1. Introduction
With the fast development of urbanization in recent years, China’s construction industry has developed rapidly, resulting in a lot of problems, such as the generation of large amounts of construction waste. In China, about 1.8 billion tons of construction waste is produced every year [
1]. By 2020, the amount of construction waste will even be more. Construction waste is the main source of municipal solid waste, mainly including residue, waste concrete, broken brick, waste asphalt, waste pipe, waste wood and so forth. At present, landfills or stacking are mainly used to deal with construction waste, which not only takes up large land area but also causes serious pollution.
Road construction needs a large amount of aggregate and therefore manufacturing recycled aggregate from building waste can significantly alleviate the shortage of natural aggregate resources and also help to solve the problem of building waste. There have been many researches on the application of construction waste in road by conducting the laboratory tests. Sobhan et al. [
2] studied the flexural fatigue behavior of a stabilized fiber-reinforced pavement base material using recycled concrete aggregates. Chini et al. [
3] evaluated the performance of recycled concrete aggregate as a base material and as an aggregate. Etxeberria et al. [
4] analyzed the influence of the order of materials used in concrete production (made with recycled aggregates). Huang et al. [
5] evaluated the feasibility of adding fired ware scrap, a waste ceramic material, into Portland cement concrete (PCC) and hot-mix asphalt (HMA). Thomas et al. [
6], Xiao et al. [
7] and Guo et al. [
8] analyzed the durability properties of concrete made from recycled aggregates. It was shown that a higher recycled aggregate content and water/cement ratio can result in poor durability. Abdulla [
9] evaluated the effect of recycled coarse aggregate type on concrete. The test results showed that the mechanical properties of recycled concrete were significantly affected by the original aggregate nature and quality.
Some clay brick particles are often found in the recycled aggregate of construction waste, which usually have effects on the performance of recycled aggregate. This has attracted the attention of many researchers. Chi et al. [
10] studied the applicability of blending recycled concrete aggregates and crushed clay brick as subbase materials by comparing to the subbase materials prepared with natural aggregates. Chi et al. [
11] also conducted an investigation on the use of recycled concrete aggregates and crushed clay brick for producing paving blocks. Khalaf [
12] evaluated the possibility of using crushed clay brick as coarse aggregate for concrete manufacturing. Debieb et al. [
13] tested the use of coarse and fine crushed bricks as aggregate in concrete. Bektas et al. [
14] studied the effects of crushed clay brick aggregate on mortar durability. Cahim [
15] conducted some tests on the mechanical properties of concrete made with crushed bricks. Syed Ishtiaq et al. [
16] conducted a comprehensive testing program to study the effect of crushed clay bricks as coarse aggregate on the creep behavior of concrete. Kenai et al. [
17] tested the durability performance of recycled concretes using coarse and fine crushed bricks. Yang et al. [
18] conducted a study on the physical and mechanical properties of concrete with recycled concrete aggregate and crushed clay bricks. Ahmad et al. [
19] investigated the creep behavior of normal strength concrete made from crushed clay bricks as coarse aggregate. Afshinnia et al. [
20] studied the effect of ground clay brick on mitigating the alkali silica reaction in mortar bars. For pavement base materials evaluation, there have also been many research, like evaluation of erosion process in Rigid Pavement [
21], performance evaluation of cement stabilized crushed stone pavement base materials [
22], mix design of semi-rigid base material [
23] and so forth.
Although there has been significant progress on the evaluation of mechanical properties of mixtures with recycled aggregate and crushed clay bricks like strength, stiffness, shrinkage and so forth., there still lacks sufficient research on some of the other performances like anti-freezing and anti-scouring performances. In this study, based on cement stabilized recycled aggregate pavement base, the effects of clay brick particles on the performance of cement stabilized recycled aggregate as pavement base were investigated by conducting the unconfined compressive strength, compressive resilience modulus, anti-freezing, anti-scouring, drying shrinkage and temperature shrinkage tests of 6 kinds of mixtures with different contents of coarse and fine clay brick particles. The test results could provide a reference for future application of cement stabilized recycled aggregate as pavement base.
4. Discussion and Conclusions
Based on the test results, it is discovered that the use of CRCBA reduced the unconfined compressive strength in a certain extent. Although the unconfined compressive strength of 7d has no significant relationship with the amount of CRCBA, the increase of CRCBA content might affect the later strength of CSM. For example, the 7d unconfined compressive strength of B-4 is almost the same as B-2 but the 28d strength of B-4 is lower than that of B-2. A little of FRCBA increases the unconfined compressive strength to a certain extent. For example, the 7d and 28d strengths of B-4-1 are increased by 9.94% and 4.74%, respectively, compared with those of B-4. Compared with B-6, the strength of B-M decreases significantly and with the increase of curing age, the decreasing rate shows an increasing trend. Although, as aggregate applied in pavement base and subbase, the RCBA only can meet the requirements of grade 2 and below highways, the early strength of cement stabilized recycled aggregates with clay brick particles could meet the requirements of all grade highways except the extremely heavy duty express highway. The strength of the mixtures and the influence of different content of clay brick particles need further investigation, to better understand the mechanism of interaction of clay brick particle in the mixtures.
A small amount of clay brick particles has little effect on the resilience modulus of CSM. With the increase of clay brick particles, the modulus increases significantly. With the addition of coarse clay brick particles, the strength loss rate of the CSM decreases significantly after the freeze-thaw cycles, the scoured weight loss rate almost remains constant. The increase of clay brick particles could significantly reduce the drying shrinkage coefficient in the early stage of construction. The drying shrinkage coefficient decreased with the increase of clay brick particles (no matter what the particle size is). The drying shrinkage coefficient in the early period of B-0 decreased 71.8% in comparison with that of B-M. At the same time, the increase of clay brick particles had almost no effect on the temperature shrinkage performance of the mixture.
The influences of clay brick particles on the performances of cement stabilized recycled aggregate as pavement base were investigated. Some conclusions can be drawn as followings.
- (1)
The clay brick particles can reduce the unconfined compressive strength of cement stabilized recycled aggregate. The reduction rate may increase with the increasing age of the mixture.
- (2)
The addition of coarse clay brick particles has small influences on the compressive resilient modulus of the mixture and the addition of fine particle could significantly increase the compressive resilience modulus of the mixture.
- (3)
The frost resistance of the mixture increases first and then slightly decreases with the increase of coarse clay brick particles. The addition of fine clay brick particles could enhance the frost resistance of the mixture.
- (4)
The addition of clay brick particles had almost no influence on the scoured weight loss of the mixture; the addition of fine clay brick particles had almost no influence on the scoured weight loss of the mixture.
- (5)
The drying shrinkage coefficient of the mixture decreases with the increase of clay brick particles no matter what the particle size is. The effect of clay brick particles on the temperature shrinkage performance of the mixture is not so significant.
- (6)
Though the test evaluation of various performances shows that clay brick particles have different effects on the properties of cement stabilized pavement base materials, it can meet the requirements of the pavement base in many cases. For actual applications, it is still necessary to be optimized considering the load and climate conditions.
It should also be noted that clay brick particles are generally considered to compromise the performance of CSM. The main purpose of this paper is to investigate the possible influences of clay brick particles on CSM and the feasibility of CSM including clay brick particles used as pavement base. Many aspects need to be determined according to the specific conditions of the whole highway. So, in the current stage, it is really difficult to determine the “optimum clay brick particle content”. Future studies should take this issue into consideration.