This research aims at analysing the mechanical performance of concrete with recycled aggregates from concrete pavements. First, the characteristics of various natural and recycled aggregates used in the concrete were thoroughly analysed. The composition of the recycled aggregates was determined and several physical and chemical tests of the aggregates were performed. In order to evaluate the mechanical performance of recycled concrete, cube compressive strength and flexural tensile strength tests were performed. The effect of recycled aggregates on the strength of recycled concrete is related to the strength of recycled aggregates, the strength of natural aggregates, and the strength of old concrete. The strength of recycled concrete decreases with increasing water-cement ratio. However, due to the water absorption of the recycled aggregate, it has a certain inhibitory effect on the strength reduction. As the replacement rate of recycled aggregates increases, the optimal sand ratio decreases. The sand ratio is controlled between 32% and 38%, which is ideal for recycled concrete. With the increase of fly ash content, the 7 d strength of recycled concrete has decreased to some extent, but the 28 d strength has been slightly improved. In addition, for compressive strength and flexural tensile strength, the optimal content of fly ash is different.
In several developed countries such as Europe and the United States, the use of recycled aggregates for concrete has been for many years [
In China, the consumption of cement is 13.71 billion tons in 2018, a year-on-year increase of 6.6%. It is estimated that approximately 246 million tons of waste concrete are currently produced annually in the mainland of China [
Recycled aggregates from concrete pavements are concrete fragments obtained by recycled concrete pavement slabs after being crushed, washed, and classified [
Strength is the most important mechanical indices of recycled concrete. The strength of recycled concrete is usually defined as the ability to resist external damage, sometimes the damage is equivalent to the occurrence of cracks [
Based on the previous research, this study replaces coarse and fine aggregate at the same time and compares the effects of recycled aggregate replacement rate, water-cement ratio, sand ratio, and fly ash content on the mechanical properties of recycled aggregate concrete, such as cube compressive strength and flexural tensile strength.
Waste concrete was directly collected from concrete pavement demolishing spot, and sorting was conducted. After secondary crushing using an impact crusher or an E-crusher aggregate crushing plant, the concrete was graded into three types of fine and coarse recycled aggregates with sizes of 10–30 mm, 5–10 mm, and smaller than 5 mm. The natural aggregate used in the test was granite, including 10–30 mm, 5–10 mm, and smaller than 5 mm. The test results of physical and mechanical indices of aggregates are shown in Tables
Physical and mechanical performance of coarse aggregates.
Aggregate types | Crushed value (%) | Apparent relative density | Water absorption (%) | Needle-chip particle content (%) | |||
---|---|---|---|---|---|---|---|
5–10 mm | 10–30 mm | 5–10 mm | 10–30 mm | 5–10 mm | 10–30 mm | ||
Natural | 18.4 | 2.650 | 2.693 | 1.25 | 1.18 | 2.4 | 3.6 |
Recycled | 24.5 | 2.627 | 2.671 | 4.88 | 4.29 | 4.2 | 6.6 |
Physical and mechanical performance of fine aggregates.
Aggregate types | Apparent relative density | Sand equivalent (%) | Water absorption rate (%) | Sturdiness (%) | Angularity (s) |
---|---|---|---|---|---|
Natural | 2.741 | 60.5 | 1.52 | 7.4 | 46 |
Recycled | 2.627 | 79.8 | 5.04 | 6.2 | 47 |
The recycled aggregate behaved higher crushed value than the granite aggregate, but it still met the requirement of not higher than 30% in Technical Specifications for Construction of Highway Base of China. Recycled aggregate had higher needle-chip particle content than the granite aggregate. Its water absorption was far higher than that of granite aggregate.
The recycled fine aggregate showed higher sand equivalent and angularity than granite fine aggregate, but was sturdy. These indicate that recycled aggregate meets the requirements of pavement slab of highway in China.
The solid content of the water reducing agent used in the test is 23%, and the pH value is 6.8. The water reduction rate is 21%, and the chloride ion content is 0%.
Coarse aggregate was the basis of forming the skeleton of cement concrete, while fine aggregate mainly fills in the internal void of the skeleton. The aggregate gradation had a direct impact on the performance of cement concrete [
In the test, eleven groups of continuous gradations’ aggregates were prepared by replacing granite with 0%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, and 100% of recycled aggregate. In each grading, fine and coarse aggregate were replaced in the same proportion. The eleven continuous gradations were shown in Figure
Grading curve.
The mechanical properties of several kinds of recycled aggregates concrete were investigated to understand the change rule. Firstly, recycled aggregate concrete was studied with 0.10, 0.20, 0.30, 0.40, 0.50, 0.60, 0.70, 0.80, 0.90, and 1.0 aggregates replacement rate. Then, the concrete was studied with 0.29, 0.31, 0.33, 0.35, and 0.37 water-cement ratio. For sand ratio, the doping amount mainly included 0.35, 0.37, 0.39, 0.41, and 0.43. For fly ash, the doping amount mainly included 0, 0.10, 0.20, 0.30, and 0.40. The mix proportion of concrete with recycled aggregates is shown in Table
The mix proportion of concrete.
No. | Water-cement ratio (%) | Content (kg·m3) | |||||||
---|---|---|---|---|---|---|---|---|---|
Water | Cement | Fly ash | Fine aggregates | Coarse aggregates | Water-reducing agent | ||||
Natural | Recycled | Natural | Recycled | ||||||
RA0 | 0.33 | 122.77 | 370.00 | 0 | 763.00 | 0.00 | 1193.00 | 0.00 | 1.11 |
RA10 | 0.33 | 122.77 | 370.00 | 0 | 686.70 | 76.30 | 1073.70 | 119.30 | 1.11 |
RA20 | 0.33 | 122.77 | 370.00 | 0 | 610.40 | 152.60 | 954.40 | 238.60 | 1.11 |
RA30 | 835.10 | ||||||||
RA40 | 0.33 | 122.77 | 370.00 | 0 | 457.80 | 305.20 | 715.80 | 477.20 | 1.11 |
RA50 | 0.33 | 122.77 | 370.00 | 0 | 381.50 | 381.50 | 596.50 | 596.50 | 1.11 |
RA60 | 0.33 | 122.77 | 370.00 | 0 | 305.20 | 457.80 | 477.20 | 715.80 | 1.11 |
RA70 | 0.33 | 122.77 | 370.00 | 0 | 228.90 | 534.10 | 357.90 | 835.10 | 1.11 |
RA80 | 0.33 | 122.77 | 370.00 | 0 | 152.60 | 610.40 | 238.60 | 954.40 | 1.11 |
RA90 | 0.33 | 122.77 | 370.00 | 0 | 76.30 | 686.70 | 119.30 | 1073.70 | 1.11 |
RA100 | 0.33 | 122.77 | 370.00 | 0 | 0.00 | 763.00 | 0.00 | 1193.00 | 1.11 |
W/C29 | 0.29 | 107.30 | 370.00 | 0 | 534.10 | 228.90 | 835.10 | 357.90 | 1.43 |
W/C31 | 0.31 | 114.70 | 370.00 | 0 | 534.10 | 228.90 | 835.10 | 357.90 | 1.30 |
W/C33 | 835.10 | ||||||||
W/C35 | 0.37 | 136.90 | 370.00 | 0 | 534.10 | 228.90 | 835.10 | 357.90 | 0.98 |
SR29 | 0.35 | 129.50 | 370.00 | 0 | 479.22 | 205.38 | 889.98 | 381.42 | 1.09 |
SR31 | 0.35 | 129.50 | 370.00 | 0 | 506.60 | 217.12 | 862.60 | 369.68 | 1.09 |
SR35 | 0.35 | 129.50 | 370.00 | 0 | 561.37 | 240.59 | 807.83 | 346.21 | 1.09 |
SR37 | 0.35 | 129.50 | 370.00 | 0 | 588.76 | 252.32 | 780.44 | 334.48 | 1.09 |
FA10 | 0.35 | 129.50 | 333.00 | 37 | 534.10 | 228.90 | 835.10 | 357.90 | 1.09 |
FA20 | 835.10 | ||||||||
FA30 | 0.35 | 129.50 | 259.00 | 111 | 534.10 | 228.90 | 835.10 | 357.90 | 1.09 |
FA40 | 0.35 | 129.50 | 222.00 | 148 | 534.10 | 228.90 | 835.10 | 357.90 | 1.09 |
It can be seen from Tables
The mixing of recycled aggregates concrete was carried out according to the test methods of cement and concrete for highway engineering (JTG E30-2005). The slump of concrete should be measured and predicted to ensure the mixing quality of recycled aggregate concrete. The vibration table should be used to vibrate, until the floating slurry appears on the concrete surface. Plastic film was used to seal the sample surface to reduce the moisture loss after forming. After 24 h, demould and store with a constant temperature of 20°C and humidity of 95%.
The mixing of recycled aggregate concrete was carried out according to the specification “test methods of cement and concrete for highway engineering (JTG E30-2005)”.
The test methods of mechanical properties such as compressive strength and flexural tensile strength of recycled aggregate concrete were carried out according to the specification JTG E30-2005.
When the water-cement ratio is the same, the apparent density of recycled aggregate concrete decreases continuously with the increase of recycled aggregate replacement, as shown in Figure
Apparent density of recycled aggregate concrete with different replacement rates.
The compressive strength and flexural tensile strength of recycled concrete with different recycled aggregate replacement rates are shown in Figure
Strength of recycled aggregate concrete with different replacement rates. (a) The compressive strength. (b) The flexural tensile strength.
A large number of studies have shown that [
Water-cement ratio is one of the key factors affecting the strength of concrete. In order to study the effect of water-cement ratio on the strength of recycled concrete, five kinds of concrete with water-cement ratio were prepared in the experiment. Among them, the replacement rate of recycled aggregates is 30%. The results of strength at 7 d and 28 d tests are shown in Figure
Strength of recycled aggregate concrete with different water-cement ratios. (a) The compressive strength. (b) The flexural tensile strength.
For the compressive strength of recycled concrete, before the water-cement ratio is less than 0.34, the decrease in compressive strength at 7 d and 28 d is greater. After the water-cement ratio is greater than 0.34, the decrease is significantly weakened. For the flexural tensile strength, there is also a turning point similar to the change law of compressive strength. After the water-cement ratio is greater than 0.32, the decrease in the flexural tensile strength at 28 d of recycled concrete is significantly weakened. The main reason for this phenomenon is that the recycled aggregate has water absorption. When the water-cement ratio is relatively small, the water is mainly absorbed by the cement and the surface of the aggregate and recycled aggregates cannot fully absorb water. As the water-cement ratio increases, a large amount of free water appears in the mixture, and this part of water is gradually absorbed by the recycled aggregate. This actually reduces the effective water-cement ratio of the recycled concrete, resulting in a decrease in the strength reduction.
Sand ratio has a greater impact on the workability of concrete, which in turn affects the strength of concrete. The test used a water-cement ratio of 0.35 and a recycled aggregate replacement rate of 30% to prepare recycled concrete that with 5 kinds of sand ratio. The effect of sand ratio on compressive strength and flexural tensile strength was measured. The results are shown in Figure
Strength of recycled aggregate concrete with different sand ratios. (a) The compressive strength. (b) The flexural tensile strength.
It can be seen from the figure that the compressive strength at 28 d, the flexural tensile strength at 7 d, and the flexural tensile strength at 28 d of recycled concrete reach maximum values when the sand ratio is 39%. Therefore, recycled concrete has the same optimum sand ratio as conventional concrete. Using this sand ratio, recycled concrete can obtain maximum compressive strength and flexural tensile strength. Through a large number of experiments, when the replacement rate of recycled aggregate is 30%, the optimal sand ratio of recycled concrete is 38%–39%. As the replacement rate of recycled aggregates increases, the optimal sand ratio decreases. The sand ratio is controlled between 32% and 38%, which is ideal for recycled concrete.
Adding fly ash to concrete can improve the workability, compactness, and durability of concrete. Several groups of recycled concrete specimens with different contents of fly ash were prepared in the experiment. The water-cement ratio of these specimens is 0.35, and the replacement rate of recycled aggregates is 30%. Then, the effect of the fly ash content on the compressive strength and flexural tensile strength was measured. The results are shown in Figure
Strength of recycled aggregate concrete with different fly ash contents. (a) The compressive strength. (b) The flexural tensile strength.
However, for recycled concrete, the more fly ash is not the better even for 28 d strength. It can be seen from the figure that, during the increase of the fly ash content from 0 to 30%, the compressive strength and flexural tensile strength at 28 d increased by 16% and 4.9%, respectively. However, when the fly ash content increased to 40%, the 28 d compressive strength and flexural tensile strength decreased to varying degrees. For compressive strength and flexural tensile strength, the optimal content of fly ash is different. Therefore, for recycled concrete, the content of fly ash should be determined according to the compressive or flexural needs of the structure and the changes in compressive strength and flexural tensile strength.
The effect of recycled aggregates on the strength of recycled concrete is related to the strength of recycled aggregates, the strength of natural aggregates, and the strength of old concrete. As the replacement rate of recycled aggregates increases, the strength of recycled concrete may increase or decrease. The strength of recycled concrete decreases with increasing water-cement ratio. However, due to the water absorption of the recycled aggregate, it has a certain inhibitory effect on the strength reduction. The greater the content of recycled aggregates, the more obvious the inhibition effect of the strength reduction of recycled concrete. Recycled concrete has the optimum sand ratio as conventional concrete. When the replacement rate of recycled aggregate is 30%, the optimal sand ratio of recycled concrete is 38%–39%. As the replacement rate of recycled aggregates increases, the optimal sand ratio decreases. The sand ratio is controlled between 32% and 38%, which is ideal for recycled concrete. With the increase of fly ash content, the strength of recycled concrete at 7 d has decreased to some extent, but the strength at 28 d has been slightly improved. In addition, for compressive strength and flexural tensile strength, the optimal content of fly ash is different.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
This work was supported by the National Natural Science Foundation of China (no. 51808329) and Science and Technology Project of Shanxi Transportation Holding Group Co., Ltd. (nos. 19-JKKJ-6 and 19-JKKJ-67).