International Center for Aggregates Research (ICAR)

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The International Center for Aggregates Research is the voice of the aggregates industry in the research community and serves as a facilitator for determining the most effective use of aggregates in design, specification and construction.

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Acceptability criteria for high fines content aggregate pavement layers
(2007-12-31) Ashtiani, Reza S.; Little, Dallas N.
The objective of this study was to evaluate the impact of increasing fines content on the performance of unbound (unstabilized) and lightly stabilized aggregate systems. The aggregate systems analyzed varied in the amount of mineral fines, the moisture state during curing and at the time of testing, and the amount of Portland cement used to stabilize the blend. The evaluation was based on measurements of anisotropic resilient properties, permanent deformation, and unconfined compressive strengths of aggregate systems. In addition, the nonlinear anisotropic resilient properties of the aggregate blends were used in a finite element program to determine critical pavement responses. Aggregate systems with higher fines contents were, as expected, more sensitive to moisture than control systems with standard fines contents. The increase in the fines content in the unbound systems when molding moisture was greater than optimum dramatically diminished the quality of performance. However, the aggregate systems with higher fines benefited considerably from low percentages of cement stabilizer. Researchers found that with the proper design of fines content, cement content, and moisture, the performance of the stabilized systems with high fines content can perform equivalent to or even better than systems with standard fines content. This was clearly demonstrated that by enhancing the resilient properties (an increase in stiffness and a decrease in the level of anisotropy), permanent deformation of the aggregate systems were significantly reduced. This finding was in conformity with unconfined compressive strength of lightly stabilized high fine systems.
Adhesion in bitumen-aggregate systems and quantification of the effects of water on the adhesive bond
(2005-12) Hefer, Arno; Little, Dallas N.
This research is intended to contribute toward the understanding, development, and implementation of a more fundamental design process for bituminous pavement materials, utilizing thermodynamic properties of the materials involved. The theory developed by van Oss, Chaudhury and Good forms the basis of this research. Optimization of techniques to characterize surface energy, as well as consideration and evaluation of additional factors that influence adhesion in the presence of water, are pursued. A synthesis of theories and mechanisms of bitumen-aggregate adhesion is presented, and existing and potential techniques for surface energy characterization are reviewed to establish firm background knowledge on this subject. The Wilhelmy plate technique was scrutinized and improved methodologies and analysis procedures are proposed. Inverse gas chromatography (IGC) is introduced as an alternative technique. A reasonable comparison of total surface energy values form these techniques with mechanical surface tension values were found. Results suggest that bitumen surface energies do not vary substantially. Inability of these techniques to detect the effect of a liquid additive is rationalized by the ‘potential’ surface energy concept. Suggestions for a more realistic characterization of bitumen polar surface energy components are presented. A static gravimetric sorption technique was employed to characterize aggregate surface energies. Dynamic vapor sorption was identified as a candidate alternative technique for aggregate surface energy characterization.
Aggregate Update
(International Center for Aggregates Research, 1996) Fowler, David W.
Aggregates in Self-Consolidating Concrete
(2007-03) Koehler, Eric Patrick; Fowler, David W.
Self-consolidating concrete (SCC) is an advanced type of concrete that can flow through intricate geometrical configurations under its own mass without vibration or segregation. A research project was conducted to investigate the role of aggregates in SCC. Although SCC can be proportioned with a wide range of aggregates, the selection of favorable aggregate characteristics can significantly enhance the economy and performance of SCC. The objectives of the research project were to evaluate the effects of specific aggregate characteristics and mixture proportions on the workability and hardened properties of SCC, to identify favorable aggregate characteristics for SCC, and to develop guidelines for proportioning SCC with any set of aggregates. The effects of aggregate grading; maximum size; shape, angularity, and texture; clay content; and packing density were evaluated. Separately, the effects of mixture proportions, cementitious materials, and chemical admixtures were evaluated. In total, 12 fine aggregates, 7 coarse aggregates, and 6 microfines were tested. Tests were conducted on paste, mortar, and concrete. Paste measurements were conducted to evaluate the effects of cement, fly ash, microfines, high-range water-reducing admixture (HRWRA), and viscosity modifying admixture (VMA) on rheological properties. Mortar measurements were conducted to evaluate the effects of fine aggregates, microfines, and mixture proportions on workability and hardened properties. Concrete measurements were conducted to evaluate the effects of fine aggregates, coarse aggregates, microfines, and mixture proportions on workability and hardened properties. Target properties for SCC workability were defined as a function of the application and in terms of filling ability, passing ability, segregation resistance, and rheology. Seven workability test methods were evaluated extensively to provide sound, engineering justifications for their use and for the interpretation of their results. Specific tests for filling ability, passing ability, and segregation resistance were recommended. Based on the results of this research and well-established principles from the literature, a mixture proportioning procedure for SCC was developed.
Alkali-Silica Reaction in Portland Cement Concrete: Testing Methods and Mitigation Alternatives
(2001-07) Touma, Wissam; Fowler, David W.; Carrasquillo, Ramón L.
Identifying the susceptibility of an aggregate to alkali-silica reaction (ASR) before using it in concrete is one of the most efficient practices for preventing damage and failure. Several tests have been developed for identifying aggregates subject to ASR, but each has its limitations. A three-year research study was initiated on January 1, 1998 at The University of Texas at Austin for investigating ASR in portland cement concrete. The scope of the study was essentially three fold: (1) investigate the predictive ability of ASTM C 1260 and C 1293, (2) develop more accurate and more efficient modifications of these procedures, and (3) investigate ASR mitigation alternatives. Aggregate samples from 14 sources from around the United States were acquired for the investigation. Aggregates were used in an extensive testing program during which guidelines for predicting the potential ASR of aggregates were developed and recommendations for minimizing concrete damage due to ASR were formulated. This report includes a review of the state-of-the-art of ASR, an evaluation of testing protocols and recommendations for dramatically shortening test time, test results, and mitigation options.
Annual Report
(University of Texas at Austin. International Center for Aggregates Research, 2000) International Center for Aggregates Research
Characterization of Unbound Granular Layers in Flexible Pavements
(2001-12) Adu-Osei, Alex
The mathematical characterization of unbound granular materials should ideally be based on the behavior of the individual constituent elements and their interaction. Until particulate mechanics are developed to a level where it can easily be applied to characterize unbound granular materials, a nonlinear and cross-anisotropic model must be used to characterize the behavior of granular materials in pavements. Existing pavement design and analysis methods have generally taken a very conservative view of the relative strength properties of granular materials used as base and subbase layers in conventional flexible pavements. The mechanical properties of unbound granular layers in flexible pavements are important to the overall structural integrity of the pavement structure. Linear elastic analysis can be used with reasonable confidence for pavements with full depth asphalt layers, but it is inappropriate for unsurfaced or thinly surfaced flexible pavements unless the nonlinear behavior of unbound granular materials are properly taken into account. Work done by several researchers suggest that incorporating a cross-anisotropic elastic model significantly improves isotropic models and drastically reduces the tensile stresses computed within granular layers. This is due to the fact that the behavior of granular materials depends on particle arrangement. The laboratory determination of cross-anisotropic properties of granular materials has been a difficult task for researchers. In this study, a new laboratory testing protocol has been developed based on the theories of elasticity to determine cross-anisotropic properties of granular materials. The testing protocol is efficient and precise. The test is also an excellent tool for comparative analysis of compacted materials. The behavior of four unbound granular materials was studied. The resilient responses of the materials obey the Uzan type nonlinear model. It was observed that under low stress levels accumulation of permanent strain could stabilize in granular layers. However, at high stress levels, permanent strain will continuously accumulate. A finite element program was modified to incorporate the cross-anisotropic material model. Pavement sections were analyzed with the finite element program. It was observed that cross-anisotropic modeling eliminates the presence of tension zones predicted by isotropic resilient models. Deflection bowls predicted by nonlinear resilient models agree with field deflection bowls.
Characterizing minus no. 200 fine aggregate for performance in concrete
(2006-03) Stewart, Jane; Norwell, Justin; Juenger, Maria C. G.; Fowler, David W.
ASTM C 33 limits the amount of microfine aggregate smaller than 75 µm (No. 200 sieve) to be used in concrete. In the past, it was believed that this fraction was clay and, therefore, a poor performer. This is not necessarily the case with manufactured fine aggregates. While work continues toward altering ASTM C 33 to allow a higher percentage of microfine aggregates, there is need for a method of determining whether these microfines will have deleterious effects or not. Fourteen aggregates were collected for evaluation in this study. Methods of characterizing the microfines to determine their effects on concrete properties were developed in this study. In addition to fully characterizing the aggregates using advanced techniques, simple tests for microfines that can be used as a criterion for their exclusion or inclusion were evaluated. For such a test to be meaningful there must be a strong correlation between its results and concrete performance. Mortar and concrete mixes incorporating microfines from fourteen different aggregates are tested in this project for a variety of performance criteria. This project fully characterizes microfines and evaluates simple tests for predicting performance in concrete.
Characterizing Surface Properties of Aggregates Used in Hot Mix Asphalt
(2006-03) Bhasin, Amit; Little, Dallas N.
Physical and chemical properties of aggregates at the micro scale strongly impact the adhesive bond (strength and durability) between bitumen and aggregate. These properties include surface free energy, chemical interaction potential, and specific surface area. This report describes testing methods developed for the Universal Sorption Device (USD), the Wilhelmy Plate (WP), and the microcalorimeter (MC) to measure these surface properties of aggregates. Test results from five different asphalt binders and nine different aggregates are presented to demonstrate how these surface properties can be used to: (1) select combinations of bitumen and aggregates that are more resistant to moisture damage, (2) select additives that can be used to improve the performance of asphalt mixtures based on the physico-chemical nature of the bitumen and aggregate, and (3) predict the resistance of the mixture to moisture-induced damage.
Determination of Aggregate Shape Properties Using X-ray Tomographic Methods and the Effect of Shape on Concrete Rheology
(2005-08) Erdogan, Sinan Turhan; Fowler, David W.
The shape of aggregate particles can significantly influence certain properties of concrete, both in its fresh and hardened states. Therefore, there is a need to be able to completely characterize the shape of aggregate particles, in three dimensions, in order to develop computational models that accurately predict properties. In the past, numerous methods have been suggested for this task. However, these methods are often only applicable to two-dimensional images of particles, they output a single or a few values, and fail to characterize the true shape of the particle. X-ray tomographic techniques allow the capturing of the true shape of particles and have been applied to concrete aggregates. Computed tomography has been used to characterize coarse and fine aggregate particles, while X-ray microtomography has been used to characterize particles passing the 75 m sieve. Sample preparation methods and scanning parameters applicable to concrete aggregates have been developed. The spherical harmonic method was used to efficiently store shape information, and to calculate useful parameters for individual particles, such as volume and surface area. Comparisons of the results to properties determined using other techniques were made and it was determined that the results of indirect or two-dimensional shape and size characterization methods can be misleading.
Development of a Portable Rheometer for Fresh Portland Cement Concrete
(2004-08) Koehler, Eric Patrick; Fowler, David W.
The purpose of this research was to identify an effective field test method for measuring the workability of concrete in general and of high-microfines concrete in particular. The workability of fresh concrete has traditionally been measured with the slump test, which provides an inadequate indication of workability. For certain concrete mixtures—such as that containing fiber reinforcement, ground granulated blast furnace slag, or high contents of aggregate microfines—the slump test can provide inaccurate and misleading results. The need for a better test method for workability is well established within the concrete industry. Based on a literature search in which 61 existing workability test methods were identified and on feedback from government, industry, and academia, criteria for an improved workability test device were developed. It was determined that the best approach to measuring workability would be to develop a new portable rheometer. The ICAR rheometer—a low-cost, fully portable test device for concrete—was developed and tested. A first generation prototype was built using off-the-shelf components. The ICAR rheometer is approximately the size of a drill and can be operated by hand or positioned above a standard container. It is capable of measuring a flow curve or performing a stress growth test and is appropriate for nearly the full range of concrete workability ranging from a slump of approximately 2 inches to self-consolidating concrete. Experimental testing on a wide range of concrete mixtures indicated that the ICAR rheometer was able to detect changes in workability and rheology successfully. As a dynamic test that adds energy to concrete, it is well suited for measuring high-microfines concrete and other highly thixotropic concrete mixtures. Field testing confirmed the portability of the ICAR rheometer. The low cost and portable form factor of the ICAR rheometer can make the routine measurement of concrete rheology in the field an economically viable solution to characterizing concrete workability.
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.
Effects of Superpave restricted zone on permanent deformation
(2001-05) Chowdhury, Arif; Button, Joe W.; Grau, Jose D. C.
The purpose of this study is to evaluate the restricted zone effect using four different aggregates: crushed granite, crushed limestone, crushed river gravel, and a mixture of crushed river gravel as coarse aggregate with natural fines. As the restricted zone is a component of Superpave, the blends prepared met most of the Superpave criteria, except the restricted zone in selected mixtures and fine aggregate angularity in three mixtures. Each type of aggregate was used for mixture design of three gradations: above, through, and below the restricted zone. The twelve mixtures designed were tested in the laboratory to evaluate their relative resistance to permanent deformation. Four types of tests were performed using Superpave equipment: simple shear at constant height, frequency sweep at constant height, repeated shear at constant stress ratio, and repeated shear at constant height. Rutting resistance of the mixtures was measured using the Asphalt Pavement Analyzer.
The effects of the aggregates characteristics on the performance of Portland cement concrete
(2003-12) Quiroga, Pedro Nel; Fowler, David W.
Aggregate shape, texture, and grading have a significant effect on the performance of fresh concrete. Aggregates blends with well-shaped, rounded, and smooth particles require less paste for a given slump than blends with flat, elongated, angular, and rough particles. At the same time, uniform gradings with proper amounts of each size result in aggregate blends with high packing and in concrete with low water demand. Optimized aggregate blends have high packing, requiring low amounts of paste. As a result, they are less expensive and will have less durability problems caused by the paste such as head generation, porosity, and drying shrinkage. The effect of shape, texture and grading of aggregates on fresh concrete was evaluated experimentally, quantified by means a proportioning method based on packing density concepts, the Compressible Packing Model (CPM), and analyzed by an empirical tool suggested by Shilstone. The effect of different types and amounts of microfines was evaluated simultaneously as well as the impact of chemical admixtures and some supplementary cementing materials can be used to improve the workability of concrete with high microfines without negatively affecting hardened concrete. Guidelines for portioning and optimizing aggregate blends were made based on Shilstone's Coarseness Chart and the 0.45 Power Chart and CPM equations and procedures.
Evaluation of Aggregate Characteristics Affecting Hot Mix Asphalt (HMA) Response
(International Center for Aggregates Research, 2004) International Center for Aggregates Research
Aggregate shape properties are known to influence asphalt pavement performance. Angularity and texture govern the frictional properties and dilation of the aggregate structure. Aggregate texture plays a major role in influencing the adhesive bond between the aggregate and the binder, while aggregate form influences the anisotropic response of asphalt mixes... Aggregate shape properties are known to influence asphalt pavement performance. Angularity and texture govern the frictional properties and dilation of the aggregate structure. Aggregate texture plays a major role in influencing the adhesive bond between the aggregate and the binder, while aggregate form influences the anisotropic response of asphalt mixes.
Evaluation of aggregate characteristics affecting HMA concrete performance
(2004-01) Masad, Eyad; Little, Dallas N.; Tashman, Laith; Saadeh, Shadi; Al-Rousan, Taleb; Sukhwani, Rajni
This report documents the outcomes of the ICAR study on the Evaluation of Aggregate Characteristics Affecting HMA Concrete Performance. This study was conducted with support from the Federal Highway Administration (FHWA) program on Simulation, Imaging, and Mechanics of Asphalt Pavements at Texas A&M University. The first outcome includes assessment of HMA sensitivity to aggregate shape characteristics. Aggregate shape is characterized through detailed measurements of angularity, form, and texture using the Aggregate Imaging System (AIMS). The shape characteristics are presented in terms of the distribution of the property in an aggregate sample rather than an average index of this property. The second outcome of this study is the development of a viscoplastic model for permanent deformation. The model accounts for the aggregate structure in the mix, which is related to the shape properties measured using AIMS. The model capabilities are demonstrated through matching the results of testing various mixes from the Accelerated Loading Facility (ALF) of the FHWA using the triaxial creep and strength tests. In addition, the model is used to predict the response of mixes that include aggregates with different shape characteristics in order to develop relationships between the model parameters and aggregate shape characteristics. As part of the model development, an experiment was conducted to capture and characterize damage evolution in HMA due to permanent deformation. HMA specimens were loaded using a triaxial compression setup to four predefined strain levels at three confining pressures. Consequently, image analysis techniques were used to analyze damage distribution. The results of the damage experiment supported the damage evolution function proposed in the viscoplastic model.
Evaluation of Superpave Aggregate Specifications
(International Center for Aggregates Research, 2002) International Center for Aggregates Research
The purpose of this study was to evaluate the restricted zone effect using four different aggregates: crushed granite, crushed limestone, crushed river gravel, and a mixture of crushed river gravel as coarse aggregate with naturals and fines. Each type of aggregate was used for mixture design of three hot-mix asphalt (HMA) gradings: above, through, and below the restricted zone.
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.
An Experimental Study on the Guidelines for Using Higher Contents of Aggregate Microfines in Portland Cement Concrete
(2001-12) Fowler, David W.; Ahn, Nam-Shik
The use of manufactured fine aggregate (MFA) in portland cement concrete becomes more common as sources of natural sands are depleted. There is a great need to utilize MFA better, particularly the minus 75µm sizes. The development of specifications that aid aggregates producers, engineers, and specifying agencies in the utilization of MFA is of interest to many parties. Better utilization of crushed aggregates has both economic and environmental benefits for the construction and materials industry. This report presents some of the effects of high-fines on the properties of cement mortar and concrete. A total of 50 sands were used in this mortar study, 10 of which were included in the concrete research. A summary of aggregate characteristics that affect the properties of mortar and concrete are presented along with the correlations evaluated between these properties.
Federal Highway Administration Research Project, Task 3, Concrete Mixture Research Related to Pavements: Final Report
(2008-10-15) Fowler, David W.
Manufactured fine aggregate (MFA) can be used as a replacement for or in conjunction with natural sand in concrete mixtures. MFA does not exhibit ideal shape or texture for fine aggregate, and the production of MFA generates high percentages of microfines, particles that pass the No. 200 sieve. Microfines are washed from the aggregate due to specification limitations, resulting in wasted aggregate and a coarser fine aggregate grading. Three manufactured sands were incorporated into concrete mixtures to determine their effects on fresh and hardened concrete properties. It was found that greater proportions of manufactured sand caused the workability to decrease, the demand for high-range water-reducing admixture to increase, the compressive strength to increase, and the modulus of elasticity to decrease.

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