International Concrete Abstracts Portal

The durability of reinforced concrete structures has been acquiring an increasing degree of emphasis in research activities. The development of concrete mixtures with regard to different attacks imposed on concrete structures by climatic influences plays a decisive role in this context. Under certain conditions, the performance of these concretes can be increased by using pozzolanic additions, such as fly ash or blast-furnace slag, as cementitious components. This publication intends to summarise the influence of these cementitious components on important processes contributing to chloride-induced corrosion. Diffusion and corrosion cells were used to examine the diffusion resistance and corrosion rate of mortars and concretes containing up to 60 mass.% of fly ash and up to 75 mass.% of blast-furnace slag in relation to the total content of cementitious binders. The results of pore structure investigations were also employed in order to clarify the influence of these cementitious components on ion transport in the microstructure of the concrete.

This paper presents extensive long-term data on chloride penetration into unprotected and protected members of a reinforced concrete structure exposed to an aggressive salt-laden environment. The test results up to 11 years of exposure show that chlorides penetrate very rapidly into the concrete from an early stage, and this penetration increases with time. There was a visible peaking of chloride concentration in the vicinity of the location of the reinforcing bars. Structural members protected with the acrylic rubber coating showed an almost total absence of penetration of chlorides throughout the exposure period, and the coating maintains long-term durability of RC structures under marine environment. The accumulated chloride ions in concrete become important for the durability of reinforced concrete and these have been calculated from the distributions of chlorides in relation to the exposure period. Analytical evaluation shows that the accumulated chlorides are expressed by a model represented by an equation with coefficients,where the coefficients are closely related to environmental conditions and characteristics of concrete.

This paper presents results from a major long-term study on chloride induced steel corrosion in concrete. Specifically, data on concrete resistivity and estimated corrosion rates of steel in concrete have been measured and compared. The performance of 50 reinforced concrete slabs made with a range of portland and blended cement binders was evaluated. A high C3A cement, a low C3A cement, a blended fly ash cement and a blended blast furnace slag cement were used. All reinforced concrete slabs were exposed to high chloride conditions by partial immersion in simulated sea water conditions. Reinforcement was cleaned and weighed prior to embedment into the concrete slabs. Periodic non-destructive measurements of concrete performance included half cell potential monitoring, concrete resistivity and electrochemical measurements of rates of corrosion of steel in concrete using potentiodynamic anodic procedures. In addition, individual slabs were broken for reinforcement recovery at predetermined times during the study and measurements made of the area of corrosion and the weight loss of steel through corrosion. As opposed to the initiation-propagation model frequently cited in the literature, three distinct segments were apparent when the estimated corrosion current data were plotted against the concrete resistivity over a period of five years for reinforced concrete slabs considered in this study. The first stage was described as the Quiescent Stage, during which it was found that resistivity increased with time and estimated corrosion current values were low. Upon reaching a maximum resistivity value, a second stage of corrosion took place. This stage was described as the Active Stage, during which resistivity values decreased and estimated corrosion current values increased. After this, a third or Breakaway Stage of corrosion was reached, during which resistivity values decreased at a lower rate while estimated corrosion currents increased significantly. The findings for each of the above stages were consistent with the exposure conditions applied to the slab, in that, after chlorides had reached the steel and caused depassivation, the concrete resistivity decreased while the steel corrosion rate increased. Using the above information, it was found that the time taken to reach the maximum resistivity value could be used as a criterion for corrosion onset as, after this time, steel corrosion rates in concrete could be expected to increase.

The paper describes the physical characteristics of pumped high-strength, high-performance concrete for a large (26,480m3) 50m high concrete slipform placement where the air content in the formwork was intentionally varied from 2-3% in the lower portion of the placement to 4-6% in the upper portions of the placement. Included are evaluations of the slump, air content and density of the unhardened concrete at the batch plant, in distribution hoppers after pumping, at the formwork before vibration, and in the formwork after vibration and re-vibration. A special tub test was developed to approximate the unhardened characteristics of the pumped concrete in the formwork without having to remove concrete from the formwork to do the testing. The concrete was a modified normal density concrete where a portion (45% by volume) of the normal weight coarse aggregate was replaced with structural lightweight aggregate to reduce the concrete density. High slumps (210-230mm) were used to facilitate pumping, and to accommodate extremely congested reinforcing bar situations. Concrete cylinder strengths of 74 to 78 MPA were obtained at 28-days age for concretes having air contents from 4 to 6%. Hardened concrete air-void parameters indicated fewer but larger air voids than what might normally be expected for durable concrete yet the freezing and thawing behavior in water (ASTM C666, Procedure A) for 500 cycles showed no change in the quality of the concrete.

Curing of concrete is essential for reliable performance of concrete structures. The recommendations concerning curing of high-strength concrete are contradictory. The traditional ways of curing fail in the case of high strength concrete. A higher porosity in the vicinity of edges, microcracks due to self desiccation and shrinkage, and reduced compressive strength affect the durability of high performance concrete. Therefore, another approach is followed which consists of a new idea for supplying curing water in the interior of the concrete, by using lightweight expanded clay aggregates. When a shortage of water in the hydrating cement paste occurs, the water from the lightweight aggregates is transported by capillary suction or by capillary condensation into the smaller pores of the cement paste, thereby permitting continuous hydration. About 25% by volume of the aggregates are lightweight. The improved durability due to the higher degree of hydration, an improved density of the hydrated cement paste, less drying shrinkage and higher com-pressive strength have been shown by experiments. Test results from differen-tial thermal analyses, X-ray diffraction, mercury porosimetry, water absorp-tion, drying shrinkage and compressive strength are presented and compared with normal weight concrete with 100% natural aggregates.