Resilient and permanent deformation behaviors of construction and demolition wastes in unbound pavement base and subbase applications

Abstract

The reuse of construction and demolition wastes (C&D wastes) as unbound granular materials (UGMs) to construct pavement base and subbase courses is of great potential, while the cyclic behaviors of unbound C&D wastes under traffic loadings still lack the support of test data. To investigate the traffic-induced resilient and permanent deformation behaviors as well as the shear and particle breakage properties of the unbound recycled concrete aggregate (RCA) & recycled clay masonry (RCM) blends, a series of consolidated drained, resilient deformation and permanent deformation tests by a large-scale dynamic triaxial apparatus were performed. The RCM content in two blends made by different mixed methods was emphasized, and the factors of gradation, confining pressure and cyclic deviatoric stress amplitude were also taken into consideration. Test results show that RCM content makes great influences on the particle breakage, resilient modulus, permanent axial strain and shakedown behavior of unbound RCA & RCM blends, and the influences show obvious differences between the two mixed methods. The peak triaxial strength, particle breakage level and resilient modulus of blends 1 (B1) are generally larger than those of blends 2 (B2), and the influence of confining pressure and cyclic deviatoric stress amplitude on resilient modulus are related to the mixed method but unrelated to RCM content. The popular modified K-θ model is employed to predict M_r of B1 and B2, and the reference ranges of the regression coefficients with respect to RCM content are obtained. Based on the results, it is evaluated that when RCM content is below a certain extent (e.g. 30%), B1 perform better (and the best at around RCM content of 10%) as pavement base and subbase courses in regard to shear strength, resilient modulus and permanent strain. However, as the RCM content continues to increase, B2 may be better than B1 considering the overall performance of cyclic properties.

Introduction

The construction and demolition sectors produce lots of solid wastes, which consume a large amount of natural resources, occupy a large area by landfills and impose huge pressures on the environment. To overcome or alleviate the problems, the reuse of construction and demolition wastes (C&D wastes) has become a global consensus during the past decades, and many effective applications in civil engineering have been extensively explored and widely used in cases such as road construction, ground improvement, backfill and concrete construction [1]. In particular, the feasibility to use C&D wastes as unbound granular materials (UGMs) of the pavement base or subbase layers has been proposed and investigated by many researches [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], and it is generally accepted that unbound C&D wastes have a great potential to construct the unbound pavement base or subbase courses.

Reviewing the literature and engineering cases, the main alternatives to natural granular materials in pavement construction are recycled concrete aggregate (RCA) and recycled clay masonry (RCM). Most of the previous researches [1], [2], [4], [7], [8], [9], [13], [14], [15] indicated that the basic physical properties of RCA and RCM, such as the density, strength, CBR (California Bearing Ratio), abrasion, water absorption and etc, were good enough to be reused as UGMs of the pavement base or subbase layers. For example, Arulrajah et al. [1] argued that RCA and RCM met the physical and shear strength requirements for aggregates of pavement base and subbase courses. Arulrajah et al. [3] reported that RCM performed satisfactorily at low moisture levels and suggested blending RCM with other recycled materials to enhance its performance in pavement subbase applications. Arulrajah et al. [4] tested blends of RCA & RCM with 5 different mixed proportions, and they carried out lots of laboratory tests on particle-size distribution, modified Proctor compaction, particle density, water absorption, CBR, Los Angeles abrasion etc, which indicated that up to 25% RCM could be safely added to the blends in pavement sub-base applications. Poon et al. [13] conducted laboratory tests on different blends of RCA & RCM and compared them with natural granular materials. It was found that the RCA & RCM blends were suitable for subbase which met the minimum requirement of CBR in Hong Kong.

Resilient modulus is one of the key engineering parameters in pavement design, which is mainly used to evaluate the pavement strains of elastic layers under wheel loadings. In addition, laboratory results of permanent deformation are widely employed to predict and control the pavement settlement under traffic loadings. Many road diseases such as rutting, fatigue cracking and etc are strongly related to the traffic-induced resilient and permanent deformations of base and subbase courses. As a result, it is expected that UGMs coming from C&D wastes should have sufficient resilient stiffness to bear and transfer the loads, and no excessive deformations are incurred during the long-term service life.

There are some studies which have involved the cyclic properties of C&D wastes under traffic loadings [2], [4], [5], [6], [9], [11], [12], [14]. The overall conclusions are that the resilient modulus and permanent deformation generally met the requirement of pavement base or subbase construction, but they were affected by several factors including the proportions of different recycled materials, gradation, dry density, moisture content and etc., and the worst situation such as saturation should be considered so as to prevent unsuitable C&D wastes from being used as base or sub-base. Moreover, it is also found from the literature that the cyclic behaviors of C&D wastes were rarely emphasized compared to other physical properties and the conclusions were based on very limited laboratory efforts. Arisha et al. [2] implemented cyclic triaxial tests on the unbound blends of C&D wastes and RCM. They found that the blends yielded high resilient modulus, which tended to decrease as the RCM content increased. All blends were recommended to be used as subbase materials except the C&D wastes & RCM blend of 100/0% that could be used as base material. Arulrajah et al. [4] conducted repeated triaxial tests on RCA & RCM blends and showed that the effects of RCM content on the deformation and resilient modulus relative to dry density and moisture content were small. Azam et al. [5] found that the matric suction of RCA & RCM blends had influences on the resilient modulus and permanent strain, and they indicated that the permanent strain rate failed to meet the South Australian requirement for base but it achieved that for subbase, and the resilient modulus was greater than the minimum requirement. However, only a blend of 20% RCM with 80% RCA was tested in their study. Bennert et al. [6] concluded that a mixture of 25% RCA with 75% natural aggregate would obtain the same resilient and permanent deformation properties as a dense-graded aggregate base coarse which was currently used in base and sub-base layers. Diagne et al. [14] showed that the resilient modulus of RCA & RCM was decreased by the increase in both moisture and RCM content. Gabr et al. [9] tested two types of RCA and one virgin aggregate product, and the results showed that RCA performed better than the virgin aggregate in terms of both resilient modulus and permanent deformation, and RCA had resilient moduli greater than minimum requirement of the South Australian specification. However, blends of RCA & RCM were not tested in their study. Leek et al. [11] studied the effects of moisture content on RCA & RCM blends and concluded that the resilient modulus decreased and the permanent strain increased as the moisture content increased. Nataatmadja et al. [12] found that the resilient modulus of well-graded RCA was affected by the compressive strength of the original concrete, the amount of softer material in the RCA, and the aggregate shape.

Summarizing, the reuse of RCA or the blends of RCA with RCM as UGMs to construct pavement base and subbase courses have drawn more and more attentions around the world, but the resilient and permanent deformation behaviors under traffic loadings are still lacking the support of test data. Meanwhile, the mixed method of RCA with RCM was rarely evaluated, the worst moisture condition of saturation was seldom considered, and the mixture proportion, confining pressure and cyclic stress amplitude compared to the factors of moisture content and dry density were less emphasized. The purpose of this study is to assess the potential applications of RCA & RCM blends as unbound pavement base or subbase layers in terms of the traffic-induced deformation behaviors, shear and particle breakage properties, by conducting a series of CD (consolidated drained), RD (resilient deformation) and PD (permanent deformation) tests through a large-scale dynamic triaxial apparatus. Besides, this study is also motivated to explore the influences of mixed method on the geotechnical properties of RCA & RCM blends, and to assess a more reasonable or provide an alternative way to blend RCA and RCM in the engineering applications.