ASPHALT CONCRETE MIXTURES Objectives: To introduce asphalt concrete To explain the asphalt concrete mixture To explain the types of asphalt concrete mixtures To explain the requirements of aggregates for asphalt concrete
ASPHALT CONCRETE: Introduction Asphalt concrete is basically a mixture of asphalt cement and aggregates, hot-mixed in an asphalt plant and then hot-laid to form the surface course of a flexible pavement The properties of asphalt concrete depend on: the quality of its components (i.e., asphalt binder and aggregates) the mix proportions construction process Asphalt concrete must provide a stable, safe, and durable road surface
ASPHALT CONCRETE: Stability Stability of the asphalt concrete depends on the strength and flexibility of the mixture and the degree of compaction during placing The strength must be sufficient to carry the load without shear occurring between particles. The structure must remain intact. The main contributor to strength is friction between grains. A dense-graded mixture, composed of particles with rough faces, with a relatively thin asphalt film between them is best for high-friction strength. Flexibility is also important as the pavement distributes the imposed load by deflecting slightly as the load is applied, without cracking or permanent deformation. To meet this requirement, a more open-graded mixture, with a higher asphalt content, is best. Strength and flexibility are evaluated by various tests, depending on the design method being used
ASPHALT CONCRETE: Safety Safety is very important for the surface course Safety is achieved by making the surface course skid resistant and able to allow quick drainage of water from the surface. Skid resistance is enhanced by using smaller sized, very hard aggregates for the surface course. This provides more points of contact for the development of friction forces. Open-graded surface courses are used in very heavy traffic areas to allow immediate drainage of rainwater before it can result in hydroplaning.
ASPHALT CONCRETE: Durability Durability of the asphalt concrete is critical to ensure that it maintains the stability and skid resistance properties for the design service life Asphalt ages, and pavements become denser (i.e., aged) with time and traffic Pavements fail (i.e., durability of pavement is lost) due to: changes in the aggregates permanent deformation or rutting cracking, either due to fatigue, or low temperatures bleeding of asphalt to the surface
ASPHALT CONCRETE MIXTURE Asphalt concrete mixture basically consists of asphalt cement, aggregates, and air. Out of the total asphalt added to the mixture, some of the asphalt is absorbed in the pores of the aggregate particles. The portion of asphalt absorbed by aggregate particles is called absorbed asphalt The net amount of asphalt available to coat and bind aggregates together is called effective asphalt
ASPHALT CONCRETE MIXTURE---contd. The mass/volume relationships of a compacted asphalt mixture are illustrated in the following figure: Mass/volume relationships for an asphalt concrete mixture: Density () Asphalt content (P B ) Effective asphalt content (P BE ) Asphalt absorption (P BA ) Air voids (AV) = M/V P B = M B / M P BE = M BE / M P BA = M BA / M G AV = V A / V Voids in mineral aggregate (VMA) VMA = (V BE + V A )/V Voids filled with asphalt (VFA) VFA = V BE / (V BE + V A ) M Total mass (= M G + M B ) M G Mass of aggregate M B Mass of asphalt (binder) (= M BE + M BA ) M BE Mass of effective asphalt, the asphalt binder between particles M BA Mass of absorbed asphalt, absorbed into the pores of the aggregate particles V Total volume of the compacted mix V G Volume of aggregate, the bulk volume including the aggregate pores V BE Volume of effective asphalt V BA Volume of absorbed asphalt V B Volume of asphalt (= V BE + V BA ) V A Volume of air between the coated aggregate particles in the mix V GE Effective volume of aggregate (= V G V BA ) V MM Volume of voidless mix (maximum mix volume)
TYPES OF ASPHALT CONCRETE MIXTURES Asphalt concrete mixtures can be classified into following two types based on whether hot-mixed, hot laid or cold-mixed, cold-laid: Hot-mixed, hot-laid asphalt (HMA) concrete mixture Cold-mixed, cold-laid asphalt concrete mixture Asphalt concrete mixtures can be classified into following two types based on whether in-situ-mixed or plant-mixed: Road-mixed or in place-mixed asphalt concrete mixture Plant-mixed asphalt concrete mixture HMA concrete mixtures can be classified into following three types based on type of aggregate grading used: Dense-graded HMA concrete mixture Stone matrix asphalt (SMA) concrete mixture Open-graded HMA concrete mixture Asphalt concrete mixtures can be classified into following three types based on type additives used: Rubber-modified asphalt concrete mixture Polymer-modified asphalt concrete mixture Sulfur-modified asphalt concrete mixture
TYPES OF ASPHALT CONCRETE MIXTURES Hot-mixed asphalt (HMA) concrete A hot-mixed asphalt (HMA) concrete is a mixture of asphalt, fine aggregate or both fine and coarse aggregates, and mineral filler (optional) The grade of asphalt to be used depends on: the type of construction climatic conditions amount and nature of traffic HMA concrete is produced and laid in the following steps: both aggregate and asphalt are heated prior to mixing to drive off moisture from the particles and make the asphalt sufficiently fluid (maximum temperatures for heating asphalt cement and emulsified asphalt are 176.6 F and 82.2 F, respectively) after heating, all the raw materials are mixed in the plant, and the hot mixture is transported to the paving site and spread on a loosely compacted layer to a uniform, even surface with the help of a paving machine while the mixture is hot it is compacted by heavy, motor-driven rollers to produce a smooth, well-compacted paving course Since the aggregates are thoroughly dried prior to mixing, stripping of asphalt (i.e., removal from the pavement) will not take place in hotmixed, hot-laid asphalt pavements
TYPES OF ASPHALT CONCRETE MIXTURES Cold-mixed asphalt concrete Like a hot-mixed asphalt concrete, cold-mixed asphalt concrete is also a mixture of asphalt, fine aggregate or both fine and coarse aggregates, and mineral filler (optional) Cold-mixed asphalt concrete is produced and laid at normal temperature, however, some heating of both the aggregates and asphalt may be required during winter season Drying of aggregates is not necessary except when the particles have surface moisture To improve bonding, commercial additives are needed in this type of asphalt concrete
TYPES OF ASPHALT CONCRETE MIXTURES Dense-graded HMA concrete A dense-graded HMA concrete is produced using well-graded aggregates, and intended for general use When properly designed and constructed, a dense-graded HMA concrete is relatively impermeable Dense-graded HMA concrete mixes are generally referred to by their nominal maximum aggregate size They can further be classified as either fine-graded or coarse-graded. Fine-graded mixes have more fine and sand sized particles than coarse-graded mixes
TYPES OF ASPHALT CONCRETE MIXTURES Dense-graded HMA concrete Fine- and course-graded definitions for dense-graded HMA concrete are presented in the following Table: Mixture Nominal Maximum Aggregate Size Coarse-Graded Mix Fine-Graded Mix 37.5 mm (1.5 inches) 25.0 mm (1.0 inch) 19.0 mm (0.75 inches) 12.5 mm (0.5 inches) 9.5 mm (0.375 inches) < 35 % passing the 4.75 mm (No. 4 Sieve) < 40 % passing the 4.75 mm (No. 4 Sieve) < 35 % passing the 2.36 mm (No. 8 Sieve) < 40 % passing the 2.36 mm (No. 8 Sieve) < 45 % passing the 2.36 mm (No. 8 Sieve) > 35 % passing the 4.75 mm (No. 4 Sieve) > 40 % passing the 4.75 mm (No. 4 Sieve) > 35 % passing the 2.36 mm (No. 8 Sieve) > 40 % passing the 2.36 mm (No. 8 Sieve) > 45 % passing the 2.36 mm (No. 8 Sieve)
TYPES OF ASPHALT CONCRETE MIXTURES Open-graded HMA concrete An open-graded HMA mixture is designed to be water permeable (dense-graded and SMA mixes usually are not permeable) Open-graded mixes use only crushed stone (or gravel) and a small percentage of manufactured sands. There are three types of open-graded mixes typically used in the U.S.: Open-graded friction course (OGFC). Typically 15 percent air voids, no minimum air voids specified, lower aggregate standards than Porous European mixes (PEM). Porous European mixes (PEM). Typically 18-22 percent air voids, specified minimum air voids, higher aggregate standards than OGFC and requires the use of asphalt binder modifiers. See Figure 2.13. Asphalt treated permeable bases (ATPB). Less stringent specifications than OGFC or PEM since it is used only under densegraded HMA, SMA or PCC for drainage
AGGREGATES FOR ASPHALT CONCRETE MIXTURES Introduction In asphalt concrete mixture, aggregates may constitute about 70-75% by volume or 90-95% by weight The roles of aggregates in a asphalt concrete pavement are to distribute wheel loads through point-to-point contact (aggregate interlock) ensuring stability of pavement (i.e., resistance to pavement deformation under load) provide resistance to abrasion and skid A number of factors, such as quality, gradation, shape, stiffness, and quantity of aggregates, determine effectiveness of aggregates in load transfer and the stability of pavement Aggregates for asphalt concrete are usually classified as coarse aggregates, fine aggregates, and mineral filler Mineral filler is often used in asphalt concrete mixtures to supply the fines (smaller than 75 m or No. 200 sizes) Fines are very important in producing a dense-graded, strong material. Many natural sands do not contain the amount or type of fines required. Limestone dust is the most common material used for mineral filler
AGGREGATES FOR ASPHALT CONCRETE MIXTURES General requirements Aggregates should be: well-graded dense, including mineral filler (if required) for strength hard for resistance to wear and to polishing due to traffic. sound for resistance to breakdown due to freezing and thawing. rough surfaced crushed rough surfaces give higher friction strength and a better surface for adhesion of the asphalt cement. free from cubical thin, elongated aggregate particles because they are broken easily. hydrophobic (or water hating ) some siliceous aggregates such as quartz are hydrophobic ( water liking ), meaning that they have a greater affinity for water than for asphalt, due to their surface charges. This may lead to stripping, as asphalt coating comes away from the particle in the presence of water. free from deleterious substances clay particles, dust, dirt, and lightweight pieces may lead to a lower quality asphalt film on the aggregate particles or may result in breaking of some of the particles.
AGGREGATES FOR ASPHALT CONCRETE MIXTURES Types of aggregate gradation Following two types of aggregate gradation are commonly used in pavement construction: (i) open gradation and (ii) dense gradation Open-graded aggregate is an aggregate containing little or no fine aggregate, or one in which the void content in the compacted aggregate is relatively large, as large as 20% Open-graded aggregate is used to make opengraded asphalt concrete, which provides good skid resistance and high permeability so as to permit good surface drainage Dense-graded aggregate is an well-graded aggregate containing fine aggregate filling the voids in coarse aggregate Use of dense-graded aggregate provides a dense and impermeable layer and does not normally require surface treatment or seal coat