STRENGTH CHARACTERISTICS OF COMPOSITE PAVEMENTS

Abstract

Acomposite pavement consists ofmultiple, structurally significant layers ofdifferent composition. It may consist ofeconomical concrete base for providing auniform support and bonded to arelatively thin upper layer ofhigher strength composite concrete matrix. With the advent of composite materials like Steel Fibre Reinforced Concrete (SFRC) and Ferrofibrocrete (FFC), the recent emphasis has been on the construction of thinner pavement sections possessing superior strength characteristics and relatively higher life expectancy. Composite pavements exhibit superior performance with respect to load carrying capacity, flexural strength and resistance against crack propagation. Ferro-fibrocrete pavements exhibit multiple crack mechanism and low crack width with cracks distributed over a larger surface area of the pavement. They display significant improvement in pavement serviceability due to reduction in damage by remarkable post cracking ductility. The composite materials may be used in pavement layers in a variety of combinations. In order to assess their suitability in field applications, it was planed to evaluate their basic strength characteristics and assess their structural behaviour from the load deformation characteristics ofsemi- full scale composite pavement sections. The present study was aimed to investigate the strength parameters of composite materials and evaluate and compare the load carrying capacity, cracking behavior and other pavement performance parameters of composite pavements with those of plain cement concrete pavements. The experimental work planed in this investigation consisted of compressive strength, static flexural strength, flexural fatigue strength, modulus of elasticity &Poisson's ratio testes on cube specimen (150 mm x 150 mm x 150 mm), beam specimen (100 mm xlOO mm x 500 mm), and cylindrical specimen ofcomposite materials, consisting of PCC, SFRC, FFC, PCC+SFRC and PCC+FFC respectively in layers. Mix design was done for dry lean concrete (DLC), plain cement concrete (PCC), steel fibre reinforced concrete (SFRC ) and ferro fibrocrete ( FFC ). The test program included workability tests for PCC and SFRC mixes, preparation and testing of composite specimens for compressive strength, flexural strength, fatigue characteristics and modulus of elasticity &Poisson's ratio, tests for microstructural behavior of DLC, PCC, SFRC and FFC, and laying and testing of SFRC, FFC, PCC- SFRC and PCC- FFC composite pavements over DLC base. Microstructural study was carried out using scanning electron microscope (SEM) to analyse the internal structure of composite matrix and assess the extent of pores, micro cracks and internal configuration. Laboratory investigations on compressive strength yielded 3%, 8.8%, 15.3% and 23% increase in respect of PCC-SFRC, PCC-FFC, SFRC and FFC mixes respectively over PCC. The improvement in flexural strength was observed to be 29.2% to 82.8% for 100 mm total thickness for various combinations composite material layers. Similarly cyclic load tests on beam specimen indicate the improvement in flexural fatigue strength by 37.1%, 45.3%, 72.9% and 87.5% in respect of PCC-SFRC, PCC -FFC, SFRC and FFC combinations respectively over PCC. Micro - graphic observations using Scanning Electron Microscope (SEM) exhibit the massive calcium hydroxide crystal formation and establish the superior strength of ferrofibrocrete matrix as compared to PCC. In FFC, the porous spaces in the well-mixed paste of li the matrix are much less than those in PCC. Further, the SEM observations reveal absence of any crack in SFRC mixes while micro-cracks were observed in PCC specimen. The composite pavements having different thickness of different materials in various layers over dry lean concrete base course were also laid. The size of pavement model slab was kept 1.8 mx 1.8 mowing to the restriction of size of test pit. The various combinations ofmaterial and thickness used in composite pavements are given below. For comparison, a PCC pavement slab was also laid. All the composite pavement sections were tested for their performance with respect to deflection, strain, crack width and load carrying capacity. (i) One 120 mm thick PCC pavement slab over 100 mm dry lean concrete (designated as slab Si) (ii) One 120 mm thick PCC+SFRC composite pavement slab composed of 40 mm of SFRC over 80mm PCC laid over 100 mm dry lean concrete (S2). (iii) One 100 mm thick PCC +SFRC composite pavement slab composed of25 mm SFRC over 75 mm PCC laid over 100mm (S3). (iv) One 100 mm thick PCC+SFRC composite pavement slab composed of40 mm SFRC over 60mmPCC laid over 100mm (S4). (v) One 100 mm thick PCC + FFC composite pavement slab composed of 25 mm Ferro fibrocrete (FFC) over 75 mm PCC laid over 100 mm dry lean concrete (S5). (vi) One 80 mm thick PCC+ FFC composite pavement slab composed of 20 mm FFC over60mmPCClaidover 100 mmdry leanconcrete (S6). (vii) One 40 mm SFRC pavement slab over 100 mm dry lean concrete (S7). (viii) One 25 mm FFC pavement slab over 100 mm dry lean concrete (S8). The above pavement slabs were tested for corner, edge and central loading using a rigid plate of 300 mm diameter. The investigation on composite mixes and pavements consisting of various combinations of PCC, SFRC and FFC establish that thin sections of 100 mmand above may in be advantageously employed in heavy duty pavements and exhibit appreciably higher load carrying capacity, lower crack widths and superior post cracking ductility as evidenced by enhanced performance under cyclic loading. From the considerations of the thickness requirements and load carrying capacity, the combinations PCC+SFRC (60 mm + 40 mm) and PCC+FFC (60 mm + 20 mm) were observed to give the optimum results, the combinations PCC+FFC being the most economical. Thin PCC+FFC (80mm) composite pavement section exhibited superior performance with appreciably low crack width at 120 kN load and has the potential advantage as an economical pavement section for heavy duty pavements. This section can also be advantageously employed in strengthening the pavement with thin overlays. The higher initial cost is compensated by better serviceability due to evidence of reduced crack width, reduced maintenance and much higher life expectancy. Finally, the composite pavement slab has been analysed besed on elastic properties of constituent materials in layers. The computer program "COMPAVE" developed on the basis of formulations of the constitutive relations for a lamina composed of general orthotropic material may be employed to evaluate the stresses and strains in a pavement section. It can also be used to design a composite pavement considering various combinations of materials & layer thickness. IV