Effects of Moisture on Flexible Pavement - Case Study Example

Moisture effects on Pavements Name Course Instructor Institution Location Date Table of Contents Effects of Moisture on Flexible Pavement 2 Moisture Rates in Victoria, Australia 4 Stress and Failure in Bound Flexible Pavement from Moisture Saturation 5 Effects of Underground Moisture on Flexible Pavement 6 Effects of Moisture on bound flexible pavement 6 Key Factors in Bound Flexible Pavement Deterioration. 9 Detachment 9 Spontaneous Emulsification 10 Pore pressure 10 Hydraulic Scour/Wheel loading 11 Effects of Temperature on Bound Flexible Pavement 12 Reducing Moisture Adsorption in Bound Layers 12 References 13 Effects of Moisture on Flexible Pavement Moisture conditions in pavement change at different times of the year. The level of moisture in both bound and unbound pavements are often set at an optimum during construction. However, these conditions adjust themselves to natural moisture levels after the completion of construction. It shares similar regimes as the temperature conditions of the pavement and is governed by the boundary conditions of the pavement. The equilibrium water levels in a pavement directly depend on the nature of the aggregates and the distance to the piezometric height, and it changes with seasons. The moisture conditions at the lower levels of the pavement are significantly high. However, the upper layers go through cycles of completely dry to saturated conditions as dictated by the environmental conditions. The degree of saturation of upper sections also depends on its surface conditions as well as its longitudinal and transverse positioning within the pavement system. Moisture conditions at the surface, areas near cracks and edges are controlled by the climatic conditions; this also includes the rainfall, percolation, and evaporation (Taylor and Khosla, 2003, p154). In relation to the moisture content, a pavement profile can be divided into distinct regions. Assuming that the bottom of the pavement in sealed from underground water, the effect of saturation can be ignored, and only the vadose zone considered in modeling. The moisture levels in the vadose section vary between two extremes, saturated zone at the bottom and natural moisture conditions at the top layer. This zone can be graded into three distinct regions, the capillary region, intermediate vadose region, and the surface region. At the bottom, the layer exerts capillary pressure on adjacent layers while retaining water from infiltration; the voids are almost filled with water, and the air phase is not continuous, as such, pore water pressure falls below atmospheric pressure. Depending on the aggregate properties and the season, the thickness of the capillary zone varies from a few centimeters to about a meter. In the intermediate zone, the remaining water from both percolation and infiltration are held, the region is characterized by continuous phases of both air and water. In the surface section, water is partially filling the pores and the air phase in continuous. The water levels of surface region and intermediate zone vary with climatic conditions, the levels increase during rainy and spring thaw season and evaporates during the dry seasons. The figure below conceptualizes the different zones in a bound flexible pavement as defined by moisture conditions. The mechanical attributes of the aggregates also affect the moisture content of the layers in a pavement. For instance, a rise in moisture contained in coarse-grained materials translates in reduced inter-particle friction because of water lubrication. In bound materials with high fineness modules, such as the crush rock, stress characteristics of the layer is affected by suction and pore pressure variations. Moisture Rates in Victoria, Australia In Australia, Country Roads Board plays a critical role in establishing standards to be observed in road construction. In 1982, Victoria Technical Bulletin no 32, they established factors applicable in determination of infiltration rates and moisture level for pavements in Victoria. They are infiltration factors, which for pavement design purposes, are multiplied by the mean intensity of rain experienced in two successive years, one-hour rainfall to determine the infiltration rates, i.e. Infiltration rate = Infiltration factor × mean 2yr rainfall × mean 1hr rainfall Where: 1. Mean Ihr rainfall - Average amount of rain that falls in one hour in a specific region 2. Mean 2yr rain intensity -Average amount of rain that falls over a period of two successive years. The table below shows the standard amount of rain expected to flow into the pavement for each aggregate type. These values can be used in prediction of infiltration rates for Wallan Region. Aggregate of Surface Type Infiltration factor Sprayed seal 0.2 - 0.25 Asphalt 0.2 - 0.4 Cement concrete 0.3 - 0.4 Unsealed shoulders 0.4 - 0.6 Stress and Failure in Bound Flexible Pavement from Moisture Saturation Moisture effects have serious impacts on the performance of bound flexible pavements; the degree of damage is only second to adverse loading conditions. Major effects come from moisture reduction of the aggregates’ resilient modulus. Research notes that resilient modulus of aggregates can decrease by fifty percent from its modulus when the pavement performs in dry conditions (Lottman, 2001, p51). According to Victory Country Roads Board, a degree of saturation between fifty and hundred percent and densities ranging between ninety-five to hundred percent results in loss of strength by sixty percent. The following equation is applicable in determining the resultant resilient modulus at different moisture saturations. Mr = 45.2 – 0.428Sr Where Mr- Resilient Modulus Sr- Moisture Saturation Other studies established that the loss of granular stiffness as the moisture level increases results into a decrease in stress-bearing capacity of the pavement at a moisture saturation of 80–85 percent and rapid loss of strength occurs with a lesser increase in saturation. Effects of Underground Moisture on Flexible Pavement While most of the water that affects pavements comes from the surface, underground water also contributes to the total effects. Underground water rises to the pavement mainly by the capillary. This can be in situations that the pavement profile intersects with the ground water table, disruption of underground flow by a pavement or rise of the water table due to heavy rains. Deep percolation results into increased concentration of water below the piezometric height, the water then begins to rise by capillary towards the surface and thus saturating the pavement’s lower section. Saturation of the pavement by underground water has similar effects to the case of water from rainfall. Effects of Moisture on bound flexible pavement Similar to all other geotechnical structures, pavement performance is dependent on the moisture conditions and other environmental factors. Some of the significant elements that affect the amount of moisture in a bound flexible pavement are infiltration and percolation of rainwater and underground water. While infiltration is the passage of water through the ground surface, percolation is the movement of water through the different layers of pavement. Water from rain enters soil through different ways, this can be through surface infiltration (cracks found on the surface of bound pavement), edge infiltration (common in inadequately drained side ditches that collect water during rainfall) and from the underlying water as underground water movement. Moisture within that pavement has detrimental effects on the performance of the pavement; the problem is worse when the moisture goes through cycles of freeze and thaw. One of the effects is the reduction in strength of crush rock sediments in the pavement, as the water infiltrates, it carries with it fine aggregates from the upper layers and saturate them at the crush rock, consolidation, and expansion. These cycles of moisture cause compromise the performance of the pavement, which manifests itself as localized stresses such as potholes or wide scale stresses as overall road roughness. Infiltration in bound pavements occurs as explained in the previous section. Water concentration is dictated by the height of the area of interest from the surface. As it rains, water that does not drain off infiltrates into the pavement. The water first saturates the upper surface immediately after rain, continued rainfall increase the water concentration at the top layer and more water migrates to the vadose zone and subsequently to the foundation. The rate at which infiltration occurs is directly proportional to the aggregates forming the bound pavement, for the crush rocks aggregates, the amount of water held tends to be lower as the crush rock have low water holding capacity. Fine textured bound pavement has high water retention and low rate of infiltration and percolation. For bound pavements with fine texture at the sides, water retention within the pavement tends to be high; more water infiltrates through the crack at the surface while lesser is discharged from the edges, as such, and the pavement reaches maximum saturation after short precipitation. After prolonged precipitation, saturation of water at the lower section of the pavement because of infiltration and percolation eventually exceeds the upper sections. The zone of saturation within the pavement begins to move upwards from the foundation into the vadose zone. The process continues if the rate of infiltration and percolation to the subgrade exceeds drainage and eventually the entire pavement becomes saturated. The air phase in all the layers becomes discontinuous. At this state, friction between the pavements aggregate is reduced, and the bound flexible pavement is susceptible to damage. The increased water content in the soil also causes expansion and widens the voids. When drying commences, water saturation decreases from the upper layers downwards. The water because of saturation is held by gravity and is at a lower pressure than the atmospheric pressure. This causes evaporation and thus decreases in water concentration at the surface. The reduced concentration of the upper layer is at a higher potential that the lower vadose layer that is still saturated this causes water from the saturated vadose layer to migrate to the upper layers and to the atmosphere through the capillary rise and eventually evaporation. The same scenario occurs as the vadose layer is drained. The changing decreasing water concentration result in it more of continuous air phase down the pavement layers. The cycle of saturation and drying depicts the extremes and intermediate levels of moisture in the soil. In most cases, this continues until the water concentration reaches optimum levels. Key Factors in Bound Flexible Pavement Deterioration. Moisture content in soil has varied impacts that range from those manifesting in small scale to wide scale manifestation. Moisture damage to pavement can be defined at the loss of strength and durability in bound flexible pavement because of extreme moisture cycles in the pavement. The loss of cohesively between particles defines the degree of damage to the pavement as a result of moisture variations in the pavement. Previous research has noted that the degree of damage is dependent caused by cycles of high and low moisture concentration, effects of moisture cycles to the pavement can be described as stripping, detachment, displacement, emulsification, pore pressure and hydraulic score (Kiggundu and Roberts, 2008, p39). Detachment Detachment in bound flexible pavement is the separation of the aggregates by a thin film of water, but there is observable evidence if separation. Understanding of these phenomena is based on the theories of adhesive bonds between the aggregates. Several processes are involvement in the detachment, the attachment between the aggregates in a bound pavement is determined by the ability of the aggregates to develop surface tension. Research has established for a two consisting of water and aggregates exist, excess water result into a reduction of the free energy of the system and creates a thermodynamically stable condition with minimum energy (Curtis, 2002). Research at Road Research Laboratory has established that in a situation of excess water, the bond development between the aggregates is chiefly as a result of small dispersion forces since aggregates have low polar activity (Curtis, 2002). Water molecules are characterized by high polarity, and this can replace the aggregate-aggregate bonding. The result is that the particles in the unconfined pavement will be dispersed at high water concentration, increased dispersion allows water that is infiltrating through the soil to carry these sediments down the sections, and the result is dispersed upper layer that permits high infiltration and percolation of water into the pavement. In addition, the upper surface becomes susceptible to cracking due to weak bonding. Spontaneous Emulsification Emulsification is one of the negative effects of moisture to bound pavements; there are different theories explaining how an emulsion is formed, however, the result of the emulsion to bonding between aggregates is detrimental to the life of pavement. The rate of emulsification is increased by the presence of emulsifiers; clay, for example, forms part of the bound pavement. Organic armines, which are found naturally in the pavement can increase the effect; they create a strong bond with water molecules during the rainy seasons. Research demonstrates that, while emulsification can occur with any type of water, this effect is accelerated by distilled water such as rainwater (Majidzadeh and Brovold, 2008, p92). This explains why pavement deterioration due to emulsification is dominant during the rainy seasons and results of weakness. However, this effect is reversible during the dry seasons due to the absence of water. Pore pressure Entrapment of water is the reason behind distress in the flexible bound pavement. When the pavement is subjected to extend loading, the pressure developed from this effect worsens the pore pressure, which in term can overcome aggregate cohesion and promote the growth of crack within the subgrade. The pore pressure can also result in strain hardening of the pavement, however, not in the same fashion as in the case of metals which are strained hardened to promote interlocking of particles and thus resistance to slip. In bound flexible pavement, it results in a lessening of interactive, cohesive, and adhesive forces. The extent of this damage is directly proportional to the concentration of water in the subgrade as water in the micro-cracks when subject to increased pressure widens the gaps. Research have established an optimum air void level, within which air pore are interconnected, and water can flow freely out of the pavement when subjected to pressure, this is called the passium air voids (DiVito and Morris, 2002, p107). Within the passium range, water can enter the pores but cannot flow freely out of the pores, between aggregate, traffic loading transferred to the crush rock by the entrapped water as pore pressure is a potential risk to the pavement, and its continuation can result in failure. Hydraulic Scour/Wheel loading Damage from hydraulic scour is often significant at the pavement surface where the top layer is stripped off or cracked and water is forced into the pavement by wheel action. The influence of osmosis and pullback plays the greatest roles in this process. The presence of salts in bound pavements creates an osmotic potential that sucks water into the pavement through the asphalt layer. Some researchers argue the low-pressure region created when the wheel leaves ground prevents the effect of moisture by pulling water back to the surface, however, some argue that water is packed by greater pressure by salt containing aggregates and asphalt. From previous research, asphalt cement hold water at greater pressures (Castan, 2008, 82). Effects of Temperature on Bound Flexible Pavement Temperature fluctuation has direct effects on the levels of moisture in the pavement. The thermal characteristics dictate how much water a pavement can hold. At constant moisture conditions, the thermal behavior of bound flexible pavement does not depend on the aggregates interaction but also on the phase relationships between solid, liquid and gaseus phase. Contribution to water movement in relation to temperature is based on its effect on the phase relationships. High temperatures mean lightness of the water molecules, and thus, vertical upward conductivity of moisture exceeds the horizontal and vertical downward flow by far. This effect is felt during the dry seasons when temperatures of the surface layers exceed that of the lower layers, the result if vapor formation at the surface and resultant evaporation, moisture in the lower regions of the pavement are then dragged upwards resulting in drying. In addition, an increase in heat results in an expansion of the gaseous phase in pavement, which contributes to the increased evaporation. At low temperatures, water condenses and sometimes forms freeze into snow depending on the season. Solidified water cannot flow, and the subsurface flow is significantly reduced. Reducing Moisture Adsorption in Bound Layers Prevention bound flexible pavements from damage by rain begins at its construction, at this stage, the exposure of the pavement to rainfall should be minimized by covering or fully constructing pavement section to levels that water infiltration is retarded by the top layer. Planning for daily work should only allow for tasks that are completed to levels that no loose pavement materials are left, in addition, windrowed construction material should permit drainage and not hold water on the pavement surface. The use of excess water during compaction leads to larger void when the water dries, these voids can act as moisture storage and results into serious damage. The material used on the pavement should be of low permissibility to avoid the intake of moisture from the sides; water would run off instead of infiltrating into the pavement. In addition, materials with high moisture sensitivity should not be used in wet sections. To determine the effect of moisture on the pavement materials, Repeated Load Triaxial from wheel plots can be used. Another research document that pavements with less moisture sensitivity materials have a longer life span (Cheng, 2001, p73). References Castan, M. 2008. Rising of Binder to the Surface of an Open-Graded Bituminous Mix. Bulletinde liaison des laboratoires routiers, No. 33, pp. 77–84. Cheng, D. Z., Little, N., Lytton, R., and J. C. Holste. 2001. Surface Free Energy Measurement of Aggregates and Its Application on Adhesion and Moisture Damage of Asphalt–Aggregate System. Proc., 9th International Center for Aggregate Research Symposium, Austin, Tex. 73 Curtis, C. W. 2002. Fundamental Properties on Asphalt Aggregate Interactions Adhesion and Adsorption. Final Report on Contract A-003B. Strategic Highway Research Program, National Research Council, Washington, D.C. DiVito, J. A., and G. R. Morris. 2002. Silane Pretreatment of Mineral Aggregate to Prevent Stripping in Flexible Pavements. In Transportation Research Record 843, TRB, National Research Council, Washington, D.C., pp. 104–111. Kiggundu, B. M., and Roberts, F. 2008. Stripping in HMA Mixtures: State-of-the-Art and Critical Review of Test Methods. Report 88-2. National Center for Asphalt Technology. Auburn University, Auburn, Ala.,. Majidzadeh, K., and Brovold, F. 2008. Special Report 98: State of the Art: Effect of Water on Bitumen-Aggregate Mixtures. HRB, National Research Council, Washington, D.C.,. Taylor, M. A., and Khosla, P. 2003. Stripping of Asphalt Pavements: State of the Art. In Transportation Research Record 911, TRB, National Research Council, Washington, D.C, pp. 150–158. Lottman R. The Moisture Mechanism That Causes Asphalt Stripping in Asphalt Pavement Mixtures. University of Idaho, 2001. Read More