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International Concrete Abstracts Portal

Showing 1-5 of 9 Abstracts search results

Document: 

SP191-01

Date: 

December 1, 1999

Author(s):

S. W. Forster

Publication:

Symposium Papers

Volume:

191

Abstract:

Durability is defined in ACI 116R as the ability of concrete to resist weathering action, chemical attack, abrasion, and other conditions of service. Concrete can certainly be used to construct durable, long-lasting pavements and structures; we see numerous examples of this behavior daily as we go about our lives. Durability remains an issue, however, because we also see some examples of concrete construction that have not been as distress-free, or lasted as long as we would have liked. In these latter instances, usually one or more aspects of the environment, materials and mix design and/or construction were not sufficiently considered for the impact they would have on the performance of the concrete used. The objective of this paper is to review the various aspects of concrete durability as considered by ACI Committee 201, particularly as to the demands placed on the concrete as a material. Current methods used to explain the durability performance of in-service concrete or predict the performance of concrete to be placed will also be reviewed. The aspects of durability that affect and interact to produce the durability performance of concrete may be placed in five broad categories: freezing and thawing; aggressive chemical attack; surface abrasion; corrosion of embedded steel; and the alkali-aggregate reaction. Tests used to estimate concrete durability include microscopic examination, chemical techniques, characterization of the concrete components, and accelerated testing to simulate the durability aspect under consideration. Interpretation of the performance of in-service concrete is made more difficult by the fact that usually the deterioration is not caused by one type of distress, but involves a combination of factors.

DOI:

10.14359/5730


Document: 

SP191-06

Date: 

December 1, 1999

Author(s):

D. Whiting and M. Nagi

Publication:

Symposium Papers

Volume:

191

Abstract:

A laboratory and field test program was undertaken to determine the perfromance of a nuclear water/cement content gauge for fresh concrete. The laboratory evaluations included study of the effects such variables as air content, pozzolans, hold time, coarse aggregate, and temperature on gauge response. The laboratory testing demonstrated that the gauge is sensitive to materials compositions and other factors, and therefore must be calibrated with exactly the same materials as will be used on the job in question. With proper calibration in a laboratory setting, the cement gauge is capable of determining cement content of fresh concrete to within approximately 10 to 20 lb/yd3 (6 to 12 kg/m3). The water gauge is capable of determining water content to within approximately 2 to 4 lb/yd3 (1 to 2 kg/m3). Field tests at two locations are described. Favorable results were acheived where calibrations were carefully carried out using the same materials as to be used in actual construction. In these cases, avearge water content determinations for a series of samples using the nuclear gauge were comparable to those obtained using a microwave oven drying technique.The gauge is well-suited for use at construction sites. Technicians (having proper radiation safety training and certification) can successfully operate the gauge after a brief period of training, and the gauge can be transported in construction vehicles and set up on-site with a minimum of effort. The test period is short, requiring approximately ten minutes per sample, including consolidating of concrete into a test bucket.

DOI:

10.14359/5735


Document: 

SP191-05

Date: 

December 1, 1999

Author(s):

K. A. MacDonald and D. O. Northwood

Publication:

Symposium Papers

Volume:

191

Abstract:

The water/cementitious ratio of concrete is an important indicator of quality of concrete in the numerous ways that have been chosen to measure it. Low water/cementitious ratio concretes are high strength, low permeability, high durability and permanent concretes. It is also a property which is not frequently measured at the time of construction or on concretes which undergo distress during their service life due to the lack of standardized test methods and cheap test procedures for determining this property. In the present study, the use of electrical resistance measurements to estimate the water cementitious ratio and chloride ion diffusivity in terms of water cementitious ratio, compressive strength of concrete is explored. Both hardened and plastic concretes were studied, over a wide range of water/cementitious ratios. Supplementary cementing materials, paste volume fractions and admixture chemistries were varied as well. The results indicate that a rapid, field portable test can be used to estimate the water/cementitious ratio of plastic concrete delivered to the site, as well as samples of hardened concrete removed from structures. Use of the formation factor analogy to describe the pore system and measurements of the total pore volume allow an estimate of the transport properties of the concrete, such as diffusivity and permeability. Properties estimated by this technique are compared to those determined using the standard determination methods. The implications of using assumed pore solution characteristics are discussed. The method developed is potentially useful in both Quality Assurance and Quality Control testing of high performance structures. Additional work is required to develop a field test.

DOI:

10.14359/5734


Document: 

SP191-03

Date: 

December 1, 1999

Author(s):

U. H. Jakobsen, P. Laugesen, and N. Thaulow

Publication:

Symposium Papers

Volume:

191

Abstract:

This paper describes a method for determining the water to cement ratio (w/c) of hardened concrete using optical fluorescence microscopy. The method is well established and has been used for many years. In Denmark the method is used for quality control of hardened concrete. The method is based on vacuum impregnation of concrete using a yellow fluorescent epoxy. During impregnation the capillary porosity, cracks, voids, and defects in the concrete are filled with epoxy. The amount of fluorescent dye entering the cement paste depends on the capillary porosity, which is determined by the w/c and the degree of hydration. After impregnation and hardening of the epoxy a thin section of concrete with a thickness of 0.020 mm (20 µm) is prepared. The thin section is analyzed under an optical microscope using a combination of a blue excitation filter and a yellow blocking filter. This is the fluorescent light mode in which epoxy filling air voids and cracks appears yellow, cement paste as shades of green, and aggregate black. The shade of green of the cement paste depends on the capillary porosity. A sample with low w/c appears dark green, i.e. has less fluorescence intensity due to a low amount of epoxy within the paste. A sample with high w/c appears light green, i.e. has high fluorescence intensity. These shades of green (fluorescence intensity) are used to determine the w/c by comparing the fluorescence intensity of the cement paste with the standards of known w/c. This paper describes the fluorescent impregnation technique, the thin section preparation, the visual determination of w/c and discusses the pitfalls in the w/c determination. Furthermore, the paper presents data from a quality assurance project and damage analysis and data of Round Robin Testing.

DOI:

10.14359/5732


Document: 

SP191-08

Date: 

December 1, 1999

Author(s):

S. J. DeSouza, R. D. Hooton, and J. A. Bickley

Publication:

Symposium Papers

Volume:

191

Abstract:

The performance needs of the Toronto Transit Commission (TTC), for the reinforced concrete tunnel liner segments for the Toronto Sheppard Line Subway required a service life of at least 100 years. Under the contract specification for chloride diffusion, compliance with this requirement was determined by the water permeability and chloride diffusion coefficients of the concrete. The specified test procedures required up to six months for final test results, after a segment was cast. From a practical point of view, what is required is confirmation that the segments are acceptable shortly after they have been cast. An in-situ rate of absorption (sorptivity) test was modified and implemented. This paper summarizes the implementation of this simple test procedure into the QC programme to non-destructively evaluate the quality of the concrete shortly after fog curing is complete. The primary research goal of this project was to develop a service life model that utilized the initial sorptivity value as determined by the in-situ sorptivity test combined with a diffusion value established for the concrete mix. The use of service life modelling to predict performance of reinforced concrete structures exposed to chlorides is the next logical step to provide assurance of longevity for owners.

DOI:

10.14359/5737


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