Chloride penetration resistance of functionally graded concrete by RCPT
[Objective]Functionally graded concrete(FGC)consists of several functional layers,each fulfilling a different role.Research has consistently shown that FGC outperforms single-layer concrete,leading to its increasing application in practical engineering.Testing methods such as the rapid chloride permeability test(RCPT)(given in ASTM C1202),non-steady-state migration test(given in Nordtest NT BUILD 492),bulk diffusion test(given in ASTM C1556 and Nordtest NT BUILD 443),and ponding test(given in ASTM C1543)are commonly used to assess chloride transport in single-layer concrete,only the RCPT is suitable for swiftly evaluating the chloride penetration resistance of FGC.In this study,we utilized the RCPT to measure the electric flux of FGC,examining the relationship between the chloride permeability resistance of FGC and the electric flux,as well as the relationship between the electric flux and the thicknesses of FGC's functional layers.The results offer a reference method for rapidly determining the chloride penetration resistance of FGC.[Methods]Drawing from the calculation formula for the thermal conductivity of double-layer thin-walled composite materials,we employed a similar formula to calculate the electrical flux of FGC.We then conducted extensive experiments on the electrical fluxes of FGCs with different types of cementitious materials and varying thicknesses of functional layers.This allowed us to verify the feasibility of this formula in calculating the electrical flux of FGC.Concurrently,we analyzed the relationship between the electrical flux and chloride penetration resistance of FGCs.The electrical flux of FGC was measured using RCPT,while the chloride permeability resistance was determined by an immersion test.Concrete specimens were immersed in a 2%NaCl solution(mass ratio)for 180 days,after which the chloride permeability coefficient was measured.A rapid chloride ion analyzer was used to determine the chloride content at different positions within the concrete.[Results]The soaking test revealed that AAS mortar exhibits superior resistance to chloride penetration,achieving a chloride penetration coefficient as low as 2.1×10-13 m2/s.Conversely,SAC mortar demonstrated poorer performance,with a chloride permeability coefficient reaching up to 7.3×10-12 m2/s.Our findings indicate that in FGC,a thicker AAS mortar layer results in lower electrical flux,while a thicker SAC mortar layer increases the electrical flux.The electrical fluxes of two-and three-layer FGCs based on AAS,P.S,or SAC,calculated using the formula,closely match the experimental results.[Conclusions]Much like the similar to the calculation formula for the thermal conductivity of double-layer thin-walled composite materials,the electrical flux of FGC can also be calculated using a similar formula.The electrical flux of FGC can accurately reflect its resistance to chloride penetration,i.e.,the lower the electrical flux,the better the FGC resistance.The RCPT allows for the quick and accurate determination of FGC's electrical flux,enabling us to ascertain the material resistance to chloride penetration effectively.
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