The Influence of Bonding between Layers on Pavement Performance, a Case Study of Malaysian Road

. This paper summarizes a theoretical study undertaken to provide a better understanding of the consequences of poor bond on flexible pavement performance. The main objective of this paper is to investigate the influence of bond on the performance of Malaysian road. The pavement structure of Malaysian road was analyzed using a layered linear elastic program, BISAR 3.0 taking into account different state of the bond at the interfaces of the pavement layers and a static horizontal load in addition to the standard vertical dual load. The results indicate that the condition of the bond between the wearing and binder course can reduce the life of the pavement by up to 64%. On the other hand, the results also indicate that the condition of the bond between the binder and road base course, which was made up from asphaltic materials can reduce the life of the pavement by up to 68%.


Introduction
The issues on flexible pavement distress have been widely highlighted by several researchers for years. The causes are also varies depending on the type of distresses occurred. However, one of the main factors contributed to the pavement failures is poor interlayer bonding between the pavement structures. Based on many research that has been conducted in the past, the interlayer bond is responsible for ensuring all layers to behave as a single entity, thus reducing cracks and deformation of the pavement [1]. In most pavement design, the pavement layers are usually assumed to be fully bonded together and no displacement is developed between them. Through different discussion, the most effective method to ensure the interlayer bonding of pavement layers are by applying a thin bituminous bond coat (or tack coat) at the interfaces [2].
However, full bonding is not always achieved since many cases of pavement distresses caused by poor bonding between pavement layers has been reported in different countries. In 1980, Peattie [3] reported that 56 cases of premature bond failures between surfacing and binder course of (mainly) newly constructed roads in the United Kingdom (UK) were reported. Shaat [4] reported that in Northern Ireland, some sections of newly constructed roads experienced bond failures soon after they were opened. Meanwhile, Hakim [5] stated in his research that de-bonding problem between bases was found in a three years old pavement structure in the UK [2]. In Malaysia, it was reported that delamination is the most common failures occurred in Malaysian road, caused by slippage at the interface between wearing and binder course [6]. From the cases reported, it can be observed that the failures commonly occur between wearing and binder course. In addition, location with high horizontal loading is more prone to failure.
In pavement engineering, a number of computer programmes have been developed in order to overcome the problems related to poor pavement bond. However, only a few of these computer programmes address different interface conditions [7]. Unlike other programmes, Bitumen Stress Analysis in Roads (BISAR) that was developed by Shell Research Gate in 1970 is most widely used software due to its capability to include shear spring compliance into the analysis [8]. Furthermore, BISAR analysis produces comprehensive calculations, produces strain and stress profile in a pavement structure resulting from different loadings and provides a value for the expected life of the pavement for each run.

Incorporating bond condition in pavement design
Nottingham Design Method is adopted for this study [9]. The method is based on the analytical pavement design approach. There are two modes of failures that were observed in pavement structure: 1. Development of permanent deformation (rut) shown in Fig. 1 (a), coming from the accumulated permanent strain in the pavement. 2. Fatigue cracking of the bituminous layer, shown in Fig. 1 (b) caused by load induced repeated tensile strain, which induced by each load. BISAR program calculates stresses, strains, and displacements in a multi-layer elastic system, defined by the following configuration, material behaviour and few assumptions that have been introduced in this program: 1. Consisting of horizontal layers of uniform thickness placed on a semi-infinite base or half space. 2. Infinite extension of layers in horizontal directions. 3. The homogenous and isotropic material in each layer. 4. The materials are linear elastic. In elastic layered system, two different interface conditions are considered: full bond (full friction) and full slip (frictionless). The interface condition is represented by Goodman's constitutive law: where τ denotes the interface shear stress, Ks is the horizontal shear (interface) reaction modulus and ∆U is the relative horizontal displacement at the interface. Within BISAR programme, the slip between pavement layers is accounted for by employing the concept of shear spring compliance (standard or reduced). The physical definition of the standard shear spring compliance (also known as the inverse of the interface's horizontal shear reaction modulus), AK, is given as follows: = relathe tive horiontal displacement between layers interface′s stress which relation is expressed mathematically through the parameter α, defined as where a is the radius of the load (m), E is the modulus of the layer above the interface (Pa), v is the Poisson's ratio of that layer and α is the friction parameter, with 0 ≤ α ≤ 1 (α=0 means full friction, α=1 means complete slip). The reduced shear compliance, ALK (m), is defined as According to Sutanto [2], among those aforementioned models to characterize the interface bond condition; the shear reaction modulus, Ks, seems to be the most widely used by researchers. Although the models incorporated into the Finite Element (FE) analysis might be more accurate in representing the bond conditions, the parameter Ks is less complicated and can be easily incorporated into BISAR programme to analyze the effect of bond on the state of stress, strain and deflection within the pavement structure.

Estimation of design life
Brown and Brunton [9] highlighted that the ultimate state of pavement at the end of their design life (usually 20 years) may either be one of a "failure" or of a "critical" condition. Failures indicate that the pavement is no longer suitable for use and, this state is distinguished when there is existence of about 20mm rut or extensive cracking in the wheel tracks. Meanwhile, the "critical" condition is defined by a 10mm rut or the first appearance of wheel track cracks.
The Nottingham Design Method estimates pavement's life according to critical strains and mixture characteristics, which can be expressed by the following equations: where N is the number of load applications to failure, εt is the horizontal tensile strain, εz is the vertical compressive strain, VB is the percentage of the binder by volume, SPi is the initial softening point of bitumen and fr is a rut factor (fr = 1.56 for DBM). Values of VB, SPi and fr are adopted from Brown and Brunton [9] and Jabatan Kerja Raya [10]. Table 1 and Table 2 show the properties and materials of pavement layers studied in this research, stiffness data refers to Jabatan Kerja Raya [10] and the effect of road geometry was not studied. The condition of the bond at the interfaces considered was evaluated by the horizontal shear reaction modulus, Ks. The interface is considered fully bonded when Ks ≥ 10,000 MN/m 3 . While for Ks ≤100 MN/m 3 , the interface can be considered as fully debonded.

Results and discussion
From Table 1 below, since the bituminous layers of the pavement structure is comprised by 3 different layers (two interfaces), the effect of the bond at the base of respective bituminous layers will be analyzed separately. Firstly, the effect of the bond between the wearing course and binder course was being analyzed (Pavement 1A), then the effect of the bond between the binder course and first layer of road base was being analyzed further by using BISAR 3.0 (Pavement 1B). The results of the analysis using BISAR 3.0 are presented in Table 3. It shows the computed maximum horizontal tensile strain εt at the bottom of bituminous layers for Pavement 1A, 1B and 2. The values range from 151.8 to 306.8 × 10 -6 , 82.6 to 344.7 × 10 -6 and 66.25 to 102.7 × 10 -6 for Pavement 1A, 1B and 2 respectively. Similarly, the computed vertical compressive strains on top of the subgrade are 290.7 to 344.7 × 10 -6 , 290.7 to 415.4 × 10 -6 and 271.1 to 333.0 × 10 -6 for the Pavement 1A, 1B and 2 respectively.  Fig. 2, 3 and 4 show the pavement's life to failure condition of different de-bonded interfaces. The life to failure due to fatigue and life to failure due to deformation is referred to as Nf and Nd, respectively. In Fig. 2  interface between layer 1 and layer 2 (wearing and binder course) while analysis for Fig. 3 is observed at the partially bonded interface between layer 2 and 3 (binder and road base course).