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12 laboratory fatigue life and then predicting pavement perform- Testing was conducted on 9.5-mm and 12.5-mm nominal ance by incorporating the effect of mixture stiffness on the maximum aggregate size (NMAS) mixtures to examine the predicted pavement strains resulting from layered elastic effect of increasing asphalt binder content. Flexural beam analysis. Using this methodology, the authors show the poten- fatigue tests conducted at 600 ms indicated a slight increase tial benefits of a rich bottom layer. It is also important to rec- in fatigue life with the 0.5% higher asphalt content. A sub- ognize that lower in-place air voids increase stiffness while stantial increase in fatigue life was noted with 1.0% higher resulting in longer fatigue lives, opposite conventional wis- asphalt content. dom that suggests that stiffer mixes have shorter fatigue lives. Monismith et al. (20) reported on the development of High Modulus Base the design and specifications for the California I-710 reha- bilitation. In their study, AR-8000 (roughly equivalent to a Europeans have used stiff binders to produce high modu- PG 64-10 or PG 64-16) and PBA-6a (PG 64-40) asphalt lus base layers (10, 14, 28, 29). Corte (28) reported on the use binders were used to prepare mixtures for testing at two com- of high modulus asphalt mixtures in France. The first use of binations of asphalt binder content and air void content. high-modulus asphalt concrete (HMAC) occurred around Mixes were prepared with 0.5% higher asphalt binder content 1980. Initially, these mixtures were used for strengthening or and 3% lower air voids. When tested at 20C using the proce- rehabilitation where pavement thickness was constrained (for dure described in AASHTO T321, the measured fatigue life of instance by bridge clearance). The use increased in 1985. It the mixes with 0.5% higher asphalt binder content (and lower was found that locally available weak aggregates could be air void contents) was approximately two times the fatigue life used with stiff binders. These mixtures were designed with of mixes prepared at lower asphalt binder content. relatively high asphalt binder content and low voids (less Anderson and Bentsen (26) reported on a study evaluating than 6%). Constant strain fatigue tests indicate that HMAC the influence of voids in mineral aggregate (VMA) on mix- mixes are more fatigue resistant than conventional base ture performance. Since VMA is related to asphalt binder mixtures. This is believed to be due to the higher asphalt con- content, mixtures with high VMA had asphalt binder con- tent and lower voids found in the HMAC mixtures. Corte's tents that were approximately 1.0% higher than the low VMA findings match the findings in similar studies where lower mixtures. Flexural beam fatigue testing conducted at 20C air voids increase stiffness, but also appear to increase fatigue and 500 ms indicated that the mixes with the higher asphalt life (20, 21). The stiffness of these layers reduce the strain at binder contents had two times greater laboratory fatigue life the bottom of the asphalt layer using less thickness than than the low asphalt binder content mixes. conventional asphalts. Cracking can be a problem with these Harvey et al. (24) reported on California's experiences with mixes. Corte (28) discusses binder tests to minimize the like- the design and construction of long-life asphalt pavements. lihood of cracking. The authors reported that most full-depth asphalt long-life designs will include a stiff, fatigue-resistant bottom layer. This Laboratory Fatigue Tests layer, termed a rich bottom layer, is designed to have a very low and Analysis Methods air void content (approximately 0% to 3%). Stiffness of the layer is a consideration since it is intended to reduce the over- The SHRP A003-A project (2, 3) evaluated seven methods all thickness of the HMA layers. The low air void content also of measuring laboratory fatigue life. Repeated load flexure reduces permeability and improves moisture resistance. The and direct tension tests received the highest rankings. A benefit of rich bottom layers is maximized with a thickness methodology was developed to evaluate fatigue life using range of 50 mm to 75 mm. Illinois has also adopted this flexural beam fatigue tests conducted in constant strain mode concept (14). at 10 Hz. Thin pavements are generally subjected to a mode Generally the stiffness of a mix can be increased with of loading best represented by constant strain. Thick pave- increased compaction. Further, increased compaction gen- ments are generally represented by a mode of loading most erally increases fatigue life at the same strain level. Thus, closely represented by constant stress. However, the SHRP increased compaction specifications for lower lifts result in A003-A researchers recommended constant strain tests for both lower strains due to increased stiffness and also increased all pavement loading conditions. This recommendation was fatigue life for a given strain level, producing a more eco- based upon the fact that if fatigue evaluations are made in the nomical pavement. The unbound layers must be sufficiently context of the pavement structure (e.g. by calculating expected stiff to allow a high degree of compaction in the bottom HMA strains at the bottom of a given pavement structure), then layers. constant stress and constant strain tests give similar rank- A study conducted by Maupin (27) examined the impact ings (3, 25, 30). AASHTO T321 is the current standard for of asphalt content on durability of Virginia surface mixtures. beam fatigue tests.