Mechanical characteristics of structural concrete using building rubbles as recycled coarse aggregate

1 Introduction

The use of recycled building waste as concrete in Iraq is much lower than in advanced countries. Recycling and reusing construction debris can save money and protect the environment by reducing waste materials. In Iraq, fine and coarse aggregates are extracted from the rivers and used in the construction industry. However, the demand for aggregates is high due to their scarcity in some parts of Iraq. Moreover, the natural resources of sand and gravel are expected to run out eventually [1,2].

Recycled aggregate is manufactured from the recycling process of waste concrete and bricks. It was previously used for low-value-added products such as filling, sub-base road materials, and minor concrete components. However, with recent technological advancements in the processing of waste concrete, it is now possible to produce recycled aggregate of quality similar to natural aggregate. As a result, the potential of using it in structural elements has increased significantly [3].

Globally, there has been a growing interest in improving the use of recycled aggregates in the construction industry for environmental and economic reasons. The availability of recycled aggregates in large quantities is more feasible than extracting natural aggregates due to advances in crushing technology, enabling more economical production of RAC. When shown to be practical, recycled coarse aggregate (RCA) may be substituted for natural coarse aggregate (NCA) in various concrete applications. The majority of expertise and knowledge in RAC emerged via the usage of substandard concrete components in comparison to newer materials. Additionally, concrete methods are significantly improved. Therefore, a study in this field might add value to the existing knowledge and extend the usage of these wastes

Various studies have investigated the effect of adding or replacing the normal aggregate in concrete mixes with the aggregate extracted from the demolished concrete. These studies have discussed the prospect of successfully recycling demolished concrete and reducing the damaging effect that construction waste has on the environment by employing a range of processes [3,4,5,6]. Furthermore, considering different factors such as type and aggregate size, they have looked at how recycled concrete may be constructed and optimized to get the ideal balance of strength and durability [7,8,9,10,11,12]. Additionally, the mechanical and physical properties of recycled concrete have been investigated in previous studies with various environmental variables and load types, including creep, shrinkage, water absorption, compression, tension, shear, and fatigue [13,14,15,16,17]. The performance and durability of recycled concrete structures in different conditions and environments have also been evaluated [18,19,20].

The objective of this study is to evaluate the characteristics of normal and recycled concrete that is made with aggregates from building rubble that are produced locally, such as clay bricks and waste of crushed concrete (CC). The physical and mechanical properties of local materials differ from those used in other countries, so studying their effects is crucial.

2 Experimental program

The experimental program focuses on determining the qualities of concrete created from recycled aggregate obtained by crushing demolished concrete and clay bricks. The demolished concrete consists of concrete samples used for lab tests in the quality control procedure and crushed bricks (CBs) obtained from laboratory test specimens. A small jaw crusher was used to smash the concrete and bricks, Figure 1. The cast concrete was made using various percentages of aggregates acquired by crushing the demolished concrete, bricks, and natural sand required for better workability. The effect of demolished concrete on the performance of the cast concrete is assessed by comparing it with reference concrete, which is made from natural aggregates with equivalent proportions. The 150 mm cubes, 150 mm × 300 mm cylinders, and 100 mm × 100 mm × 400 mm prisms were cast from all the batches of concrete. The specimens were water-cured for 28 days and then tested for compressive, tensile, and flexural strengths by compression, splitting, and two-point flexural tests.

Figure 1

Small jaw crusher.

3 Materials

In this work, ordinary Portland cement, RCAs, NCA, and natural sand were used, as shown in Figure 2.

Figure 2

RCA made of (a) crushing concrete and (b) clay bricks.

4 Mix proportions

The RCAs were presoaked before mixing because of their high water absorption. The presoaking duration ranged between 2 and 3 h depending on the aggregate type and followed by airing until full surface drying was noticed. The water-to-cement W/C ratio was adjusted at 0.5. Two groups of mixes were produced. CC is used as recycled aggregate in the first group, whereas CB is used in the second. Varying RCA replacement percentages are used for each group, which are 0, 35, 65, and 100%, respectively. The normal concrete that acted as the reference concrete in the case of an RCA replacement is equal to zero. The concrete mix proportions are presented in Table 1.

Tam et al. [21] proposed mixing recycled aggregate concrete (RAC) in two stages to improve compressive strength and eliminate strength fluctuation. Studies by Etxeberria et al. [22] revealed that recycled aggregates must be saturated surface dry (SSD) to control fresh concrete’s W/C ratio. The suggestion provided by Etxeberria et al. [22] is adopted in this work.

5 Preparation of specimens

Slump testing was carried out to evaluate the workability. Each group’s mixture was cast in 150 mm cubes, 150 mm × 300 mm cylinders, and 100 mm × 100 mm × 400 mm prisms in four steel molds, and a vibration table was used to compact the samples. They were removed from the mold 1 day after casting and cured for 28 days in water. The cube compressive strength was obtained using cube samples, the tensile strength was determined using cylinder samples, and the flexural strength of the RAC was determined using prism samples.

6 Test results

Physical and mechanical tests were carried out to evaluate how RCA affects the compressive, tensile, and flexural strength of RAC.

6.1 Slump test

The slump test results for each batch of concrete are presented in Table 2 and Figure 3. RCA of 100% replacement yielded slump values of 78 mm for CC aggregate and 64 mm for CB aggregate, 24 and 38% lower than the control mix’s slump of 103 mm. The slump decreased as the RCA mixing ratio increased, and the brick aggregate resulted in a less workable mix. This could be due to the significant amount of crushed cement mortar in RCA, which was weak and porous. During mixing, brick particles and crushed cement mortar might break, increasing the proportion of fine particles. The mixture’s fine particles and crushed mortar affected the aggregate size distribution and reduced water absorption capacity. The loss of free water and the decrease in cement hydration reduced the workability of the mix. It was also observed that the RAC surface was hard to compact and finish after casting.

Table 2

Slump test results

RAC %		0%	35%	65%	100%
Slump (mm)	CC	103	91	80	78
	CB	103	86	72	64

Figure 3

Influence of RCA replacement on slump.

6.2 Density of concrete

The average density of concrete with natural aggregates used was 2,390 kg/m3, as listed in Table 3, while the density of concrete of recycled aggregates was considerably lower, with about 2,116 and 2,019 kg/m³ for 100% RAC replacement for both CC and CB, respectively. The reduction in density reached approximately 11 and 15% of the control mix for both CC and CB replacement, respectively, as shown in Figure 4. The crushed aggregates have a lower bulk-specific density compared to natural aggregates [23].

Table 3

Apparent density of concrete (kg/m³)

No.	NA	CC 35%	CB 35%	CC 65%	CB 65%	CC 100 %	CB 100%
1	2,408	2,240	2,189	2,213	2,101	2,160	1,982
2	2,382	2,251	2,207	2,219	2,120	2,178	2,071
3	2,355	2,234	2,175	2,210	2,163	2,056	2,012
4	2,414	2,225	2192.3	2,207	2,198	2,074	2,012
Average	2,390	2237.4	2,191	2,212	2,145	2,117	2,019

Figure 4

Influence of RCA replacement on density.

6.3 Compressive strength

Using an ELE International hydraulic press, compressive strength is typically tested using 150 mm cubes by BS EN 12390-3 [24]. The compression strength test measured the samples’ ultimate stress under uniaxial compression. Four cube samples were made per batch of concrete mix for this test by BS EN 12390-1 [25]. Various parameters influencing the compressive strength of RAC were studied. According to the results of the tests, the RCA content substantially affects the compressive strength of concrete. The compressive strength of test samples is defined as the peak stress in the samples when subjected to uniaxial compression. The cube with various RCA substitution percentages is shown in Table 4, from which it can be revealed that the RCA content considerably affects the compressive strength; as RCA increases, the compressive strength drops.

Table 4

Compressive strength (MPa)

No.	NA	CC 35%	CB 35%	CC 65%	CB 65%	CC 100%	CB 100%
1	32.9	29.3	18.0	17.5	11.3	11.9	10.9
2	33.4	29.5	18.6	16.2	13.8	12.2	10.6
3	32.7	29.1	17.0	16.8	11.5	12.0	11.1
4	33.7	28.7	17.1	17.2	11.3	12.0	10.4
Average	33.2	29.2	17.7	16.9	12.0	12.0	10.8

The compressive strength of concrete reduces as the ratio of the RCA increases in aggregate proportions, as seen in Figure 5. The decrease was around 64% when the RCA replacement was 100% for both the CC aggregate replacement and the CB aggregate replacement. The CB aggregate gives lower strengths than CC aggregates, but this difference will diminish as the replacement percentages increase to 100%. Various factors, including increased concrete porosity and a weak aggregate-matrix interface connection [26], might be responsible for RAC concrete’s loss in compressive strength. The other reason for the drop in compressive strength may be owed to adding prior amount of water to recycled aggregate to substitute the higher porosity in comparison with natural aggregate.

Figure 5

Influence of RCA% content on the concrete compressive strength.

6.4 Splitting tensile strength

The tensile test was conducted using the splitting method according to BS EN 12390-6 [27]. The tensile strength was measured using the ELE International apparatus. Each concrete batch had four-cylinder samples tested at 28 days, just like compressive strength samples. Compressive load is applied to the longitudinal axis of a concrete cylinder (Figure 6). The test used 150 mm × 300 mm cylinders. The results of the tensile splitting strength test are listed in Table 5. The splitting strength was dropped as the substitution of RCA in the mixture increased.

Figure 6

A cylinder failed by splitting test.

Table 5

Tensile strength (MPa)

No.	0%	35%	65%	100%
1	3.48	2.50	1.40	1.10
2	3.35	2.40	1.70	1.30
3	3.41	1.90	1.60	0.90
4	3.62	2.20	1.80	0.78
Average	3.47	2.25	1.63	1.02

The relative tensile strength test results, established as the RAC’s splitting tensile strength compared to the control concrete, are displayed in Figure 7. When the RCA rises, the splitting tensile strength drops. The tensile strengths of concrete with 100% RCA are only 29% of control concrete for both CC and CB aggregate substitution. Using the CC aggregate as the recycled aggregate is a somewhat better enhancement in strengths than using CB aggregate. The strength decrease can be again due to extra water used in RAC and high porosity. However, the effect of using RCA on tensile strength is less than that in compressive strength, which can mean that the irregular shape with the unsmooth surface of RCA will increase the bond between RCA and the cement paste.

Figure 7

Influence of RCA content on the concrete tensile strength.

6.5 Flexural test

This test measures the flexural strength of concrete using simple prisms under two-point loading. After 28 days, four prism samples of 100 mm × 100 mm × 400 mm were adopted from each concrete batch according to BS EN 12390-5 [28], as shown in Figure 8.

Figure 8

Prisms failed by flexure test.

The results of the flexural strength test, shown in Table 6, revealed that the flexural strength of concrete made with 100% RCA content at 28 days was about 38% of that of the control mix, as seen in Figure 9. It can be noted that when the replacement ratios are increased, the RAC with CB aggregate gives 10% fewer strengths than the RAC with CC aggregate.

Table 6

Flexure strength (MPa)

No.	0%	35%	65%	100%
1	5.67	4.80	3.45	3.15
2	5.25	3.75	3.75	3.02
3	5.00	4.50	3.60	3.90
4	5.40	4.65	3.45	3.06
Average	5.33	4.43	3.56	3.28

Figure 9

Influence of RCA content % on the concrete flexural strength.

Choi et al. [29] observed that the replacement of RCA has little effect on the flexural strength of RAC. Topçu and Şengel [30] revealed that the flexural strength dropped when RCA replacement increased. It was revealed in this work that increasing the RCA proportion had a noticeable effect on flexural strength, although not as significantly as on compressive strength.

7 Comparison between flexural and splitting strength

The flexural and splitting strengths were 16–27 and 8–10%, respectively, relative to the compressive strength. However, compared to the guidelines in ACI-363R [31], they are rather high for splitting tensile strength and within the acceptable range for flexural strength. The following equations are recommended to estimate the relations between the compressive ( f c ′ ), flexural (f _r), and splitting (f _t) strengths of concrete:

where f c ′ is assumed to be

The flexural and splitting strengths were 16–27 and 8–10%. Figure 10 displays the relations between compressive, flexural, and splitting strengths and the lines indicate the ACI relationships mentioned before. The figure shows that when the RCA content increases, the projected splitting strengths increase compared to the experimental values.

Figure 10

Relative splitting tensile and flexural strengths vs RCA content %.

However, for low percentages of replacement, i.e., less than 20%, the RAC showed slightly higher values of strengths than that predicted by Eq. (2). For higher percentages of replacement, the gap increases between experimental and calculated strengths and the ACI formula gives 45% higher strength at 100% replacement. On the other hand, the flexural strengths are well predicted by Eq. (1) for replacement percentages less than 80%. The experimental flexural strength increases as replacement percentage increases and it is 15% higher than that predicted by ACI formula for 100% replacement percentage. From above, it is clear that RAC is more beneficial in case of flexural strengths while they are weaker for tensile splitting strengths.

8 Conclusion

The study presents and discusses the experimental data obtained for RAC. The following findings may be taken from this investigation:

1. RCA replacement percentage has a considerable influence on the workability of RAC. It might be owing to RCA’s high water absorption capacity.

2. Increasing the substitution percentage of recycled aggregate in RAC leads to decreased apparent concrete density. With 100% replacement, the density will decrease to 12%.

3. The compressive strength will be reduced to 64% when using 100% replacement for both CC and CB aggregates. However, the CB aggregate provides less strength if compared with CC aggregates, but this will lessen as the replacement percentages increase to 100%.

4. Tensile splitting strength will be lowered by increasing the substitution content of the RCA in the mixture to achieve a reduction of 29% for 100% replacement. However, the CC aggregate as recycled aggregate gives higher strengths than the CB aggregate.

5. The flexural strength of concrete with 100% RCA was about 38% of control natural aggregate concrete specimens after 28 days. Increasing replacement percentages of CB aggregate in the RAC gives 10% lower strengths than RAC with CC aggregate.

6. The compressive strength is related to the flexural and splitting strengths using ACI formulae. It is observed that enhancing the RCA concentration results in increasing the estimated splitting strengths compared to experimental values. Expect lower results than anticipated for lightweight aggregate concrete, which exhibits certain tensile strength characteristics with the concrete constructed with recycled aggregates. However, the enhancement will be on flexural strength, even at high RCA concentrations.