Assessing the Impact of Recycled Concrete Aggregates on the Fresh and Hardened Properties of Self-Consolidating Concrete for Structural Precast Applications

Abstract

This study explores the influence of different concentrations of recycled concrete aggregate (RCA) on the fresh and hardened properties of self-consolidating concrete (SCC) in order to assess the structural suitability of the use of RCA in a precast concrete plant. The study particularly emphasizes the early strength of the produced concrete. The RCA was sourced from crushed concrete used in roadway applications and was sieved to replicate the characteristics of natural aggregate. Five different SCC mixes were produced, with RCA substituting 0%, 10%, 30%, 50%, and 70% of the natural coarse aggregate (NCA) by weight. For each different mix design, the hardened properties tested were the compressive strength and tensile strength. The fresh properties investigated were the passing and filling ability. Additionally, aggregate properties including grain size distribution and absorption of coarse aggregate were studied. The selected mix design follows a typical well-graded self-consolidating concrete mix with 28-day strength of 8000 psi (55.16 MPa). It was found that replacing up to 50% of the NCA with RCA improves the early strength of concrete without a significant impact on the fresh and hardened concrete properties.

1. Introduction

Due to its exceptional flowability and high-performance characteristics, self-consolidating concrete (SCC) is generally the preferred choice in modern construction practices. SCC has been widely adopted in the precast concrete industry due to its superior properties compared to conventional concrete. Although, in precast applications, SCC offers less labor cost and requires less time overall, there is an increase in cost due to the high content of cement, admixtures, and quality control procedure. The high use of cement also creates an environmental impact due to the higher CO₂ emissions and extraction of raw materials. In the process of renewing and replacing our aging and deteriorating infrastructure, reinforced concrete structures end up being demolished. This creates a vast supply of crushed concrete, which is mainly used as a subbase for roadways [1]. The crushed concrete may also be processed into recycled concrete aggregate (RCA). Utilizing RCA in SCC production holds immense promise for sustainable construction practices which could help the concrete industry reduce CO₂ emissions. Since aggregate sources and landfill have become limited, the use of RCA is a great environmental and sustainability alternative. Several studies are available in the literature on concrete mixes containing RCA, including SCC mixes [2,3,4,5,6,7,8,9,10,11]. These investigations indicate that concrete with as much as 40% RCA replacement ratio can be produced without impairing the concrete strength, or workability. However, SCC used for precast applications needs to meet special requirements over conventional concrete that need to be studied. Precast concrete manufacturers cast new concrete every day, and they remove formworks as early as 16 h after casting. Additionally, most precast concrete members are pretensioned; therefore, they require very high compressive strength at the time of formwork removal.

While the incorporation of Recycled Concrete Aggregates (RCA) has been demonstrated to enhance the mechanical properties of concrete, including increased strength and stiffness [12,13], it is worth noting that higher replacement ratios can lead to a reduction in strength and stiffness [14,15]. Additionally, the utilization of a significant amount of RCA has been shown to adversely affect the workability of fresh concrete due to its increased porosity and higher absorption [16,17,18]. Moreover, long-term durability concerns have been raised regarding the use of RCA in concrete [19,20,21].

The great majority of research focused on RCA involved laboratory-produced RCA, from crushing of concrete members of known origin and mechanical properties [22,23,24]. Other researchers have studied the effect of the parent concrete on subsequent mixes containing RCA [25,26,27]. However, crushed concrete as found in the field has different origins that oftentimes cannot be tracked. The novelty of the previous studies is in sample preparation, testing methods, and how much concrete strength can be achieved after incorporating recycled materials. However, a significant gap exists in studying the influence of RCA derived from diverse sources. In this research study, our primary objective was to investigate the impact of field-sourced recycled concrete aggregates, commonly used for road subbases, on both the fresh and hardened properties of self-consolidating concrete. Our comprehensive analysis covers the assessment of various aspects, including the properties of the aggregates themselves, the workability of the fresh concrete, and the compressive and tensile strengths of concrete mixes at different replacement ratios. Particular emphasis has been placed on evaluating the early-age strength.

2. Experimental Program

An experimental program was undertaken to assess the effects of recycled concrete aggregate on the Fresh and Hardened Properties of Self-Consolidating Concrete for Precast Concrete Applications. First, the properties of both RCA and NCA aggregates were determined. Then, the fresh and hardened properties of concrete with various replacement ratios of recycled concrete aggregate were tested. Figure 1 shows a summary of the experimental program.

3. Selection of Recycled Aggregates

Most of the existing research on RCA predominantly relies on laboratory-produced RCA, which involves creating concrete specimens solely for the purpose of crushing them to produce RCA. However, this approach does not accurately represent the current availability and use of RCA in practical applications. Our study takes a unique approach by focusing on the RCA that is commonly available. We specifically investigate crushed concrete derived from diverse sources, often found in landfills, and typically employed for road base applications. In essence, our research objectively assesses the utilization of RCA derived from multiple origins, as it would be encountered in real-world scenarios, reflecting a more representative and practical approach to the integration of recycled concrete aggregate into common building materials.

Recycled concrete aggregate exhibits several notable characteristics that distinguish it from natural aggregate, as summarized in

Table 1. Coarse Aggregate Properties.

	Natural #57	RCA (#57)
Moisture Content (%)	2.7	5.3
Water absorption (%)	3.9	9.4

. The most prominent distinction is the RCA’s significantly higher absorption rate, measuring 9.4% as opposed to 3.9% absorption rate observed in crushed limestone aggregate (natural #57 stone). This heightened absorption is a consequence of RCA’s notably higher porosity when compared to its natural counterpart. What sets RCA apart even further is its composite nature. RCA contains not only the rock component but also remnants of concrete paste from the original source. This accentuates RCA’s angularity and surface roughness. The precise composition of RCA can vary depending on the makeup of the parent concrete, making it a dynamic material. In instances where the parent concrete was fiber-reinforced, RCA may also contain residual fibers. A visual comparison of natural aggregate and RCA is shown in Figure 2 and Figure 3.

4. Gradation Curves

One of the primary challenges associated with the utilization of landfill RCA lies in its gradation curve. Crushed concrete sourced from landfills, commonly used for road base applications, exhibits a gradation profile closely resembling that of a #610 aggregate, characterized by a substantial content of fine particles, typically less than 4.75 mm. #610 aggregate has a size ranging from 1.5 inches (38 mm) to powder, rendering it unsuitable for concrete applications. For this study, natural #57 and #89 aggregates were used. Self-consolidating concrete mixes require well-graded aggregate in order to avoid any segregation or bleeding issues. To address this challenge, it was crucial to align the gradation of the recycled concrete aggregate with that of the natural aggregate. The gradation curves for both natural coarse aggregate and RCA are shown in Figure 4.

5. Mix Design

All concrete mixes were prepared with both sustainability and performance in mind. The overarching objective was to formulate Self-Consolidating Concrete (SCC) that met the stringent criteria necessary for precast concrete applications. These criteria encompass achieving a high early strength, substantial strength at 28 days, and excellent self-compacting properties. The mixes had a water-to-cement ratio (W/C) of 0.35. Amount of water in

Table 2. Mix Design Proportions (Note: 1 lb./yd³ = 0.59 kg/m³).

Mix ID	Cementitious Materials (lb./yd3)		Water (Gal/yd3)	Coarse Aggregates (lb./yd3)		Fine Aggregates (lb./yd3)
	PC	FA	W/C: 0.35	NA #57	RCA #57	NA #89	RCA #89	Sand
0% Replacement	688	172	36	943	0	629	0	1393
10% Replacement	688	172	36	848	94	556	73	1393
30% Replacement	688	172	36	660	283	440	189	1393
50% Replacement	688	172	36	471	471	314	314	1393
70% Replacement	688	172	36	283	660	189	440	1393

refers to the amount of free water. It should be noted that the aggregates were in a saturated surface-dry (SSD) condition. The cementitious materials used were composed of 80% ASTM C150 Portland cement and 20% of ASTM C618 Fly Ash class C. #57, #89, and sand were used as main aggregates. Different SCC mixes were produced with RCA substituting 0%, 10%, 30%, 50%, and 70% NCA by weight (Table 2). Water reducer and set retarder admixtures were used to enhance the fresh properties of the concrete mixes. A set accelerator admixture was used to ensure the concrete would have high strength 24 h after mixing.

6. Fresh Properties Tests

In this study, a comprehensive analysis of the workability of the fresh concrete was undertaken. Both the filling ability and passing ability were examined across all concrete mixes, encompassing varying RCA replacement ratios ranging from 0% to 70%. The filling ability was tested using the slump flow (Figure 5) according to ASTM C 1611/C1611M [28]. This standardized test measures the ability of fresh self-consolidating concrete to flow under the influence of its own weight and fill every corner of a formwork without requiring manual consolidation. The passing ability of concrete was measured using the J-Ring test, according to ASTM C 1621/C 1621M [29]. This test is used to measure the ability of fresh concrete to pass through confined conditions, such as congested reinforcement (Figure 6). The results of these tests are expressed as the average diameter of the spread of the concrete. An important aspect of these tests is the comparison between the diameter measurements obtained from the slump flow and J-Ring tests. A smaller difference in diameter between these two measurements indicates a superior passing ability of the concrete.

Furthermore, the segregation resistance of the concrete mixes was tested. To measure the segregation resistance, the static segregation using the column technique was measured for the mix designs containing 0% replacement and 50% replacement. This test was performed according to ASTM C1610 [30]. Additionally, all the concrete mixes were subjected to visual inspection of segregation when the cylinders were split on the tensile splitting test, to determine if significant segregation had taken place.

7. Hardened Properties Tests

In this study, both compressive and tensile strength were tested for all the mix designs with a focus on early strength development. For instance, the compressive and tensile strength of the concrete mixes were measured at 1, 3, 7, and 28 days of curing (Figure 7). Standard cylindrical specimens of 4 inches in diameter and 8 inches in height were cast and then cured in immersed conditions with saturated limestone water 73 degrees Fahrenheit. To evaluate the compressive strength, the cylinders were tested in compression until failure per ASTM C39 [31] (Figure 8). Three cylinders were used for each test and the average strength was recorded. To measure the tensile strength, the split cylinder test was performed according to ASTM C496-96 [32] using a universal testing machine. Two specimens were tested per mix and the average strength was recorded. Overall, over 100 cylindrical specimens were used in this study.

8. Results and Discussion

8.1. Fresh Properties Results

Throughout this study, the workability of the fresh concrete was evaluated using both the slump flow and J-Ring tests. Notably, all concrete mixes demonstrated slump flow spreads well within the limits established by ACI 237 R-07. The highest recorded slump flow was 28 inches (711 mm) for mixes with 0%, 10%, and 30% RCA replacement, followed by the 50% RCA replacement mix with 27 in. slump flow, and the 70% RCA replacement mix with 25 in, (641 mm), as shown in Figure 9. Although all of them are within the limit, the decrease in workability with the increase in RCA content can be owing to the increased rate of absorption of the aggregate due to the increase in RCA content. The J-Ring test reduced the spread of concrete by less than 2 in. (50 mm) for all mix variations. This outcome indicates that all mixes exhibited satisfactory passing ability, and that the quantity of RCA employed did not significantly affect the passing ability of the concrete. Furthermore, as the replacement ratio of RCA increased, a notable trend emerged: the diameter of the concrete spread decreased. This can be attributed to the high absorption capacity and surface roughness of RCA. Importantly, none of the mixes experienced segregation or severe bleeding. As the replacement ratio increased, the visual stability index improved. It is worth noting that with increasing replacement ratios, the visual stability index improved. Remarkably, mixes with 50% and 70% RCA replacement experienced the least visual segregation or bleeding. However, it is noteworthy that the mix with 70% replacement partially lost its self-compacting properties. This is owing to the large amount of RCA which may have increased the amount of aggregate absorption.

Additionally, the segregation resistance was determined for some of the mix designs. The static segregation of the 0% and 50% replacement ratio mixes was measured using the column technique. The mix with 0% replacement experienced 19% segregation, whereas the mix with 50% replacement had 16% segregation of aggregate retained by the sieve #4. As the replacement ratio increased, the amount of paste decreased, making the concrete more viscous and less prone to segregation. Figure 10 shows the cross sections of split cylinders with different replacement ratios. There is no significant segregation observed in any of the cylinders.

8.2. Early Strength Results

One of the goals of this study is to determine how suitable self-consolidating concrete with RCA replacement is for precast concrete applications. Therefore, early strength is crucial. Both compressive and tensile strength were measured with all the replacement ratios 24 h after casting (Figure 11). The compressive strength 24 h after casting dramatically increased when higher replacement ratios were used. An increase of up to 73% was found in the compressive strength on day 1 when 30% of natural aggregate was replaced. Once the replacement ratio surpassed 30%, the compressive strength slightly decreased and plateaued to around 3050 psi (21 MPa) compared to 1955 psi (13.5 MPa) of the mix with 0% replacement. From the literature [15], It should be noted that the variation in concrete compressive strength is affected by maturity age and it was suggested in the literature that the concrete compressive strength increases with the increase in the size of aggregates.

Tensile strength on day 1 was also increased with the utilization of RCA following the same pattern of compressive strength. An increase of 57% was observed when there was 30% replacement compared to the benchmark mix with 0% replacement (Figure 12).

The increase in early strength could be due to early hydration, aggregate interlocking, and high-water absorption of RCA. In fact, recycled concrete aggregate is more porous than natural aggregate allowing for extra moisture absorption, and the rough edges of the aggregate increase the friction between aggregates and the bonding of aggregate and paste. In general, higher early strength for concrete with RCA may be owing to a number of reasons. The RCA naturally has a higher amount of cement, especially since these recycled aggregates are sourced from the precast industry. In general, targeting a good quality RCA can result in structural concrete with competitive characteristics when compared to a regular concrete.

8.3. Strength Results

The compressive and tensile strength were also studied at later stages. Figure 13 and Figure 14 show the compressive and tensile strength at 3, 7, and 28 days. Although there is some variation at days 3 and 7, the compressive strength was almost equal at 28 days when using up to 50% RCA content. Once the replacement ratio was increased past 50%, a significant decrease in the compressive strength was observed. It should be mentioned that, as discussed before, although all samples achieved acceptable workability, the workability decreases as the RCA content increases. The compressive strength on day 28 decreased by 23% when using 70% replacement compared to the rest of the samples. This can be explained due to the high content of RCA as well as its effect on workability, and the nature of high strength concrete. RCA provides a larger surface area than regular aggregates and has rough surfaces, resulting in less workability due to the larger water demand. This in turn affects the concrete compressive strength as the percent of replacement increases. RCA is generally weaker than natural aggregate, and aggregate strength is essential to achieve high strength concrete as cracks propagate through the aggregate rather than only through the paste. However, the strength reached by all the concrete mixes was still higher than the average strength specified for precast members (5000 psi [34.47 MPa]). The aggregate size plays a factor since it affects the adherence of the aggregate with the mortar. Thus, the concrete tensile strength decreases beyond the 30% replacement ratio.

8.4. Strength Development

One of the most important properties of concrete used in the precast concrete industry is early strength. Figure 14 shows the effects of RCA on the early strength development of concrete. In Figure 15, the strength was normalized with the specimen’s 28-day strength being 100% of its compressive strength. It was found that the concrete mixes with high RCA replacement ratios gain its initial strength much faster than the mixes with low replacement ratios. In the first 24 h, the mix with 70% replacement developed 50% of its 28-day strength, and the mix with 0% replacement developed 24% of its 28-day strength. All the mixes developed most of their strength in the first 7 days, ranging from 86% to 99%.

9. Conclusions

This experimental research was performed to determine the effects of recycled concrete aggregate on the properties of high strength self-consolidating concrete for precast concrete applications. This research focuses on the precast industry, since recycled concrete as well as mix design are relevant to what is used in the precast concrete plant. This is expected to have a great economic impact since it will result in a reduction in use of regular coarse aggregates since a percent of the aggregates can be replaced with RCA. Also, less environmental impact can be achieved since a concrete with a high content of a recycled concrete can be produced, which will in turn make the amount of released CO₂ smaller. With the aging infrastructure nationwide, concrete’s rate of disposal is higher than ever. Using recycled concrete as an aggregate (RCA) in concrete production could be a way for the precast concrete industry to lower the concrete environmental impact. Tests were conducted on the fresh concrete to determine its passing and filling ability, as well as the segregation resistance. Several concrete cylinders were cast to study the effects of RCA on the compressive and tensile strength of self-consolidating concrete at different ages. More research in this area focusing on precast applications is needed. The following conclusions were drawn:

• Using up to 30% RCA replacement does not affect the filling ability of self-consolidating concrete. A slight reduction occurred when using 50% replacement.
• Increasing the amount of RCA improves the segregation resistance of SCC mixes.
• Using high ratios of RCA without increasing the amount or water or chemical admixtures significantly reduces the filling ability and self-compacting properties of concrete, due to its high porosity and absorption.
• Increasing the amount of RCA does not affect the passing ability of SCC mixes.
• Using up to 50% replacement enhances early strength of concrete without hindering strength at later stages. After 24 h following the casting of concrete with 30% replacement, that concrete had 73% higher compressive strength and 53% higher tensile strength than concrete without RCA.
• Using mixes with high RCA replacement ratios have rapid strengthening but have significantly lower compressive and tensile strength at 28 days.
• Using RCA improves early strength development. This is due to early hydration and high surface roughness.
• RCA can be used successfully to produce SCC for precast concrete applications.

Future Work

• Special attention should be given to the durability of concrete containing RCA. The high porosity of RCA may hinder its ability to resist long term weathering. More research is needed in this area.
• Landfill crushed concrete contains large amounts of fines (<4.75 mm). The performance of concrete replacing fine aggregate with crushed concrete should be studied.