The workability of green star concretes in terms of measured slump values are shown in Figure 3
and is bench marked with respect to control concrete. It can be seen that the slump value of two green star concrete is higher than that of control concrete and can be attributed to the use of 40% slag as partial replacement of OPC. The relatively lower specific gravity of slag than that of cement increases the paste volume in concrete containing 40% slag, which cause the improvement in workability. The slump of mix 2 is decreased from 175 mm to 160 mm when 40% RCA is used as partial replacement of NCA in mix 3. This reduction can be attributed to the presence of 40% RCA whose water absorption is higher than NCA and contain more 10 mm size aggregates than the NCA. The workability is slightly affected in three green star concrete (mix 4) due to addition of 50% reclaimed wash water as partial replacement of tap water. High turbidity and pH of wash water are attributed to this slight reduction in workability. Nevertheless the workability of all green star concretes is better than the control concrete.
The measured compressive strength of all concretes at 7 and 28 days are shown in Figure 4
. It can be seen that the compressive strength of green star concretes is lower than the control concrete at both ages especially at 7 days. The partial replacement of OPC by 40% slag in two green star concrete exhibited about 31% and 12% reduction, respectively at 7 and 28 days compared to control concrete. The slow pozzalonic reaction of slag is the reason for higher compressive strength loss at 7 days in this concrete than at 28 days. However, when 40% RCA is used as partial replacement of NCA in mix 3 a slight reduction in both 7 and 28 days compressive strength is observed compared to mix 2. A slight reduction in both 7 and 28 days compressive strength is also observed in three green star concrete (Mix 4) when 50% wash water is used as partial replacement of tap water compared to Mix 3. The presence of weak interfacial transition zone between old mortar of RCA with new matrix and the higher turbidity of wash water could be the reason of the observed slight reduction in compressive strength in mixes 3 and 4 than mix 2. Nevertheless the three green star concrete containing 40% less OPC, 40% less NCA and 50% less fresh water exhibited a 28 days compressive strength of 38 MPa, which is 18% lower than the control concrete but still adequate enough for structural application. Similar reduction trend in indirect tensile strength and elastic modulus is also observed in all green star concretes compared to control concrete. It can be seen in Figure 5
and Figure 6
that the reduction of indirect tensile strength of all green star concretes is slightly lower than that of compressive strength at both ages, however, in the case of elastic modulus the reduction was higher than the compressive strength. It is also interesting to observe that indirect tensile strength and elastic modulus of three green star concrete at both ages are similar and slightly higher, respectively than those of mix 3 concrete.
The measured durability properties of green star concretes are shown in Figure 7
, Figure 8
, Figure 9
and Figure 10
. Figure 7
shows the water absorption of green star concretes. No changes in water absorption of two green star concrete compared to control concrete after 7 days of wet curing can be seen, however, about 15% reduction is observed after 28 days curing. This can be attributed to the pozzolaanic reaction of SiO2
of slag with Ca(OH)2
of hydration reaction which densified the matrix through formation of additional hydration products and pore filling by the slag particles. The three green star concrete and mix 3, as expected, showed much higher water absorption than control and two green star concretes due to the presence of 40% RCA, whose water absorption capacity is much higher than NCA as shown in Table 3
. The effect of wash water however is not affected the water absorption of three green star concrete than of mix 3 concrete. Unlike water absorption, the rate of water absorption of all green star concretes is lower than that of control concrete at both ages. Among green star concretes, the two green star concrete containing 40% slag as partial replacement of OPC exhibited the lowest rate of water absorption at both ages and is believed to be due to the pore refinement of its matrix due to additional hydration product formed through pozzolanic reaction and pore filling. After 28 days of curing this rate further reduced. A summary of water sorptivity, which is the slope of the rate of water absorption lines in Figure 8
, is shown in Figure 9
. It can be seen that the sorptivity of mix 3 and mix 4 concretes is very similar, where no adverse effect of wash water on sorptivity is observed in three green star concrete. Similar result is also observed in chloride ion penetration in green star concretes in RCPT test. It can be seen in Figure 10
that the chloride permeability in two green star concrete is significantly reduced by 62% after 28 days of wet curing. This can be contributed by the pore refinement of matrix in two green star concrete due to pozzolanic reaction of slag and pore filling. However, after 7 days of curing the reduction in chloride permeability is much lower than that at 28 days due to slow pozzolanic reaction of slag. The addition of 40% RCA in mix 3 concrete, however, increased the chloride permeability possibly due to higher porosity of RCA than NCA and the presence of more interfacial transition zone with matrix and RCA than with matrix and NCA. Interestingly, the three green star concrete shows lower chloride permeability than the other two green concrete mixes, which is not observed in water absorption and sorptivity tests, where a slight increase in water absorption and sorptivity is observed in three green star concrete than mix 3. While all three measured durability properties are affected by the pores in the concrete, the water absorption and sorptivity are mostly depend on overall porosity of the concrete as the specimens in those tests are kept in water or in contact with water for long time. However, in RCPT test Cl−
is forced to pass from one side of the concrete specimen to other side. Therefore, the observed lower chloride ion penetration in three green star concrete than other two green star concretes indicate that the pores in that concrete are not interconnected rather disperse.