Durability Of Fly Ash Concrete Research Articles

Influence of nano silica on impact resistance and durability of fly ash concrete in structural buildings

Influence of nano silica on impact resistance and durability of fly ash concrete in structural buildings

Impact of Silica Nanoparticles on the Durability of fly Ash Concrete

In the present study, the mechanical and durability properties of silica nanoparticle (SNP)-incorporated fly ash (FA) concrete mix were examined after 365 days of exposure. The dosages of FA replaced by cement in the present study were 30%, 40%, and 50%, while 3% SNPs were added by the weight of cement in the FA incorporated mix. For a comparison of SNPs with silica fume (SF), 6% SF was added (by the weight of cement) and entire casting was performed at a constant water to binder (w/b) ratio of 0.29. The present work is the extension of a previous study wherein durability properties of the same mixes were reported for up to 180 days of exposure. Compressive strength results show that in the presence of SNPs, the enhancement in compressive strength was in the range of 10–14%, while, in presence of SF, 8–10% of the enhancement was observed as compared to control. However, exposed samples in a carbonation environment showed that the compressive strength of the control and SF incorporated mix increased, while SNP-incorporated samples showed negligible enhancement. Further, sulphate exposed mix show that compressive strength decreases, however, the SNP-incorporated mix showed the lowest reduction compared to other mixes. Therefore, the study shows that the SNP-incorporated mix has higher mechanical properties and more durability compared to other mixes in a severe environment.

Carbonization Resistance of Reinforced Concrete under Bending Load

Fly ash has been used more and more often to take the place of cement as the admixture of concrete in the construction of concrete buildings. However, with the increase of the carbon dioxide (CO2) concentration in the atmosphere, carbonization damage has become an essential factor affecting the durability of fly ash concrete. Here a long-term bending load device was developed to explore how the pouring surface and the bending load affect the carbonization resistance of reinforced concrete under rapid carbonization. In addition, the relationship between the bending-tension and bending-compression loads with respect to the carbonization damage of test blocks was also investigated. Due to the differences in the concrete compactness, the carbonization depth of the pouring surface was found to be greater than that of the bottom at the same position. To a certain extent, with the increasing bending-load stress, different carbonization resistances were observed in the bending-tension zone and the bending-compression zone of the concrete test blocks. Meanwhile, to study the relationship between the carbonization damages in the bending-tension zone and the bending-compression zone of concrete test blocks, a carbonization influence coefficient of bending tension-compression load was proposed, which provides a convenient and scientific guidance for the detection and evaluation of concrete carbonization damages in practical engineering.

DURABILITY AND POZZOLANIC REACTION OF FLY ASH CONCRETE EXPOSED TO VARIOUS OUTSIDE ENVIRONMENTS

In this study, outside exposure tests of fly ash concrete were started in 2009 at various places in Japan in order to assess the effect of the differences of the exposure environment on the variation of long term durability of fly ash concrete and the degree of pozzolanic reaction of fly ash. This paper shows the results of durability monitoring over seven years from the start of exposure testing and the degree of pozzolanic reaction after seven years. The durability of fly ash concrete is improved due to pore structure complicated by the pozzolanic reaction. The degree of pozzolanic reaction of fly ash in concrete exposed under any conditions ends up being the same when enough time has elapsed, such as seven years, regardless of differences in exposure conditions.

Enhancement of strength and durability of fly ash concrete in seawater environments: Synergistic effect of nanoparticles

Fly ash is used in concrete industry to reduce the amount of cement and to enhance the durability of concrete. Nuclear industry recently adopted fly ash concrete as a government initiative to reduce carbon footprint and also to provide an impermeable skin for the several structures in the cooling water system exposed to marine environment for enhanced durability. Earlier detailed studies on fly ash concrete has shown that despite its superiority over conventional concrete with respect to strength and durability, some concerns such as delayed setting and hydration, low early-age strength, and higher carbonation may pose a problem for its wider application. The present work is an attempt to overcome these deficiencies by nanophase modification of concrete with the incorporation of nano-titania and nano-calcium carbonate at 2% by weight of cement. Four different types of concrete mix were arrived and specimens of different sizes were cast in order to explore the various properties of concrete. After 28 days of curing in potable water, the samples were exposed to seawater at Nuclear Desalination Demonstration Plant (NDDP) sump at Kalpakkam and were withdrawn for testing at different ages like 56, 90, 180 and 365 days. Nanophase modification increased the pozzolanic activity resulting in faster hydration, early-age strength and long-term compressive and split tensile strength, permissible electrical resistivity for corrosion, lower chloride ion penetration, carbonation depth biofilm formation and higher internal pH. Among the different mixes, the synergy of 1% NC and 1% NT emerged superior with excellent concrete properties related to mechanical properties, pore structure, durability and optimum antibacterial activity.

Effect of Rapid Hardening Cement and Setting Accelerator on the Freeze-Thaw Durability of Fly Ash Concrete

This paper presents the results of the second phase of an experimental research, which was carried out to investigate the freeze-thaw durability of fly ash (FA) concrete subjected to slow freeze-thaw cycles. The results of the first phase showed that FA had a significant detrimental effect on the freeze-thaw durability concrete. The lower resistance to freezing condition was attributed to the low early strength of FA concrete, at early ages, compared to Portland cement concrete. Therefore, rapid hardening Portland cement (RHPC) and setting accelerator (SA) were introduced, in the second phase, in a bid to modify the early age properties of FA concrete and, hence, enhance its freeze-thaw durability. The weight loss and residual strength of the specimens were assessed after specific number of freeze-thaw cycles. The results showed that the use of rapid hardening Portland cement (RHPC) and/or setting accelerator (SA) enabled earlier demoulding to take place and enhanced the early age strength of FA concrete. However, this did not necessarily improve the freeze-thaw durability of FA concrete. The effect RHPC and SA on the deterioration and residual strength of concrete containing fly ash was indifferent. Therefore, it is suggested that further research is needed to confirm the effect of RHPC and SA on the freeze-thaw durability of fly ash concrete.

Nanoparticles for enhancing mechanical properties of fly ash concrete

Fly ash is used in concrete industry to reduce the amount of cement and to enhance durability of concrete. Some early-age performance issues of fly ash concrete are, delayed setting time, low early-age strength, and a more stringent curing conditions. To overcome this deficiency, nano modifications are adopted. This study is attempted to understand the effect of nano TiO2 (NT), nano CaCO3 (NC) particles and a combination of NT and NC particles (NTC) on various properties like compressive strength, workability and durability of fly ash concrete by partial replacement of cement. NT and NC particles with average diameter of 50-70 nm were used with six different contents ranging from 0.5 to 3% by weight of cement, with an increment of 0.5%. The results showed that addition of NT particles up to a maximum replacement level of 3.0% produced concrete with improved strength, whereas addition of NC and NTC particles in the mix showed higher strength up to 2% and thereafter it decreased. Workability of fresh concrete increased at 0.5% addition of all the three nano particles and thereafter, though it decreased for given water to powder content, the mixes were workable. This was confirmed with consistency tests. The fresh and hardened properties along with low rapid chloride penetration values and high pH values highlight the advantages of nano modification of flyash concrete.

Resistance Performance of Concrete Using both Blast Furnace Slag Cement and Fly Ash against Chloride Attack and Carbonation

From the viewpoint of effective utilization of industrial wastes and reduction of CO2 emission, the use of concrete mixed with blast furnace slag and fly ash has been promoted. However, the durability of fly ash concrete using blast furnace slag cement has not been clarified enough. About chloride attack, the resistance against the penetration of chloride ions could be improved due to the dense pore structure formed by the synegetic effect of mixing both ground granulated blast-furnace slag and fly ash into concrete. In this study, resistance performance of concrete using both blast furnace slag cement and fly ash against chloride attack and carbonation was experimentally investigated. The relationship between such resistance performance and pore structure of the concrete was also examined. As a result, the combination of blast furnace slag cement and fly ash type II resulted in the decrease of pore volume over 50 nm in the diameter and reduction of the apparent diffusion coefficient of chloride ion, but the resistance performance against carbonation of the concrete was lower than the case of the normal fly ash concrete.

Experimental study on durability improvement of fly ash concrete with durability improving admixture.

In order to improve the durability of fly ash concrete, a series of experimental studies are carried out, where durability improving admixture is used to reduce drying shrinkage and improve freezing-thawing resistance. The effects of durability improving admixture, air content, water-binder ratio, and fly ash replacement ratio on the performance of fly ash concrete are discussed in this paper. The results show that by using durability improving admixture in nonair-entraining fly ash concrete, the compressive strength of fly ash concrete can be improved by 10%–20%, and the drying shrinkage is reduced by 60%. Carbonation resistance of concrete is roughly proportional to water-cement ratio regardless of water-binder ratio and fly ash replacement ratio. For the specimens cured in air for 2 weeks, the freezing-thawing resistance is improved. In addition, by making use of durability improving admixture, it is easier to control the air content and make fly ash concrete into nonair-entraining one. The quality of fly ash concrete is thereby optimized.

Effects of Curing and Test Conditions on the Salt Scaling Durability of Fly Ash Concrete

Some specifications currently limit the amount of fly ash used in concrete exposed to de-icer salts and freezing conditions because of concerns over salt scaling durability. There is concern that the standard test method commonly used to assess concrete salt scaling performance, ASTM C672, is overly severe and not representative of field conditions. There is also concern that the use of fly ash as a mass replacement could lead to additional damage because of the higher paste volume that results from the lower specific gravity of fly ash relative to Portland cement. In this study, concrete mixtures with 40 % replacement by mass were tested according to ASTM C672 using a constant cementitious material mass and a constant paste volume. Specimens were tested at an outdoor field exposure site to test the severity of ASTM C672 and the effects of specimen size on salt scaling resistance. The experiments showed that the fly ash source has a large effect on the salt scaling resistance in laboratory testing, although the increase in cement paste volume from the mass replacement of cement did not correlate with decreased performance. Laboratory-size specimens showed much higher salt scaling than larger companion slabs cast from the same concrete batches.

Investigation of Durability of Wall Materials Concrete Prepared with Fly Ash

Based on durability of fly ash concrete, the freezing and thawing, impermeability and cracking resistance of fly ash concrete were researched. The results show that the frost resistance of concrete was reduced, impermeability and crack resistance were improved with increasing the fly ash content.

Research on Improving the Durability of Fly Ash Concrete

The purpose of this study was to improve the durability of fly ash concrete. As a result, by making fly ash concrete into non-air-entraining type and using durability improving admixture, the compressive strength of fly ash concrete increases 10%~30%, reducing initial compressive strength defects; drying shrinkage is controlled at 60% compared to when the mixture is not added; carbonation of fly ash concrete can be considered roughly proportional to water-cement ratio regardless of water-binder ratio or fly ash replacementratio; the freeze damage resistance improves for 2 weeks curing in air (drying process). Finally, by making fly ash concrete from non-air entraining type and using durability improving admixture, the difficulty of controlling air content in fly ash concrete is reduced and quality management is simplified.

Sorptivity of fly ash concretes

A factorial experiment was designed to measure the sorptivity of cement and fly ash concretes in order to compare the durability of fly ash concrete against the cement concrete. Sorptivity measurements based on the capillary movement of water was made on three grades of cement concrete and six grades of fly ash mixes. The effect of curing was also studied by treating the samples in two curing conditions. A functional relationship of sorptivity against the strength, curing condition and fly ash content has been presented. The results were useful to analyse the factors influencing the durability of cement and fly ash concretes and to explain why some of the previously reported findings were contradictary. Curing conditions have been found to be the most important factor that affected the durability properties of fly ash concrete. When proper curing was provided, a mix with 40% fly ash was found to reduce the sorptivity by 37%. Under inadequate curing the sorptivity was found to increase by 60%. The influence of curing on cement concrete was found to be of much less importance.