International Journal of Innovative Research in Advanced Engineering (IJIRAE) ISSN: 2349-2763 Issue 08, Volume 3 (August 2016) www.ijirae.com _________________________________________________________________________________________________ IJIRAE: Impact Factor Value – SJIF: Innospace, Morocco (2015): 3.361 | PIF: 2.469 | Jour Info: 4.085 | Index Copernicus 2014 = 6.57 © 2014- 16, IJIRAE- All Rights Reserved Page -43 An Investigation of the Interlayer Adhesion Strength in Deeper Layers of the Pavement Structure Naphtal NTIRENGANYA * Denis KALUMBA Marius De Wet Department of Civil Engineering Department of Civil Engineering Department of Civil Engineering Integrated Polytechnic Regional Centre Kigali, University of Cape Town, RSA Stellenbosch University, RSA Rwanda Abstract— A road pavement structure is typically composed of different layers arranged one on top of the other, all supported by a natural or improved subgrade. The main purpose of this configuration is to provide the most effective structure with adequate potential to spread traffic loading from the surface to the subgrade with minimum damage. In spite of material properties and construction techniques, researchers have shown that the overall pavement performance is significantly influenced by the interlayer adhesion condition throughout the pavement structure ([8], [11], [21], [22]). Lack of intimate contact between layers results in them acting as individuals rather than as a thick bonded unit. This therefore induces overloading of layers which leads to premature deterioration of the entire structure due to traffic induced distresses. This work, therefore investigated the interlayer adhesion characteristics between the granular base and the lightly cemented subbase of a typical South African pavement structure. The influence of bonding condition on the overall pavement performance was also studied. A series of interlayer direct shear tests was run on 300 x 300 mm samples made of two layers: a 100mm G2 Granular Base (GB) compacted on top of the 100mm Cement Treated Subbase (CTSB) composed of a G5 material stabilised with 1.8% of cement. Effects of the CTSB scarification, normal pressure and moisture conditions were analysed whereby the results of the interlayer strength tests were compared with those of intra-layer strength tests. The comparative analysis showed that scarifying the CTSB before laying the GB enhances intimate contact between two layers and stimulates the unison interaction which, according to structural modelling results, improves the overall pavement performance. Keywords— Interlayer adhesion, Direct shear test, Cement treated subbase, Granular base, Pavement life. I. INTRODUCTION AND BACKGROUND The multi-layered structure adopted as a typical South African pavement structure dates back to the Roman era. This structure was made of a large stone foundation with a surfacing course of smaller stones and gravel, confined between raised stone kerbs. The modern structure is not far from that and it comprises a surfacing layer made of asphaltic materials or concrete, base and subbase courses which might be natural or stabilised, selected layers and natural subgrade. Basically, road pavement performance is governed by the strength and stiffness of the materials available in each individual layer. The failure mechanism of main pavement materials has been well documented from many years ago and fairly well understood by pavement engineers ([9], [17], [6], [18] and [10]). In most cases, structural failure is generally linked to lack of maintenance and pavement overloading. However, the lack of mutual interaction between layers has also been identified to influence the pavement response against traffic induced stress and strain across the entire structure, and consequently affects the pavement performance. Researchers like Kruntcheva in [11] used theoretical analysis to establish the impact of interlayer adhesion on stress- strain distribution throughout the pavement structure. They analysed a pavement structure using a multi-layered linear elastic program by considering various degrees of interface adhesion between pavement layers. The results indicated that the condition of interlayer adhesion can reduce the life of a pavement structure by up to 80%. They also conducted static linear and nonlinear two-dimensional finite element analyses and similar results were found. After realizing the influence of the interface condition on the pavement performance, different laboratory and in situ testing methods have been developed in different countries to assess the degree of adhesion between pavement layers. Since the interface was admitted to fail by shear, most of the testing approaches were typically based around shear testing. From the comparative study conducted by Raab as illustrated in [14], the direct shear test was selected as the most reliable and effective method for testing interlayer adhesion strength in the pavement structure.