1. Introduction
Concrete is considered one of the most used materials in construction. This is owed to its versatility in application and properties. The endless demand for natural resources for concrete production creates concerns for the environment. The construction sector is responsible for the emission of CO
2 at a global level [
1]. Related activities include concrete mixing, cement manufacturing, and other chemicals that generate high atmospheric emissions of CO
2 [
2]. Thus, in the last decade, the research on using alternatives to replace conventional quarried materials is tremendously increasing [
3]. On the other hand, the production of a significant amount of construction wastes increases the environmental burden. The use of recycled waste construction materials in concrete fabrication is considered a key solution that could lessen the burden placed on the environment and the economy [
4]. Alternatives for quarried aggregates usually take the form of recycled materials (i.e., recycled concrete aggregate). These materials could alter the fresh and hardened concrete properties.
Concrete compressive strength is considered the key and main indicator of concrete quality which expresses the mechanical and durability performance of concrete. In the construction industry practice, the destructive technique is used to evaluate compressive strength using coring samples that have certain limitations in application. However, non-destructive testing methods are an effective solution for assessing compressive strength. The Schmidt hammer test, developed in the late 1940s, is considered a very popular non-destructive testing tool that has been used to assess the in situ strength of concrete [
5]. Once the plunger of the Schmidt hammer is pushed against the surface of concrete, the material hardness is recorded to estimate the compressive strength of concrete. Based on regression analysis, several empirical relationships have been established between rebound values (RVs) and compressive strength measurements to predict the strength. The condition of the concrete surface has a significant impact on the RVs. Consequently, the corresponding RV is dependent on the energy absorbed in the spring. Hack and Huisman [
6] showed that RVs are influenced by the tested material with a depth up to numerous inches. All factors that can affect the surface roughness and regularity must be considered during the assessment of strength with the Schmidt rebound hammer test. These factors include the type of formwork, consistency of the concrete mix, curing condition, and other factors. Earlier studies had revealed that the water-to-binder (w/b) ratio, type of aggregate, type of cement, moisture content, and carbonation are factors that can alter the surface hardness of concrete [
7]. For instance, Breysse [
8] states that a special calibration is needed when applying the Schmidt hammer test since the RVs differentiate with six points between careous and siliceous aggregates, with the latter being considered a harder type of aggregate.
The ultrasonic pulse velocity (UPV) test is another popular and widely used non-destructive evaluation technique used for evaluating concrete quality in situ through analyzing the homogeneity of concrete ingredients and assessing its integrity [
9]. The deterioration that occurs to the concrete structures due to the harsh environment such as freeze–thaw cycles can be detected by the UPV test. UPV measurements of concrete, just like its hardened properties, are affected by the mix design, namely, the characteristics and the type of aggregates, cement type, porosity and properties of cement paste, and the bond of aggregates and cement paste [
10].
It is worth mentioning that a limited number of experimental studies were performed to assess the compressive strength of recycled aggregate concrete (RAC) using non-destructive testing techniques. The physical characteristics of coarse aggregates affect the surface hardness of concrete considerably [
11]. NA and RA are characterized by different physical properties including water absorption and roughness. These factors could impact the response of RVs. Kazemi et al. [
12] carried out an experimental program to assess the strength of concrete specimens made utilizing RA. Their study showed that RVs of RAC were on average about 44.9% and 46.1% lower than that of the control mixes under wet and dry curing conditions, respectively. This was attributed to the porous interfacial transition zone between the new mortar and RA due to the RA’s characteristics. Thus, a poor interlock between the cement matrix and RA is achieved. Latif Al-Mufti and Fried [
13] examined the early age properties of RAC compared to natural aggregate concrete. The corresponding results revealed a positive effect of RA on the RVs. This was associated with higher water absorbency of RA resulting in a harder concrete surface, compared to natural aggregate concrete. Soares et al. [
14] reported that RAC mixes tend to have higher RVs than the control mixes. The superficial texture of RAC mixes and higher water absorption properties justified the cause for its harder surface. The use of RA and additives in concrete has shown their impact on UPV results [
15]. Kurda et al. [
16] carried out an indirect evaluation of the compressive strength of RAC with a high content of fly ash. Their experimental results indicated that UPV values worsened as the incorporation of RA and fly ash increases, owing that to the physical characteristics of RA and fly ash when compared to NA and cement, respectively. Khatib [
17] evaluated the effect of using RA on the UPV of concrete mixes at various ages (1, 7, 28, and 90 days). Their results revealed that UPV measurements decreased with the increasing rate of use of RA.
The unique features of SCC led to a noticeable rise in its use in the last decade. It requires high dosages of powder materials to ensure adequate rheological properties. The use of supplementary cementitious materials (SCMs) in concrete has been promoted due to the benefits of reducing the emission of CO
2 during cement fabrication. By-product materials, including fly ash, blast furnace slags, metakaolin, and silica fume are the most commonly used SCMs in the fabrication of SCC, yet these materials are not cost-effective considering their environmental impact [
18]. For that reason, it is a necessity to test other types of absolute waste SCMs that could be used in the casting of SCC. The simultaneous use of RA and waste SCMs in SCC may contribute to a cleaner production of concrete, resulting in an eco-efficient SCC [
19].
The widespread application of SCC on-site and in pre-casting demands a better understanding of its behavior when its mechanical properties are assessed using non-destructive testing methods. The present study investigates the performance of potentially eco-efficient self-compacting high-performance concrete (SCHPC) in a long-term period (up to two years). There is a lack in the literature of the studies which evaluate the performance of concrete incorporating RA and/or waste SCMs at ages up to a two-year period. However, waste materials could impact concrete properties at longer ages due to their proceeding hydration. Moreover, considering the aforementioned variables, the non-destructive evaluation of eco-efficient SCHPC is still not investigated, especially after such exposure environments as freeze–thaw cycles.
5. Conclusions
This research paper was a part of a comprehensive experimental work undertaken to evaluate the use of RA and absolute waste SCMs (WPP, WFA, and WCC) on the long-term mechanical and durability performance of potentially eco-efficient SCC (up to two years). The main objective of this work was to allow a better understanding of the non-destructive methods (RVs and UPV) of expectably eco-efficient SCHPC.
SCHPC containing RA exhibited higher compressive strength than equivalent mixes prepared with NA, as with the increase of age, mixes containing RA continue to gain strength due to the hydration of the attached mortar on the RA itself. Mixes prepared with WFA and WPP govern the compressive strength performance of SCMs, as compared to mixes prepared with WCC. This was attributed to the pozzolanic reactions promoted by these materials. In contrast, the WCC is characterized by its large particle size that causes internal micro-cracking in the concrete, supporting loss in strength with time.
Given the fact of the good quality of RA and similarity in density with NA, mixes prepared with RA showed slight variation in RVs and UPV results. Based on the obtained results, relationships between compressive strength and RVs or UPV have been proposed to assess the compressive strength at different ages and after exposure to freeze–thaw cycles. The proposed relationships showed that BS EN 13795 [
40] is conservative for predicting SCHPC compressive strength. Non-destructive evaluation methods provide an acceptable prediction for compressive strength of SCHPC at different ages and in different environments.