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Moisture Damage in Fibre-Reinforced Bituminous Road Materials Ibrahim Kamaruddin and Madzlan Napiah Universiti Teknologi PETRONAS, Seri Iskandar, Perak, Malaysia Abstract Water related damage in bituminous pavements is a major distress form in many South-east Asian countries like Malaysia. Highway engineers acknowledge that stripping or loss of adhesion between the aggregate surface and the bitumen film coating the aggregates and other forms of moisture damage in bituminous pavements are shortening the pavement life as a result of this damage. There is thus a need to evaluate the damage caused by water to pavements. The use of the retained strength ratios obtained from laboratory moisture damage tests is a useful tool in making quantitative predictions of the related damage caused by water. This study involved laboratory work on the effect of water on the performance of bituminous mixtures. Comparisons are made between the performance of Hot-rolled Asphalt (HRA) bituminous mixtures containing a base bitumen of 50pen grade to that of a polymer-fibre reinforced HRA mixtures. Keywords Fibre-reinforced bituminous mix, stripping, tensile test, saturation, voids and porosity. 1. Introduction The damaging effects of water on the physical properties and mechanical behaviour of bituminous mixtures have been the focus of many studies. Many laboratory tests have been developed in order to evaluate and quantify the amount of damage caused by water on bituminous mixtures. The most widely used laboratory method in conducting these tests appear to be the immersion-mechanical tests which measures the changes in mechanical properties of the bituminous specimens after exposure to water. Typically the results are reported in terms of percentage retained strength of the specimens. This paper is based on some laboratory work and addresses the damaging effect of water in polymer fibre-reinforced bituminous mixtures. 2. Stripping in Bituminous Mixtures The unfilled void spaces in compacted bituminous mixtures are able to hold sufficient quantities of water that can cause distress and damage and reduce the mixture performance. The volume of these voids varies depending on the nature of the voids, characteristics of the mix and the degree to which they are compacted in the pavement (Lottman, 1982). The damaging effect of water will give rise to stripping, de-bonding or loss of adhesion of the bitumen from the aggregate surface. This is brought about by the loss of cohesion and softening of the binder due to the action of the moisture within the bitumen or bituminous mix. Both stripping and softening can occur in the same mixture. Stripping is the physical separation of bitumen from the aggregate produced by the loss of adhesion between the bitumen and the aggregate surface primarily due to the action of water or water vapour. It is an aggregate interfacial phenomenon and induces the loss of stability in the bituminous mixture that in turn promotes failure (Fromm, 1974). The phenomenon can be further aggravated by the presence of aggregate surface coatings and by aggregates with a smooth texture surface. Stripping is primarily an aggregate problem but that the type of bitumen used is also important (Kennedy et. al, 1983). Softening is the general loss of stability of a bituminous mixture that is brought about by a reduction in cohesion within the bituminous mix matrix due to the action of water. The resulting damage brought about by water can be assessed quantitatively by mechanical tests. Properties such as loss of tensile strength or decrease of resilient and stiffness moduli have been measured. Water induced damage is usually easy to identify when stripping is evident. Where a loss of pavement stiffness or moduli occurs without the visual evidence of stripping, the cause of the problem is less easily recognised. There are a number of laboratory methods and tests that are cited in the literatures that enable the determination of the moisture susceptibility and thus the proneness of bituminous mixtures to stripping or de-bonding. Based on previous work (Tunnicliff and Root, 1983 and Gilmore et. al, 1983), it appears that vacuum saturation of the bituminous specimens followed by conditioning and testing using the Indirect Tensile test show promise for introducing moisture into the specimens and measuring their strength to predict the moisture susceptibility of the mixtures. Specimens were moisture conditioned before the determination of their structural strengths as those mixtures most susceptible to moisture damage would have the lowest structural strengths following conditioning. The retained strength is determined by comparing the dry tensile strength to the wet conditioned tensile strength of the bituminous mixtures. 3. Adhesion Properties of Bituminous Mixtures Loss of adhesion is especially common in bituminous mixes which utilises hydrophilic aggregates i.e. aggregates which have an affinity for water. An example of this type of aggregate is granite which is widely used in road pavement construction in Malaysia. Ishai and Craus (1977) listed two ways in which modifications are possible should the use of hydrophilic aggregates prove to be unavoidable: 1. modification of the adhesion properties of the bitumen by additives 2. modification of the surface properties of the aggregates by treating them with cement- water solution or hydrated lime-water solution. Ishai and Craus (1977) also acknowledged that of the two modifications, the first is more useful in asphalt paving technology. However, high quality additives are quite expensive in the mass production of bituminous materials. 4. Materials Used in the Investigation 4.1 Mineral Aggregates, Filler and Bitumen Limestone aggregates and Ordinary Portland cement (OPC) filler and a binder of nominal penetration Grade 50 pen were used in the preparation of the Hot-Rolled Asphalt (HRA) bituminous mixtures in this study. Some relevant properties of the material used are shown in Table 1. 4. 2 Synthetic Fibres Two types of synthetic fibres namely polypropylene and polyester were used in this study. The fibres were used as a partial replacement of the filler; on an equal volume basis; at two concentrations of 0.5% and 1% by weight of the mix. The fibres; in chopped form; were the by- products of the textile industry and thus their potential use was desirable on environmental grounds. Some characteristics of the fibres used are shown in Table 2. Table 1: Properties of Mineral Aggregates, Filler and Bitumen Material Percentage by Weight Relative Density Absorption % BS Specification Coarse Aggregate 35 2.75 0.47 BS 594: Part 1: 1992 Table 3, Type F Wearing Course designation 30/14 Sand 55 2.65 1.37 Filler (Ordinary Portland Cement) 10 3.15 Penetration (0.1 mm) Softening Point (°C) Penetration Index, PI Bitumen 52 48.5 -0.37 Table 2: Characteristics of Fibres Used in Study Specific Gravity Denier Length (mm) Average Diameter (µ µµ µm) Degradation Temperatur e (° °° °C) Polypropylene (PP) 0.91 6 6 22* 160 Polyester (POL) 1.41 3 6 17* 250 *Values obtained from 20 readings using a light microscope at 400X magnifications. The preparation of the bituminous samples in the laboratory is an energy intensive process. In order to maintain thermal stability when using the polypropylene fibres, it was decided that the mixing temperature when preparing the Hot Rolled Asphalt (HRA) mixture will not exceed 140°C and compaction be done at 130°C. 5. Wet-Dry Indirect Tensile Test The wet-dry indirect tensile test was adopted as a principal measure of the bituminous mix response to water damage. Most evaluations of water damage have been assessed quantitatively by mechanical tests in which such properties as loss of tensile strength or decrease of resilient and stiffness moduli have been measured. These are then given in the form of a tensile-strength ratio and a modulus of elasticity ratio, for which the tensile strength and modulus of the dry specimens served as references. The tensile strength ratio (TSR) and modulus of elasticity ratio (MER) are dimensionless numbers used to represent the portion of tensile strength and modulus retained following conditioning. Low values indicate high moisture damage. These ratios are given as: Tensile Strength Ratio (TSR) dry wet ITS ITS TSR = Modulus of Elasticity Ratio (MER) dry wet MER MER MER = Lottman (1982) used the static indirect tensile strength test to study the effect of water on bituminous mixtures and recommended a minimum tensile strength ratio of 0.7 to differentiate between a stripping and a non-stripping bituminous mix while Maupin (1982) reported values of between 0.7-0.75. Ishai and Nesichi (1988) cited values of 60-75 percent retained stability values for roads and highway pavements and 75 percent for airfield pavements as the quality criteria used in Israel. Kennedy and Anagnos (1984) were also of the opinion that mixtures with less than 70 percent retained strength are moisture susceptible and would require treatment before being used as bituminous material. 6. Degree of Saturation The degree of saturation gives a measure of the amount of water that is absorbed by the specimen into its permeable voids. The degree of saturation is thus defined as the ratio of the volume of water in the wet specimen to the total volume of voids in the specimen. The creation of a degree of saturation in the laboratory high enough without damaging the specimens and that the retained strength can be determined involves a moisture conditioning process. Static soaking seems to provide ideal condition for stripping to occur while both pressure and vacuum saturation procedures may create damage to the specimens. For practical laboratory purposes however, static soaking may require too much time. Saturation by partial vacuuming for short period of time has therefore been used in moisture damage studies on bituminous mixtures. If the volume of absorbed water exceeds the volume of voids, the specimen has been supersaturated and damage. A number of researchers have come up with various regimes for vacuuming and saturating the bituminous specimens. Lottman (1982) for example used 26-inches of mercury to vacuum the specimens while lower levels of vacuuming (4 and 15-inches of mercury) have been cited in the literature (Gilmore et. al, 1983, Graf, 1986). In addition to vacuuming, Lottman (1982) also subjected the specimens to an advanced moisture conditioning in which thermal cycles or a cycle of freezing-soaking was carried out. Ishai and Nesichi (1988) subjected the specimens to hot water immersion (at 60°C) for up to 14 days and testing the specimens at different immersion period to determine the retained strength of the specimens while Kennedy et. al. (1983) subjected the specimens to the boiling test, a freeze-thaw cycle before conducting the indirect tensile test on dry and conditioned specimens. 7. Void Structure in Bituminous Mixtures The moisture conditioning process attempts to allow water to penetrate and occupy the air voids in the specimen. An appreciation of the void structure in bituminous mixtures is thus very vital. Kumar and Goetz (1977) conducted a laboratory study to examine the influence of asphalt film thickness, voids and permeability on asphalt hardening in asphalt mixtures and came up with a hypothetical model of the air voids system in a compacted bituminous mixture. Different water saturation techniques were employed in their study that included a 24 hours soaking and vacuuming at different absolute pressures. The model divides the air voids system into three categories; through passage accessible air voids, dead end accessible air voids and non- accessible air voids. The 24 hour soaking allows water to only occupy the through passage accessible voids. The 24 hour soaking and hand pumping allowed the water to occupy the through passage accessible voids as well as a small portion of the dead end accessible voids. Vacuuming with an absolute pressure of 2.5cm mercury allowed the water to occupy the through passage accessible voids and most of the dead end accessible voids. It is not possible for the water to occupy the non-accessible voids unless the specimen is damaged in the process of saturation. Therefore it is important in studies on water damage on bituminous mixtures to attain a degree of saturation high enough without damaging the specimens to enable the retained strength of the specimens to be determined. 8. Experimental Procedures As the water susceptibility of the mixes was to be determined, the specimens were tested in both the dry and wet condition. The dry conditioning involves curing the specimens at room temperature for two days prior to testing. The wet conditioning involved subjecting the bituminous samples to a combination of air vacuum, vacuum saturation and static soaking. Air vacuuming is to evacuate all the accessible pores from air and water. The objective of vacuum saturating the specimens was designed to accelerate the moisture damage process. It is extremely important that the process of vacuum saturating the specimens do not result in a degree of saturation greater than 100% which is indicative that more water was introduced into the voids than there are void space, making comparison between samples no longer valid. Figure 1 is a schematic diagram of the vacuum saturation apparatus. In this study, the specimens were placed in a thick-walled desiccators jar. Valves (W) and (A) are closed and valve (V) which led to the vacuum pump is opened. The air vacuuming process takes about half an hour to drive out all the air trapped in the accessible voids. Distilled water was then used to fill the jar to about 2 cm. above the specimens and about 3 cm. below the top rim of the jar. This was followed by a 1-hour vacuum saturation period at 1 atmospheric pressure which was considered a pre-treatment of moisture conditioning and a means of water-saturating the specimens, during which time the jar surfaces was gently agitated. After the one hour of vacuuming, the vacuum was removed and the inside of the jar was allowed to reach ambient atmospheric pressure and the specimens undergoing static soaking for a period of 24 hours for the purpose of achieving a constant weight of the specimens (fully saturated condition). Earlier studies by Soelistijo (1995) and Celik (1996) on moisture damage in bituminous materials reveal that this saturation regime produced a degree of saturation in the region of 75-100% of the bituminous sample voids and was therefore also adopted. After the immersion process, the specimens were weighed in water. They were then weighed in air in the saturated surface dry condition. The volumes of the saturated specimens were determined by subtracting the mass of the saturated specimen from its mass in the saturated surface-dry state. The volume of absorbed water was determined by subtracting the air-dry mass of the specimen from its saturated surface-dry mass. The degree of saturation is given by: %100(%) x resVolumeofPo rsorbedWateVolumeofAb turationDegreeofSa = Alternatively, the degree of saturation can be determined from the following relationship: %100 )( (%) x CBV AB turationDegreeofSa − − = where: A = dry weight of specimen in air (gm) B = weight of surface-dry specimen after saturation (gm) C = weight of saturated specimen in water (gm) V = porosity of the specimen (%) The moisture conditioning process using the vacuum saturation technique can also be used to measure the porosity of the specimens which is determined by dividing the volume of absorbed water by the volume of the saturated specimens. Lottman (1982) suggested the following equation for the calculation of measured porosity. %100 )( )( (%) x CB AB rosityMeasuredPo − − = where: A = weight of dry specimen in air (gm) B = weight of surface-dry vacuum saturated specimen in air (gm) C = weight of vacuum saturated specimen submerged in water (gm) 1. Vacuum Pump 2. Vacuum Gauge 3. Trap 4. Vacuum Desiccator 5. Three-way Valves 6. Water Container Figure 1: Schematic Diagram of Vacuum Saturation Apparatus 9. Discussion of Results Figure 2 shows the relationship between the degree of saturation and bitumen content for the control Hot-Rolled Asphalt (HRA) mixture in comparison with the fibre modified mixes. The general trend is for the degree of saturation to decrease with increasing bitumen content. The addition of fibres appears to bring about an increase in the degree of saturation, this increase being more pronounced at the higher fibre concentration. This may be the result of the higher porosity that is associated with the higher fibre content mixes. The polypropylene mixes appear to exhibit better result than the polyester fibre mixes. For all the mixes, the saturation lines obtained appear parallel to one another with a somewhat similar slope. This is indicative that the degree of saturation in all the mixes decreases consistently with increasing bitumen content. Figures 3 and 4 show the variation between the calculated and measured porosity between the control and the polypropylene fibre modified mixes and the polyester fibre modified mixes respectively. The calculated porosity gives a measure of all the voids in the specimens that include both the accessible and non-accessible voids while the measured porosity as was obtained from the moisture conditioning process determined only the accessible voids. The calculated porosity therefore is always greater than the measured porosity as shown in the figure. The general trend is the porosity decreases with increasing bitumen content. The result of adding fibres to the HRA mix resulted in higher porosity in the resulting mix. 40 50 60 70 80 90 100 110 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10 Bitumen Content (%) Degree of Satur ation (%) Control 0.5PP 1PP 0.5 POL 1 POL Figure 2: Degree of Saturation vs Bitumen Content Bitumen Content for Control and PP Fibres 0 2 4 6 8 10 12 14 16 18 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10 Bitumen Content (%) Porosity (%) Control (M) Control (C) 0.5PP (M) 0.5PP (C) 1PP (M) 1PP (C) Figure 3: Calculated and Measured Porosity vs Bitumen Content for Control and PP Fibres Mixtures The relationship between the measured and calculated porosity is plotted graphically in Figure 5 resulting in a linear relationship between the two. The point of intercept of the lines with the calculated porosity axis gives an indication of the percentage of non-accessible or unconnected pores in the respective mixes. The addition of fibres appears to reduce the percentage of non- accessible pores in the mix; this reduction was seen to be more pronounced in mixes with greater fibre content. A general trend shown from the figure suggest that increasing the bitumen content resulted in an increase in the unconnected voids as the bitumen fills up the void space or continuous channels in the specimen. Subsequently, this reduces the connectivity of the voids. The presence of the fibres also suggests an increase in the connectivity of the voids as the porous nature of the fibres gave a continuous channel (path) of void space. It must also be remembered that the control mix gave lower porosity than the fibre incorporated mixes, thus justifying its behavior as in Figure 5 0 2 4 6 8 10 12 14 16 18 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10 10.5 Bitumen Content (%) Porosity (5) Control (M) Control (C) 0.5PP (M) 0.5PP (C) 1POL (M) 1POL (C) Figure 4: Calculated and Measured Porosity vs. Bitumen Content for Control and POL Fibres Mixtures The indirect tensile strength (ITS) ratio is effectively an indication of the amount of strength loss due to the effect of water. The variation of indirect tensile strength ratio and the degree of saturation is shown in Figure 6. The ITS ratio shows it decreasing with increasing degree of saturation for all the mixes. The lines obtained are rather parallel to one another indicative that the decrease is somewhat similar. The fibre-modified mixes exhibited higher ITS ratios of around 11-25% over the control mix. It is appropriate to be reminded that the fibre incorporated mixes had higher porosity and permeability than the control mixes that will permit easier access to water and increase the potential for stripping. It may thus appear that the more viscous binder of the fibre- incorporated mixes had a better cementing and adhesive property at the binder-aggregate interface that resulted in a reduction in stripping. It is believed that de-bonding may not have been solely responsible for the decrease in wet tensile strength values but other moisture damaging factors such as binder matrix softening may have been responsible as well. 10. Conclusions Based on this study, the following conclusions can be drawn: 1. Changes in both the cohesive properties of the bitumen and the adhesion of the bitumen to the aggregate surfaces may occur as a result of exposing the bituminous mixtures to moisture. Polymer fibre incorporation into bituminous mixtures helps reduce the high level of moisture damage that was noted from the control mix. The polyester fibre modified mixes also showed lower moisture susceptibility than those of the polypropylene mixes at the same fibre concentration. However, the 0.5% fibre concentrated mixes showed better resistance to water damage than that at the 1% concentration. 0 2 4 6 8 10 12 14 16 18 0 2 4 6 8 10 12 14 Measured Porosity (%) Calculated Porosity (%) Control 0.5PP 1PP 0.5 POL 1 POL Figure 5: Calculated and Measured Porosity for Different Mixtures Degree of Saturation 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 1.05 1.1 50 60 70 80 90 100 Degree of Saturation (%) Indirect Tensile Strength Ratio Control 0.5PP 1PP 0.5POL 1POL Linear (Control ) Linear (0.5PP) Linear (1PP) Linear (0.5POL) Li near (1POL) Figure 6: Indirect Tensile Strength Ratio vs Degree of Saturation 2. It is important to remember that mixes with polymer fibres had greater bitumen content and yet greater void contents than the control mix. Regarding resistance to moisture damage, these two parameters would be expected to oppose each other. Suffice to say that the additional bitumen in the fibre mixes increased the film thickness on the aggregate particles thus affording additional protection from moisture. The 0.5% fibre concentration may have provided enough reinforcement across the plane of failure in the mixtures while the 1% fibre [...]... Moisture Damage to Bituminous Paving Mixtures by Long Term Hot-Immersion”, Transportation Research Record No 1171, 1988, pp 12-17 Ishai, I And Craus, J., “Effect of Filler on Aggregate-Bitumen Adhesion Properties in Bituminous Mixtures”, Proceedings of the Association of Asphalt Paving Technologists, Vol 46, 1977, pp 228-258 Kamaruddin, I., “The Properties and Performance of Polymer Fibre- Reinforced...concentration may have far too high void contents that allowed for more water penetration into the mixtures 3 The incorporation of polymer fibres in bituminous mixtures also acts to decrease the moisture sensitivity of the bitumen to aggregate bonding This may be due to the strengthening of the wetted binder matrix which promote both adhesion and cohesion retention References Celik,... Maupin, Jr., G.W., “Result of Indirect Tensile Test Related to Asphalt Fatigue”, Highway Research Record No 404, Highway Research Board, 1972, pp 1-7 Soelistijo, A., “Stability and Tensile Strength of Bituminous Mixtures”, Unpublished MSc (Eng) Thesis, University of Leeds, England, 1995 Tunnicliff, D.G and Root, R.E., “Testing Asphalt Concrete for Effectiveness of Anti-stripping Additives”, Proceedings . water in polymer fibre- reinforced bituminous mixtures. 2. Stripping in Bituminous Mixtures The unfilled void spaces in compacted bituminous mixtures. performance of bituminous mixtures. Comparisons are made between the performance of Hot-rolled Asphalt (HRA) bituminous mixtures containing a base bitumen of