Browsing by Subject "Air voids"
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Item Clustering of air voids around aggregates in air entrained concrete(2007-05) Naranjo, Andy, 1977-; Folliard, Kevin J.Recent research reports have suggested that one of the potential causes of low compressive strengths in air-entrained concrete is the clustering of air voids around the aggregates. The phenomenon of clustering of air voids has been associated with the use of non-vinsol resin air-entraining admixtures and retempering (i.e. the late addition of water). The clustering is thought to reduce the bond strength at the aggregate-paste interface resulting in substantial reduction of compressive strength. The purpose of this research was to try and reproduce the clustering of air voids around aggregates in the laboratory by retempering, determine the severity of the clustering, and to evaluate its effects on concrete strength properties. Review of low strength field cases was also performed to determine if any correlations exist. This research evaluated three different types of air-entraining admixtures (wood and gum rosin, vinsol resin, and a synthetic formulation) and two types of coarse aggregates (siliceous river gravel and crushed limestone). A total of 43 concrete mixtures were made in the laboratory. The mixing procedure focused on retempering the concrete with water followed by an agitation period, and was successful at producing clustering of air voids around the aggregates. However, compressive, splitting tensile, and flexural strengths all showed no signs of strength loss that could be attributed to clustering of the air voids. Review of available field data and petrographic reports found that the phenomenon of air void clustering has a good correlation with the use of non-vinsol resin air-entraining admixtures and retempering of field concrete.Item Effects of Aggregate Gradation and Angularity on VMA and Rutting Resistance(2001-06) Park, Dae-Wook; Chowdhury, Arif; Button, Joe W.The Superpave system adopted the voids in mineral aggregate (VMA) criteria developed by McLeod using the 75-blow Marshall compactor for conventional dense-graded hot mix asphalt (HMA) mixtures. This VMA criteria is a function of only the nominal size of aggregate regardless of shape, texture, or gradation. The Superpave volumetric mixture design process contains a required minimum value for fine aggregate angularity (FAA) as a function of traffic level and position of the layer within the pavement structure. This parameter is reported as the percentage of uncompacted air voids, with larger values generally indicating increased aggregate angularity and, thus, higher VMA and better resistance to permanent deformation. The purpose of this study was to evaluate the effects of FAA and gradation on the resulting VMA of certain HMA mixtures. The effect of FAA was evaluated using mixtures containing coarse limestone combined with six different fine aggregates. Mixtures with three gradations which pass through, above, and below the restricted zone; three different mineral filler contents; and four different values of FAA were analyzed to evaluate the effects of these parameters on VMA of Superpave mixtures. Based on analyses of these tests, mixtures containing fine granite or limestone showed less permanent deformation than mixtures containing fine river gravel or natural rounded sand. FAA values and permanent deformation did not correlate well. Gradations that pass through the restricted zone did not significantly affect mixture VMA. Mineral filler contents and FAA value did affect mixture VMA significantly. Higher FAA values yielded higher VMA.Item Evaluation of Superpave Fine Aggregate Angularity Specification(2001-05) Chowdhury, Arif; Button, Joe W.; Kohale, Vipin; Jahn, David W.The validity of the Superpave fine aggregate angularity (FAA) requirement is questioned by both the owner agencies and the paving and aggregate industries. The FAA test is based on the assumption that more fractured faces will result in higher void content in the loosely compacted sample; however, this assumption is not always true. Some agencies have found that cubical shaped particles, even with 100 percent fractured faces, may not meet the FAA requirement for high-volume traffic. State agencies are concerned that local materials, previously considered acceptable and which have provided good field performance, cannot meet the Superpave requirements. Researchers evaluated angularity of 23 fine aggregates representing most types of paving aggregates used in the USA using seven different procedures: FAA test, direct shear test, compacted aggregate resistance (CAR) test, three different image analyses, and visual inspection. The three image analyses techniques included Hough Transform at University of Arkansas at Little Rock (UALR), unified image analysis at Washington State University (WSU), and VDG-40 videograder at Virginia Transportation Research Council (VTRC). A small study was performed to evaluate relative rutting resistance of HMA containing fines with different particle shape parameters using the Asphalt Pavement Analyzer (APA). The FAA test method does not consistently identify angular, cubical aggregates as high quality materials. There is a fair correlation between the CAR stability value and angle of internal friction (AIF) from the direct shear test. No correlation was found between FAA and CAR stability or between FAA and AIF. Fairly good correlations were found between FAA and all three image analysis methods. Some cubical crushed aggregates with FAA values less than 45 gave very high values of CAR stability, AIF, and ‘angularity’ from imaging techniques. Moreover, the three image analysis methods exhibited good correlation among themselves. A statistical analysis of the SHRP-LTPP (Strategic Highway Research Program-Long-Term Pavement Performance) database revealed no significant evidence relationship between FAA and rutting. This lack of relationship is not surprising since many uncontrolled factors contribute to pavement rutting. The APA study revealed that FAA is not sensitive to rut resistance of HMA mixtures. Image analysis methods appear promising for measuring fine aggregate angularity. Until a suitable replacement method(s) for FAA can be identified, the authors recommend that the FAA criteria be lowered from 45 to 43 for 100 percent crushed aggregate. Analysis of the FAA versus rutting data should be examined later as the amount of data in the SHRP-LTTP database is expanded.