Experimental Study On Early-age Shrinkage And Crack Resistance Of Steel Slag Powder Concrete Based On Elliptic Ring Method

Shrinkage cracking performance affects the safety and use of concrete structures,which is the hot and difficult point in concrete research,also the pain point in concrete engineering structures.ASTM,AASHTO and other standards recommend the restrained ring method to evaluate the crack resistance of cement-based materials.However,the traditional ring method has problems such as slow cracking speed,uncertain location,and difficult to accurately measure the cracking time.Steel slag is a common solid waste material,and the ground steel slag powder is used as concrete mineral admixture,which conforms to the current low-carbon environmental protection concept.In this paper,the self-made elliptical ring test device,combined with the traditional shrinkage test method,has been used to conduct beneficial research and exploration on the early shrinkage and crack resistance of steel slag powder concrete under multiple factors,including:（1）Through the research on the influence of different water binder ratio,mineral admixtures（steel slag powder,slag powder,fly ash）and content（0～30%）on the physical and mechanical properties,shrinkage and cracking properties of steel slag powder concrete,the optimal mix and appropriate content range of mineral admixtures that are conducive to improving the crack resistance of steel slag powder concrete are discussed.The results show that the concrete can effectively exert the"superposing effect"and has good mechanical,shrinkage,and crack resistance properties when three kinds of steel slag powder,slag powder and fly ash are mixed;When the total content of mineral admixture is30%,and the optimal proportion of steel slag powder,slag powder and fly ash is 1:2:1,the steel slag powder concrete has good mechanical properties.The 28d compressive strength,splitting tensile strength,axial compressive strength and static elastic modulus are 62.9MPa,6.5MPa,48.8MPa and 29.8GPa respectively,and the 60d shrinkage rate is only 213×10^-6,with low cracking sensitivity.（2）The influence of different factors such as size effect of elliptic ring（length of elliptic ring axis,thickness of concrete ring and thickness of restrained steel ring）on cracking sensitivity of steel slag powder concrete was studied by using homemade restrained elliptic rings with different specifications,and the stress gradient and variation rule of elliptic ring at different positions were analyzed by numerical simulation.The test results show that when the concrete wall thickness is large,the specimens are less sensitive to small cracks,the crack drive energy is low,and the cracking sensitivity is poor.However,the thicker steel ring can provide a higher degree of constraint on the concrete,resulting in increased interface pressure,earlier cracking time,and higher cracking sensitivity.（3）The influence of multiple factors on the early hydration characteristics of steel slag powder cementitious material system is studied to reveal the micro mechanism of early shrinkage cracking of steel slag powder concrete and provide theoretical support for exploring the inhibition measures of early shrinkage cracking of steel slag powder concrete.Through the hydration heat test,it is found that when the steel slag powder,slag powder and fly ash are mixed three times and the proportion is 1:2:1,the hydration reaction is full,the induction period is long,the heat release rate is reduced,and the cumulative heat release is reduced,thus effectively reducing the risk of shrinkage cracking of steel slag powder concrete.In addition,the calculation shows that the steel slag powder concrete has good economic and environmental benefits when it is mixed with steel slag powder,slag powder and fly ash three times and the mixing ratio is 1:2:1.