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An interlayer stress-absorbing composite (ISAC) was developed and evaluated for the purpose of effectively alleviating or mitigating the problem of reflection cracking in an asphalt concrete (AC) overlay. ISAC materials and performance properties were carefully selected through comprehensive theoretical studies and laboratory evaluation programs. The ISAC system consists of a low-stiffness geotextile as the bottom layer, a viscoelastic membrane layer as the core, and a very high stiffness geotextile for the upper layer. In order to evaluate the effectiveness of the ISAC layer to control reflective cracking, a laboratory pavement section with an AC overlay placed on a jointed portland cement concrete slab was constructed and tested in an environmental chamber. A mechanical device was used to simulate thermal strain in the slab, and the joint was opened and closed at an extremely slow rate. The testing was conducted at −1.1°C, and strain in the overlay was monitored using a sensitive linear variable differential transducer device. The force required to pull and push the slab was monitored using a load cell placed between the slab and a hydraulic ram. Performance of ISAC was evaluated by comparing the cycles to failure of an ISAC-treated overlay with the performance of a control section without ISAC and of test sections containing two commercially available products. The base isolation properties of the ISAC system were demonstrated in the laboratory evaluation studies, which indicated that the ISAC system vastly outperformed the control section and the sections with the two commercial products tested. Several years of field performance testing have show that the ISAC system is highly effective for mitigating reflective cracking in AC overlays used on both airport and highway pavement systems.

The Illinois Department of Transportation (IDOT) spends $2 million annually on reflective crack control treatments; however, the costeffectiveness of these treatments had not been reliably determined. A recent study evaluated the cost-effectiveness of IDOT reflective crack control System A, which consists of a nonwoven polypropylene paving fabric, placed either in strips longitudinally over lane-widening joints or over the entire pavement (area treatment). The study was limited to projects constructed originally as rigid pavements and subsequently rehabilitated with one or more bituminous overlays. Performance of 52 projects across Illinois was assessed through crack mapping and from distress and serviceability data in IDOT's condition rating survey database. Comparisons of measured reflective cracking in treated and control sections revealed that System A retarded longitudinal reflective widening crack development, but it did not significantly retard transverse reflective cracking, which agrees with earlier studies. However, both strip and area applications of these fabric treatments appeared to improve overall pavement serviceability, and they were estimated to increase rehabilitation life spans by 1.1 and 3.6 years, respectively. Reduction in life-cycle costs was estimated to be 4.4 and 6.2 percent when placed in medium and large quantities, respectively, and to be at a break-even level for small quantities. However, life-cycle benefits were found to be statistically insignificant. Limited permeability testing of field cores taken on severely distressed transverse joints suggested that waterproofing benefits could exist even after crack reflection. This was consistent with the observation that, although serviceability was generally improved with area treatment, crack reflection was not retarded relative to untreated areas.

SUMMARYAn investigation was conducted to develop a comprehensive moisture model for predicting non-isothermal moisture conditions in soils. An extensive literature review indicated that a model based on the Philip and de Vries equations for non-isothermal moisture movement and heat conduction would give the best results. By using numerical methods, the implicit finite difference approximations to the moisture movement and heat-transfer equations were programmed for computer solution of water content and temperature in the soil with time.Validation studies indicate that the moisture model can be used to predict accurately moisture conditions in the soil. The model was validated by using hydraulic data from laboratory studies conducted on soil columns compacted with AASHO A-3 and AASHO A-4 soil.The application of the moisture model to the study of non-isothermal moisture movement in the field is demonstrated. The influence of parameters such as water table depth, precipitation, and soil hydraulic properties on soil moisture content are shown by use of the moisture model. The model is shown to be applicable to a wide range of boundary conditions and that it predicts the moisture-temperature regime with time in soils utilizing climatic input data.

Engine failure caused by bird strikes can be particularly perilous for today's typical twin-engine aircraft. Although large-bird populations have increased substantially since the 1970s, modern-day turbofan engines are not tested for large birds. Instead, it is acceptable for contemporary turbofan engines to lose all power because of large-bird ingestion. With the increasing use of turbofan engines and air traffic, not only are more bird strikes expected in the near future, but also more bird strikes are anticipated to result in engine failure. This study identified the factors that were statistically associated with the probability of engine failure in the event of a bird strike. A large sample of more than 42,000 U.S. bird strikes was used. The missing data in the sample were multiply imputed by using an approximate Bayesian bootstrap method. With the multiply imputed data, 15 factors were statistically analyzed. Six of those factors were found to be significantly associated with the probability of engine failure in the event of a bird strike: altitude above ground level, bird size, number of birds struck, flight phase, daylight conditions, and sky conditions. A logistic regression model was developed, and a detailed probabilistic interpretation of the model is given for practitioners. By using the findings, aviation authorities can improve bird strike hazard mitigation strategies, flight crews can reduce the potential of bird strikes resulting in engine failure, and researchers can better understand the nature of bird strikes and develop a scientific approach to minimize the likelihood of engine failure in the event of a bird strike.