Aldosteron

When concrete is subjected to a compressive load, the modulus of elasticity (EC) will describe the concrete’s instantaneous deformation, based on the stress-strain relationship and its secant slope (Dhir Obe et al., 2019). Because this property is used in structural members of buildings to estimate its deflection and buckling, it is highly important. When there is a replacement level of 100% of coarse NA with RCA, the EC can show a reduction of 6% to 40% and an average value of 30% (K. W. Anderson, Uhlmeyer, & Russell, 2009; Dhir Obe et al., 2019; K. Sagoe-Crentsil & Brown, 1998).

For concrete in compression the relationship between stress and strain is not truly linear, but for the low levels of strain and stress, it is considered to be. This relationship is dependent of the concrete’s stiffness, constituents and testing methods. Several studies have been done to compare the concrete with coarse NA and that with coarse RCA. The comparison between the two is shown in the following picture and can be divided in three regions:

Figure 10: The stress-strain relationship for concrete with coarse NA (red) and coarse RA (green) (G. M. Chen, He, Yang, Chen, & Guo, 2014; Folino & Xargay, 2014; 陈宝璠 & Chen, 2013)

• Region 1: the stress-strain relationship is more or less linear for up to 30% of its eventual strength, for the concrete with coarse NA, as for the concrete with coarse RCA. The stress of the NA concrete will be higher at a given strain compared to that of the concrete with RCA.

• Region 2: the relationship between stress and strain is more parabolic. It can be noted that the RAC with coarse RCA has a higher peak strain (= strain corresponding to the peak stress) in comparison to that of concrete with NA.

• Region 3: the curves of both the RAC and NAC will soften after the peak, this means that they will descend after the peak. The rate of this descending is normally faster for the RCA concrete than for the NA concrete. Unfortunately, it can also be deduced from the figure that the ultimate strain of the concrete with coarse RCA is less than that of the concrete with coarse NA. The strain where the crushing failure of the concrete occurs, is called the ultimate strain of the concrete.

Regardless that the test to determine the EC is simple, to study the coarser RCA’s behavior, the variables that are used for doing this, often vary in terms of type of test method, the type of specimen used, the curing conditions and the properties of the aggregates (Dhir Obe et al., 2019;

Lye, Dhir, & Ghataora, 2016). Because the attached cement paste on the coarse recycled concrete aggregates is weak and porous, the EC of the RAC was theoretically higher than that of the corresponding concrete with coarse NA for 4.5% of all the data. If all other properties were kept equal, the EC could not be increased by the usage of RCA. A study by (Dhir Obe et al., 2019) showed a decreasing rate in the decrease of the EC of the concrete as the content of

coarse RCA increased. If the concrete’s compressive strength increases, the drop in the relative value of EC will decrease at a given content for coarse RCA. As the strength of the concrete increases, the volume of coarse aggregates will decrease. This causes a reduction in the overall effect on the concrete’s properties. At a higher strength, a relatively smaller decrease in the concrete’s EC will occur due to the relatively smaller impact on the attached cement paste where coarse RCA is used. High-quality coarse aggregate is required to make concrete with high strength. It is expected that the quality of the coarse RCAs will be better to produce concrete with high strength. As a result, the decrease in the EC will probably be smaller due to the use of RCA.

To estimate the concrete’s EC, the rock types of the used aggregates will be taken in by the Eurocode 2 who has limited them to basalt, quartzite, limestone and sandstone. For concrete that is made with basalt, quartzite, limestone and sandstone, the different EC values are shown in the figure 11.

The line for concrete with NA where the strength is less than 60 MPa, can be found between sandstone and limestone, but for a strength more than 60 MPa, the line can be found between limestone and quartzite. If at a given strength, the concrete’s EC reduces while the content of coarse RCA increases, the trend lines will move from between limestone-concrete and quartzite-concrete progressively to the area between sandstone-concrete and limestone-concrete.

Another property is the dynamic elastic modulus ED to measure the concrete’s damage evaluation after an exposure to weathering (e.g., freeze-thaw). If coarse NA is replaced with coarse RCA, the concrete’s ED will result in a reduction.

The ratio under axial load of the axial strain to the corresponding transverse strain within the elastic range is called the Poisson ratio. This ratio will marginally increase when the coarse NA is replaced with coarse RCA in concrete in most cases (Ajdukiewicz & Kliszczewicz, 2002, 2007; Dhir Obe et al., 2019; S. Huda & Alam, 2015) et al.

Figure 11: Ec in relation to the concrete with coarse NA's / coarse RCA's compressive strength (Dhir Obe et al., 2019)