Journal of Materials in Civil Engineering

The goal of this research was to study the relationship between cracking and concrete permeability and to support accounting for permeability and cracking resistance to other factors besides strength, as criteria to be considered in mix design to achieve a durable concrete. The effect of material composition [normal-strength concrete (NSC) and high-strength concrete (HSC) with two different mix designs] and crack width (ranging from 50 to 400 μm) on water and chloride permeability were examined. Cracks of designed widths were induced in the concrete specimens using a feedback-controlled splitting tensile test. Chloride permeability of the cracked samples was evaluated using a rapid chloride permeability test and the water permeability of cracked concrete was then evaluated by a low-pressure water permeability test. Uncracked HSC was less water permeable than NSC, as expected, but cracking changed the material behavior in terms of permeability. Both NSC and HSC were affected by cracking, and the water permeability of cracked samples increased with increasing crack width. Among the tested materials, only HSC with a very low water-to-cement ratio chloride permeability was sensitive with respect to cracking. Results indicate that the water permeability is significantly more sensitive than the chloride permeability with respect to the crack widths used in this study.

Using the details of a previous study in which the flexural response of steel fiber reinforced concrete (SFRC) was accurately predicted from a constitutive model, explicit expressions for the first crack and the ultimate flexural strength are derived in this paper. In the present analytical model, the direct tensile strength, residual strength, and the product of bond strength τ_d and fiber aspect ratio (L/d) are used as parameters in the investigation. The “bond-aspect ratio” index τ_d(L/d) and fiber volume fraction play important roles in influencing the flexural behavior of SFRC. Calculated flexural strengths using the derived explicit expressions are compared with experimental data on SFRC beams and slabs. Good agreement between calculated strengths and experimental data is obtained. To further simplify the ultimate strength prediction, studies on the behavior of basic parameters [f_ult, f_t, and τ_d(L/d)] for a range of practical fiber volume concentration V_f were conducted, and from which a simple expression is derived. Predictions using the simplified ultimate strength expression are compared with results obtained from the above analytical procedure and with experimental data. The approaches serve as an effective tool for flexural strength assessment of SFRC.

This paper presents experimental results on porosity and pore size distribution of fly ash and silica fume modified cement pastes and mortars using mercury intrusion porosimetry. It is found that replacement of cement by fly ash increases the porosity but decreases the average pore size of the pastes at the ages of 28 and 56 days. The additional replacement of cement by up 5% silica fume did not significantly change the pore size distribution and porosity of either the plain cement pastes or the fly ash cement pastes. Interfacial porosity, however, was significantly influenced by incorporating fly ash and silica fume in the mortars. The interfacial porosity was much reduced when fly ash and silica fume were incorporated. Compared with the porosity and average pore size of the pastes and mortars, the interfacial porosity better explains the results of a chloride diffusion test of the fly ash and silica fume concrete in most cases. However, for the concrete with 55% fly ash replacement at 28 days, the drop of ionic concentration of the pore solution seems to be more responsible for the low result of coulomb passed in the test.

The use of recycled tire rubber in a portland cement concrete (PCC) mixture is investigated as a possible alternative for nonconventional PCC mixtures. This study is focused on the determination of the practicality of producing such mixes and evaluating their engineering properties. An experimental program was developed to use two types of tire rubber (fine crumb rubber and coarse tire chips) in PCC mixtures. A control PCC mix is designed using American Concrete Institute mix design methods, and three groups of rubberized PCC mixes were developed by partially replacing the aggregate with rubber. Eight tire rubber contents were used in each group. Mixes were tested in compressive and flexural strength in accordance to ASTM standards. Results show that rubberized PCC mixes can be made and are workable to a certain degree with the tire rubber content being as much as 57% of the total aggregate volume. However, strength results show that large reductions in strength would prohibit the use of such a high rubber content. It is suggested that rubber contents should not exceed 20% of the total aggregate volume. A characteristic function that quantifies the reduction in strength for rubberized concrete mixes was developed that could be useful for mix design purposes. Rubberized concrete mixes may be suitable for nonstructural purposes such as lightweight concrete walls, building facades, and architectural units. They could also be used as cement aggregate bases under flexible pavements. Fire hazards are of major concern and need to be thoroughly investigated before recommendations for practical implementation are drawn.

Ring-type tests were performed for studying restrained shrinkage cracking of high-performance concrete. The effects of silica fume, fly ash, ground granulated blast furnace slag, and calcium nitrite inhibitor to restrained shrinkage cracking were investigated. The study shows that the crack width increases with increasing silica fume, fly ash, and calcium nitrite inhibitor. Even though the replacement of 50% cement by ground granulated blast furnace does not significantly change the shrinkage strain, it does change the restrained shrinkage cracking manners of the concrete. A crack growth model has been proposed to analyze the problem. The quantitative explanation for the relation between crack width and shrinkage strain in the restrained ring-type concrete has been obtained by introducing the concept of damage of a restrained surface. The numerical analysis of the experimental results indicates that the damage of the restrained surface makes a significant contribution to the crack width due to the restrained shrinkage. The decrease of the degree of damage at the restrained surface suggests that friction resistance and/or pozzolanic reaction could exist near the surface of the concrete.

Fiber-reinforced-polymer (FRP) composites are currently being developed and used in bridge decks. This report examines the life-cycle cost-effectiveness of three FRP bridge decks, using a life-cycle cost method specifically tailored for comparing new materials with conventional ones. The method includes a cost classification scheme for capturing all project-related costs and for comparing the cost advantages and disadvantages of different materials. The uncertainty of new-material costs is assessed using the Monte Carlo simulations. Analysis indicates that one of the FRP decks is life-cycle cost-effective for a particular class of bridges and traffic levels.

A research project has been undertaken in Kuwait University to investigate the long-term strength development and durability of lightweight aggregate concretes (LWAC's) and a normal weight concrete (NWC) under hot dry and hot humid coastal exposure conditions. This paper reports the strength development and durability characteristics of 35- and 50-MPa NWC specimens exposed to the seaside up to an age of 9 months. The water penetrability of LWACs seems to be more sensitive to the extent of initial curing than it is for NWC. The water penetrability of LWC50 after 7 days of initial curing and subsequent exposure to the seaside was about 1/3 more than that of the NWC50 in the same exposure condition. The depth of carbonation, sulfate, and chloride penetration, and concentration in LWC50 were somewhat higher than that in NWC50 under similar exposure regimes.

The distresses of asphalt pavement, which include fatigue, rutting, and low temperature cracking, are related to the elastic modulus of asphalt concrete (AC). In addition, the elastic modulus of AC is a design variable for asphalt pavement structural design when elastic-layer system theory is employed. However, in the most commonly used AC design methods (the Marshall, Hveem, and Superpave methods), the elastic modulus is not used as a control variable. Therefore, these design methods do not ensure that the desired elastic modulus of AC will be obtained. In this paper, AC is treated as a two-phase composite with aggregates dispersed in the asphalt matrix. Based on this treatment, a two-layer built-in micromechanical model of AC is developed by embedding an asphalt-coated circular aggregate into an equivalent AC medium. Using this model, an equation predicting the elastic modulus of AC is derived. The elastic modulus of AC predicted by the present model is compared with the Hashin and Shtrikman theoretical bounds and the Heukelom and Klomp equation. Using these comparisons, the proposed model is shown to be reasonable and applicable for predicting the elastic modulus of AC. Thus, the existing mix design approaches can be improved by using the modulus prediction model presented in this paper.

The current procedures for the design of concrete mixes are based on specifying target workability and strength. Graphs are then used to determine the masses of constituent materials. Small changes in initial water content will result in the concrete mix not achieving its target, as water is necessary for the workability and hydration of cement, to produce cement paste that acts as the binder. Therefore, it is important to understand the distribution of water in fresh concrete to achieve the required workability, strength, and durability. This paper considers the use of electrical measurements to study the movement and special distribution of water within freshly mixed concrete. The technique is used to determine moisture content and rates of water absorption and adsorption of individual components forming the concrete. A theoretical model is developed, and simplified, using electrical formulas. The results from this investigation show that the cement paste is the primary governing factor in determining the electrical resistance of a fresh concrete. However it also shows that all constituents have some influence.

Earth block construction offers a low energy alternative to conventional low-rise building materials such as clay fired masonry units, concrete, processed timber, and steel framed construction. Used to some extent in areas such as New Mexico, further development of the technology has been hampered somewhat by very limited understanding of material performance. In this paper, work undertaken to study the flexural bond strength characteristics in pressed earth block masonry is summarized. Cement stabilized pressed earth blocks were fabricated using a manual press and masonry piers were built using both cement:lime:sand and soil:cement mortars. Flexural strengths were determined using the bond wrench method. Bond strength is shown to be a function of both block and mortar strength, as well as block moisture content during construction, and mortar clay content. Bond strengths decreased significantly with saturation of the masonry but showed litle development beyond 1 day after construction. Characteristic bond strengths in pressed earth masonry are typically much less than values generally assumed in conventional masonry. In conclusion the paper outlines recommendations for mortar usage with pressed earth blocks.

Low-grade laterite and pit-run gravel aggregates available in abundance in eastern India may be turned into a cost-effective base for low-volume roads when ordinary portland cement, river sand, and aggregates are mixed in a 1:6:12 proportion by weight. 152 × 152 × 152 mm³ cubes were cast at several water-cement ratios and the mix was compacted by a vibratory plate compactor. The compressive strength test was determined for different water-cement ratios, and the one that gave the maximum strength was used to prepare 102 × 102 × 508 mm³ beam specimens. A number of cubes and beams were kept inside airtight polyethylene bags for accelerated curing at a temperature of 50°C. Modulus of rupture and flexural fatigue tests were carried for the evaluation of pavement design parameters. A methodology of design of a lightly cemented pavement along with an example are also presented.

Three hundred eighty-five joints made with elastomeric construction adhesive were tested in short-term loading in shear. The ultimate stress and slip were recorded for each specimen. The purpose of this study was to evaluate the two previously evaluated mechanical properties as a function of: chemical formulation of the adhesive (acrylic-, urethane-, and rubber-base); treatment with chromated-copper-arsenic preservative; and moisture condition at the time of joint fabrication and testing (fabricated/tested = dry/dry, wet/dry, and wet/wet). Application of the elastomeric construction adhesives was performed according to manufacturer's specifications. The mean and standard deviation were collected for all specimens (≈13) evaluated for each combination of the aforementioned variables. In addition, analysis of variance statistical procedures were used to determine if mean properties varied as a function of the experimental variables. It was found that the inherent variation between test specimens contributed substantially to the total variation of the joint mechanical behavior. In addition, joints that were dry at testing had superior properties to those that were wet. Rubber-based (styrene-butadiene rubber) adhesives generally demonstrated poorer mechanical behavior than those made from acrylic and urethane bases. The latter two had somewhat similar behaviors. The effect of chromated-copper-arsenic treatment was somewhat masked by the large variation in joint fabrication, although no definitive effect of this variable was apparent.