-and-thawing cycles. After casting, all the molded specimens were covered with plastic sheet and left in a casting room for 24 hours. They were then demolded and stored in 20 water until the beginning time of tests. The exceptions were the specimens for drying shrinkage test and carbonation test. The drying shrinkage specimens were stored in water for 7 days prior to being exposed to drying conditions at 20 and a relative humidity of 60%. Specimens for carbonation test were stored in water for 28 days and then exposed to drying conditions at 20 and a relative humidity of 60% for 28 days before testing. The compressive strength was determined on three cylinders at 3, 7, 28, and 91 days. The drying shrinkage was measured using contact gages. Carbonation of concrete was tested using 100x100x400mm prisms, exposed to an environment of 20, 60% relative humidity, and 5% carbon dioxide. The prisms were cut at the designated age and phenolphthalein solution of 1% was sprayed onto the cut surface of sample in order to determine the depth of carbonation. The resistance to freezing-and-thawing cycles according to JIS A 1148 was determined with changes in mass and resonant frequency. 6. Results and Discussion 6.1 Properties of fresh concrete Figure 6 shows the change in slump with time. There was little difference in fluidity of concrete due to the types of mixer. Figure 7 shows the change in air content with time. Concrete manufactured by a truck mixer had larger amount of air content than that made by a stationary mixer immediately after mixing, consequently a truck-mixing concrete with a water-cement ratio of 35% did not satisfy the target value of air content. This might be explained by the fact that the air bubbles in truck-mixing concrete were almost the air bubbles Slump (cm) Elasped time after mixing (min.) Fig.6: Change in slump with time after mixing Air content (%) Elasped time after mixing (min.) Fig.7: Change in air content with time after mixing

6 entrapped in concrete during mixing. However, the entrapped air was not stable, in fact air content of a truck-mixing concrete with a water-cement ratio of 35% met the target value at 30 minutes after mixing. 6.2 Compressive strength Figure 8 shows the compressive strength of concrete as a function of water-cement ratio (W/C). Effect due to the type of mixer was observed for all ages from 3 days to 91days; Compressive strength of concrete made by a stationary mixer was a little higher than that of concrete made by a truck mixer; from 2 MPa to 3 MPa at 28 days and from 3MPa to 6 MPa at 91 days. It was also found that when water-cement ratio became lower, the long-term compressive strength of concrete with recycled aggregate did not increase in proportion to the increase of water-cement ratio. This tendency could be found regardless of the kind of the mixer. Hence, the reason why the long-term strength at low water -cement ratio did not develop, was caused by the difference in water-cement ratio between the original concrete of recycle aggregates and new recycled concrete; it might be possible that the water-cement ratio of original concrete was less than 35%. Compresive strength (Mpa) Truck mixer Stationary mixer k d Age 91 days 28 days 7 days 3 days Water cement ratio (W/C) Fig.8: Effect of type of mixing on the relationship between water-cement ratio and compressive strength of concrete 6.3 Drying shrinkage Figure 9 shows the measurement results of drying shrinkage. The drying shrinkage was found to be less than 750 x10-6 for drying periods of 6 months. These values were a little smaller than that of the conventional concrete used in Japan today. Moreover, there was almost no difference between the concretes made by a truck mixer and a stationary mixer. From these results, it seems that when the high quality recycled coarse and fine aggregates are used, the drying shrinkage of concrete using these aggregates is hardly enhanced. 6.4 Carbonation depth Carbonation is a typical problem for concrete with conventional recycled aggregates. Figure 10 shows the results of accelerated carbonation test. It could Strain ( 10 ) Duration of drying (days) Fig.9: Change in drying shrinkage with time

7 be found that carbonation depth of truck-mixing concrete was a little larger than t hat of concrete mixed by a stationary mixer. Figure 11 shows a relationship between compressive strength and carbonation depth. There was a good correlation between them. Truck-mixing concrete had a higher depth of carbonation compared with one made by the stationary mixer. This can be due to the effect of the type of mixer on concrete strength. 6.5 Resistance to Freezing-and-thawing cycles Figure 12 shows the results of freezing-and thawing test. Both concrete manufactured by truck mixer and stationary mixer, showed high resistance against freezing and thawing; more than 90% of the relative dynamic modulus of elasticity. In general, by considering the results of strength, drying shrinkage and carbonation depth, where the high quality recycled aggregates were used, it can be concluded that the concrete produced by the truck mixer had sufficient quality to be applicable to actual concrete structure when water-cement ratio in the range of 35 to 55%. 7. Conclusions In this study, the quality of truck-mixing concrete produced made of high quality recycled aggregate was Depth of carbonation (mm) Depth of carbonation at 26weeks (nm) Duration of carbonated test (weeks) Fig.10: Change in carbonation depth with time Compressive strength (N/mm 2 ) Fig.11: Effect of compressive strength on carbonation depth Relative dynamic modulus of elasticity (%) Stationary Truck Freezing and thawing cycle Fig.12: Result of freezing-and-thawing test examined in order to establish a closed recycling system for concrete in urban area. The results indicated that there was no difference between truck mixing concrete and concrete made by a

8 stationary mixer in slump, air content, drying shrinkage, and freezing and thawing resistance. Meanwhile, the strength test showed that for the truck mixer concrete, the compressive strength was about 3 to 6 MPa less than that of the stationary mixer. The carbonation depth of truck-mixing concrete was slightly larger corresponding with the decrease in strength. Accordingly, by controlling strength, good durability of the recycling concrete using the truck mixer can be obtained. From these results, it could be concluded that the quality of recycled concrete manufactured by a truck mixer did not differ from that of concrete made by a stationary mixer. Therefore, truck mixing is useful in achieving a closed recycling system for concrete in urban areas. References [1] H. Shima et al., New technology for recovering high quality aggregate from demolished concrete, Proc. of international symp. of the Univ. of Dundee, London, November, 1998,

TACAMP 2014 CONCRETE. Presented by Rick Wheeler

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