4.2.2. Tensile Strength

Tensile strength showed behavior similar to compressive strength, although, according to a study carried out by Gonzalez-Corominas [111], if the curing process is steam, the tensile strength tends to improve. This behavior is closely linked to the addition of fly ash in partial replacement to Portland cement that results in a concrete with lower cement content, and consequently lower availability of calcium hydroxide in the concrete matrix (FILHO [112]), as shown in Figure 4.

**Figure 4.** Results of tensile strength of concrete with recycled aggregate of concrete (RCA), mixed recycled aggregate (RMA), and ceramic bricks (RA) from some researchers. Data from da Silva and Andrade [17], Kou and Poon [32], Shaikh [33] and Sunayana and Barai [34].

Regarding the tensile strength, the results of several research studies are convergent. Kou and Poon [32] evaluated such property by employing replacement levels of 50% and 100% of natural aggregate by the coarse recycled concrete aggregate, with the addition of fly ash at three replacement levels (25%, 35%, and 50%) to the Portland cement ASTM Type I. The sum of the principal oxides of the fly ash employed in this study (SiO2 + Al2O3 + Fe2O3) was 90.31%. The w/b ratio was 0.55, the slump was kept constant for all mixtures (120 mm), and the tensile strengths were estimated at ages of 28 days, 1, 3, 5, and 10 years. It was observed that the concretes produced with natural aggregate and fly ash in the proportions 0%, 25%, 35%, and 55% (R0F0, R0F25, R0F35, and R0F55) showed an increase in splitting tensile strength by 38.9%, 43.0%, 44.1%, and 39.8%, respectively, in the period between 28 days and 10 years. The concretes with 100% coarse recycled aggregate and fly ash in the same proportions showed an increase in strength of 57.8%, 62.5%, 67.2%, and 70.9%, respectively, in the period between 28 days and 10 years. After 10 years, the concrete with 100% coarse recycled aggregate and 25% fly ash (R100F20) presented the highest tensile strength. However, the concrete with 100% coarse recycled aggregate and 55% fly ash (R100F55) was the one that showed the highest resistance gain. According to the authors, this behavior is closely related to the incorporation of fly ash into the recycled aggregate, which improves the microstructure of the ITZ, which, in turn, increases the adhesion between the aggregates and the paste. Mehta and Monteiro [1] suggest that the concentration of calcium hydroxide crystals in the ITZ may be reduced by chemical reactions when a pozzolanic mixture or a reactive aggregate is present. The authors suggest that possible chemical interaction between calcium hydroxide and the calcareous aggregate is probably the reason for the increased tensile strength of concrete.

In another study, Kou et al. [113] produced concretes with different w/c ratios (0.45, 0.50, and 0.55), Portland cement ASTM Type I and fly ash whose sum of the principal oxides (SiO2 + Al2O3 + Fe2O3) is 90.31%. The absorption of the recycled aggregate varied between 4.26% and 8.69%. In this study, the authors produced reference concretes with 20, 50, and 100% recycled aggregate content and concretes with 25% addition of fly ash in partial replacement of Portland cement and 0%, 20%, 50%, and 100% of recycled aggregate in partial replacement of natural aggregate.

According to the authors, the tensile strength at 91 days of concrete with 100% recycled aggregate for w/b ratios 0.50, 0.45, and 0.40 were 12%, 10%, 9%, and 10% lower than that of the reference concrete, respectively. By using the addition of fly ash as a partial replacement for Portland cement, the strength of concretes with 100% recycled aggregate increment by 3%, 6%, and 8%, respectively, contrasted with cement without fly ash. Kou et al. [113] suggest that such an increase in strength in the concretes with fly ash can be credited to the densification of the concrete because of the possible reduction in porosity and to the pozzolanic reaction of the fine fly ash particles.

The result of the influence of fly ash in concretes with mixed recycled aggregate in the tensile strength in concretes was produced by da Silva and Andrade [17], between the period of 28 days and 91 days. The authors observed that the addition of fly ash in concretes with recycled aggregate would, in general, constrict the adverse effects that the recycled aggregate may cause in the tensile strength of concretes.

The authors observed that the negative effect in concretes with 25% of RCA and with an increasing amount of fly ash was smaller than in concrete with 20% of fly ash when the content of replacement of RCA increased. This behavior is due to the reduction of concrete porosity due to the filling of voids by fly ash particles. The addition of fly ash in concretes with recycled aggregate, by containing very fine materials, has a pore plugging effect in the recycled aggregates and makes a denser paste, and consequently reduces the harmful effects of the incorporation of recycled aggregate.

#### 4.2.3. Elastic Modulus

According to Neville [96], the elastic modulus of concrete depends on the elastic modulus of the aggregate and the volume proportion of the aggregate in concrete. Studies show that the elastic modulus decrease in concretes with fly ash and recycled aggregate in the early ages. Sunayana and Barai [34] and Kou and Poon [32] observed similar behavior of the elastic modulus of concrete produced with different fly ash contents and with 50% recycled aggregate, as shown in Figure 5.

However, the improvement in elastic modulus was observed in concretes with RCA in the older ages by some authors. Sunayana and Barai [34] used two groups of recycled concrete aggregate as coarse aggregate for the production of fly ash concretes. The first group of recycled concrete aggregate consists of a particle packing method (PPM), which consists of adopting a continuous particle size range between 4.75 mm and 20 mm because, according to the authors, the voids between larger particles are filled by the smaller particles to achieve the lower amount of voids in a concrete mix. The mixing method used for the production of concrete with recycled aggregate and fly ash was in the following steps:


**Figure 5.** Elastic Modulus of concrete with recycled aggregate of concrete (RCA). Data from Kou and Poon [32] and Sunayana and Barai [34].

Cylindrical samples were fitted with a compressometer to measure the displacement at each load increment, which was subsequently converted to strain. The load was applied for three load cycles up to 1/3 compressive strength of similar cylinders and converted to strain. The stress–strain relationship in the linear elastic region was used to find the modulus of elasticity (E) and to minimize the effect of compressive strength. The parameter (E/fc0.5) was found to be in the range of 4461–5507. Based on the authors' results, the reference concrete presented a compressive strength of approximately 43 MPa, while the concretes with 20 and 30% fly ash and RCA presented an average of 41 MPa and 42 MPa, respectively. The modulus of elasticity of the reference concrete was 36,000 MPa, while concretes with 20 and 30% fly ash and RCA had an average of 34,808 MPa and 31,340 MPa, respectively. The relationship between the modulus of elasticity and the compressive strength (E/fc0.5) for the reference concrete was approximately 5505 and for the concretes with 20 and 30% fly ash and RCA, it was around 5443 and 4764, respectively.

Based on the results, the authors observed a 5 to 10% reduction in elastic modulus in concretes with NAC compared to natural aggregate for 20% and 30% of fly ash replacement presented a decrease for RAC + FA20 compared to NAC for the same w/b ratio of 0.45. This behavior is due to the lower elastic properties of recycled aggregate due to the presence of adhered mortar compared to natural aggregates.

Kou and Poon [32] produced concretes with coarse recycled concrete aggregate in the replacement of 50% and 100% of coarse natural aggregate. The PC used in this research was equivalent to ASTM Type I. The sum of the principal oxides of the fly ash employed (SiO2 + Al2O3 + Fe2O3) was 90.31%, and the proportions of the addition of fly ash in replacement of PC were 25%, 35%, and 50%. The w/b ratio was 0.55, and the concrete was kept constant at 120 mm. The static elastic modulus was determined according to ASTM C 469 (2002) in specimens at 28 days, 1 year, 3 years, 5 years, and 10 years.

Based on the results, the authors observed that in the first year of curing, there was a gain in the elastic modulus for R100 R100F25, R100F35, and R100F5 mixtures of 7.2%, 7.9%, 11.2%, and 14.8%, respectively. However, at higher ages, the increase in percentages in the elastic modulus was much higher, corresponding to 31.3%, 33.9%, 40.7%, and 46.1%, respectively, between 28 days and 10 years. The authors also observed that, compared to the control mixture, the utilization of a considerable quantity of recycled aggregate in the concrete diminished the modulus increment following 10 years of curing.

According to Neville [114], the modulus of elasticity increases with concrete strength, but the growth in the concrete modulus of elasticity is smaller than the growth in compressive strength. The elastic modulus of concrete is closely linked to the elastic modulus of the binder matrix and aggregates. As recycled aggregate presents lower stiffness when compared to natural aggregate due to its composition and highly porous internal structure, it is expected that the higher the content of recycled aggregate substitution for natural aggregate, the lower the concrete elasticity modulus will be [42]. In the study by Kou and Poon [32], a good correlation was observed between compressive strength and modulus of elasticity for all concrete mixes with recycled concrete aggregate and fly ash. That is, as the compressive strength increases, the modulus of elasticity increases. For this analysis, the authors employed an equation based on ACI 318-08 to estimate the modulus of elasticity in terms of the compressive strength of natural concrete.

Thus, since the elastic modulus of concretes depends on the aggregate characteristics (considering the same properties for cement paste), it can be challenging to determine an adequate correlation between the elasticity modulus and concretes produced with recycled aggregate since the elasticity modulus of aggregates presents a significant variability, depending mainly on their type and origin.
