that the annual worldwide generation of concrete and masonry rubble is approximately one billion tones, while only a small percentage of them are recycled.[2] While local RCA could be successfully used in Portland cement concrete,most of the RCA used currently were only used in less cost-effective applications such as backfill, base, and subbase of pavement. More efficient manners of using RCA in construction are in need. Due to concerns of higher fine content and absorption, use of fine recycled concrete aggregates (FRCA) in new concrete is restricted or even prohibited. While it is generally expected that the use of FRCA could have negative impact of concrete properties, such as the decrease in strength and an increase in drying shrinkage rate, there are studies that suggest the use of FRCA is not necessarily inauspicious and it is feasible to obtain concrete with acceptable performance with FRCA.[3–9]
Although using recycled aggregate in concrete has been extensively documented, there are only a limited number researches conducted on SCC with different kinds of recycled aggregate,including recycled concrete, bricks, and glass.[10–21] Different test methods were used to evaluate flowability, passing ability, and stability of fresh SCC, together with harden concrete properties, including compressive strength, tensile strength, and drying shrinkage. Results show that while the performance of SCC is highly dependent on quality of recycled aggregate used in the mix, it is possible to obtain SCC with RCA of acceptable flowability and strength.[13–15] It is also found that most of the works are focused on replacing coarse aggregate, and there is not adequate study on the influence on SCC properties of different rates of FRCA replacement. A systematic research is needed to evaluate performance of SCC with FRCA. The obtained results can be very significant from both technical and environmental prospective.
2. Experimental details
2.1. Materials
Type I Portland cement that meets ASTM C150 (Standard Specification
for Portland Cement) [22] and class C fly ash that meets ASTM C618 (Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete) [23] were used as cementi-tious materials in the concrete mix. The chemical compositions and physical properties of cement and fly ash used in the study are reported in Table 1. Crushed limestone, limestone-based manufacturing sand, and silica sand were used as aggregate in the concrete mixtures. RCA with nominal maximum size of 25mm (1 inc.)was obtained from a local recycled concrete plant and further crushed with a laboratory jaw crusher with an opening of approximately 15mm. A sieve was then used to screen out particles larger than the sieve
size of 2.36mm (#8). FRCA with maximum size of 2.36mm (#8) obtained, as shown in Figure 1, was used as substitute of natural fine aggregate, i.e. manufacturing sand and silica sand. Apolycarboxylate-based high-range waterreducing(HRWR) admixture (Glenium? 7700) and viscosity-modifying admixture (VMA) (Rheomac VMA 362) were used to adjust the workability of SCC mixtures.
Sieving analyses were performed on all four aggregates used in the study according to ASTM C136 (Standard Test Method for Sieve Analysis of Fine and Coarse Aggregates).[24] Gradation curves of all four different kinds of aggregate are shown in Figure 2. Fineness modulus of manufactured sand, silica sand, and FRCA were calculated as 3.22, 1.08, and 2.52, respectively.
Specific gravities and absorptions of aggregates were measured based on ASTM C127 (Standard Test Method for Density, Relative Density [Specific Gravity] [25], and Absorption of Coarse Aggregate) and ASTM C128 (Standard Test Method for Density, Relative Density [Specific Gravity], and Absorption of Fine Aggregate) [26], respectively, and the results are shown in Table 2. It should be noted that due to the high amount of residual-hardened cement paste, FRCA has much higher absorption (7.2%) and relatively low-specific gravity comparing to manufactured sand and silica sand.
2.2. Mix proportions
Two series of SCC mixes as shown in Table 3 were prepared in this study.
With series A contains class C fly ash and series B contains only portland cement as cemen-titious material. The water-to-cement ratio of series A was 0.41 and the water-to-binder ratio was 0.34; the water-to-cement ratio of series B was 0.41. Based on the two reference mixes (A FRCA0 and B FRCA0), other eight mixes with FRCA replacement of 25, 50, 75, and 100% (of mass of natural fine aggregate used in reference mixes) were designed to evaluate the effect of FRCA. Since this study is focus on effect of FRCA, in order to simplify the mix design matrix, amount of cement, fly ash, water, HRWR, and VMA were intentionally kept constant within each of these two series. Masses of aggregate used in the table were presented in saturated surface dried conditions. 2.3. Concrete mixing
A MP 75 SICOMA Laboratory Mixer was used to mix concrete based on procedure as described in ASTM C192 (Standard Practice for Making and Curing Concrete Test Specimens in the Laboratory).[27]Moisture contents of coarse aggregate, manufactured sand, silica sand, and FRCA prior to being placed into the mixer were at 1.28, 2.93, 4.33, and 3.58%, respectively. Amount of water in each mix were adjusted accordingly. During the mixing, coarse aggregate was first introduced into the mixer and mixed with approximately half of the water for 30 s. After that, fine
aggregates, cement, fly ash (if applies), and approximately half of the remaining water with HRWA and the rest of water with VMA were placed into the mixer and mixed for three minutes. The mixtures were rested in the mixer for 3 min, followed by another 2 min of mixing before the completion of the whole mixing procedure.
Table 1. Chemical compositions and physical properties of cement and fly ash. Oxide (%) Cement Fly ash SiO2 20.4 35.8 Al2O3 4.6 20.37 Fe2O3 4.5 5.54 CaO 64.4 26.04 MgO 0.8 4.49 SO3 3.7 1.52 Loss in ignition 1.8 0.31 Blaine fineness (M2/kg) 433 NA Specific gravity NA 2.74
Figure 1. FRCA used in the study.
Figure 2. Particle size distribution of Aggregates.
Table 2. Aggregate specific gravity and absorption.
Aggregate type Specific gravity(OD) Specific gravity(SSD) Absorption(%) Fineness modulus Crushed limestone 2.384 2.475 3.80 NA Manufactured sand 2.482 2.556 2.98 3 .23 Silica sand 2.648 2.664 0.60 1.00 FRCA 2.158 2.313 7.20 2.52
Table 3. Mix proportion.
A FRCA0 A FRCA25 A FRCA50 A FRCA75 AI FRCA100 Cement 390 (657) 390 (657) 390 (657) 390 (657) 390 (657) Fly ash 83 (140) 83 (140) 83 (140) 83 (140) 83 (140) Water 160 (270) 160 (270) 160 (270) 160 (270) 160 (270) Crushed limestone 917 (1546) 917 (1546) 917 (1546) 917 (1546) 917 (1546) Manufactured sand 555 (935) 402 (678) 253 (427) 103 (173) 0 (0) Silica sand 98 (165) 73 (123) 44 (75) 18 (30) 0 (0) FRCA 0 (0) 163 (275) 326 (550) 489 (825) 600 (1012) HRWR 1043 (16) 1043 (16) 1043 (16) 1043 (16) 1043 (16) VMA 652 (10) 652 (10) 652 (10) 652 (10) 652 (10) B FRCA0 B FRCA25 B FRCA50 B FRCA75 B FRCA100 Cement 415 (700) 415 (700) 415 (700) 415 (700) 415 (700) Fly ash 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) Water 170 (287) 170 (287) 170 (287) 170 (287) 170 (287) Crushed limestone 831 (1400) 831 (1400) 831 (1400) 831 (1400) 831 (1400) Manufactured sand 659 (1111) 480 (809) 301 (507) 123 (207) 0 (0) Silica sand 116 (196) 85 (143) 53 (90) 21 (36) 0 (0) FRCA 0 (0) 194 (327) 388 (654) 581 (980) 714 (1203) HRWR 978 (15) 978 (15) 978 (15) 978 (15) 978 (15) VMA 522 (8) 522 (8) 522 (8) 522 (8) 522 (8)
Notes: Cement, fly ash, water, and aggregates are in kg/m3 (pcy), HRWR and VMA are in ml/100 lb (fl oz/cwt).
2.4. Test methods
2.4.1. Fresh concrete properties
After concrete mixed, a slump flow test based on ASTM C1611 (Standard
Test Method for Slump Flow of Self-Consolidating Concrete) [28] was performed to evaluate the lateral flow and filling potential of different SCC mixtures. The testing apparatus included a standard slump cone and a 900mm by 900mm (35.4 inc. By 35.4 inc.) stainless steel plate. With this
相关推荐：
loading