The most obvious evidence of moisture sensitivity is the formation of potholes after a rainstorm. The new, unpatched pothole past the patched sections was deep enough to cause a number of vehicles to lose their hub caps. The owner of the nearby house had stacked hubcaps found in his front yard next to the mail box.
There are three general ways that the presence of moisture can damage HMA mixtures. The first is a function of asphalt binder-water interactions (loss of cohesion). Strength can also be lost as a result of the loss of bonding at the asphalt binder-aggregate interface. The third way water damages HMA is by the degradation of the aggregate because of less than desirable aggregate properties and/or severe environmental conditions.
Water droplets become dispersed in the asphalt binder film. This is referred to as an inverted emulsion; a typical emulsion is asphalt binder droplets suspended in a continuous water phase. This moisture sensitivity problem is amplified by the presence of emulsifiers. This could occur if an emulsion was used in the HMA mix). It may also occur if emulsifiers such as clays are present. The end result of a loss of cohesion is a loss of strength and stiffness of the HMA.
Overly wet stockpiles or damp stockpiles that are not thoroughly dried during production can cause problems. The moisture in the aggregate becomes trapped under the asphalt binder coating. Since aggregate tend to prefer water to asphalt binder, this can disrupt adhesion.
Hydraulic scour occurs when there is water in the pavement and traffic loads move the water back and forth over the coated surface. This happens as a result of the stress reversal from compression to tension when a load passes over a given point in the pavement.
In addition to hydraulic scour, traffic loads can actually force water into the asphalt binder film or through small pinholes in the film. This allows the water to displace the asphalt binder at the aggregate surface. It can also promote a loss of cohesion.
There are four major theories that cover the adhesion of the asphalt to the aggregate surface. The first one is purely mechanical. The next three listed on this slide are in some fashion governed by chemistry. Each of these theories will be discussed in more detail in the next few slides.
The mechanical theory of adhesion is a function of the characteristics of the aggregate surface. Smoother surface textured aggregates do not provide as good a “grip” by the asphalt binder to the surface of the aggregate. On the other hand, some researchers have reported that rougher textured aggregates may make it difficult to achieve a uniform (complete) coating of the aggregate, thereby allowing water to disrupt the asphalt binder film. The pore size and volume of pores (porosity) helps control the amount of asphalt absorbed as well as promoting selective absorption. In general, deeper penetration of the asphalt binder into the aggregate is considered to improve the mechanical interlock. It is thought that pore size may be more important than pore volume. A dirty surface (i.e., surfaces coated with clay-sized particles) prevents the asphalt binder from forming a good contact with the aggregate surface. Dust is thought to reduce the rate of spreading (coating) of the asphalt binder on the aggregate surface. It also prevents the penetration of the asphalt binder into the aggregate, thereby limiting the mechanical interlock. Greater aggregate surface areas or larger quantities of finer aggregates require greater amounts of asphalt binder. Appreciable amounts of fines (minus No. 200) require increasingly larger quantities of asphalt binder to completely coat all of the aggregate particles. The amount of asphalt binder needed for this coating is usually in excess of the amount selected during mix design. This leaves a less than desirable film thickness on the finer aggregates. Mixes with larger amounts of fines are generally considered to be more prone to stripping.
There is no firm agreement between researchers on the influence of combinations of asphalt binder-aggregate chemistries on moisture sensitivity. However, a number of researchers have indicated that stripping may be more of a problem with acidic aggregate mixes than with basic mixes.
This figure provides some general guidelines for what types of aggregates will have either positive or negative surface charges. According to this, the granites would tend to strip. However, there are some cases where these aggregate perform well. Similar exceptions can be found in practice with regards to limestone aggregates. Aggregates that are hydrophobic (don’t like water) tend to have a greater attraction for asphalt. In general, hydrophobia aggregates are basic and have low silica contents. Hydrophilic (water loving) aggregates are usually acidic and have a high silica content.
This slide provides some examples of how aggregate types would be ranked for stripping potential.
Weathered aggregate is considered less prone to stripping than freshly crushed aggregates. This is because the weathered aggregate surfaces have time to absorb oils from the atmosphere. These contaminates actually help the asphalt to coat the aggregate surface.
Wetting ability is defined as the ability of the asphalt binder to make a very close contact with the asphalt binder surface (Hicks, 1990). The resistance to flow (i.e., viscosity) is associated with molecular friction. Wetting ability is also related to surface tension which is the stress that tends to hold a drop of liquid in spherical shape.
There are two general types of antistripping additives used to reduce the moisture sensitivity of asphalt mixtures: liquid antistrips and lime. These will be discussed in more detail in the following slides.
These are surface-active agents that reduce the surface tension of the asphalt binder. A liquid antistripping additive is chosen so that the electrical charge is opposite to that of the aggregate. There are two methods by which liquid antistrips are added to the mix: mixed with the asphalt binder prior to mixing with aggregate, or added directly to the aggregate surface. While the last is conceptually the best for altering the chemistry at the asphalt binder-aggregate interface, the first is widely preferred for each and economical considerations. When the liquid is added to the asphalt binder, it is thought to migrate to the aggregate surface because of the differences in polarity at the interface. When the additive reaches the surface, is should displace any water molecules present, thus promoting adhesion. However, when the asphalt binder cools, the viscosity of the asphalt binder increases substantially. This tends to limit the ability of the liquid antistrip to migrate to the aggregate surface. The normal time available for migration is about 3 hours while the time needed for full migration has been estimated to be up to 12 hours.
Lime has been used since the early 1900’s, initially as a mineral filler. While there are two types of lime that can be used as an antistrip additive, the hydrated lime is preferred for safety reasons. Quickline produces a highly exothermic reaction in the presence of moisture. Hydrated lime reacts with most silicate aggregates to form a strongly-bonded crust of hydroxy silicate on the aggregate surface. This crust then attracts the carboxylic acids in the asphalt binder and strongly absorbs them onto the surface. Methods of application include: dry hydrated lime added to the cold feed belt, hydrated lime slurry mixed with the aggregate, dry hydrated lime mixed with water, and hot (quicklime) slurry. These are briefly discussed in the next two slides.
One of the major problems with the dry method is holding the lime on the surface of the aggregate until it is coated with asphalt binder. There tends to be more loss in drum plants which tend to pick up some of the lime in the gas flow. Also, the loose lime will act as a mineral filler in the asphalt binder rather than purely as an aggregate treatment. This method of adding lime has not proven to be as consistently effective as some of the other methods of addition. Hydrated lime, when mixed with water prior to use, is called a slurry. One of the main disadvantages to this method is that additional water is added to the aggregate stockpile and must be removed during drying. As an alternative, the aggregate can be treated with the lime slurry, then stockpiled (referred to as marinating) for up to 30 days. This allows time for the majority of the water to evaporate but requires that an additional aggregate handling process be added. One advantage is that it provides a good coating of the aggregate surface with much less lime lost to gas flow and filler.
Dry lime added to damp aggregate on the cold feed belt is probably one of the most common methods of adding lime. Mixing of the aggregate on the scalping screens and on belt changes helps coat the aggregate; the damp aggregate helps mitigate the loss of the lime. Quicklime costs about the same as hydrated lime. However, when it is mixed with water, the hydrated lime yield is about 25% greater. The exothermic reaction results in an increase in temperature which helps somewhat in removing the additional water.
This slide summarizes the main aggregate characteristics that are considered important by industry. Desirable characteristics are shown in parentheses.
There is no consensus within the HMA industry as to optimum asphalt properties. There is some evidence that higher viscosity asphalt binders tend to have less of a stripping potential. However, higher viscosity asphalt binders don’t coat the aggregate as well as a lower viscosity asphalt binder would. Asphalt binder with higher percentages of carboxylic acids and certain sodium compounds have been found to be more susceptible to moisture problems. Both low percentages of air voids and a dense gradation reduce the permeability of the mix. This limits the ability of the water to enter the mix, thereby improving its resistance to moisture damage.
If the weather is wet and cool during construction, the aggregate stockpiles tend to be overly damp and not thoroughly dried during HMA production. The cool weather can make it difficult to get the best in-place density. Both of these factors will increase the potential for moisture damage. The amount of rainfall and the number of freeze/thaw cycles the pavement is subjected to after construction can also have a significant affect on the ability of water to damage the pavement. Significant numbers of wet days increases the probability of problems with hydraulic scour or pore water pressure. These problems are accentuated with increasing traffic levels. A large number of freeze/thaw cycles tends to damage the pavement due to the expansion of the water when frozen.
The boiling water method is described in ASTM D3625. Originally, the sample was subjected to boiling water for one minute. However, several agencies did not feel that this was sufficient and increased the time to 10 minutes. Several researchers also indicated that after 10 minutes, the general effectiveness of any antistripping additives can be assessed. Once the samples have been in the boiling water, the loss of asphalt binder coating of the aggregate is evaluated. These subjective results are expressed in terms of the % coating retained. Because of the subjective nature of the this test, efforts to establish a precision statement have been unsuccessful. Most users recommend that this test method be limited to either an initial pass-fail evaluation or to process control during production.
A small sample 1-5/8 in in diameter by ¾ inches thick is prepared with a uniform sized aggregate then placed on a pedestal inside a container, covered with water, then subjected to repeated freeze/thaw cycles. The freeze/thaw cycles are continued until the sample cracks. If the sample cracks within the first 10 cycles, then the mix is considered to be moisture sensitive. More than 25 cycles, the mix is resistant to moisture damage. Disadvantages to this test are that only one aggregate fraction is considered in the assessment and the length of time needed to get a result (25 cycles needs 25 days).
Terrel and Swailmi proposed the concept of pessimum voids which indicates that the void system plays an important role in moisture resistance. Mixtures with low voids are relatively impermeable to water intrusion are as such not affected by the presence of water. Mixtures at some void content that is greater than about 14% because, although they are very permeable, they are also free-draining. That is, water does not stay in the material long enough to damage the material. In between these ranges, there is a critical range of voids which allows sufficient water intrusion but does not easily drain. This combination results in the greatest moisture damage. The range of voids associated with this region is referred to as the pessimum (as opposed to optimum) void range.
Typical mixes that fit these three void ranges are shown in this slide.
3HMA Characterization Moisture SensitivityReasons for Damage• Loss of cohesion in asphalt binder film• Failure of adhesive bond• Degradation of aggregate• Freeze/thaw
4HMA Characterization Moisture SensitivityLoss of Cohesion(Spontaneous Emulsification)• Inverted emulsion• Aggravated bypresence of emulsifiers• e.g. clays, additives• Loss of stiffness andstrength in asphaltbinder
5HMA Characterization Moisture SensitivityLoss of AdhesionMoist Aggregates• Internal moisturedisrupts asphaltbinder film• Most states havemax. moisturecontent requirementon fresh HMA
6HMA Characterization Moisture SensitivityLoss of AdhesionHydraulic Scour• Traffic-induced movement of water “scrubs” asphaltbinder off of aggregateStress as tire passes
7HMA Characterization Moisture SensitivityLoss of AdhesionPore Water Pressure• Usually traffic-related• Voids decrease andwater is trapped• Moisture gets“pressed” to aggregatesurface throughbreaks in film
8HMA Characterization Moisture SensitivityTheories of Adhesion• Mechanical adhesion• Chemical reaction• Surface energy• Molecular orientation
10HMA Characterization Moisture SensitivityChemical Reaction• Better adhesion with basic rather than acidicaggregates• Basic•pH > 7•Positive charge• Siliceous aggregates tend to strip• Mixed results and findings in literature
12HMA Characterization Moisture SensitivityExamples of Stripping Potential forVarious Minerals and AggregatesSlight Moderate SevereHornblende Quartz QuartzBasalt Basalt GraniteSiliceous river gravel QuartziteGranite GneissLimestone Limestone ChertSandstone
13HMA Characterization Moisture SensitivityWeathering of Aggregate• Over time, aggregate is exposed to numerous cycles ofvarying temp. and humidity• Outermost adsorbed water molecules are partiallyreplaced or covered by organic contaminates• Absorbs oils from air• Helps improve wetting by asphalt binder• Stockpiled agg more resistant to stripping than freshlycleaved rock
14HMA Characterization Moisture SensitivitySurface Energy Theory• Wetting ability of asphalt binder• Related to viscosity• Water better than asphalt binder• Higher surface tension = better adhesion• Water will displace asphalt binder due to highertension
16HMA Characterization Moisture SensitivityLiquid Antistripping Additives• Surface-active agents• Reduce the surface tension between aggand asphalt binder• Give surface charge opposite of aggregate• Amines most commonly used•Form positive ion (R-NH3+) whencombined with water or acid
17HMA Characterization Moisture SensitivityLime as an Additive• Lime• Hydrated lime Ca (OH)2• Quick lime CaO• Hydrated lime reacts with most silicate agg.• Crust of calcium hydroxy silicate• Strong bond with agg• Sufficient porosity to allow pen. of AC• Carboxylic acids and 2-quinlenes of ACabsorbed• Forms insoluble calcium salt
18HMA Characterization Moisture SensitivityMethods of Adding Lime• Dry• Loss of lime in plant• Results not consistent• Hydrated lime slurry• Additional water needed•Reduces production rates•Increases fuel costs
19HMA Characterization Moisture SensitivityMethods of Adding Lime (Continued)• Dry hydrated lime added to damp agg• 3-5% moisture• Mixing in pug mill or tumble mixer• Quicklime slurry• Yield of hydrated lime approx. 25% greater forsimilar cost• Exothermic (very)
20HMA Characterization Moisture SensitivityHMA Factors Which InfluenceMoisture• Aggregates• Surface texture (Rough)• Porosity• Mineralogy• Coatings (Clean)• Surface moisture (Low)• Surface chemical composition( ) = Desirable, no info = no consensus
21HMA Characterization Moisture SensitivityHMA Factors Which InfluenceMoisture(Continued)• Asphalt cement• Viscosity (High)• Surface chemistry• Composition• HMA• Voids (Low)• Gradation (Dense)( ) = Desirable, no info = no consensus
22HMA Characterization Moisture SensitivityHMA Factors Which InfluenceMoisture(Continued)• Weather Conditions• Temperature during construction (Warm)• Rainfall during construction (None)• Rainfall following construction (Min.)• Freeze/thaw following construction (Min.)( ) = Desirable, no info = no consensus
23HMA Characterization Moisture SensitivityBoiling Water Test• Subjects HMA loose mix to boiling water for 10minutes• Evaluation of stripping based on subjectiveevaluation of loss of asphalt binder coating• Potential for quick field use or initial pass-fail test• < 95% retained = problem• Mixed results - no precision statement
24HMA Characterization Moisture SensitivityTexas Freeze / Thaw Pedestal Test• Subjects 41 mm diameter by 9.5 mm thicksample to repeated freeze and thaw cycles• Uses fine portion only• One Cycle: 10oF for 15 hr, 75oF for 45 min.,120oF oven for 9 hr.Repeat until sample cracks< 10 cycles = moisture sensitive> 20-25 cycles = resistant to moist.
25HMA Characterization Moisture SensitivityHMA Voids• Relationship between strength and air voidsRetained Mix Strength, %Air Voids, %0 5 10 15 201000ImpermeablePessimumVoidsFree Draining
26HMA Characterization Moisture SensitivityDefinitions(Terrel and Swailmi, 1994)• Impermeable or low void mixtures• High asphalt binder or mastics• Offset instability with crushing, large stone, andmodified AC• Pessimum void range• Conventional dense-graded HMA in US• Free draining or open graded• Modified asphalt binder and higher % for thicker films• Remain open under traffic