The role of cracks and chlorides in corrosion of reinforced strain hardening cement-based composite (R/SHCC)

[摘要] ENGLISH ABSTRACT: By using various kinds of fibre-reinforced concrete (FRC), new dimensions of structuralperformance have been developed. Strain-hardening cement-based composite (SHCC) is abranch of these FRCs and show remarkably improved mechanical and durabilityperformance. FRCs provide ductility and through fibre-bridging show remarkable strainhardening behaviour of up to 3% and in some cases, beyond 5% tensile strain for SHCC withespecially-graded fine sand (particle size less than 0.3 mm). SHCC forms multiple fine cracksthat are closely spaced together when subjected to tensile or flexural loads. This behaviour isa key feature of the material's ability to potentially reduce the ingress rates of harmfulsubstances such as water, oxygen and chlorides which are the key ingredients that causecorrosion of steel in reinforced SHCC (R/SHCC). This dissertation reports on a researchstudy where the fibre-controlled crack widths and spacings are investigated to determine ifthese fine cracks delay or prevent chloride-induced corrosion in R/SHCC. Therefore, themain aim of this research was to determine a relationship between the crack widthdistribution, cover depth, chloride level and corrosion.The mechanical characteristics of the SHCC and reference mortar specimens are reported onwhere the material's behaviour in compression, direct tension and flexural load are discussed.For the purpose of this research work, quite a large number of different types of SHCC andmortar specimens such as cubes, cylinders, small prisms and beams were tested to determinethe mechanical properties. The crack widths and crack distribution under uni-axial tensionand flexural testing were measured on the surfaces of the specimens made with bothreinforced and un-reinforced SHCC. In the case of corrosion testing, a total of about 100beam specimens of R/SHCC having two different sand types, two different reinforcing barlayouts and three different cover depths, were cracked and exposed to a 3.5% NaCl solution(by wt of water) representing sea water. The copper/copper-sulphate half-cell was used torecord the corrosion potential in the specimens periodically in order to indicate changes in thecorrosion process. The Coulostatic method (as part of the polarization resistance technique)was also used to measure the corrosion rate of steel bars inside SHCC. Little corrosiondamage was seen in the specimens after about 2 years of testing. A relationship between thecracks, cover depths, chloride content and corrosion rate was then documented for the SHCCmaterial used in this research work.The chloride ion content in SHCC and mortar specimens was determined by means of X-rayfluorescence (XRF) and chemical analysis. The presence of chloride in concrete can be in theform of free chloride and bound chloride. Therefore, XRF was mainly used to determine thetotal chloride (free plus bound) content at different depths of the specimens while chemicaltesting was performed for both total and free chloride. A link was established between theserecorded values and the rate of corrosion of the steel reinforcement. The chloride diffusioncoefficient of un-reinforced SHCC and mortar was also determined by doing rapid chloridemigration testing. Steady-state chloride penetration profiling by means of capillary andponding suction was also done in finely-cracked SHCC specimens. It was found that the ingress of chloride ions in an average crack width of about 50 μm occurred up to full crackheight of 60 to 80 mm in under an hour of exposure. Some other durability tests such asfreeze-thaw attack, capillary water absorption and electrical resistivity of SHCC and mortarwere also investigated.Finally, it was found that in cracked R/SHCC specimens in the early stage of testing, a higherchange (from passive to active) in corrosion potential reading could be observed due to theelectro-chemical reaction. Nevertheless for this higher potential value, no major corrosiondamage can be seen in the specimen in the early stage of testing. A 25 mm cover depth wasfound to be an approximate threshold for chloride penetration in this specific mix of SHCCmaterial. In addition to corrosion potential and rate readings, actual corrosion-induced pittingdepth and area, mass loss and loss of tensile resistance were measured after removal of thesteel from the specimens at the end of the tests.Based on the detailed experimental results obtained from this research work, empiricalformulas are proposed to predict the corroded depths and loss of steel force due to pitting andmass loss. A number of recommendations are made for the corrosion rate measurementmethodology used here of how to improve the variations in experimental and actual results.Some observations and suggestions are also proposed based on the mechanical and durabilitytests performed in SHCC. In the final conclusions, some approaches are suggested for futurestudies on durability of SHCC, which could help researchers in increasing their knowledge ofSHCC properties and which may lead to the optimal use of SHCC in a sustainable way. Theuse of SHCC may be feasible in the protection of concrete structures from severe chlorideinducedcorrosion or in delaying such corrosion.

[发布机构] Stellenbosch University