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International Concrete Abstracts Portal

Showing 1-10 of 26 Abstracts search results

Document: 

SP189-25

Date: 

January 1, 2000

Author(s):

J. R. Casas, R. Gettu, L. Agullo, and B. Toralles-Carbonari

Publication:

Special Publication

Volume:

189

Abstract:

The influence of the type of placing of concrete (free fall or compacted layers) in vertical elements on the mean value of the compressive strength, as well as its variability is investigated in the present work, with comparisons of normal (NSC) and high-performance (HPC) concretes. The experimental program consists of the fabrication of NSC and HPC columns with steel reinforcement placed under two extreme conditions, and the extraction and testing of cores from different heights. From these results, the corresponding strength reduction factor to be applied to concrete is obtained for NSC and HPC with the condition that the same safety level is to be obtained in both cases. The study covers the range of target reliability indices from 3.5 to 5. The extreme values of the sensitivity factor of the random variable concrete strength are also considered. The main conclusion that can be drawn is that the reduction factors used in design codes or normal concretes do not apply to HPC since its final strength is more sensitive to the placing conditions. However, due to the limited number of tests performed, further and more extensive results are necessary to calibrate a value of the strength reduction factor for the design codes.

10.14359/5866


Document: 

SP189-24

Date: 

January 1, 2000

Author(s):

S. M. C. Diniz and D. M. Frangopol

Publication:

Special Publication

Volume:

189

Abstract:

ACI Code-95 recommendations for column design are based mostly on test results on concretes with strengths up to 42 MPa. Due to differences between high-strength concrete (HSC) and normal-strength concrete (NSC) material and structural behavior, using these recommendations for HSC columns does not mean that the same level of safety as for NSC is obtained. As a consequence, the reliability that the same level of safety as for NSC is obtained. As a consequence, the reliability of HSC columns must be assessed. Since most of the variables involved in column design (material properties, geometric characteristics, loads, etc.) are random, a basic step in the reliability assessment of HSC columns is the modeling of uncertainties associated with both column strength, as well as, load effects is presented. Regarding the computation of the statistics of the HSC column strength, many issues have to be resolved: (a) the scarcity of information on the variability of the compressive strength of HSC; (b) the unavailability of a closed form solution to express column strength; and © the compatibility with the assumed failure criterion. Regarding the computation of the load effect statistics, special attention given to the case of slender HSC columns.

10.14359/5865


Document: 

SP189-23

Date: 

January 1, 2000

Author(s):

R. E. Loov and L. Peng

Publication:

Special Publication

Volume:

189

Abstract:

The shear stress detrmined using ACI 318-95 is limited to the stress determined for 10,000 psi (69 MPa) concrete. This limit has been set because it was considred that there was not enou data to justify the extension of existing equations to higher strengths. Modification are suggested which adapt the ACI equations so thy apply to the full range of concrete strengths. The proposed equations predict the shear strength of beams more accurately than the current ACI equations. For the beams investigated, the coefficient of variation is only 2/3 of that obtained using the ACI equations. The proposed equations apply to concrte stengths as high as 18,000 psi (124 MPa). Aoart frin tgeur simplicity, the proposed equations have a number of other advantages. Unlike ACI equations 11-1 to 11-3, they correctly predict the increased strength of beams with small shear-span to depth ratios. Because the proposed equations assume the most pessimistic location of cracks, the maximum stirrup spacing requirements are satisfied sirectly so that adtitional spacing limitations appear to be extraeous. To illustrate the improvement that can be obtained using the proposed equation, published test results are compared with predicted values.

10.14359/5864


Document: 

SP189-22

Date: 

January 1, 2000

Author(s):

W. J. Weiss, W. Yang, and S. P. Shah

Publication:

Special Publication

Volume:

189

Abstract:

High strength concrete (HSC) typically exhibits improved surface abrasion resistance, reduced chloride penetrability, and improved resistance to freezing and thawing damage. For these reasons, HSC use in transportation structures is increasing due to the potential for increased service life. Although several potential benefits are associated with the use of HSC, these mixtures may exhibit increased sensitivity to early-age shrinkage cracking. In addition to weakening the structure, cracks increase the rate at which corrosive agents can penetrate the concrete, thereby accelerating the potential deterioration of the reinforcing steel and concrete. For this reason, it is essential that the concrete which is used to build transportation structures exhibits sufficient resistance to early-age cracking, in addition to the aforementioned benefits, to produce durable structures. The objective of this paper is to demonstrate that a holistic design approach is required to specify material composition for durable concrete structures. Experimental results and theoretical modeling predictions are used to illustrate the characteristics of higher strength concrete that result in increased cracking potential. Theoretical simulations demonstrate the role of both material properties and the surrounding structure on the tensile residual stress developemnt and cracking potential. In addition, results demonstrate that a shrinkage-reducing admixture (SRA) may be used to decrease the potential for early-age shrinkage cracking in HSC while sustaining the advantageous mechanical and durability properties associated with HSC.

10.14359/5863


Document: 

SP189-21

Date: 

January 1, 2000

Author(s):

J. J. Schemmel, J. C. Ray, and M. L. Kuss

Publication:

Special Publication

Volume:

189

Abstract:

The impact of a Shrinkage Reducing Admixture (SRA) on the properties and performance of conventional concrete was investigated. The SRA studied was a commercially available glycol ether blend. Tests were conducted on two structural air-entrained concretes commonly used in bridge deck construction. The SRA dosage rate was varied between 1.0% to 2.0%, by weight of cement. Influence of the SRA on fresh concrete properties, compressive strength development, unrestrained shrinkage, and freezing and thawing durability was examined. It was determined that the SRA has a slight effect on initial workability and a moderate impact on compressive strength development. Free shrinkage was reduced on the order of 50%. The freeze-thaw durability of some SRA mixtures was found to be below generally accepted limits. This behavior was traced back to problems associated with maintaining entrained air. It was discovered that the SRA mixtures tended to lose air more rapidly than the control mixtures. Further, petrographic analyses suggest that the air content in hardened SRA concrete may be noticeable less than that measured in fresh concrete. Strategies to over come this problem are proposed.

10.14359/5862


Document: 

SP189-20

Date: 

January 1, 2000

Author(s):

R. C. A. Pinto, S. V. Hobbs, and K. C. Hover

Publication:

Special Publication

Volume:

189

Abstract:

The hardening of concrete mixtures is caused by chemical reactions occurring in the Portland cement paste fraction of the mixture. The extent of these reactions is related to the non-evaporable water content in the reaction products, and determine hardened properties of the concrete mixture. High-performance concrete mixtures often contain cementitious materials in addition to portland cement that increase strength and /or durability. These cementations materials can modify the rate of reactions and thus the development of mechanical properties. This work studied non-evaporable water content in low w/c mixtures with and without silica fume in relation to the development of compressive strength and ultrasonic pulse velocity over time. Results were compared with those for a conventional mixture with a moderate w/c. It was observed that the long-term non-evaporable water contents were higher for the conventional mixture than for the low w/c mixtures. The addition of 10% silica fume in the low w/c mixtures caused an even lower long-term non-evaporable water content. This addition of silica fume did not affect the relationship between non-evaporable water content and ultrasonic pulse velocity, but did affect the relationship between non-evaporable water content and compressive strength beyond a certain degree of cement hydration.

10.14359/5861


Document: 

SP189-19

Date: 

January 1, 2000

Author(s):

C. Aldea, J. Shane, T. Mason, and S. P. Shah

Publication:

Special Publication

Volume:

189

Abstract:

One of the main causes of premature deterioration of concrete structures is the corrosion of steel reinforcement. Corrosion is mainly caused by the ingress of chloride ions into concrete. The Rapid Chloride Permeability Test (RCPT) has been developed as a quick test able to measure or predict the rate of transport of chloride ions in concrete. The goal of the research presented here was to study the microstructural changes induced by the RCPT using impedance spectroscopy (IS) measurements. Test series included one-and-a-half year-old materials: (1) paste, (2)Mortar, (3) normal strength concrete (NSC) and (4) high strength concrete (HSC). IS was used to investigate the effect of high electric fields, increased temperatures due to the RCPT, as well as the impact of RCPT sample preparation on the microstructure and transport properties of the materials tested. 2-point electrode-configuration measurements were taken before and after RCPT for all the materials tested. Experimental results showed that paste and mortar exhibited large changes in electric resistance as a result of RCPT, whereas NSC and HSC presented moderate changes. As electrical properties of the HSC were affected by RCPT sample preparation, this material required additional analysis. 4-point electrode-configuration and 4-point direct current resistance were measured for HSC. Based on this study it is suggested that demonized water treatment induces layering low conductivity surface layers). Using the theoretical correlation between initial current and total charge measured during the RCPT, resistance/conductivity measurements can be used to assess RCPT performance and resistance measurements can replace the RCPT.

10.14359/5860


Document: 

SP189-18

Date: 

January 1, 2000

Author(s):

K. Kovler, I. Schamban, S.-I. Igarashi and A. Bentur

Publication:

Special Publication

Volume:

189

Abstract:

Statistical relationships between compressive strength, tensile splitting strength and autogenous shrinkage of early-age high-performance concrete (HPC), on one hand, and water/binder ratio, silica fume (SF) incorporated in the mix and concrete age, on the other hand, are drawn. The methods of experimental design are used. It is shown that the tensile and compressive strength obtained is higher for higher SF content. However, the ratio of tensile to compressive strength decreased with increase in SF. These trends triggered the authors to study the complex behavior of early-age HPC, including the phenomenon of autogenous shrinkage and the cracking damage that it may induce. Convenient nomograms for use by practicing engineers for the prediction of both strength and shrinkage properties, were developed.

10.14359/5859


Document: 

SP189-17

Date: 

January 1, 2000

Author(s):

M. D. A. Thomas and M. H. Shehata

Publication:

Special Publication

Volume:

189

Abstract:

This paper presents data on the performance of concrete containing ternary combinations of Portland cement, silica fume and fly ash. Porosity measurements show that the individual effects of silica fume and fly ash on pore size distribution are cumulative when both materials are added to Portland cement. Chloride diffusion and rapid chorine permeability test on concrete indicate significant improvements can be achieved through the use of these ternary combinations. There is a synergistic effect attributed to the early-age benefits of incorporating silica fume and the long-term improvements normally associated with fly ash. The final product is concrete with very low initial directivity values that continue to reduce with time. Simplistic service-life calculations indicate that high-performance concrete with ternary cementations blends may provide protection to steel reinforcement way beyond the normal expectations of engineers today. These results are not inconsistent with recent studies of marine exposed concretes, which indicate that the penetration of chlorides may eventually decrease to almost insignificant rates in concretes containing 30% to 50% fly ash.

10.14359/5858


Document: 

SP189-16

Date: 

January 1, 2000

Author(s):

J. Roncero, R. Gettu, P. C. C. Gomes, and L. Agullo

Publication:

Special Publication

Volume:

189

Abstract:

The composition of the cement paste system of concrete consisting of cement, water, mineral admixtures and superplasticizers, practically govern its flow behavior, influencing workability, slump loss and other phenomena. Obviously, it also provides the cohesion necessary for the mechanical integrity and durability of concrete. The optimization of the composition of high performance concretes should therefore include the design of the paste phase, which should incorporate the selection of superplasticizer type and dosage. The present work deals with studies of the flow behavior of superplasticized pastes using the Marsh cone test. Saturation superplastcizer dosages and loss of fluidity with time have been determined suing this test. In order to validate the use of data obtained from pastes for proportioning mortar and concrete, fluidity test have been performed on mortars and concrete, and the results compared with those of the corresponding paste phases. It is seen that the superplasticizer saturation dosage may increase slightly due to the incorporation of sands with significant coefficients of absorption and that the loss of fluidity is much higher in mortar than in paste. The behavior of the concrete, characterized by slump and DIN flow test, is similar to that of the mortar. However, when the concrete is maintained in movement (or remixed) as in truck transportation, the slump loss is more dramatic. Never theless, the loss in DIN flow is not very high indicating that some of the loss in workability can be compensated by vibration. The K-slump test data and those of compactability based on DIN flow show peak values that coincide with the paste composition with the superplasticizer saturation dosage. In general, the results demonstrate that for practical purposes the optimization of high performance concrete in terms of its flow behavior can be based on the behavior of its cement paste phase, permitting the selection of superplasticizer type and dosage from simple fluidity tests on pastes.

10.14359/5857


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