THE APPLICATION OF RECYCLED AGGREGATES IN SCC

Transcription

1 THE APPLICATION OF RECYCLED AGGREGATES IN SCC Tsung-Yueh. Tu (1), Yuan-Yuan Jann (2), Chao-Lung Hwang (1) (1) CMRL, Taiwan University of Science and Technology (NTUST), Taiwan (2) HWA-SHA Institute of Commerce and Technology, Taiwan Abstract The utilization of recycled aggregates can reduce the over-development of the environment and slow the huge consumption of natural resources for concrete use. However, recycled aggregates are not suitable in the production of Self-Consolidating Concrete (SCC) due to their relatively high absorption capacity, unstable properties and the weaker strength of the recycled aggregates. Such dilemma can be overcome through carefully examining the characteristics of recycled aggregates and then adopting proper mixture proportions. In this paper, recycled aggregates generated from demolished construction wastes were examined and the Densified Mixture Design Algorithm (DMDA) was applied for batching SCC. Results show that most DMDA applied SCC specimens can have a slump of more than 18mm and a slump flow larger than 55mm during the initial stage. As a result of the high amount of recycled aggregates and cement added into the mixture, several DMDA specimens were not high-flowing and self-consolidating after 1 hour had lapsed. As to durability, both concrete resistivity and chloride ion penetration of DMDA specimens were all much better than those of traditional concrete at the age of 91 days. Keywords: SCC, recycled aggregates, DMDA, durability, sustainable development 1. INTRODUCTION Taking the concept of sustainable development into consideration, the concrete industry has to implement a variety of strategies in regards to future concrete use, which for instance, requires better durability of concrete and the use of recycled materials in concrete. The durability of concrete is elemental, but also the most significant; as it represents the ability of concrete to resist the degradation factors that lead to the concrete deteriorating. However, The excellent durability of concrete remains un-reachable as a result of natural factors, which are far beyond the present capabilities of science to over-come and produce a sustainable alternative. Therefore, it seems the utilization of recycled materials is an easier and a more effective way for the concrete industry to move toward the goal of sustainable development at Page 1

2 the present time. In general, 55% ~ 8% of concrete components in volume are aggregates. Without proper alternative aggregates being utilized in the near future, the concrete industry will consume 8 ~ 12 billion tones of natural aggregates after the year of 1 globally [1]. Such consumption of natural aggregates will be destructive to the environment. Therefore, to find and apply suitable substitutes for natural aggregates is an urgent task. When the deadly earthquake hit the central region of Taiwan in 1999, it produced more than 15 hundred million m 3 of demolished construction wastes. Among the wastes, recyclable concrete and bricks represented 28% and 25% respectively. If proper production methods can be developed, the utilization of these recycled aggregates will not only be one solution to the over-development of the environment and contribute to slowing of the huge consumption of natural resources by the concrete industry, but also help in the disposal of wastes for the local authorities and the environment. Accordingly, this paper will discuss the utilization of recycled aggregates that have originated from demolished construction wastes in SCC. Properties of concrete both at the fresh and hardened stage will then be illustrated and further analysis of the influence of recycled aggregates on the properties of SCC will be presented. 2. RESEARCH PLAN ASTM C15 type I cement, ASTM C618 Class F fly ash, and ASTM C595 blast furnace slag with the finess of cm 2 /g, which is being produced in the local factories of Taiwan were used in this research project. The mixing water was normal tap water. Demolished construction wastes, mostly generated during the 1999 earthquake, were processed and used as recycled coarse aggregate. After the basic procedures of breaking and shattering, the aggregate was seived and then (due to the materials poor gradation formation originally) remixed in order to meet ASTM C33 standard specifications. Two types of fine aggregates were used in this research. One was from demolished construction wastes and its processing proceedure was similar to the procedure used for producing the recycled coarse aggregate, except for the diameter size, while natural fine aggregates from Mainland China were used for comparative and analytical reasons. The aggregates basic properties in this research project are shown in Table 1. A naphthalene lingo-sulfonate based ASTM C494 type G (HPC-1) SP was also used. 1 concrete mixtures were prepared using the Densified Mixture Design Algorithm (DMDA). The Densified Mixture Design Algorithm (DMDA) had 2 groups of mixtures, DR and DN. The difference between DR and DN, was whether or not recycled fine aggregate was used. 3 various w/b ratios with the same water content of 16kg/m 3 were used in the production of DMDA designed concrete specimens. In addition, another 2 concrete specimens for Group DR and DN were designed, in which w/b was.32, but the water content was 15 kg/m 3 and 17 kg/m 3, respectively. The result of each concrete mixture is shown in Table 2. Page 2

3 Table 1: Basic characteristics of applied aggregates Physical Properties Recycled Coarse Aggregate Recycled Fine Aggregate Natural Fine Aggregate Specific Gravity (SSD) Specific Gravity (OD) Absorption Capacity (%) Dmax (in) 1/2 #4 #4 FM Unit Weight (kg/m 3 ) Soil Content (%) Soundness (%) Los Angeles Test (%) Brick Content (%) Ceramic Content (%) Table 2 : Mixture proportions of concrete(kg/m 3 ) Set Water Cement Coarse Agg. Fine Agg. Fly Ash Slag SP DR DR DR DR DR DN DN DN DN DN A represents the ACI mixture design, D represents the DMDA design, R, represent the utilization of recycled coarse and fine aggregates, N represent the utilization of recycled coarse aggregate and natural fine aggregate. 2. The number after AR and AN represents w/c ratios, the first two digits after DR and DN are w/b ratios, the other 3 digits are the water content in the specimen. 3. RESULTS 3.1 Fresh Properties Slump: The initial slump of Group DR and DN specimens can meet the SCC requirements, 23 ~ 27 mm. After 1 hour, most Group DR and DN specimens can still be highlyflowable as SCC requires, except for specimens with a higher aggregate content (DR16 & DN16), a higher cement content (DR3217) and a lower water content (DN3215). Slump Flow: Similarly, as with the slump performance, Group DR and DN specimens can perform as the SCC requires at the initial time. However, specimens with a higher aggregate content (DR16 & DN16), a higher cement content (DR3217) and a lower water content (DN3215) are unable to reach the SCC standards after 1 hour. In short, the higher absorption capacity affects the workability of concrete designed with recycled aggregates. Page 3

4 Table 3: Properties of Fresh Concrete Set Initial 1 hour Slump(mm) Slump Flow Slump(mm) Slump Flow time(sec) flow(mm) time(sec) flow(mm) DR DR DR DR DR DN DN DN DN DN Hardening properties Compressive Strength: As to the DMDA specimens, as the water content is fixed, the lower the w/b ratio, the higher the compressive strength (Figure 1). On the other hand, DR s specimens with a higher water content have better performance in compressive strength than DR s with the lower water content, when the w/b ratio is constant and the amount of mixing water is varied. For Group DN specimens, the one with a water content of 16kg/m 3 indicates a slightly higher performance in compressive strength than those with a water content of 15 and 17kg/m 3, when the w/b ratio is fixed. Normally, concrete utilizing the DMDA design with a fixed w/b should indicate that the lower the water content, the higher the compressive strength is. This different findings result from the complete utilization of recycled aggregates in Group DR and DN. Recycled aggregates need a relatively larger water content to satisfy their higher absorption capacity and hydration behavior, without sufficient water content, compressive strength develops poorly (Figure 2). Comparing the compressive strength of Group DR and DN, Group DN is generally higher than Group DR based on the same water content and w/b ratio at a given age. This proves the utilization of natural aggregates is able to enhance the compressive strength of concrete, when recycled aggregates are substituted for aggregates. Table 4: Properties of hardened concrete Test Compressive Strength (Mpa) Concrete Resistivity (KΩ-cm) Chloride Penetration (Coulombs) Age (days) DR DR DR DR DR DN DN DN DN DN Page 4

5 Compressive Strength (Mpa) days 7 days 28 days DR 56 days 91 days DN 5 3 Compressive Strength (Mpa) w/b w/b 1 Figure 1: The development of compressive strength vs. the age and w/b ratio (dmda concrete with the same water content of 16kg/m 3 ) 5 DR DN 5 Compressive Strength (Mpa) The Water Content (kg) 3 days 7 days 28 days 56 days 91 days The Water Content (kg) 3 1 Compressive Strength (Mpa) Figure 2: The development of compressive strength vs. the age and the water content (dmda concrete with the same w/b of.32) Concrete Resistivity: For Group DR and DN, when the same water content is used, the lower the w/b ratio, the higher the performance in concrete resistivity. For example, DR3216 can reach 5.1 kω-cm (Figure 3), when the DN3216 reading is 77.9 kω-cm a time period of 91 days had lapsed. On the other hand, when the w/b ratio is fixed and the water content varies, Group DN s specimens with a lower water content will have a higher concrete resistivity than those with a higher water content (Figure 4). DN3215 can even reach 8.6 kω-cm at 91 days. This outcome is a similar trend to the DMDA findings with regards to concrete resistivity[3~8]. However, Group DR does not have the same tendency, as its specimen with lowest water content does not have the highest development of concrete resisitivity at a given age. When the water content is relatively lower, insufficient water supply consequently influences the extent of the hydration reaction and is the possible cause that limits the densification of Group DR. After a time period of 28 days has lapsed, the concrete resistivity of specimens using the DMDA design, both Group DR and DN, will still be developing. This is because the densification of DMDA designed mixes still grows after 28 days. Page 5

6 1 DR DN 1 3 Days 56 Days 8 7 Days 91 Days 8 28 Days w/b w/b Figure 3: The Relationship between concrete resistivity and the age and w/b ratio (DMDA concrete with the same water content of 16kg/m 3 ) 1 DR DN 1 3 Days 56 Days 8 7 Days 91 Days 8 28 Days The Water Content (kg) The Water Content (kg) Figure 4: The relationship between concrete resistivity and the age and the water comtent (dmda concrete with the same w/b of.32) Chloride Penetration: For DMDA designed mixes, when the water content is constant, the lower the w/b ratio, the lower the value of chloride penetration for both Group DR and DN. When the w/b ratio is fixed, specimens with lower water content have the lower measurement of chloride penetration for both Group DR and DN. As the parameters are the same, such as the water content and the w/b ratio, chloride penetration of Group DN is better than that of Group DR. In short, for DMDA designed specimens under the same conditions, specimens containing natural fine aggregates performs better with regards to chloride penetration than those that do not utilize natural aggregates, as the adding of natural fine aggregate can resist chloride penetration and increase the durability of concrete further. Even chloride penetration of Group DR is at a moderate level based on ASTM C12 Standard Specifications at 56 days of age, all DMDA designed specimens are lower than coulombs at 91 days in terms of chloride penetration. According to ASTM C12, this reading represents a low level of chloride penetration and also means SCC concrete using recycled aggregates have good performance with regards to durability (Figure 5). This also further demonstrates that the DMDA design can strengthen durability in terms of chloride penetration. Page 6

7 56 Days Chloride Ion Penetration (Qs) Days 3 1 Chloride Ion Penetration (Qs) DR3216 DR3616 DR16 DN3216 DN3616 DN16 DR3215 DR3216 DR3217 DN3215 DN3216 DN3217 DMDA concrete with 16 kg of mixing water DMDA concrete with.32 w/b Figure 5: Results of DMDA specimens on chloride penetration 4. CONCLUSION The higher absorption capacity and impurity of recycled aggregates do have a direct impact on properties of concrete both at the fresh and hardened stage. Concrete designed with recycled aggregates might suffer a long term downgrade in strength development with regards to both ACI and DMDA designed mixes due to weaker interfaces and bindings. In DMDA designed production, the proper water demand, the use of higher SP and fly ash content are used to compensate for the defects of recycled aggregates. Despite the absorption capacity of recycled aggregates, less water and cement content with lower w/b ratio are able to ensure the durability of concrete when using the DMDA design. The application of natural fine aggregate and proper design algorithms is able to advance the property of concrete, which is basically designed with recycled aggregates. All results make it clear that recycled aggregates can be used to produce SCC as long as the proper design algorithms are applied. Further researches, such as modulus of elasticity, creep and shrinkage, have to be conducted for structural applications. REFERENCE [1] WWF, 'The living planet report', (Taipei, 1998). [2] Mindess, S. & J. F. Young, 'Concrete', (Prentice-Hall, N.J. USA, 1981). [3] Mehta, P. K., 'Concrete Structure, Properties, and Materials', (Prentice-Hall, N.J. USA, 1986). [4] DePauw, C., 'Fragmentation and Recycling of Reinforced Concrete Some Research Results', Chapter in P. Kreijger 'Adhesion Problems in the Recycling of Concrete', (Nato Conference Series IV (Materials Science) Plenum Press, New York, NY 1981) 311~317. [5] Lin, T. Y., 'The Application of Electric Arc Furnace Reduction Slage to High Performance Concrete', The Master Thesis, (NTUST, Taipei, 3). [6] Hwang, C. L., ' Properties and Behaviors of Concrete', (Chan s Arch Books, Taipei,1999). Page 7

8 [7] Lu, H. C., 'The Effect of Paste Amount on the Volume Stability of Eugenic Concrete', The Master Thesis, (NTUST, Taipei, 1998). [8] Lin, Z. Q., 'The Effect of Fly Ash LOI on the Properties of Eugenic Concrete', The Master Thesis, (NTUST, Taipei, 1998). [9] Hansen,T.C., and H. Narud, 'Strength of Recycled Concrete Made from Crushed Concrete Coarse Aggregate', 'Concrete International', Vol.5, No1, (1993) 79~83. [1] Hasaba, S., Kawamura, M., Toriik, K. and K. Takemoto, 'Drying Shrinkage and Durability of the Concrete Made of Recycled Concrete Aggregate', 'The Japan Concrete Institute', vol.3, (1981) 55~6,(additional information obtained from background report in Japanese). [11] Back, A. D., 'Recycle concrete as a source of aggregate', 'ACI Journal', Title No , (1977) 212~219. [12] Li, L.S. and C.L. Hwang, 'A Quality Assurance System of SCC in Taiwan', ' The Proceedings of First North American Conference On the Design And Use of Consolidating Concrete', (ACBM, 2) 275~28. Page 8