Risk Management of Low Air Void Asphalt Concrete Mixtures

Various forms of asphalt pavement distress, such as rutting, shoving and bleeding, can be attributed, in many cases, to low air voids in the mixtures during production and placement. The occurrence of low air void contents during plant production may originate as a result of an accidental increase in binder content or mix fines (or both). When low air voids are encountered during production, the specifying agency must decide whether to require the material that has already been placed to be removed and replaced or whether it can be left in place with a reduction in pay. Consequently, the Indiana Department of Transportation (INDOT) initiated this research project to develop a decision-support tool for dealing with such events that is based on projected rutting performance of the pavement system. The study was conducted along three paths. In the first, INDOT sponsored two pavement test sections at the National Center for Asphalt Technology (NCAT) Test Track. The second path involved testing mixes in the INDOT Accelerated Pavement Testing (APT) Facility. In both cases, mixtures were produced in local hot mix plants by either increasing the fines content or the binder content. The NCAT test sections included low void mixes in the surface course only and performance was measured by the progression of rutting. Significant rutting developed in the low void mixes. The results suggested that removal be considered for mixtures with air voids below 2.75% but that no pay adjustment was necessary for air voids above this level. However, the NCAT results were limited to one pavement structure, one set of materials, one climate and low voids in the surface mix only. In the APT, low air void mixtures were placed in either the surface or the intermediate course and different materials were used. The pavement response (permanent deformation of the top pavement layers) resulting from repetitive APT wheel passes was measured using a laser based system. Lastly, a simplified mechanistic analysis, using a software program called QRSS (Quality Related Specification Software) was used in an attempt to simulate the effects of low void mixtures on pavement performance and service life with different materials in different pavement layers and under different traffic loads. The results of these efforts were used along with engineering judgment to formulate the desired decision-support tool

Brillouin optical correlation domain analysis in composite material beams

Structural health monitoring is a critical requirement in many composites. Numerous monitoring strategies rely on measurements of temperature or strain (or both), however these are often restricted to point-sensing or to the coverage of small areas. Spatially-continuous data can be obtained with optical fiber sensors. In this work, we report high-resolution distributed Brillouin sensing over standard fibers that are embedded in composite structures. A phase-coded, Brillouin optical correlation domain analysis (B-OCDA) protocol was employed, with spatial resolution of 2 cm and sensitivity of 1 °K or 20 micro-strain. A portable measurement setup was designed and assembled on the premises of a composite structures manufacturer. The setup was successfully utilized in several structural health monitoring scenarios: (a) monitoring the production and curing of a composite beam over 60 h; (b) estimating the stiffness and Young’s modulus of a composite beam; and (c) distributed strain measurements across the surfaces of a model wing of an unmanned aerial vehicle. The measurements are supported by the predictions of structural analysis calculations. The results illustrate the potential added values of high-resolution, distributed Brillouin sensing in the structural health monitoring of composites