**1. Introduction**

With the development of the construction industry has come a myriad of construction waste and great damage to the environment [1]. In China, 2.36 billion tons of construction waste was generated annually in the most recent decade [2]. Consequently, it is urgent to use recycled construction and demolition materials. The use of recycled aggregate (RA) in concrete opens a whole new range of possibilities for reusing materials in construction. Reuse of waste concrete as RA in new concrete is beneficial from the viewpoint of environmental protection and preservation of resources, as it reduces the use of natural materials used in concrete production [3]. It presents a more environmentally friendly alternative destination for this waste [4]. Using recycled aggregate could save about 60% of limestone resources and reduce CO2 emissions by about 15–20% [5]. However, the strength of RAC decreases with the increase of the replacement percentage of recycled aggregate due to the presence of micro-cracks and old cement paste that has adhered to

**Citation:** Tu, K.; Wu, J.; Wang, Y.; Deng, H.; Zhang, R. Uniaxial Compressive Stress–Strain Relation of Recycled Coarse Aggregate Concrete with Different Carbonation Depths. *Materials* **2022**, *15*, 5429. https://doi.org/10.3390/ma15155429

Academic Editors: Daniel Ferrández Vega, Jean-Marc Tulliani and Carlos Morón Fernández

Received: 6 April 2022 Accepted: 3 August 2022 Published: 7 August 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

the original aggregates [6–8]. Azevedo pointed out that the substitution of more than 25% of construction and demolition waste (CDW) for sand requires relatively larger amounts of water in the mortar, and the water absorption of the mortar increased with the levels of incorporated CDW [9]. Because of the high variability in the characteristics of recycled aggregates, in order to obtain better properties, a deeper classification of the constituents should be carried out [10]. The premix process can fill up some pores and cracks, resulting in a denser concrete, an improved interfacial zone around recycled aggregate, and thus a higher strength when compared with the traditional mixing approach [11].

In addition, many researchers have argued that the carbonation resistance of RAC was worse than that of natural aggregate concrete (NAC) [12–14], which may lead to the corrosion of reinforcement, causing safety hazards and great economic losses [15]. There are many factors affecting the carbonation of RAC, such as the cement matrix, aggregate, mixing method, external load, and external environment, etc. [14]. Consequently, it is necessary to strengthen recycled coarse aggregate (RCA) or recycled aggregate concrete (RAC) itself in order to obtain RAC with better properties.

Some researchers found that the properties of RAC could be improved by adding pozzolan, silica fume, and rubber particles and fibers into RAC [16–20]. Other researchers found that the properties of RAC can be improved by carbonation which includes carbon conditioning and carbon curing. Carbon conditioning is the injection of CO2 into recycled aggregate, accomplished with the assistance of a sealable carbonation chamber [21]. Zhang et al. [22] and Zhan et al. [23] found carbonation can improve RCA in physical and mechanical properties. The apparent density of RCA was significantly increased, whereas the water absorption and crushing value of RCA were significantly decreased after carbonation. Carbon curing relies upon carbonation reaction between CO2 and cement paste, and it carbonates entire concrete blocks after concrete mixing [18]. Zhan et al. [24] found that carbon curing improved rapidly the compressive strength of RAC with strength gains ranging from 108% to 151% higher than conventional moisture curing. Zhan et al. [25] and Xuan et al. [26] found that carbon-curing conditions will lead to improvements of RAC strength.

The widespread adoption of RAC requires not only a better understanding of its mechanical properties and durability but also the availability of guidelines on designing reliable RAC structures [27]. In order to guide the design of RAC structures, the stress–strain relationship of RAC has been preliminarily investigated [28–31]. In addition, Luo et al. [32] investigated stress–strain curves of fully carbonated RAC and found that carbonated RAC improved the compressive strength and elastic modulus of RAC. However, less information is available on the effect of carbonation depth on the stress–strain relation of RAC.

The objective of this study is to evaluate the peak stress, elastic modulus, strain, relative toughness and establish the stress–strain model of RAC with different carbonation depths according to experimental results.

#### **2. Experimental Method**

#### *2.1. Materials and Mixture Proportions*

Ordinary Portland cement of grade 42.5 was used in this study. The cement properties are shown in Table 1, and river sand with particle size less than 4.75 mm was used as fine aggregates. NCA was from gravel, and RCA was produced from Nanjing Shoujia Renewable Resources Utilization Company (Nanjing, China). After the concrete residues were transported to the company, a jaw crusher was employed in order to reduce the size of the large pieces of concrete residues. After being crushed, the residues were cleaned and graded ready for the planned tests. The crushing values of NCA and RCA were measured according to Pebbie and crushed stone for building (GB/T14685-2011) in China. The size grading of RCA and NCA were similar to ranging from 5 mm to 20 mm, and satisfied the standard for technical requirements and test method of sand and crushed stone (or gravel) for ordinary concrete (JGJ52-2006) in China. Figure 1 shows the sieving results of aggregates, and the properties of NCA and RCA are shown in Table 2.


**Table 1.** Properties of cement.

**Figure 1.** Grading curve of aggregates.

**Table 2.** Properties of the NCA and RCA.


Due to the high water absorption of RCA, pre-soaking was used to make RCA reach the saturated surface dry (SSD) condition. The mix design proportions of RAC and NAC are listed in Table 3. Note that the replacement ratio of specimens is calculated by volume. The water–cement ratio and sand ratio were adjusted to obtain similar 28-day compressive strengths of NAC and RAC based on the technical code on the application of recycled concrete (DG/TJ08-2018) in China.

**Table 3.** Mix design proportions of NAC and RAC.


The specimens with 70-mm diameter and 140-mm height cylinders were cast in a PVC mould. After curing for 28 days, the compressive strengths of RAC cubes (100 mm × 100 mm × 100 mm) with replacement ratios of 0%, 50% and 100% were 37.5 MPa, 36.4 MPa and 31.1 MPa, respectively.

#### *2.2. Accelerated Carbonation Procedure*

The acceleration of concrete carbonation was performed in a carbonation experiment chamber. The temperature was 20 ◦C, the humidity was 60% and the CO2 concentration was 20%. All specimens were wax-sealed at both ends to conduct side-only carbonation. To achieve different carbonation depths, carbonation periods were set to 0, 14, 28, 42, 56, 70, and 84 days.

The carbonation depth is a quantity index that characterizes the degree of carbonation. A 1/3-length cylinder was split when measuring carbonation depth, and the cross-section of the remaining piece was sealed with wax for further carbonation and measurement. For the piece after splitting, drop 1% phenolphthalein alcohol solution (alcohol solution contains 20% distilled water) on the cross-section. Then a measurement point was marked every 45◦ on the circular section and the depth of carbonation was measured at each measurement point. The average value of carbonation depth was taken as the carbonation depth of the specimens,

$$\overline{d\_t} = \frac{1}{n} \sum\_{1}^{n} d\_i \tag{1}$$

where *dt* is the average carbonation depth (mm) after carbonation *t* (*d*) of the test, *di* is the carbonation depth of measuring point (mm), and *n* is the total number of measuring points.
