Fernando Tejeda-DominguezDavid A. LangeMatthew D. D'Ambrosia
p.1-7页
查看更多>>摘要：The growing interest in the use of self-consolidating concrete (SCC) for a wide variety of structural applications has initiated a reexamination of its properties and current construction practices and how they compare with those of conventional concrete. One property of interest is the form-work pressure of SCC and how it relates to that of conventional concrete. This work presents the results for three tall walls (28,21.7, and 13 ft tall) cast slowly with SCC and a 10.6-ft-high column poured quickly by using the same concrete used in one of the walls. The research demonstrates that the pressure of SCC against the formwork drops quickly just after the concrete material is placed. Measurements from the walls poured slowly show that the maximum recorded pressure falls far below the hydrostatic pressure and is closely related to the pouring rate. The experiments also reveal that the formwork pressure exerted by SCC can be revitalized if the SCC is vibrated, even if stiffening is already in progress.
查看更多>>摘要：Crumb rubber is a material produced by shredding and commutating used tires. There is no doubt that the increasing piles of used tires create environmental concerns. The long-term goal of this research is to find means to dispose of the crumb rubber by placement of the rubber in port-land cement concrete and still provide a final product with good engineering properties. The Arizona Department of Transportation and Arizona State University have initiated several crumb rubber concrete (CRC) test sections throughout Arizona over the past few years. Laboratory tests were conducted to support the knowledge learned in the field and enhance the understanding of the material properties of CRC. Concrete laboratory tests included compressive, flexural, indirect tensile strength, thermal coefficient of expansion, and microscopic matrix analyses. The unit weight and the compressive and flexural strengths decreased as the rubber content in the mix increased. Further investigative efforts determined that the entrapped air, which caused excessive reductions in compressive strength, could be reduced substantially by adding a deairing agent. The higher tensile strains at failure observed from the tests were indicative of more ductile, energy-absorbent mix behavior. The coefficient of thermal expansion test results indicated that CRC was more resistant to thermal changes. The CRC specimens tested remained intact after failure and did not shatter as a conventional mix did. Such behavior may be beneficial for a structure that requires good impact resistance properties. If no special considerations are made to maintain higher strength values, the use of CRC mixes in places where high-strength concrete is not required is recommended.
查看更多>>摘要：The current empirical model for the Florida Department of Transportation (FDOT) concrete modulus of elasticity (E) is a function of concrete compressive strength and unit weight. Recent testing shows that this model consistently underestimates E for FDOT concrete. This underestimation may lead to construction problems caused by the overprediction of camber and the deflection of concrete structural members. £ values for the typical concrete mix used in FDOT projects were experimentally determined at various concrete ages. Regression analyses were used to find the £ models that best fit the generated data, and such models were compared with existing E models from the literature. It was found that the aggregate type and specific gravity play significant roles in influencing the E value of concrete. The best-fit £ models for FDOT concrete, together with suggested modification factors for various aggregate types, are recommended.
查看更多>>摘要：The purpose of this study was to compare the cumulative damage to pavement concrete under splitting tensile, variable amplitude fatigue loadings by splitting tensile and flexural tensile tests. The cumulative damage theories adapted in this study included the linear damage theory, the nonlinear damage theory, and the equivalent damage theory. The results were as follows. The sums of the cumulative damage were greater by the flexural tensile test than by the splitting tensile test, even though the difference between them was small in each load case. The sums of the cumulative damage at higher stress levels showed values farther from one than those at lower stress levels. This suggested that Miner's rule might be applied to plain concrete with little error, provided that the stress level remained low. The cumulative damage obtained by the nonlinear damage theory was closer to one than that obtained by the linear damage theory in all load cases except one. This indicated that nonlinear cumulative damage could consider the effects of the magnitude and the sequence of variable amplitude fatigue loadings. The theoretical results calculated from the equivalent damage theory were similar to the experimental results except in one case, even though the equivalent damage theory was simple. The results obtained by the splitting tensile test were equivalent to or better than those obtained by the flexural tensile test for application to the equivalent damage theory. Thus, the splitting tensile test and the equivalent damage theory could be used to estimate the damage and the remaining life of pavement concrete under variable amplitude fatigue loadings.
Christopher Y. TuanMichael T. KellyHaosu SunMark E. Buss...
p.34-44页
查看更多>>摘要：The study presents the findings from a 2-year field trial in which lithium nitrate was applied at a rate of 0.006 gal/ft~2 twice a year on an existing concrete pavement in Norfolk, Nebraska, in an attempt to arrest ongoing alkali-silica reaction (ASR) distress. Various destructive and nondestructive means were used to measure the effectiveness of the treatments. Concrete cylinders were cored for petrographic examination and split tension testing. Powder samples were taken to determine the lithium content. Nondestructive evaluations used crack mapping, a Schmidt hammer, a velocity meter, and an impact echo apparatus. The amount of lithium that penetrated into the pavement by gravity soaking has been limited. The results to date have not shown a definitive benefit of application of the lithium material in controlling or mitigating the ASR process; presumably, the pavement has not reached the state of deterioration optimal for lithium penetration. The feasibility and effectiveness of other application techniques on hardened concrete, such as electrolysis, surface pressurization, and vacuum impregnation, should be investigated for comparison with the feasibility and effectiveness of gravity soaking.
查看更多>>摘要：Roller-compacted concrete (RCC) is widely used for industrial and heavy-duty pavements. An economical and durable material for paving and other applications, RCC has a high strength and a high density. Although the strength of RCC has been thoroughly investigated, the freeze-thaw durability of RCC remains a topic of contention. RCC has often performed well in harsh field environments, but it frequently fails laboratory freeze-thaw durability testing. In this study, 24 specimens were fabricated for freeze—thaw durability testing with a gyratory compactor. The study variables were the degree of compaction, the amount of water, and the water-to-cement (w/c) ratio (and, hence, the amount of cement). The specimens replicated the concrete found in typical RCC pavement and dam construction. The RCC specimens were subjected to up to 300 cycles of rapid freezing and thawing and were tested for fundamental transverse frequency and mass loss. Because of the low aspect ratios of the specimens, fundamental transverse frequency testing was not reliable. Several specimens completed the 300 cycles with a mass loss of less than 10%. The freeze-thaw durability of RCC was found to be dependent primarily on the amount of cement paste and the w/c ratio and to a lesser extent on the degree of compaction.
查看更多>>摘要：This paper presents the results of a laboratory study conducted to determine the influence of cement composition on the expansions observed in standard and modified ASTM C1260 tests. Also, the effectiveness of selected mitigation measures for alkali-silica reactivity was studied by using a modified ASTM C1260 test. Two different cements with substantially different alkali levels were used in combination with a Class F fly ash and a Class C fly ash. The results obtained from tests with 89 different sources of fine aggregate were analyzed. The results from this study indicate that cement composition has a distinct influence on the expansion levels observed in the ASTM C1260 study. For a majority of the aggregates tested in this study (88%), the use of a specific cement low in alkali content resulted in higher expansion than a cement with a relatively higher alkali content, regardless of the level of reactivity of the aggregate in the standard ASTM C1260 tests. Class F fly ash was found to be more effective than Class C fly ash in reducing the expansions in the modified ASTM C1260 tests. However, the cement used in the mixture influenced the percent reduction in expansion offered by both Class F fly ash and Class C fly ash. The use of low-alkali cement with Class F and Class C fly ashes yielded larger reductions in percent expansion.
查看更多>>摘要：Antiwashout admixtures (AWAs) are among the agents recently used to minimize the adverse effects associated with underwater concrete placement. However, relatively little work on the influence of AWAs on the performance of underwater concrete and evaluation of the validity of the conventional and nondestructive test methods used has been conducted. Thirty mixtures were prepared with several water-to-cementitious material ratios, admixture types, and dosages. Concrete specimens were placed in air and at two underwater depths. The testing program involved fresh and hardened concrete testing as well as nondestructive testing. A simple method was used to assess the concrete and the corrosion damage on exposure to wetting and drying cycles in aggressive saline solution. The impacts of the type and the dosage of the AWAs admixtures, mix proportions, and placement depth on fresh and hardened concrete properties are highlighted. The results suggest that the quality of the surface of underwater concrete is inferior to that of its core and that there is a need to adjust and calibrate the nondestructive testing methods when such concrete is evaluated. Underwater AWA concrete exhibited relatively high degrees of susceptibility to damage when it was exposed to saline solutions.
查看更多>>摘要：Self-compacting concrete (SCC) can be achieved by adjusting mix proportions and incorporating adequate admixtures, such as viscosity-modifying (VM) admixtures, with no vibration. Yet, the impact of the mix proportions and the role played by the VM admixtures as opposed to those played by superplasticizer admixtures are not fully understood. There also seems to be no unique means for assessment of such concrete, and the correlation between various tests needs further work. This study aims to achieve a better understanding of the influence of mix proportions on the properties of SCC. Twenty-six mixtures were prepared with various water-to-cementitious material ratios, fine aggregate-to-coarse aggregate ratios, and admixture dosages. Fresh concrete tests, including slump flow, L-box, U-box, and hardened concrete tests, including hardened density, compressive and flexural strength, as well as the rebound hammer tests, were performed. The uniformity of SCC properties among the column specimens was examined. The results revealed that adjustment of the mix proportions has a major impact on the fresh and hardened properties of SCC. The fresh concrete tests were found to correlate well with one another, and the SCC strength was comparable to that of reference concrete made with mechanical vibration. Yet, the components of the SCC examined exhibited a remarkable uniformity of properties. Further work that should involve large specimens in which concrete properties are evaluated in various locations is needed.
查看更多>>摘要：This paper summarizes the work conducted by the Virginia Department of Transportation on self-consolidating concrete (SCC). Initially, a laboratory study with locally available materials was conducted to develop and evaluate the properties of SCC, including flow, segregation, strength, permeability, resistance to cycles of freezing and thawing, and drying shrinkage. Studies at two precast plants followed. Then, one of the precast plants furnished SCC for an arch bridge. The other plant fabricated two bulb T-beams with SCC. Studies also were conducted with cast-in-place SCC. A structures pad at the FHWA Turner-Fairbank Highway Research Center, a median on I-64, and a column in Norfolk, Virginia, were constructed with SCC. The results show that SCC can be produced successfully and provide many benefits to transportation agencies and the construction industry.
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