Showing papers in "Journal of Materials in Civil Engineering in 2008"

Mechanical, Fracture, and Microstructural Investigations of Rubber Concrete

Abstract: Waste tire rubber constitute a serious worldwide problem due to the lack of landfills and the health hazards associated with these landfills. In addition to the environmental motivation for providing a means of recycling large quantities of waste tire rubbers, the use of tire rubber particles provides a new type of concrete that has unique mechanical and fracture criteria. This paper presents the results of recent experimental investigations on rubber concrete. Chipped and crumbed tire rubber particles were used to replace coarse and fine aggregate with different volume replacement levels. The mechanical and fracture properties of rubber concrete were examined. Quasi-brittle fracture mechanics models are used to determine the effect of incorporating tire rubber particles on the fracture performance of rubber concrete. Finally, some microstructural features of rubber concrete are also reported. It is concluded that the choice of the optimal replacement ratio of the tire rubber particles can yield concretes with desirable strength and fracture toughness criteria for different applications.

FRP Retrofit of Low and Medium Strength Circular and Rectangular Reinforced Concrete Columns

Abstract: In this study, 68 reinforced concrete columns were tested under uniaxial compression after being jacketed externally with carbon fiber-reinforced polymer (CFRP) sheets. Forty specimens were cast using low strength concrete and inadequate internal transverse reinforcement, while 28 specimens were cast with medium strength concrete and adequate internal transverse reinforcement. Thickness of the CFRP jacket, cross-section shape, concrete strength, amount of internal transverse reinforcement, corner radius, existence of predamage, loading type (monotonic or cyclic), and the bonding pattern (orientation, spacing, anchorage details, additional corner supports) of CFRP sheets were the main test parameters of this extensive experimental work. Test results showed that external confinement of columns with CFRP sheets resulted in an increase in ultimate strength and ductility. While the strength enhancement was more pronounced for specimens with circular cross section, specimens with square and rectangular cross sections exhibited larger ultimate axial deformations without a substantial loss in strength. The efficiency of retrofitting was much more pronounced in the case of relatively lower strength concrete. The proposed model, together with two other available models, were used for predicting the strength and corresponding axial deformations of more than 300 specimens tested by other researchers, as well as more than 100 specimens tested by the writers during this study and before. It was shown that the predicted results by the proposed model were in reasonable agreement with this extensive database of experimental studies.

Strength and Stiffness Response of Coir Fiber-Reinforced Tropical Soil

Abstract: Use of natural fibers in civil engineering construction practice is often advantageous as they are cheap, locally available, biodegradable, and ecofriendly. Among the available natural fibers, coir is produced in large quantities in South Asian countries, such as India, Ceylon, Indonesia, Philippines, etc. and has better mechanical properties, such as tensile strength. In this paper, results on the strength and stiffness behavior of soil reinforced with coir fibers are presented. Soil samples reinforced with coir fibers of different sizes, and made into cylindrical soil specimens were tested in triaxial shear apparatus to determine the strength and stiffness of soil response due to fiber inclusion and the results were compared with that of unreinforced soils. The results show that addition of coir (1-2%) as random reinforcing material increases both strength and stiffness of clay soil considered in the study. In addition, available theoretical models for prediction of strength of fiber-reinforced soil are examined in relation to the results of the present investigation. Analysis shows that the available models are not adequate to capture the strength and stiffness response of coir fiber-reinforced soil. A nonlinear regression model for strength and stiffness response is proposed in the present study.

Stress-Strain Behavior of Steel Fiber-Reinforced Concrete in Compression

Abstract: Good structural design demands high quality experimental data and reliable modeling of the mechanical properties of the constituent materials. Although several theoretical models and much experimental data on the behavior of fiber-reinforced concrete in compression are available in published literature, there are considerable reservations on the general applicability of these models for design. This paper presents the results of tests in compression of steel fiber-reinforced concrete carried out according to standard procedures, and a critical evaluation of the models proposed to define the stress-strain behavior in compression. The tests reported were carried out on cylindrical specimens of plain and steel fiber-reinforced concrete with fiber volume of 1, 1.6, and 3%. To evaluate the reliability of the models available in literature, a critical comparative study was carried out between the experimental data and the various proposed theoretical stress-strain relationships. It is shown that while many of the models showed good agreement with test results from which the model equations were derived, there was no such good agreement when the models were applied to other published test data.

Strength, Modulus of Elasticity, and Brittleness Index of Rubberized Concrete

Abstract: This paper presents a study of rubberized concretes designed by replacing coarse aggregate in normal concrete with ground and crushed scrap tire rubber in various volume ratios. The objective of the study was to investigate the effect of rubber types and rubber content on strength and deformation properties. The compressive strength, static, and dynamic modulus of elasticity of rubberized concrete were tested and studied. The stress-strain hysteresis loops were obtained by loading, unloading, and reloading on specimens. Brittleness index values were calculated based on the hysteretic loops. The experiments revealed that strength and modulus elasticity of rubberized concrete decreased with the increasing amount of rubber content. Compressive strength and modulus of elasticity of crushed rubberized concrete were lower than that of ground rubberized concrete. An American Concrete Institute equation could reasonably predict modulus of elasticity of rubberized concrete. Brittleness index values of rubberized concrete were lower than that of normal concrete, which means that rubberized concrete had higher ductility performance than that of normal concrete.

Simulation of Fracture Behavior in Asphalt Concrete Using a Heterogeneous Cohesive Zone Discrete Element Model

Abstract: With increasing traffic loads and changes in crude petroleum refining techniques, cracking in asphalt pavements continues to be a major cause of structural and functional deterioration of these systems, particularly in cold climates. Although modern design tools such as the AASHTO Mechanistic Empirical Pavement Design Guide have recognized the need to predict pavement cracking in pavement life cycle cost analyses, the development of true fracture tests and associated models is hampered by a lack of fundamental knowledge of the physical nature of cracking in asphalt concrete materials. A clustered discrete element method (DEM) was employed as a means to investigate fracture mechanisms in asphalt concrete at low temperatures. The DEM approach was first verified by comparing elastic continuum theory and the discontinuum approach using uniform axial compression and cantilever beam models. A bilinear cohesive zone model was implemented into the DEM framework to enable simulation of crack initiation and propagation in asphalt concrete. Verification of the cohesive zone fracture model was carried out using a double cantilever beam. The main advantage of the DEM approach was that a mesoscale representation of the morphology of the material could be easily incorporated into the model using high-resolution imaging, image analysis software, and by developing a relatively simple mesh generation code. The simulation results were shown to compare favorably with experimental results, and moreover, the simulations provide new insight into the mechanisms of fracture in asphalt concrete. The modeling technique can provide more details of the fracture process in laboratory fracture tests, the influence of heterogeneity on crack path, and the effects of local material strength and fracture energy on global fracture test response.

Fresh State Characteristics of Foam Concrete

Abstract: This paper deals with the study of the influence of a mixture composition on the properties of foam concrete at the fresh state. The stability of the base mix in terms of the water–solids ratio for getting a stable foam concrete mix of design density was determined for different mixture parameters and their influence on the water–solids ratio requirement was studied. The consistency of the foam concrete mix was measured in terms of the spread percent using a flow cone test and flow time using a Marsh cone test. The fresh state properties of foam concrete, viz; (1) stability of foam concrete and (2) consistency of the foam concrete, were very much affected by the water content in the base mix and the amount of foam added along with the other solid ingredients in the mix. Regression equations for predicting the spread flow value of the foam concrete based on the experimental results were reported. This will help in arriving at the water content of a foam concrete mix for a given consistency requirement. An ...

Early-Age Properties of Cement-Based Materials: I. Influence of Cement Fineness

Abstract: The influence of cement fineness on early-age properties of cement-based materials is investigated using a variety of experimental techniques. Properties that are critical to the early-age performance of these materials are tested, including heat release, temperature rise, chemical shrinkage, and autogenous deformation. Measurements of these properties for two cements of widely different fineness are supplemented with other performance measures, specifically acoustic emission measurements to listen for microcracking occurring in high performance w/c=0.35 mortars and dual-ring paste shrinkage measurements conducted under sealed conditions to assess residual stress development. The measured properties are observed to be quite different for the coarse and the fine cement. The current emphasis on high early-age strength within the construction industry may result in the specification of cements that are more prone to early-age cracking.

Aggregate Morphology Affecting Strength and Permanent Deformation Behavior of Unbound Aggregate Materials

Abstract: This paper presents findings of a research study focused on investigating influences of coarse aggregate morphologies on the strength and permanent deformation behavior of unbound aggregate materials. Angularity and surface texture properties of six different types of aggregates were evaluated using two shape indices, angularity index (AI) and surface texture index (ST), based on image analysis. Twenty-one unbound aggregate blends having the same gradation and voids content but different crushed percentages were prepared by mixing the imaging evaluated aggregates and tested using triaxial tests for strength and permanent deformation. The laboratory permanent strain—load application ( ep –N) curves evaluated using a phenomenological power model showed the A and b parameters to be heavily dependent on applied stress levels. Aggregate blends with higher AI and ST indices were consistently associated with lower magnitudes of A and b . The angularity property was found to contribute mainly to the strength and ...

Rheology of Cement Paste: Role of Excess Water to Solid Surface Area Ratio

Abstract: Although many attempts have been made in previous research to identify the various parameters governing the rheology of cement paste, there has been little progress in evaluating the combined effects of these parameters. In this paper, a new parameter, the excess water to solid surface area ratio, is proposed to evaluate the combined effects of water content, packing density, and solid surface area on the rheological properties of cement paste. For the purpose of determining the value of this new parameter, a new wet packing method has been developed to measure the packing density of cementitious materials so that the voids content of the cementitious materials and the amount of excess water in the cement paste can be quantified. A number of cement paste samples containing different proportions of cement, pulverized fuel ash and condensed silica fume, and different water contents have been tested and their rheological properties, as measured by a rheometer, correlated to the new parameter. The correlation revealed that the excess water to solid surface area ratio, which serves as an indicative measure of the average water film thickness, is the single most important factor governing the rheology of cement paste.

Structural Capacity of Rammed Earth in Compression

Abstract: Rammed earth walls are formed by compacting subsoil in thin layers inside temporary supporting formwork. An ancient form of construction, rammed earth has in recent years, together with other earth building methods, been increasingly used structurally in a range of contemporary buildings in many countries around the world. Though current structural design procedures for earth walls, including rammed earth, in general use provisions based on structural masonry standards, this approach has never been satisfactorily validated. This paper presents experimental results from material and large-scale testing and develops a simple theoretical model, applied to rammed earth columns subject to concentric and eccentric axial compression loading. An analytical model, using a basic strut theory, shows favorable correlation with the experimental results for all load eccentricities.

Correlation among Fresh State Behavior, Fiber Dispersion, and Toughness Properties of SFRCs

Abstract: Effective structural use of steel fiber-reinforced concrete (SFRC) relies on the assumption of uniform dispersion of fibers within the elements. Fiber dispersion related issues hence stand as a cutting edge research and technology development topic. The use of self-consolidating concrete (SCC), thanks to its rheological stability and self-placability, which leads to the elimination of compaction by vibration, may be helpful in guaranteeing a uniform dispersion of fibers. With reference to the latter, several techniques [e.g., based on alternate current impedance spectroscopy (AC-IS), microwave reflectometry etc.] have been developed in the last few years for its nondestructive monitoring. Investigation into the connections between fiber dispersion and the performance of the composite in the fresh and hardened state stand as the natural completion for a thorough assessment of the FRC properties, aimed at promoting its wider use for full load bearing structural applications. This paper presents the results of a research project aimed at the above said purpose. Based on a previously calibrated mix-design methodology, suitable specimens were cast with SFRC characterized by different performance in the fresh state (vibration-, self-, and segregation consolidating) and tested in four-point bending, in order to assess the connections among fresh state behavior and fiber dispersion, herein investigated by means of AC-IS, and the performance in the hardened state. The results, highlighting this correlation, point out their importance for a design of the material composition “tailored” to the anticipated mechanical performance and to the specific structural application.

Soil Property Criteria for Rammed Earth Stabilization

Abstract: This study relates value ranges of natural soil properties (plasticity, texture, and shrinkage) to the degree of predisposition of soils to stabilization for rammed earth wall construction. A total of 219 strength determinations were made on 104 soils compacted and stabilized with cement and/or lime and/or asphalt. Using a 2 MPa compressive strength criterion as the measure of stabilization success, soil property value ranges were related to the proportion of samples exceeding the criterion. Linear shrinkage (LS) and plasticity index (PI) are found to be the best discriminators of soil predisposition, with textural variables being useful secondary discriminators. “Favorable” soils, with stabilization success rates of ≥80% , include those with: (1) LS<6.0% and PI<15% ; and (2) LS 6.0–11.0%, PI 15–30%, and sand content <64% . These soils were stabilized with treatments averaging 4.2% cement and 1.8% lime, with individual treatments ranging from 4–8% total cement and/or lime. “Unfavorable” soils, with stabil...

Shear Transfer across a Crack in Reinforced High-Strength Concrete

Abstract: In this study, the shear transfer behavior of reinforced concrete across a crack is investigated both analytically and experimentally by conducting tests on 19 precracked push-off specimens. The major parameters considered are the compressive strength of concrete and reinforcement parameter through the shear plane. Test results indicate that the behavior of a crack (in terms of stress-displacement relations) during shear transfer is characterized by four significant events. The mechanisms involved in each of these events are described. It has been shown that an independent increase either in concrete strength or reinforcement parameter stiffens the initial (after taking off the slack created by the precrack) straight portion of the curve, raises the linear response to a higher load level and increases the ultimate strength and the corresponding deformation, but the slope of the branch where frictional slip occurs remains relatively unchanged. A large pool of test data on reinforced concrete has been collated from the available literature. These, together with the test data generated in this study, have been analyzed and modeled embracing concrete strength up to about 110 MPa for predicting the ultimate shear transfer strength across a crack. A comparison of theoretical predictions with available test results shows good agreement.

Biodeterioration of Construction Materials: State of the Art and Future Challenges

Abstract: Deterioration plays an important part in the life cycle of infrastructure systems. Among all causes of deterioration (aging, chloride ingress, etc.) the action of live organisms has shown to be critical in, for example, underground structures, sewage systems, and at-sea structures. This phenomenon is usually overlooked, in part, because in most cases live organisms accelerate other processes that may eventually lead to unacceptable structural performance or cause failure (e.g., corrosion, cracking). By the direct or indirect action, it has been estimated that biodeterioration-related structural problems cost billions of dollars a year in infrastructure maintenance and repair. The paper shows that for all major civil engineering materials, in the long term and under the appropriate environmental conditions, biodeterioration may severely affect infrastructure components and their ability to perform as designed. This is particularly relevant given the large amount of existing infrastructure that has been exposed to aggressive environments for long periods of time. This paper presents an overview of the activity of live organisms on three widely used construction materials: wood, concrete, and metals. A description of the main organisms that affect each material and the associated biodeterioration mechanisms are described. In addition, this paper discusses the uncertainties associated with modeling biodeterioration and outlines the main areas for further research.

Engineering Behavior of a Remolded Expansive Clay Blended with Lime, Calcium Chloride, and Rice-Husk Ash

Abstract: This paper presents experimental results obtained from tests conducted on remolded expansive soil specimens blended with rice-husk ash (RHA) and stabilized with lime and calcium chloride. The amounts of RHA, lime, and calcium chloride were varied from 0 to 16%, 0 to 5%, and 0 to 2%, respectively, by dry weight of the soil. The effect of additives on unconfined compressive strength (UCS) and California bearing ratio (CBR) is reported. It was found that the stress-strain behavior of expansive clay improved upon the addition of up to 5% lime or up to 1% calcium chloride. A maximum improvement in failure stress of 225 and 328% was observed at 4% lime and 1% calcium chloride, respectively. A RHA content of 12% was found to be the optimum with regard to both UCS and CBR in the presence of either lime or calcium chloride. An optimum content of 4% in the case of lime and 1% in the case of calcium chloride was observed even in clay-RHA mixes.

Contribution to the Concrete Strength versus Water-Cement Ratio Relationship

Abstract: The first rarely mentioned fundamental assumptions for the strength versus water-cement ratio relationship are discussed, namely, that: (a) the strength of structural concrete is controlled by the strength of the cement paste in it; (b) the strength of a cement paste depends strongly on the porosity in it; and (c) the porosity (capillary) is a function of the water-cement ratio. This is the foundation of the relationship between concrete strength and water-cement ratio. Numerous empirical formulas, so-called strength formulas, have been developed for this relationship; the Abrams' formula for instance. These formulas estimate the concrete strength from the water-cement ratio only, and they are usually simple but have restricted limits of validity. For improvement, a new type of strength formulas is offered in this paper, formulas that have a second independent variable beside the water-cement ratio, such as the cement content, or water content, or paste content, etc. Such augmentation (a) improves the accuracy of the strength estimation; (b) shows that if two comparable concretes have the same water-cement ratio, the concrete having the higher cement content has the lower strength; and (c) shows that the magnitude of the changes in concrete strength depends on how the water-cement ratio is changed, by changing the cement content or the water content. Experimental data support these expectations. A formula is also presented for the effect of the other kind of porosity, the air content on the concrete strength. The combination of this with any good strength formula gives good fit with experimental results both of air-entrained and nonair-entrained concretes.

Hydration and Mechanical Properties of Magnesia, Pulverized Fuel Ash, and Portland Cement Blends

Abstract: A study was conducted to determine the quantity of water bound by hydration, the products formed during hydration, the microstructures, and the mechanical properties as a function of the relative content of reactive magnesium oxide (magnesia, MgO), pulverized fuel ash (pfa), portland cement (PC), and water used to prepare the mixes. It was confirmed that hydration of MgO leads to the formation of brucite and that PC forms its normal hydration products, even when both materials are present together. It was found that calculated changes in porosity and consumption of water during hydration based on the proposed hydration reactions agree well with the measurements. It was also found that the stiffness and strength improve directly proportional to the PC content. It is proposed that the lower strength of mixes with high reactive MgO contents is due to a combination of their high water demand and the difference in morphology between brucite and C–S–H gel.

Stress-Strain relationship of frost-damaged concrete subjected to fatigue loading

Abstract: This study attempted to develop a model for the stress-strain relationship in compression of frost-damaged concrete subjected to fatigue loading. Concrete specimens were prepared and exposed to freeze-thaw cycles followed by application of static and fatigue loading. The strains induced during the freeze-thaw test were carefully measured as well as during a mechanical loading test. It was found that the static strength and the fatigue life of concrete decreases as increasing irreversible tensile strain was induced by frost action. A stress-strain model for frost-damaged concrete under application of static and fatigue loading based on the degradation of initial stiffness caused by frost damage was presented. The degradation of initial stiffness for damaged concrete was empirically formulated as a function of remaining expansion caused by freeze-thaw cycles. The plastic strain under the application of mechanical static and fatigue loading for frost-damaged concrete is higher than that for original concrete. Therefore, plastic strain for damaged concrete was formulated as not only the function of strain level under mechanical loading, but also the function of irreversible strain caused by frost action. The unloading and reloading stiffness factors were introduced to explain the change of stiffness as increasing the number of loading cycles by considering the effect of the degree of frost damage.

Evaluation of Moisture Damage Mechanisms and Effects of Hydrated Lime in Asphalt Mixtures through Measurements of Mixture Component Properties and Performance Testing

Abstract: This study presents an approach to help understand moisture damage mechanisms and to evaluate the effects of hydrated lime as moisture damage resisting agents. To this end, various performance testing of hot mix asphalt (HMA) samples induced by moisture damage and several fundamental property measurements (stiffness, strength, toughness, and bonding energy) of mixture components were conducted. Testing data and analyses demonstrated that hydrated lime contributed to moisture damage resistance due to synergistic effects of mastic stiffening, toughening, and advanced bonding characteristics at mastic-aggregate interfaces. However, a well-controlled lime treatment is required to maximize distribution and dispersion of lime particles on aggregate surfaces. In addition to the clear effects of hydrated lime, mineral filler in the HMA samples showed its effects on damage resistance in an early stage of moisture damage due to substantial stiffening-toughening effects from filler addition. Properties and damage characteristics of mixture components measured in this study were related to macroscopic performance behavior of asphalt concrete samples, which infers that the approach herein based on the mixture components can be effectively used to evaluate (or predict) moisture damage of asphalt mixtures and pavements with much less effort.

Rubber Concentrations on Rheology of Aged Asphalt Binders

Abstract: Three different concentrations of crumb rubber were mixed with two different asphalts by means of a wet process to investigate the effect of rubber particles on the fundamental internal structure of asphalt binders with respect to their long-term aging performance. The rheological properties of unaged and aged unmodified and rubber-modified asphalt binders were studied as a function of long-term oxidative aging. Master curves were constructed by shifting the aging time-frequency data to assist in the interpretation. A generalized power-law model used in neat asphalt based on the time–temperature superposition principle was applied to construct the master curves of the rheological parameters. Based on a limited study, the results indicate that the amount of rubber particles added into the asphalt binder is asphalt dependent. The results also show that the stiffness effect of crumb rubber to one less-compatible asphalt is higher than the other highly compatible asphalt. Addition of crumb rubber to both asph...

Dynamic Modulus Prediction of HMA Mixtures Based on the Viscoelastic Micromechanical Model

Abstract: Dynamic modulus (|E*|) of hot-mix asphalt (HMA) mixtures is one of the fundamental engineering properties measured by the simple performance tester and has also been incorporated as a basic input into the American Association of State Highway and Transportation Officials Mechanistic-Empirical Design Guide for flexible pavements. Although direct laboratory testing and empirical equations (such as the Witczak model and the Hirsch model) provide two ways to obtain the values of dynamic modulus of HMA mixtures, a predictive model based on the microstructure of HMA mixtures is more desirable. This paper presents a viscoelastic micromechanical model to predict the dynamic modulus of HMA mixtures based on the elastic-viscoelastic correspondence principle. In this model, HMA mixtures are treated as a composite by embedding the mastic (or asphalt binder)-coated aggregate particles into the equivalent medium of HMA mixtures. Using the proposed model, a solution was obtained to predict the elastic modulus of HMA mixtures. Based on the elastic-viscoelastic correspondence principle, a viscoelastic equation was derived to predict the complex modulus and subsequently the dynamic modulus of HMA mixtures. The developed equations had the capability of taking into account both aggregate gradation and air void size distribution. Laboratory experiments were conducted to verify the developed model. The dynamic modulus values of mastics and HMA mixtures were obtained through direct laboratory testing. The dynamic modulus of mastic was then used to predict the dynamic modulus of laboratory-prepared HMA mixtures with the newly developed model. Laboratory test results showed that a discrepancy exists between the calculated and measured dynamic moduli. The reason for the discrepancy between measured and predicted dynamic moduli and the factors affecting the dynamic modulus were also explored in the paper.

Design Aspects on Steel Fiber-Reinforced Concrete Pavements

Abstract: A numerical study on the fracture behavior of steel fiber-reinforced concrete (SFRC) slabs on grade for industrial pavements is presented. Finite element analyses have been carried out by using a commercial finite element code where user subroutines based on nonlinear fracture mechanics have been implemented to describe the progressive cracking behavior of SFRC. The model is capable of simulating both primary and secondary cracks when they occur in concrete and it has been validated by comparing numerical predictions with experimental observations of full-scale tests on slabs on elastic subgrade under point loads in different positions. A real pavement with contraction or construction joints has been numerically simulated to investigate the stress and the strain fields for the most significant positions of point loads.

Effect of Corncob, Wheat Straw, and Plane Leaf Ashes as Mineral Admixtures on Concrete Durability

Abstract: In this study, the effects of the use of corncob, wheat straw, and plane leaf ashes (CA, WSA, and PLA) as mineral admixtures on concrete durability were investigated. Ten concrete mixtures were produced in three series with control mixes having 400 kg cement content. The control mixes were modified with 2, 4, and 6% of CA, WSA, and PLA in place of fine aggregate. To establish the durability of concrete, the compressive strengths were measured after 7, 28, 90, 180 days, and 18 months under sodium sulfate solution. In the meantime, abrasion resistance and water penetration were investigated. Test results indicate that CA, WSA, and PLA addition provides good workability and abrasion resistance compared to conventional concrete. Test results also showed that minimum abrasion resistance is obtained from the control specimen, while maximum abrasion resistance is obtained from CA3 (6%) specimens. Abrasion resistance is increased as the rate of fine CA, WSA, and PLA is increased. The results indicate that the increase in ash content caused a significant increase in the sodium sulfate resistance of the concretes. Hence, concrete with CA, WSA, and PLA addition can be recommended for the production of durable concrete.

Study of the Effectiveness of Cement Kiln Dusts in Stabilizing Na-Montmorillonite Clay

Abstract: The feasibility of using cement kiln dusts (CKDs) as potential stabilizing agents for Na-montmorillonite clay was investigated using a suite of four CKDs with different chemical and physical characteristics. The Atterberg limits, pH values, unconfined compressive strengths (UCSs), stiffnesses (stress-strain behavior), and stability of the CKDs-modified Na-montmorillonite clay were measured as functions of CKDs content and curing periods. These properties were compared with those of the untreated clay to determine the extent of enhancement, which was used as a measure of effectiveness of CKDs as stabilizers. All CKDs considerably decreased the plasticity index, thereby improving the workability of the clay, while they also considerably increased the initial pH value of clay, providing a favorable environment for further chemical (pozzolanic) reaction. The addition of CKDs and subsequent compaction substantially increased the UCS and the stiffness of the clay, thus improving its structural properties. The extent of improvement of the clay characteristics was found to be a function of the chemical composition of the particular CKD, specifically its free lime content. It was also found that the length of curing period after compaction had a major role in the stabilization process.

Bond Behavior of Self-Consolidating Concrete with Mineral and Chemical Admixtures

Abstract: Self-consolidating concrete (SCC) is known for its excellent deformability, high resistance to segregation, and use in congested reinforced concrete structures characterized by difficult casting conditions without applying vibration. Research has been conducted on the development of SCC using high volumes of supplementary cementing materials (SCM) (such as fly ash and slag) and viscosity modifying admixtures (VMA). The bond characteristics of such SCCs are very important for their application in practical construction. An extensive investigation was conducted to determine the bond strength between deformed reinforcing steel bar and SCM and VMA based SCC as well as normal concrete (NC). Bond tests were conducted using a specially developed pullout test. The SCC pullout specimens were cast without applying any consolidation, whereas the NC specimens were cast by conventional practice with consolidation and vibration. It was found that the reduction in bond strength due to bleeding and inhomogeneous nature was less in SCC compared to NC. Although the variation in bond strengths at different casting elevations was observed in SCC, the extent was less significant than that of NC. SCC also exhibited a less significant top-bar effect compared to NC. This can be attributed to the more consistent nature of SCC and its superior filling capability. Performance of various code based and other existing bond equations are validated through experimental results illustrating the influence of concrete types (either SCC of different types or NC).

Determination of Accurate Creep Compliance and Relaxation Modulus at a Single Temperature for Viscoelastic Solids

Abstract: Creep compliance (or relaxation modulus), which is a fundamental property that determines the strain (or stress) development in flexible pavements or damage evolution in asphalt mixtures, can be determined from either a creep compliance test using static loading or a complex modulus test using cyclic loading. Since the nature of each test is different, creep compliance determined from the complex modulus test was significantly different compared with that determined from the creep compliance test. From this rigorous experimental and analytical study, it was concluded that the creep compliance or complex modulus test alone is not capable of providing complete information over the typical time or frequency range used in single-temperature tests. In general, the complex modulus test provides accurate creep compliance at short loading time, while the creep compliance test provides accurate creep compliance at longer loading time. An approach to determine accurate creep compliance from the combination of creep and complex modulus tests at a single temperature was developed. The method provided accurate creep compliance values that corresponded to short-term and long-term experimental data.

Chloride Penetration and Binding in Recycled Concrete

Abstract: Recycled concrete use is necessary in order to decrease the environmental impact of the construction industry. Coarse aggregates from crushed concrete may lead to relatively high porosity in the resultant concrete. For this reason, recycled concrete may seem detrimental to the durability of reinforced concrete structures, particularly when they are exposed to marine environments. The objective of this paper is to analyze the influence of coarse aggregates from concrete on the chloride penetration rate and binding capacity of concrete under marine exposure. Concrete specimens were molded and exposed to a natural marine atmosphere. After exposure periods of 6, 12, and 18 months, ingress profiles of total and water soluble chlorides were measured. Results indicate that recycled aggregate incorporation causes two opposed effects on concrete: It increases its chloride penetration rate and chloride binding capacity. Therefore, water soluble chloride contents in conventional and recycled concrete series have similar values.

Effects of Temperature and Pressure on Hot Mixed Asphalt Compaction: Field and Laboratory Study

Abstract: A field and laboratory study was carried out to study the effect of pressure and temperature on the compaction of asphalt pavements. Density, temperature, number of passes, and type of roller were recorded in the field at four different paving projects in Wisconsin, with binder and mixture samples obtained from each project. Compaction in the field occurred at temperatures that ranged from 125 to 60°C. Densification was achieved only for temperatures above the 70-80°C range. Roller contact pressures were estimated between 300 and 700 kPa during the compaction process. Laboratory compaction was performed with Superpave gyratory compactor using the field compaction temperatures and contact pressures. The lab compaction results confirmed that below a certain temperature limit densification decreased significantly. Binder testing was performed to determine the binder viscosities in the full range of field compaction temperatures observed. The upper limit of viscosity for significant reduction in densification was estimated to be 50 Pa s. A procedure for identifying the lowest temperature at which acceptable densification rate can be achieved is proposed based on testing using the dynamic shear rheometer. The determination of the lower temperature limit for compaction is an important task that is absent in the current specifications.

Study of the Durability of OPC versus GGBS Concrete on Exposure to Silage Effluent

Abstract: Ordinary portland cement OPC has been traditionally used in the construction of concrete silos in Ireland. However, the aggressive nature of the effluent produced by silage leads to severe degradation of the concrete. GGBS is a common addition to PC composites. It has been demonstrated that GGBS improves the general performance of PC concrete, decreasing chloride diffusion and chloride ion permeability, reducing creep and drying shrinkage, increasing sulfate resistance, enhancing ultimate compressive strength, and reducing heat of hydration and bleeding. It has also been suggested that GGBS may increase concrete durability in the aggressive environment of silos. In order to investigate this theory, a simulation study was carried out by immersing samples of mortars incorporating increasing amounts of GGBS in a silage effluent solution and a magnesium sulfate solution. Over the course of an experiment consisting of three, 28-day cycles of immersion in the silage effluent, the sample performance was evaluated by testing permeability, porosity, water absorption, capillary suction, compressive strength, and mass loss. According to the results obtained, the OPC samples suffered the highest rise in permeability and porosity, and the greatest loss in both mass and compressive strength. In addition, the durability of the mortars, when subjected to both salt crystallization and silage effluent cycles, increased with increasing amounts of GGBS. The significant rise in capillary suction, water absorption, and permeability over the course of the experiment indicates that the damage induced by the effluent is not as superficial as previously reported. Loss in mass and increase in permeability were found to be the most reliable indicators of corrosion, as they gave the most dramatic and uniform results.