Water Absorbing And Evaporating Characteristics Of Cement Asphalt Mortar And Effect Of Wetting-drying Cycles On Its Properties

Abstract:Cement asphalt (CA) mortar is one of key engineering materials in slab ballastless tack, which is one of the main tack structure types of China high-speed railway. Engineering practice shows that the water attack is one of main reasons for the degradation of CA mortar layer in slab ballastless tack structure. Cyclic wetting-drying phenomena can cause the changes and transmission of internal moisture content in CA mortar layer, thus resulting in the damage in the microstructure of CA mortar layer and the decrease in its mechanical properties.In this paper, water absorbing and evaporating characteristics of CA mortar and effect of wetting-drying cycles on its properties were investigated under laboratory conditions. The major experiments and conclusions are as follows:(1) The advancing contact angle is measured by sessile drop method on the surface of membrane condensed from an aqueous emulsified asphalt dispersiones with a little different additives such as cement, inorganic salts etc. Experiments results show that some degree hydrophilicity present the surface of emulsified asphalt membrane-condensed, absorbing the emulsifier molecular layer with hydrophobic group toward asphalt and hydrophibic group outside orienting on asphalt membrane, thereby the contact angle with water is less than90°; the faster the speed of the emulsified asphalt membrane forming, the greater the contact angle; cement and inorganic salts can make the contact angle of emulsified asphalt membrane-condensed increase, and bentonite can increase that of cationic emulsified asphalt membrane; the contact angle of emulsified asphalt cement membrane is related to varieties of cement and emulsified asphalt; and after water soaking the contact angle of emulsified asphalt membrane increases, and that of the emulsified asphalt cement membrane becomes smaller. Therefore the hydrophilic of emulsified asphalt membrane is related to the content and structure of emulsify molecule adsorbed, the reduction of the content or the damage to the orientation of the molecule can reduce its hydrophilicity.(2) Applying the vacuum-water-saturated method, water absorption rate and water absorption of CA mortar were tested, and it was discussed that the effect of composition and thickness of specimen on water absorption. Experiments results show that under vacuum conditions saturated water absorption characteristics of CA mortar were that water-time curve exists an obvious inflection point, in the early of which (within24h) water absorption showed a rapid increase in straight line with time, accounting for70%of total even much higher, then two days later very slow growth. The results showed that the hydrophilic of the capillary pore walls resulted in absorbing water quickly in the large capillary pore of CA mortar; the thinner the thickness of CA mortar specimens, the smaller sand-cement ratio, and the greater water-cement ratio, the greater the saturated water absorption and the faster the water absorption rate; and vice versa. That indicated water absorption was mainly attributed to the capillary pore in cement phase, therefore the saturated volume water absorption changed between about4to12percent with changes of the composition of CA mortar. It can be concluded that water depth of CA mortar filling layer of slab track structure was about10mm or so in the service processes of the high-speed railway.(3) Applying the constant-humidity-drying method with saturated salt solution, water loss characteristic of CA mortar were tested, and it was discussed that water loss velocity, water loss rate and the influence factors. The results showed that in different humidity conditions the water loss characteristics are that the water loss rate accounted for more than70%of total at85%relative humidity environment, when the relative humidity reduced to75%, the water loss rate increased slightly, the relative humidity to33%, increased further, and the total loss rate is slightly less than the saturated water absorption in vacuum; the influence rules of specimen thickness, sand-cement ratio, water-cement ratio and asphalt-cement ratio on the water loss rate is basically accordance with that of saturated water absorption.(4) With the vacuum-drying method and vacuum-water-saturated method, a experiment simulating drying-wetting cycles was conducted and tested the mechanical properties of the two groups of CA mortar after three drying-wetting cycles, and the changes of water absorbing and evaporating characteristers before and after drying-wetting cycles. The results showed that:the water absorption and rate of water absorbing in vacuum water saturation instrument decrease, water loss rate and evaporating rate in constant humidity environment of salt solution increase after3times of wetting-drying cycles. The characteristic of water absorbing and water loss is independent of asphalt/cement ratio and fineness of sand. It means that hydrophilia of capillary water in CA mortar weaken after drying in vacuum water saturation instrument of40℃and the effect of organic-inorganic interface in CA mortar with wetting-drying cycles is light. (5) The rupture strength and compressive strength decrease after water absorbing. The extent of decreasing relate to fineness of sandandasphalt/cement ratio. Analysis showed that the decreasing of CA mortar’s strength is caused by the erosion effect of organic-inorganic interface after water absorbing. The organic-inorganic interfacein CA mortar of same sand/cement ratio is wider when the sand is tinier, so that the extent of strengthdecreasing is larger. The organic-inorganic interface increases with the increasing of asphalt-cement ratio when the asphalt-cement ratio is less than0.7. So the extent of strength decreasing increases with the increasing of asphalt/cement ratio. The strength of CA mortar after3times of wetting-drying cycles is3～8%less than the strength of CA mortarafter water absorbing. It can be seen that the strength effect of wetting-drying cycles is light.Therefore, the major consideration is the "softening effect" of water on CA mortar in practical engineering. An effective way to improve resistance ability against water attack is to enhance the organic-inorganic interfacial adhesion of the structure and the hydrophobic of emulsified asphalt-cement membrane, reducing water absorption of the capillary pore.