Resistance To Chloride Corrosion Of ECC And The Application In Functionally Graded Material Beams

Engineered Cementitious Composites(ECC)have good compactness and excellent strain hardening and crack controllability,and have broad application prospects in replacing the concrete as the protective layer in the tensile zone of reinforced concrete beams to make ECC-concrete functionally gradient beams,and improve the structural stress and the durability.This paper through the natural immersion test to test the ECC chloride corrosion depth,chloride content and other indicators,the influence of crack width(0.1 mm,0.2mm,0.3mm),wetting-drying cycle and corrosion time(30 days,60 days,90 days,120 days)on the transport and binding law of chloride in ECC is analyzed.Combined with XRD and MIP microscopic tests,the corrosion mechanism of chloride in ECC was revealed.The apparent chloride diffusion coefficient and its time dependence of ECC were analyzed by theoretical calculation.The application of ECC in functionally graded beams is studied.Through the four-point bending loading tests of two types of functional gradient beams,the deformation and crack development of different types of beams are analyzed.Through the natural immersion test,the resistance to chloride corrosion and reinforcement corrosion of the beams were compared.The main conclusions are as follows:(1)With the increase of crack width,the chloride corrosion depth,the chloride content and the apparent diffusion coefficient n value increase,and the time dependence constant decreases.The corrosion depth of ECC specimens with 0.1mm,0.2mm and 0.3mm cracks perpendicular to the crack wall is about 20.4%,60.4%and 97.0%of the corrosion depth of the main corrosion surface.At each depth,the chloride content increases obviously with the increase of crack width.When the depth is greater than 10mm,the chloride content of the 0.1mm crack ECC is basically the same as that of the non-crack ECC,and the chloride content of the 0.3mm crack ECC is about 1.5-2.5 times that of the non-crack ECC.The apparent chloride diffusion coefficient of 0.1mm crack ECC is similar to that of non-crack ECC.The n values of ECC with 0.1mm,0.2mm and 0.3mm cracks decreased by about 6.6%,15.5%and 20.3%compared with non-crack ECC.(2)Both chloride corrosion depth and chloride content increased with the increase of corrosion time,the increase is faster in the early stage and slower in the later stage.The corrosion depth of 0.1mm crack ECC perpendicular to the crack wall increased by 0.26mm from 30 days to 60 days,while it only increased by 0.08mm from 90 to 120 days.The corrosion depth of 0.3mm crack ECC perpendicular to the crack wall increased by 0.98mm from 30 days to 60 days,and increased by 0.32mm from 90 days to 120 days.The apparent chloride diffusion coefficient decreases with the increase of time,and the decreases is faster in the early stage and slower in the later stage.(3)Under the wetting-drying cycle,the chloride corrosion depth,chloride content and apparent diffusion coefficient are all greater than those under the natural immersion condition.The coupling effect of wetting-drying cycle and crack is more significant,the corrosion depth perpendicular to the crack wall of 0.1mm crack ECC under the condition of wetting-drying cycle is about 1.5 times that of natural immersion condition.Under the wetting-drying cycle,the chloride content increased first and then decreased along the depth direction,and there was an obvious"chloride condensation phenomenon" in the shallow range.At the four corrosion time points,the chloride content of the chloride condensation position increased by 58.2%,93.7%,112.3%and 122.6%compared with the chloride content of the same position of ECC under natural immersion conditions.(4)The chloride binding law in ECC was more consistent with the Freundlich adsorption method.Friedel salt and CaCO3 generated by chloride corrosion make the porosity of ECC decrease.With the increase of crack width and wetting-drying cycle time,the porosity decreases more.At 60 days,the ECC of 0.1mm,0.2mm,and 0.3mm cracks decreased by about 2.5%,10.0%,and 17.4%,which compared with non-crack ECC.At 120 days,the ECC with 0.1mm,0.2mm,and 0.3mm cracks decreased by about 3.1%,13.6%,and 23.3%,which compared with the non-crack ECC.(5)The cracking forms of the two types of gradient beams are obviously different under the load.The bond beam cracks first in the ECC layer,while the non-bond beam cracks first in the concrete layer.Due to the existence of interlayer bonding force,concentrated cracks are formed through ECC layer and concrete layer under the load.The crack width is 2.52mm,and ECC layer loses its protective effect on reinforcement.Because of the separation between layers,the ECC layer can give better performance to the non-bonded ECC-concrete functionally gradient beam.During the loading process,multiple microdense cracks are formed in the ECC layer,and the width of the main crack is 0.16mm,and the ECC layer still has a protective effect on the reinforcement.Compared with the bonded beam,the ECC layer significantly improves the stress of the gradient beam.Due to the better dispersion of the cracks in the ECC layer,the non-bonded beam has a much better resistance to chloride corrosion.After immersion,the reinforcement does not corrosion,while the reinforcement of bonded beams corrosion obviously.Through comparative analysis,the non-bonded ECC-concrete functionally graded beam is more excellent.