Research On Degradation And Failure Mechanism Of Cement Emulsified Asphalt Mortar Layer In Slab Track

Abstract:as one of key engineering materials in slab ballastless track, the long-term service performance of cement emulsified asphalt mortar (SL mortar) plays important role on the smoothness and durability of slab track, and the safety and ride comfort of passenger train. Such degradation and invalidation phenomena as interfacial opening gaps between SL mortar and concrete slab, vertical cracks, surface erosion and crush in SL mortar were observed in early testing section and construction site. Rainwater, dynamic load and temperature variation were determined as the primary influence factors on the degradation and invalidation of SL mortar.Some SL mortar specimens with different mass ratio of asphalt to cement (A/C) were prepared using cation emulsified asphalt (Al), compound ion emulsified asphalt (A2) and anion emulsified asphalt (A3), P.?. silicate cement, fine sand and some additive. To obtain the effect of rainwater, dynamic load and temperature variation on the degradation and invalidation mechanism of SL mortar, the major experiments and conclusions are as follows:(1) The simulated test result of high-low temperature cycling on superposed specimen of SL mortar layer bond with concrete lay shows that under a certain high-low temperature cycling, surface crack in SL mortar and interfacial opening gaps are occurred, as well as interfacial bonding strength of superposed specimen decreases. It is indicated that temperature variation is the primary cause of interfacial opening gaps in superposed specimen. Because SL mortar thermal behavior is obviously differeent from concrete due to asphalt material characteristic. SL mortar has higher thermal deformation dependent on time and temperature, and that of concrete is lower. Under alternate temperature variation, concrete layer has constraint effect to SL mortar layer leading to reciprocating compressive or tensile stress. So some degradation phenomena appear including cracks on SL mortar surface, interfacial adhesive strength dropping and interfacial debonding and opening gaps. Hence, it is the two primary reasons for degradation and invalidation of SL mortar that environment temperature variation and thermal behavior differences between SL mortar and concrete material.(2) The absoption of SL mortar was studied by contact angle test and capillary absorption test. Results show that the contact angle of SL mortar former surface is less than90°, increasing with increase of A/C, which presents some degree hydrophilicity of SL mortar. Besides, SL mortar has considerable capillary absorption and per area mass of capillary absorption of SL mortar and t1/2demonstrates linear correlation. Its capillary absorption velocity depends on emulsified asphalt type, A/C, capillary porosity and capillary pore characteristic etc. The capillary absorption velocity decreases with increase of A/C, decrease of capillary porosity and increase of capillary pore tortuous. With the same proportion, the capillary absorption velocity of Al mortar specimen is higher than that of A3mortar specimen. It is analysed that there is lot of condensate of asphalt particles and capillary pores distributing inside hardened SL mortar. The matter of capillary pore wall is rich asphalt membrane, on which emulsifier molecular layer with hydrophobic group toward asphalt and hydrophibic group outside orienting. Surface wettability effect of emulsifier molecular leads to certain hydrophilicity of capillary pore wall, and easily transportation of water into SL mortar through capillary pores.(3)The static compression absorption test indicates that the static compression absorption velocity is higher, and an obvious inflection point exists in per area mass of static compressive absorption and t1/2relation curve. The compressive absorption velocity before the inflection point is hundreds times more than capillary absorption velocity, and the one after that are orders of magnitude greater. No matter before and after the inflection point, under the same condition, the static compression absorption velocity of the3type SL mortar are all relation to A/C and asphalt type. Due to the static compression quickens the compressive absorption velocity and drives water intrude into asphalt-cement interface and asphalt-sand interface. The interface number increases with the increase of A/C. Asphalt type plays effect on the interface number and interface adhesion strength, which results in the permeability-resistance decreases with increase of A/C. With the same proportion, the compressive absorption velocity order is A1>A3>A2, and the anti-permeability time order is A2>A3>A1.(4) The experiment results from influence of water on static mechanical properties demonstrates that the compressive strength and elastic modulus of hardened SL mortar decreases with increase of saturation, and the maximum reducing degree of compressive strength is up to40%. On account of water invading into organic-inorganic interface, the hardened SL mortar is weakened, the degree of which is correlation to A/C and the type of emulsified asphalt. The greater A/C of a mortar specimen is, the more reducing degree of its mechanical properties and the worse its water resistance are. With the same proportion, the water resistance of SL mortar with A2asphalt emulsion occupies first, then anion emulsified asphalt of A3comes and finally cation asphalt emulsion of A1takes place. So, the water resistance of SL mortar efficiently improves by advancing prevention interface from water intruding.(5) Fatigue experiment results of reference specimen and various saturation specimen show that the ultimate fatigue strength of suction specimens badly decreases, with increase of saturation. Vertical fatigue cracks can appear in reference specimen meaning splitting failure, but sloping cracks appear in most various saturation specimens meaning shear failure. Results obtained from hydrodynamic fatigue testing indicate that water detained on SL mortar surface will produce hydraulic fracture effect under alternate load of high frequency, which makes cracks quickly emerge and movement in the tips of pores or gaps. So under water-dynamic load coupling effect, the ultimate fatigue strength notably reduce and harden SL mortar damage.In conclusion, both environment temperature variation and thermal behavior differences between SL mortar and concrete are the primary reason to cause interfacial opening gaps and crack. When rainwater penetrates into SL mortar meanwhile exists on its surface, excess pore pressure of capillary water and hydraulic fracture of high velocity flow result in premature degradation and invalidation of SL mortar under alternate load of passenger train.