Research On The Property Of High Fly Ash Content Concrete Under The Coupling Effect Of Freeze-thaw And Seawater Corrosion

Since the application environmentof marine engineering concrete is very harsh, the favorable concrete durability have to be required. Especially in the coastal areas of high latitudes, the reinforced concrete structure is severely damaged by the salt erosion andfreeze-thaw. At present, the service life of marine engineering is generally less than 30 years, some even less than ten years and it causes huge economic losses. Therefore, how to ensure marine engineering concrete structure has both high frost resistance and seawater corrosion resistance is very important. Improving the corrosion resistance performance of steel bar and concrete service life is the key to the safety of ocean engineering.Fly ash is commonly used as a mineral admixture in high performance concrete. Its morphological effect, active effect and micro aggregate effect has significant influence on workability, strength, hydration heat and durability of marine engineering concrete. From late 80s until now, with the development of high efficiency water-reducing agent and a large numbers of high-quality fly ash supplied in market, High Fly Ash Content Concrete (HFCC) is widely used in practical engineering. Because of its good environmental benefit, economic benefit and social benefit, HFCC has become a kind of high-quality marine engineering construction materials.In view of the middle and high latitude coastland engineering, air-entraining agent is a main measure to enhance the freezing-thawing resistance. But air-entraining agent can decrease the concrete structure density and reduce the chloride-penetration resistance and strength of concrete. In terms of HFCC, fly ash has a distinct adsorption to reduce the frost resistance. This paper studied the effect of quality and dosage of fly ash, seawater curing condition and curing period on properties of the HFCC affected by freeze-thawing and chlorine salt corrosion. Mechanical property test, frost resistance test, air void parameters measure and SEM analysis were applied to analyze the durability of HFCC under the coupling effect of freeze-thaw and seawater corrosion. As a basic research for application, this paper attempts to provide experimental data and academic references for marine concrete desigin and preparation. The main work and achievement are as follows:(1) The fly ash cementitious coefficien β is a parameter to reflect fly ash’s contribution to concrete strength and it can be used in HFCC intensity formula and mixture ratio design directly. According to 28d compressive strength of the cement mortar with different fly ash dosage, the cementitious coefficient ft of three kinds of fly ash were confirmed. The experiments reveal that the fly ash cementitious coefficient β is a basic parameter decided by fineness, loss and water requirement and it is unconnected to the dosage of fly ash. Fly ash can decrease the brittleness coefficient of cement mortar specimen and the early strength of cement mortar specimens.(2) According to the service condition of marine concrete, fly ash is regarded as an independent component to design the mixture proportions of high fly ash content concrete. The performance of the HFCC mixture was studied. And the effect of fly ash dosages and quality on air entraining agent impact was analyzed. The experiments reveal that with the dosage of fly ash increased from 40% to 70%, the fluidity of concrete mixture has an obviously increasing trend. The effect of different quality (Fineness, Loss.ect) of fly ash on liquidity is slightly different. With the same dosage of water reducing agent, the dosage of fly ash is the main factor affecting the HFCC liquidity.The dosage and fineness of fly ash is the main factor to affect the air content of concrete mixture with the certain air-entraining agent dosage. The loss of fly ash can not accurately reflect the adsorption capacity. Both the air content of concrete mixture and the spacing facter of harden concrete can reflect the frost resistance of concrete, but there is no accurate corresponding relation between air content and spacing facter.(3) In view of the characteristic of marine service condition, freezing-thawing resistance to seawater of HFCC was studied. And the durability of HFCC under the coupling effect of freeze-thaw and seawater corrosion was analysed. The experiments reveal that either after standard curing or seawater curing, ordinary performance concrete(OPC) with about 6% air content has good freezing-thawing resistance to seawater. Fly ash can reduce internal porosity of the concrete and stabilize the bubbles. However, when the dosage of fly ash is too large, there is an adverse effect on the frost resistance of concrete.(4) In view of the low early strength of HFCC, the strength of OPC and HFCC which cured in seawater and standard condition were compared. From the micro-perspective, the influence on hydration product by seawater curing and freezing-thaw erosion was analyzed. The experimental results reveal that seawater curing decreased the strength of OPC. While the strength of HFCC increased slightly after seawater curing. In seawater, the deterioration of concrete is mainly due to the erosion of chlorine salt, magnesium salt and sulphate. But seawater can enhance the pozzolanic reaction ratio. With the increase of the pozzolanic reaction degree, the densification of cement stone was enhanced and the strength of HFCC was incresed. Both after seawater curing and standard curing, the strength of HFCC at 90d age surpasses the tensile strength of design value.