International Concrete Abstracts Portal

The present paper preliminarily illustrates the mechanism of damages caused by the alkali-silica reaction (ASR) between the high alkali content of the dry shake-hardener due to the high cement content on the top of the concrete industrial floors and the alkali-reactive coarse aggregate in the concrete substrate. To mitigate or prevent these damages a special dry shake-hardener, based on the partial replacement of the Portland cement by siliceous fly ash, is used. The beneficial influence of the fly ash, as well as that of other fine pozzolanic materials, is due to the distribution of a very large number of amorphous silica-based fine particles which can potentially react with the alkali in the same way as the amorphous or badly crystallized silica of the alkali-reactive coarse aggregates. The introduction of a very high number of pozzolanic particles significantly reduces the alkali availability for the reaction with the few alkali-reactive coarse aggregates. In other words, the alkalis instead of concentrating their aggression on a few grains of the alkali-reactive coarse aggregates, usually 5 to 15 mm (2 to 6 in.) in size, spread their action on a large number of very fine pozzolanic particles so that their expansive and destructive power is lost. However, another problem can arise when the Portland cement is partially replaced by fly ash due to the longer setting time, particularly in cold weather, of the dry shake-hardener, so that the workers must wait a very long time before the mechanical troweling and the opening of the finished surface to the pedestrian traffic. To avoid this drawback a combined use of the siliceous fly ash and a setting accelerator, based on tetra-hydrate calcium nitrate in powder form [4H₂O∙Ca(NO₃)₂ > 4H₂O∙CaO∙N₂O₅ > H₄CN₂] has been studied at three different temperatures: 35°C (95°F), 20°C (68°F) and 5°C (41°F). In warm weather, at temperatures as high as 35°C (95°F), there is no need for H₄CN₂ since the Portland cement hydration occurs at a very great rate and only the dry shake-hardener containing fly ash without H₄CN₂ can be applied within few hours and incorporated into the concrete substrate. At 20°C (68°F) the delay in the setting times caused by the partial replacement of Portland cement by fly ash can be compensated by the use of H₄CN₂ at 1% by weight of the cementitious materials. In cold weather, such as that caused by a temperature as low as 5°C (41°F), a much higher percentage of H₄CN₂, up to 5% by weight of the cementitious materials, must be used to reduce the setting times at approximately the same values as those recorded at 20°C (68°F) when the dry shake-hardener without fly ash is used.

In cold regions, freezing temperatures limit the construction season to few months, usually between May and September. The use of nanoparticles, which have high specific surface and vigorous reactivity, may potentially enhance the performance of concrete placed at low temperatures. Therefore, this study focused on developing concrete mixtures incorporating nano-silica which were mixed, placed and cured at -5°C (23°F) without any insulation or protection targeting field applications in late fall and early spring periods. Eight mixtures incorporating general use (GU) cement, fly ash (up to 25%), and nano-silica (up to 4%) were tested for this purpose, with water-to-binder ratios of 0.32 and 0.4. All mixtures contained a combination of calcium nitrate and calcium nitrite as an antifreeze admixture. Testing involved concrete setting time (placement), 7 and 28 days compressive strengths (hardened properties) and resistance to freezing-thawing cycles (durability). Moreover, mercury intrusion porosimetry, thermal analysis and scanning electron microscopy were performed to corroborate the trends from the macro-scale tests. It was found that nano-silica significantly improved the overall performance of concrete placed and cured at -5°C (23°F), which implicates its promising use for construction applications under low temperatures.

It is well-known that cooling of fresh concrete to a subfreezing temperature interrupts the structure formation and can lead to serious damages of constructions. Most of the existing antifreeze additives reduce this destructive effect, however it should be acknowledged that the processes of cement hydration is still interrupted to an extent that the strength gain in these conditions is simply negligible. When using these admixtures, it is merely expected that concrete will not lose its integrity during the phase of cooling and that strength will be gained after the ambient temperature will reach positive values. However, in our work we aim at proving the possibility of rapid strength gain of UHPC with reduced water-cement ratio even at subfreezing temperatures. The following article presents analysis of the influence of various in-house developed admixtures on kinetics of strength gain of UHPC at negative temperatures.

The quality of hardened concrete is severely degraded if the concrete is frozen at its setting or hardening stage. To prevent the degradation of materials, heating of the materials, equipment and facilities is widely utilized in ready-mixed concrete plants in cold regions of Japan. The heating is widely mentioned and conditionally recommended in specifications and recommendations. While these documents refer to material types to be heated and maximum temperature that will affect the quality of concrete, they do not refer to the energy sources or heating method for equipment and facilities. These factors, however, do have influences on the environmental performances at production stage of concrete. The difference in energy sources, types of heating and operation design of heating make significant difference in the emission of carbon dioxide and other global warming substances. Considering these importance, this paper surveys the types of energy sources and its system for concrete production at ready-mixed concrete plants in cold regions of Japan. The comparison of plants by the difference in seasonal temperature and the location is analyzed through monthly use of energy and on-site observation in various plants.

Portland Cement Pervious Concrete (PCPC) is a material of increasing interest for parking lots and other applications. PCPC typically consists of coarse aggregates, portland cement, water, and various admixtures. In this research, in-service PCPC pavements were inspected in the field, and cores were removed in order to investigate properties in the laboratory. Field evaluation methods included visual inspection, two surface drainage measurements, and indirect transmission ultrasonic pulse velocity (UPV). Laboratory testing methods included void ratio, unit weight, compressive strength, splitting tensile strength, hydraulic conductivity, and direct transmission UPV. Because it is compacted on the surface with screeds or rollers, PCPC generally has higher strength, lower void ratio, and lower hydraulic conductivity at the surface than at the bottom. Therefore, the properties of the tops and bottoms of core samples were compared. Generally, the PCPC installations evaluated under this research project have performed well in freeze-thaw environments with little maintenance required. No visual indicators of freeze-thaw damage were observed. With the exception of some installations where the pore structure was sealed during construction with wet mixtures or over compaction, nearly all sites showed fair to good infiltration capability based on drain time measurements.