FEASIBLE USE OF RECYCLED CONCRETE POWDER AS A KIND OF MINERAL ADMIXTURE

Transcription

1 FEASIBLE USE OF RECYCLED CONCRETE POWDER AS A KIND OF MINERAL ADMIXTURE Xiaoyan LIU (1) (2), Junqing ZUO (1) (3), Kaiqiong LIU (1), and Aihua LIU(1) (1) School of Mechanics and Materials, Hohai University, Nanjing, China (2) Center for Advanced Cement- Based Materials, Northwestern University, IL, USA (3) School of Materials Science and Technology, Tongji University, Shanghai, China Abstract Reusing the waste concrete can not only resolve the problem of dealing with the garbage, but also realize saving the energy and reducing the pollution. In this paper, recycled concrete powder is obtained by crushing and grinding the waste concrete. Three different concrete mixes are studied and compared: normal concrete, concrete containing fly ash and ground granulated blast-furnace slag, concrete containing fly ash and recycled concrete powder. Experimental results of strength test show that recycled concrete powder has a similar pozzolanic activity to ground granulated blast-furnace slag. The results of the slab test show that the recycled concrete powder can help to delay the initial time, decrease the maximal width and length, and decrease the total area. The results of shrinkage test also show that concrete containing recycled concrete powder has the minimal shrinkage strain. So recycled concrete powder can help to improve the early anti-crack performance of concrete. All of these experimental results show that recycled concrete powder is feasible to be used as a kind of mineral admixture. Keywords: Recycled concrete powder; Slab test; Shrinkage test; Mineral admixture 1. INTRODUCTION With the acceleration of urbanization, construction of new towns and renovation of old towns, the demand of concrete is growing in China. Meanwhile, it causes a lot of waste 722

2 concrete. The frequent geological earthquakes disasters also cause a lot of waste concrete. Now, the waste concrete produced every year is over one hundred million tons in China. As the largest quantity construction materials, waste concrete brings enormous pressure to environmental protection. On one hand, the growing demand of concrete in construction should be satisfied. On the other hand, ways should be found to deal with waste concrete. It is a good idea to use recycled materials derived from construction and demolition waste in producing the new concrete. The interest in using recycled concrete is growing all over the world [1-5]. In addition to environmental protection, conservation of natural resources, and shortage of land for waste treatment prior to disposal are the principal factors driving the recycling concept [6]. However, from the existing papers it can be found that research mainly focus on the recycled concrete aggregate. Actually, as a result of many constructions, the normal mineral admixtures are in short supply in some areas. So in this paper the feasibility of recycled concrete powder as a kind of mineral admixture will be experimentally studied. 2. MATERIALS AND EXPERIMENTAL METHOD 2.1 Materials The cement used is Portland cement. Its chemical composites are shown in Table 1. The fine aggregate is natural standard sand. The water-reducing agent is UC-Ⅱprovided by Youxi Co. Ltd. Some mineral admixtures are used including fly ash (FA) and ground granulated blast-furnace slag (GGBS). The recycled concrete powder (RCP) is obtained through grinding waste concrete. Their chemical composites are also shown in Table 1. The diameter distribution of particle is given in Table 2. The maximal diameter of the gravel is 20mm. Table 1: Chemical composites of cement and admixtures wt.% Material SiO 2 Al 2 O 3 Fe 2 O 3 CaO MgO K 2 O Na 2 O TiO 2 P 2 O 5 MnO SO 3 IL Cement FA GGBS RCP Table 2: Diameter distribution of particles Type Specific surface /(cm 2 g Diameter distribution/vol.% -1 ) <10μm 10~37μm 37~60μm >60μm FA GGBS RCP Specimens and measuring method Three concrete mixes are studied and compared: plain concrete, concrete containing fly ash and ground granulated blast-furnace slag, concrete containing fly ash and recycled concrete powder. Comparison will be done between ground granulated blast-furnace slag and recycled concrete powder. A plain concrete without any mineral admixture is also casted for evaluating 723

3 the strength growth and anti-crack property differences caused by mineral admixtures. If the properties of recycled concrete powder are no worse than those of ground granulated blast-furnace slag, then recycled concrete powder may be used to take place of ground granulated blast-furnace slag in concrete. By this, the feasibility of recycled concrete powder as a kind of mineral admixture will be experimentally studied. Specimens are cast from each mix to assess compressive strength, splitting tensile strength, drying shrinkage and early- age anti-crack property. Details of the mix proportions are given in Table 3. The slump values of the mixes are designed at mm. Table 3: Mix proportions of concrete Sand ratio FA dosage GGBS dosage RCP dosage Mix proportion/(kg m -3 ) Water Cement Sand Gravel 1 # # # To get compressive strength and splitting tensile strength, concrete specimens are prepared and tested according to the requirements of GB/ T [7]. Drying shrinkage measurement is tested according to the requirements of GBJ 82-85[8]. There are several test methods to assess the anti- crack performance of concrete. In this paper, slab test is chosen to simulate the restrained status and study the of concrete in accordance with the requirements of CCES [9]. The sectional view of the mould is shown in Figure 1. Figure 1: Diagram of the mould Two specimens are prepared of each mix ratio. After mixing, concrete is cast into the 724

4 mould (600mm 600mm 63mm). To decrease the friction, a plastic sheet is put on the bottom of the mould. After being vibrated and smoothed, the specimens are cured for 2 hours. Then the specimens will be given a wind with a speed of 0.8m/s. During the test, the temperature is maintained at 20±1, the relative humidity is maintained at 60±5%. The specimens are observed just in the moulds. At the early stage, is measured once every 5 minutes. After the first is observed, is measured once every 10 minutes. When develops very slowly, it is measured once every 30 minutes. After 3 days, it is measured twice one day. The initial time, numbers of cracks, width of cracks, and length of cracks are recorded. The average area a can be calculated using Eq. (1). N 1 a = W i L i (1) 2N i Where, N is the numbers of the cracks, W i is the maximal width of the crack i, L i is the length of the crack i. The crack numbers per area b can be calculated using Eq. (2). N b = (2) A Where A is the surface area of specimen, it is 0.36 m 2. The total area per area C can be calculated by Eq. (3). C = a b (3) The anti-crack grade can be obtained based on these parameters. The anti-crack performance can be evaluated by the grade together with the results of drying shrinkage test. 3. RESULTS AND DISCUSSION 3.1 Strength of concrete Results of the strength test are shown in Table 4. Table 4: Strength test results Compressive strength Tensile strength slump (MPa) (MPa) 7d 28d 7d 28d 1 # # # From the results, it can be seen that the addition of admixture cause the concrete strength grows less slowly comparing to the plain concrete. Concrete containing fly ash and recycled concrete powder has similar strength as concrete with fly ash and ground granulated blast-furnace slag, which indicates that recycled concrete powder has almost the same pozzolanic activity as ground granulated blast-furnace slag. As to the activity, recycled concrete powder can be used as a kind of mineral admixture. 725

5 3.2 Drying shrinkage Figure 2 shows the drying shrinkage strain of three concrete mixes. Concrete containing fly ash and recycled concrete powder exhibits the minimal drying shrinkage strain. That means recycled concrete powder is better in respect of reducing the drying shrinkage strain, comparing with ground granulated blast-furnace slag. Figure 2: Drying shrinkage of different concrete mixes 3.3 Slab test Results of slab test are shown in Table 5. From the results, it can be seen that the initial time of concrete containing fly ash and recycled concrete powder is delayed, and both the maximal width and the maximal length have an obvious decrease, comparing with the plain concrete and concrete with fly ash and ground granulated blast-furnace slag. So recycled concrete powder can help to improve the anti- property at the early age. Table 5: Results of slab test Initial time (h) Initial length Initial width Maximal width Maximal length Numbers of cracks 1 # # # According to Eq. (1) - Eq. (3), anti-crack index can be calculated and then the anti-crack grade can be obtained [9]. The results are shown in Table 6. Concrete containing fly ash and recycled concrete powder shows the best anti-crack performance. 726

6 Table 6: Anti-crack grade of concrete a b C Anti-crack (mm 2 /n) (n/ m 2 ) (mm 2 / m 2 ) grade 1 # Ⅲ 2 # Ⅲ 3 # Ⅱ Images of width are given in Figure 3. It shows the same results. (1) 1 # (2) 2 # (3) 3 # Figure 3: Images of width 4. CONCLUSIONS Recycled concrete powder is studied in this paper. The Results of strength test show that the activity of recycled concrete powder is close to ground granulated blast-furnace slag. From the results of the slab test, it can be seen that recycled concrete powder can delay the initial time, decrease the maximal width and length. It can also decrease the total area. Comparing with plain concrete and concrete containing fly ash and ground granulated blast-furnace slag, the results of shrinkage test also show that concrete containing recycled concrete powder has the minimal shrinkage strain. So recycled concrete powder helps to improve the anti-crack property of concrete. There is still a lot of work left to be done about the recycled concrete powder. However, the experimental results have shown the feasible use of recycled concrete powder as a kind of mineral admixture. ACKNOWLEDGMENTS The authors are grateful for the funding support from the Student Science and Technology Foundation of Hohai University (grant K200805) REFERENCES [1] M. Barra de Oliveira, E. Vazquez, 'The influence of retained moisture in aggregates from recycling on the properties of new hardened concrete', in 'Waste Management', Vol. 16, Issues 1-3, 1996: [2] K. K. Sagoe-Crentsil, T. Brown and A. H. Taylor, 'Performance of concrete made with commercially produced coarse recycled concrete aggregate', in 'Cement and Concrete Research', Vol. 31, 2001(5): [3] Amnon Katz, 'Properties of concrete made with recycled aggregate from partially hydrated old concrete', in 'Cement and Concrete Research', Vol. 33, 2003(5): [4] C. S. Poon, Z. H. Shui, L. Lam, H. Fok and S. C. Kou, 'Influence of moisture states of natural and 727

7 recycled aggregates on the slump and compressive strength of concrete', in 'Cement and Concrete Research', Vol. 34, 2004(1): [5] L. Evangelista, J. de Brito, 'Mechanical behavior of concrete made with fine recycled concrete aggregates', in 'Cement and Concrete Composites', Vol. 29, 2007(5): [6] PK Mehta, 'Advancements in concrete technology', in 'Concrete International', 1999(6): [7] Standard for test method of mechanical properties on ordinary concrete, GB/ T [8] Test method of long-term performance and durability on ordinary concrete, GBJ [9] China Civil Engineering Society, Guide to durability design and construction of concrete structures, CCES , China Architecture & Building Press, 2005(1):