International Concrete Abstracts Portal

This paper presents results of a study that compared ASTM C 1202 test, a steady-state migration test, a conductivity test, and a non-steady-state diffusion test for deter-mining the chloride-ion penetrability of control portland cement concrete and concretes incorporating fly ash, ground granulated blast-furnace slag, silica fume, metakaolin, and rice-husk ash. The water-to-cement ratios of the control portland cement concretes ranged from 0.31 to 0.60. The content of fly ash and slag was 20, 40, or 60% by mass of total cementitious materials as cement replacement. Silica fume, metakaolin, and rice-husk ash was used as 8% cement replacement. Most of the concrete mixtures were made from normal-weight aggregate except for two concrete mixtures that were made from light-weight aggregate. In general, the results from the four test methods led to similar conclusions regarding the penetration of chloride ions into concrete. Therefore, ASTM C 1202 test method can be used for evaluating the resistance to chloride-ion penetration for concrete with or without supplementary cementing materials in spite of its short comings. The conductivity test is another rapid and convenient method, and has potential for wide acceptance by the construction industry for determining ingress of chloride ions into concrete.

Macro- and micro-cell corrosion of steel bars in pre-cracked prism specimens ex-posed to marine environment for 15 years are summarized here. The size of the specimens was 100xlOOx600 mm. W/C were 0.45 and 0.55. The specimens were made with ordinary portland, slag (Types A, B and C), and fly ash (Type B) cements. A round steel bar of diameter 9 mm was embedded at the center in each specimen. Crack widths were varied from 0.1 to 5.0 mm. Chloride concentrations in the concrete, micro- and macro-cell corrosion, passivity grade, anodic polarization curve, deposits in the crack, and pit depths over the steel bars were investigated. Dense microstructure of concrete made with a large amount of slag (SCB, SCC) causes accumulation of more chloride in the vicinity of the unhealed cracks (>0.5 mm). However, it does not lead to a remarkable amount of corrosion at the cracked region compared to the other cements after 15 years of exposure. Narrower cracks (5-0.5 mm) as well as the debonded areas in the vicinity of the root of the crack over the steel bars heal irrespective of the cement types. It improves the passivity of the steel bar at the cracked region. Relations between pit depth and crack widths; and macro-cell and micro-cell corrosion are proposed.

Self-compacting concretes (SCC) represent a move toward a sustainable material since they encourage the use of waste and recycled materials. The high volume of very fine powder necessary to achieve deformability and passing ability properties, in fact, permits SCC to consume large amount of fly-ash, very fine particles generated by the re-cycling of demolished concrete structures, and huge amount of calcareous filler avail-able from the marble quarries. Moreover SCC turn out to be materials with an extended durability with respect to conventional concretes. Since fresh properties of self-compacting concretes (SCC) are significantly different from those of conventional concretes (CC) durability can be significantly improved when a SCC is used due to a modification of the microstructure of the interfacial transition zone between aggregates and cement matrix. This paper presents results of an experimental study carried out to evaluate changes in microstructure of interfacial transition zone (itz) and of bulk paste for both SCC and CC. Data on the influence of the calcareous filler, a fundamental ingredients to achieve self-compactability, on the hydration process of cement are also presented. Data indicate that the decrease in internal bleeding, when self-compacting concrete is used, seems to favour the formation of a stronger transition zone characterized by a less porous structure and with a limited amount of microcracking responsible for higher compressive strength values for SCC with respect CC. No differences were detected by EDAX analysis in the chemical nature of itz with respect the bulk matrix both for SCC and CC. Finally, observations of the cement hydration by analysis of the temperature pro-file vs time seem to indicate the calcareous grains promote formation of heterogeneous nucleation responsible for the increased crystallinity of ettringite, for a shorter normally dormant period and, hence, for higher strength values at early ages, when the calcareous filler is used.

A technology for comprehensive utilization of high volumes of coal ashes for production of lightweight concrete was developed at The Research Institute of The College of Judea and Samaria, Ariel, Israel. The technology is based on the combined use of bottom ash and fly ash in lightweight concrete, which enables the advantages of each kind of the ashes to be realized. The features of the suggested technology were con-firmed under laboratory and field studies. The obtained binary concrete (with respect to coal ashes) is a beneficial material for production of masonry units. The current paper describes the results of testing the effect of concrete proportions on properties of the binary lightweight concrete and of combined use of coal ashes with waste fines from stone quarries as additive for ternary lightweight concrete. The use of these materials both controls the strength of the lightweight concrete containing highly porous bottom ash and lowers the concentrations of radionuclides 226Ra, 232Th and 40K in the lightweight concrete.

For almost 70 years fly ash has been used as a constituent of concrete by replacing (substituting) a minor proportion of normal portland cement (PC). However, in this process the two complementary materials have simply been combined without much consideration being given to the specification of each to provide concrete with the best possible performance. Cements based on lime and pozzolanic materials such as volcanic ash have been used successfully for more than 2000 years and it is timely to re-view this experience in the context of contemporary technology. The paper firstly discusses the need to reconsider the chemical composition and particle size distribution of normal PC clinker and the requirement to incorporate gypsum in the final product. Secondly, the beneficiation of fly ash, normally discarded as a non-hazardous waste material from electricity generating power stations fired with pulverised coal, is considered with particular reference to the removal of carbon (char), cenospheres and sulfates. In-formation is given on the specification and performance of concretes designed to exploit and maximise the technical advantages and sustainability of PC clinker and beneficiated fly ash combinations. The test method for the determination of pozzolanic activity index has been successfully adapted using the concept of cement-factor for the evaluation of the effectiveness of a fly ash as a cementitious material when combined with a normal PC. A similar procedure has been used to evaluate the water demand of combinations of normal PC and fly ash using the concept of water demand-factor. The cement-factor and water demand-factor concepts can be used in concrete mixture proportioning procedures to achieve strengths no less than that required for structural design and a free-W/C of no greater than the maximum specified for durability. The concepts can be used to optimise concrete mixtures for price and environmental impact.