Test methods

The compressive strength of concrete was tested using a compression machine with the loading capacity of 2000 kN. The loading rates applied in the compressive tests were 5 kN/s, in accordance with GB/T 50081 (MHURD, 2002). The static modulus of elasticity of concrete was determined using the same machine as that used in the compression tests according to GB/T 50081 (MHURD, 2002). The shrinkage of concrete was determined by measuring the specimen length change according to GB/T 50082 (MHURD, 2009). In particular, Demec gauges with a gauge length of 200 mm were used to measure the shrinkage deformation on the 100×100×400 mm prisms. The initial lengths of the specimens were measured at 24 h after concrete casting and then the specimens were conveyed to the chamber with a constant temperature of (23±1)℃ and a constant relative humidity of (50±5)% for 224 days.

Experimental tests and results 3.3

3.3.1 Compressive strength

The test results of the compressive strength for each concrete mixture are reported in Table 3-5. The presented results include the mean values of the measurements from three

samples as well as the corresponding coefficient of variation (COV). It can be observed that the coefficient of variation for the compressive strength of concrete was basically independent on the service time of RCA and on the mixing methods. In this context, all the concrete mixtures tested in this chapter may be considered to have the same degree of homogeneity. In the following, the mean value of the compressive strength was adopted for all the investigation. As expected, when the RCA r ratios increased from 0% to 100%, the compressive strength decreased accordingly. Taking concrete containing RCA18-I as an example (Table 3-5), the corresponding reductions in the 28-day compressive strength of concrete with RCA r ratios of 30%, 50% and 100% were 12.0%, 12.5% and 23.0%, respectively, when compared to the NAC-I measurements.

The development of concrete compressive strength over time is presented in Figure 3-2 for concrete prepared using PS method. Based on 28d compressive strength value, the compressive strength development was more slowly before 24 h for RAC when compared to that of NAC. For example, the 1 d compressive strength of NAC-I was 12.8 MPa (Table 3-5), which was 32.6% of the 28 d value, while the 1 d compressive strength of concrete made with 100% of RCA18-I were 23.8% of the 28 d compressive strength. This observation could also be found by Kou et al. (2012), in which the 1-day compressive strength was 34.2%~37.5% of the 28 d value for NAC, while the percentage was only 20.6%~25.4% for RAC with r ratio of 50%. This is probably because the interfacial transition zone (ITZ) between the original virgin aggregate (OVA) and residual mortar (RM) was fairly weak due to the low quality of RCA used in this study and in reference (Kou et al., 2012). Actually, in this study all the recycled concrete cubes failed with compression cracks developing along the ITZ at 24 h after casting. The quality of the ITZ would be improved by the hydration of new mortar over time (Zhang and Zhao, 2015), and hence both NAC and RAC in this study had the compressive strength developed to 70.4%-78.5% of the 28 d values at 7 days. From 7 days to 28 days, similar strength development was observed for all the concrete with different r ratios. For RAC with RCA sourced from high strength parent concrete, similar strength development could be observed as that of the reference NAC all through the 28 days after concrete casting, e.g. Kou and Poon (2015), which was due to the good quality of ITZ between OVA and RM. After 28 days of curing, on the other hand, the gains in the RAC

compressive strength were larger than those of the reference NAC. Despite that slight variation was obtained in the test results, NAC presented a gain of 17.1% from 28 days to 90 days in this study, while RAC made with 100% of RCA18-I and RCA40-I had gains of 28.8% and 20.8%, respectively. A longer test period of 5 years was selected by Kou and Poon (2008), in which the concrete mixture with 100% of RCA of different qualities experienced gains ranging from 46% to 62% in comparison with the 28 d compressive strength, while only 34% increase was observed for NAC during the same period. Similar observation was reported by Zhang and Zhao (2015), in which the compressive strength gains from 28 days to 720 days were 32.3%, 47.0% and 50.6% for r ratios of 0%, 50%

and 100%, respectively. This is attributed to the RCA in SSD condition, in which case the absorbed water transported from RCA to ITZs over time, and provided ―internal curing effects‖ (Kou and Poon, 2008). In this context, the current model, which is used for NAC strength development, may be no longer capable for the prediction of RAC.

1 10 100

(a) Concrete made with RCA1-I (b) Concrete made with RCA18-I

1 10 100

Figure 3-2Development of compressive strength of concrete using PS method

Table 3-5 Compressive strength and static modulus of elasticity

Notation r (%)

Compressive strength (MPa) Elastic modulus (GPa)

1 d 3 d 7 d 28 d 90 d 28 d 90 d

Mean value (COV ^a)

Mean values (COV)

Mean values (COV)

Mean values (COV)

Mean values (COV)

Mean values (COV)

Mean values (COV)

NAC-I 0 12.8(7.0%) 26.3(1.9%) 28.8(2.4%) 39.2(2.8%) 45.9(2.9%) 26.4(0.53%) 30.7(2.76%)

RAC1-I-100%-PS 100 11.6(4.3%) 23.8(3.4%) 27.4(2.6%) 38.9(0.3%) 45.1(3.7%) 20.1(2.11%) 23.2(5.80%) RAC18-I-30%-PS 30 9.7(11.3%) 21.9(3.7%) 27.1(4.1%) 34.5(4.3%) 44.0(1.5%) 23.3(3.48%) 26.6(0.27%) RAC18-I-50%-PS 50 7.7(3.9%) 22.1(1.4%) 26.2(5.7%) 34.3(4.1%) 41.5(3.3%) 22.4(3.48%) 24.6(1.72%) RAC18-I-100%-PS 100 7.2(11.1%) 19.4(4.1%) 23.3(3.9%) 30.2(5.0%) 38.9(2.1%) 21.2(6.67%) 24.0(3.73%) RAC40-I-30%-PS 30 5.5(7.3%) 17.4(5.2%) 23.0(3.9%) 30.8(1.0%) 37.6(1.5%) 24.1(1.76%) 26.7(5.04%) RAC40-I-50%-PS 50 5.2(1.9%) 18.1(7.7%) 23.3(3.0%) 30.1(2.3%) 33.8(1.2%) 21.8(1.95%) 23.8(6.64%) RAC40-I-100%-PS 100 2.9(6.9%) 11.8(6.8%) 19.5(4.6%) 25.9(4.6%) 31.3(0.8%) 19.9(4.97%) 21.2(4.75%)

NAC-I 0 12.8(7.0%) 26.3(1.9%) 28.8(2.4%) 39.2(2.8%) 45.9(2.9%) 26.4(0.53%) 30.7(2.76%)

RAC18-I-100%-PS 100 7.2(11.1%) 19.4(4.1%) 23.3(3.9%) 30.2(5.0%) 38.9(2.1%) 21.2(6.67%) 24.0(3.73%) RAC18-I-100%-EMV 100 5.6(7.1%) 16.0(4.4%) 19.1(6.8%) 25.3(3.3%) 37.3(2.3%) 20.2(13.6%) 21.5(2.95%)

NAC-II 0 — — 36.0(4.6%) 49.5(6.1%) — 33.0 —

RAC18-II-30%-PS 30 — — 35.3(2.9%) 44.5(4.6%) — 31.3 —

RAC18-II-50%-PS 50 — — 34.3(2.0%) 43.7(3.9%) — 28.7 —

RAC18-II-70%-PS 70 — — 30.4(5.6%) 40.5(1.9%) — 24.7 —

RAC18-II-100%-PS 100 — — 26.0(4.4%) 38.8(4.4%) — 24.3 —

RAC18-II-30%-EMV 30 — — 34.2(2.7%) 45.9(1.0%) — 32.8 —

RAC18-II-50%-EMV 50 — — 33.2(6.8%) 43.4(7.2%) — 33.2 —

RAC18-II-70%-EMV 70 — — 32.8(5.6%) 41.1(2.2%) — 31.4 —

RAC18-II-100%-EMV 100 — — 29.9(2.7%) 40.0(2.3%) — 30.0 —

a. COV is the coefficient of variation of three test results in each concrete mixture.

Figure 3-3 illustrates the decrease of the compressive strength of the RAC induced by the incorporation of recycled aggregates with different service time. It can be noted that for the recycled concrete with the same r ratio, the compressive strength decreased more significantly when older RCA was adopted. Taking RCA replacement ratio (r) of 100% for example (Figure 3-3 (c)), recycled concrete using RCA1-I presented a 28 d compressive strength of 38.9 MPa, which was only 1.0%

lower than the NAC-I measurement. Compared with RAC1-100%, concrete made with 100% of 18-year-old RCA had the 28 d compressive strength decreased by 23.0%, while RAC using 40-year-old RCA had the 28 d compressive strength decreased by 34.0%. This may be because the mechanical behaviours of morta r and ITZs in parent concrete deteriorate over time due to the combined effects of weathering (e.g. wetting-drying and freezing-thawing cycles) and erosion (e.g.

chloride ingress and carbonation), e.g. Maruyama et al. (2014). In this context, RCA with longer service time contains weaker residual mortar and ITZs, leading to lower compressive strength. Actually, in terms of the index of crushing that can be used to evaluate the compressive strength of coarse aggregate (e.g. Butler et al., 2013), the RCA1-I had the index of crushing of 12.8% while RCA18-I had slightly larger value of 13.4%, despite that the original virgin aggregate in RCA18 -I was Andesite aggregate (Table 3-4), whose compressive strength was commonly accepted to be slightly higher than the Limestone aggregate in RCA1-I and NCA-I. In addition of the above possible explanation, the less significant influence on the strength of recycled concrete with younger RCA may also lay in the uncompleted hydration reactions in the residual mortar which can create new chemical bonds between the new cement paste and the residual one. These new chemical bonds strengthened the ITZs (Laneyrie et al., 2016) and led to an increased compressive strength for recycled concrete. Based on these investigations, it was suggested that the fresh construction waste concrete with short service time and old demolition concrete crushed after their service life may be grouped separately in the RCA plant to prevent large scatter in the compressive strength of recycled concrete. This is important considering that recycled aggregate with different service time could present similar water absorption and density values (Table 3-4).

0

(a) RCA placement ratio of 30% (b) RCA placement ratio of 50%

0

(c) RCA placement ratio of 100%

Figure 3-3Influence of service time of parent concrete on the compressive strength of RAC

Figure 3-4 compares the compressive strength of recycled concrete made with pre-saturation (PS) method and equivalent mortar volume (EMV) method and using different particle size distributions of the RCA (i.e. RCA18-I and RCA18-II). As expected, all the recycled concrete prepared using PS and EMV methods presented lower compressive strengths in comparison with the companion NAC, regardless of the particle size distributions of the RCA. Taking recycled concrete with r ratio of 100% for example, the 28-day compressive strength of RAC prepared with PS method using RCA18-I and RCA18-II was 22.9% and 21.6% lower than the reference NAC measurements, respectively. Such reduction in the compressive strength of concrete was consistent with that reported in literature, e.g. Xiao et al., (2012). Compared to PS method, a reduction of 16.2% in the compressive strength

of EMV concrete was observed by using 100% of RCA18-I. This may be because the sand ratio (defined as sands weight over that of aggregates) in EMV concrete was not high enough to maintain the viscosity of concrete, and therefore weaker ITZs were obtained in the concrete. One of the proof is that by using AS -1 superplasticizer in this study, the slump values for RAC made with PS method was 160 mm, while that for RAC made with EMV method was only 40 mm despite that a little more superplasticizer was added in the concrete mixture. This reduction in the compressive strength can be compensated when using finer RCA. By means of using RCA18-II, slightly higher compressive strength was obtained for EMV concrete in comparison with that using PS method (Figure 3-4). When 100% of finer RCA (i.e. RCA18-II) was used instead of RCA18-I, the EMV concrete increased its compressive strength by 58.1%. In this context, a stricter limitation on the sand ratio or on the particle size distributions may be required when using EMV method to achieve desired strength of RAC. Figure 3-4 also highlights the necessity of using same particle size distribution for both natural concrete and recycled concrete when investigating how the incorporation of recycled aggregates affects the compressive strength of concrete; otherwise the influence of the recycled aggregates on the compressive strength of the concrete may be considerably overestimated or underestimated.

0% 20% 40% 60% 80% 100%

0 10 20 30 40 50

RAC18-I RAC18-I-EMV RAC18-II RAC18-II-EMV

Percentage of RCA

Compressive strength at 28d (MPa)

58.1%

Figure 3-4 Influence of mixing method and particle size distribution on compressive strength