International Concrete Abstracts Portal

A test method was developed to predict the risks of ASR expansion for actual field concrete compositions. Concrete prisms (7 x 7 x 28 cm) were cast, demolded, and measured for their initial length. They were then immersed in an alkaline solution at 150 C for 3 weeks in individual stainless steel containers and their lengths were monitored weekly. The concentration of the solution was adapted to match as closely as possible the composition of the interstitial pore solution by summing up the contribution of the binder constituents to the effective sodium and potassium contents, respectively. Despite this fact, the alkali balance before and after treatment of the prisms shows that the concrete is enriched in Na 2O and/or K 2O during the cure. The expansions reached at 150 C after 3 weeks were compared to those obtained at 100 percent relative humidity in air at 38 and 60 C after 12 and 4 months, respectively. Good correlations were obtained for the 67 different concrete mixes tested. Consequently, an expansion value of 0.11 percent was proposed as a provisional limit, which is rather conservative. Reaction products were studied by SEM, optical microscopy, and electron microprobe as gels, and semi-organized and crystallized compounds, and were shown to be present in cracks and pores as well as within the paste. The composition of the products formed at 150 C seems to be restricted to a narrower range of Ca, Si, Na, and K concentrations than what has been reported at lower temperatures.

Petrographic examination and the South African mortar bar test have been performed at SINTEF--Structures and Concrete during the last 2 to 3 years to evaluate the reactivity of Norwegian aggregates to be used in concrete structures. Paper presents the relationships between these two test methods. The purpose of the petrographic examination is to identify, quantify, and group different rock types in an aggregate. These groups are: reactive (with known reactive field performance), potentially reactive, and innocuous aggregates. In Norway, further testing by the mortar bar test is recommended when petrographic examination indicates 20 percent of reactive or potentially reactive rock types in the aggregates. The mortar bar expansion after 14 days of exposure is used for the evaluation of potential expansivity of the aggregates. One main conclusion from the investigation is that mortar bar expansion increases to an upper level with increasing content of reactive rocks in the aggregates. Beyond a "marginal" amount of reactive rocks in aggregates, the mortar bar expansion increases no further. A significant difference in mortar bar expansion between different reactive rock types has not been found. The established limit of 20 percent of reactive rocks in aggregates appears, in most cases, sufficient for classifying aggregates as innocuous; however, no verification of the limit has been made.

This paper describes the results of a ten-year marine exposure test of reinforced concrete. Sixteen pre-cracked test specimens were examined. The target crack width was 0.2mm. The dimensions of the specimens were 15x15x100cm. Ordinary deformed bars and epoxy-coated deformed bars, as well as normal portland cement and portland blast-furnace slag cement were used. The water-cement ratio in the mixture proportions ranged from 0.320I to 0.483%. The effect of nitrite-based corrosion inhibitor was also examined. From the exposure test results, the following conclusions were drawn: When the water-cement ratio was low, the penetration of chloride ions into the concrete was low; the chloride-ion content on the surface of blast-furnace slag cement concrete was greater than on the surface of concrete made with ordinary cement, but was smaller inside; there was a tendency for the chloride-ion content around the reinforcing bars in concrete portions with small cracks to be greater than in portions with large cracks; ten years of exposure caused an increase in crack width due to the corrosion of the reinforcing bars; although the effectiveness of epoxy-coated reinforcing bars in preventing corrosion was obvious, severe corrosion was found in one coated bar. The epoxy-coated bars used were produced for the first time in Japan, and test results indicate that there were problems with the early production technology; there was no beneficial effect from corrosion inhibitor after ten years.

Inspection and service reports from concrete platforms up to 20 years old in the hostile environment of the North Sea are positive. The need for remedial work has been minimum and these high-quality concretes demonstrate excellent performance under marine conditions. To meet the demands for durability and development in structural design and construction methods, a continuous effort has been made to advance concrete materials. Improved concrete properties are predominantly a consequence of improved cement qualities, more efficient admixtures, and better controlled processing of aggregates. The soundness of the aggregates has been verified from the start. Examinations of concrete specimens drilled out from different elevations of some of the platforms have revealed rather high concentrations of chlorides close to the concrete surface. For most of the specimens, the chloride content is, however, negligible at the reinforcement. Epoxy coating applied to the concrete surface in the splash zone of some platforms has shown to be efficient in preventing ingress of chlorides.

Despite the excellent resistance of high-performance (HP) concretes in the presence of aggressive agents, instances of application have shown that the microstructure of the concrete surface can be greatly disturbed by the curing method, thereby compromising durability on the part covering the reinforcement. Paper presents results of a study on three concrete design mixes (one reference concrete and two HP concretes with and without silica fume), each subjected to three curing methods and three durability tests. Results on carbonation, variation in free lime, and microcracking indicate that HP concrete with silica fume is more sensitive to the curing method than the reference concrete or concrete without silica fume, as evidenced by increased carbonation and a larger reduction in alkalinity. The study of microcracking in the various concretes showed that desiccation causes more microcracking in the HP concrete with silica fume than in the HP concrete without silica fume. Results of microstructural inspection and physical and chemical tests explain these variations in mechanical properties and carbonation behavior of various concretes, depending on the curing method.