Research On Semi-flexible Material Cracking Performance Evaluation And Improvement Measures

Semi-flexible pavement (SFP) material combines the characteristics of both concrete andasphalt pavement and involves filling the voids of a porous asphalt mixture of between25-32%with specially designed cementitious grout. The idea is to combine the two conventionalpavements-asphalt and concrete materials so as to eliminate the weaknesses possessed by thetwo conventional pavements. The Semi-Flexible Pavement was originally developed in latesixties and later became effective as a fuel and abrasion resistant surfacing material. However,due to the high bearing capacity and rut resistance properties of this pavement material it ishighly suited for heavy duty areas.The major setback of the SFP material performance however is related to cracking. Thedifferences in the elastic modules of the asphalt and the cement have been identified as amajor cause of cracks and other defects in the SFP material. The cracking of the SFP materialis mostly caused by the stiffness of the cementitious grout in particular which acts as the rigidcomponents of the pavement material. Even though SFP has been used since the sixties andhas aroused the interest of many researchers, presently there are no well prescribed andreadily available standards for designing the porous asphalt skeleton and the cementitiousgrout for the SFP. What is available are only technological guide for application,manufactures technical manuals and some researchers work. Therefore objective of this paperis to design an appropriate cementitious grout that can effectively coop with the differences inthe elastic modulus between itself and the asphalt mixture as to control the crack.The SFP material has two (2) important design components: the porous asphalt skeletonmixture and the cementitious grout. In designing the void content of the porous asphaltmixture, most researchers stay within a percentage of between25-32%. In designing thecementitious grout, researchers combine different admixtures and or additives in order toobtain the desired grout. The admixtures includes fly ash, latex, emulsified asphalt, carboxylicether, some superplasticizers and resin admixtures etc. The differences in the workingperformance (fluidity) of the cementitiou grout lies with the proportioning and use of thesedifferent types of admixtures and or additives. In order to design the most desiredcementitious grout to be used to fill the porous asphalt mixture, this research work used flyash, emulsified asphalt and carboxylic styrene-butadiene latex as the main admixtures and oradditives to design three (3) different grouts.A series of try and error experiment was conducted on the grout without admixtures toascertain the most suitable water-cement (w/c) ratio to be used in the mixture. After analyzing and evaluating the mechanical performance of these grouts, w/c ratio of0.54,0.60and0.66was used to design initially nine (9) different cementitious grouts with different admixturesand or additives proportion. The selection of these w/c ratios was based on producing amixture that can easily flow into the voids of the porous asphalt skeleton without excessivevibration and to produce a grout that is strong enough to resist the stresses and strains thepavement is likely to experience during it life time without failing unexpectedly. After theanalysis of the specimen’s mechanical performance test results, the following were used todesign the final highly flowable cementitious grout mixture:0.63as w/c ratio;5%fly ash;4%emulsified asphalt; and9%carboxylic styrene-butadiene latex.A total of ten (10) mixture were produced with the first mixture been ordinary cementgrout mixture without any admixture. This was to study the mechanical behaviour of mixtureswith and without admixtures. Fly ash, Emulsified asphalt and Carboxylic butadiene-styrenelatex was added in the second, third and fourth mixtures respectively. Three different mixproportions were used for each admixture, and for each mixture six (6) cubes of specimenwere obtained. This was to enable us perform the dry shrinkage test, flexural strength test, andcompressive strength test.Three (3) highly porous asphalt mixtures having voids content of24%,28%and32%were designed and each filled with three (3) different cementitious grout. This allowed forvarious tests flow, flexural, compressive, shrinkage etc. to be conducted to ascertain itsmechanical performance properties. Comparative analysis of the effect of the differentadmixtures’ cementitious grout and the varying porosity of porous asphalt mixtures on themechanical performance of the SFP material was conducted. The research established amongothers that; the void percentage of the porous asphalt mixture did not have any significanteffect on the difference in the performance of the type of admixtures used in the cementitiousgrout. However it was noticed that, the higher the void in the asphalt mixture, the better themechanical performance of the SFP materials. This was attributed to the increase in thecementitious grout content as the cement forms the rigid part of the SFP material–makingthe material behave more like a concrete pavement.Irrespective of the type of test and the porous asphalt mixture’s void percentage, thecarboxylic styrene-butadiene latex mixed cementitious grout (SFP material) showed signs ofgreater stability (Dynamic stability, Marshall Stability and flow value, Tensile splittingstrength) than the other admixtures.The cohesive zone model concept was introduced to access the cracking performance ofthe semi-flexible pavement material types. Using linear elastic beam theory on a double cantilever beam, the analytical solution was obtained. This was used to establish the relationship between "the crack length and displacement" in terms of the Young’s modulus (E), the end displacement (Δ), the beam height (H), and the cohesive fracture energy (Gc).The cracking phenomenon as observed from this model’s graphs indicates that, the higher the fracture energy Gc (cohesive strength) of the SFP material the better it resistance to external deformation and hence the lower the crack length. The Carboxylic butadiene-styrene latex cementitious grout was the most consistent performer among three (3) admixtures. The emulsified asphalt cementitious grout was quite stable but the fly ash cementitious grout was the weakest in almost all the test performed.