The role of cracks and chlorides in corrosion of reinforced strain hardening cement-based composite (R/SHCC)
Thesis (PhD)--Stellenbosch University, 2015.
ENGLISH ABSTRACT: By using various kinds of fibre-reinforced concrete (FRC), new dimensions of structural performance have been developed. Strain-hardening cement-based composite (SHCC) is a branch of these FRCs and show remarkably improved mechanical and durability performance. FRCs provide ductility and through fibre-bridging show remarkable strain hardening behaviour of up to 3% and in some cases, beyond 5% tensile strain for SHCC with especially-graded fine sand (particle size less than 0.3 mm). SHCC forms multiple fine cracks that are closely spaced together when subjected to tensile or flexural loads. This behaviour is a key feature of the material’s ability to potentially reduce the ingress rates of harmful substances such as water, oxygen and chlorides which are the key ingredients that cause corrosion of steel in reinforced SHCC (R/SHCC). This dissertation reports on a research study where the fibre-controlled crack widths and spacings are investigated to determine if these fine cracks delay or prevent chloride-induced corrosion in R/SHCC. Therefore, the main aim of this research was to determine a relationship between the crack width distribution, cover depth, chloride level and corrosion. The mechanical characteristics of the SHCC and reference mortar specimens are reported on where 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 and mortar specimens such as cubes, cylinders, small prisms and beams were tested to determine the mechanical properties. The crack widths and crack distribution under uni-axial tension and flexural testing were measured on the surfaces of the specimens made with both reinforced and un-reinforced SHCC. In the case of corrosion testing, a total of about 100 beam specimens of R/SHCC having two different sand types, two different reinforcing bar layouts 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 to record the corrosion potential in the specimens periodically in order to indicate changes in the corrosion 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 corrosion damage was seen in the specimens after about 2 years of testing. A relationship between the cracks, cover depths, chloride content and corrosion rate was then documented for the SHCC material used in this research work. The chloride ion content in SHCC and mortar specimens was determined by means of X-ray fluorescence (XRF) and chemical analysis. The presence of chloride in concrete can be in the form of free chloride and bound chloride. Therefore, XRF was mainly used to determine the total chloride (free plus bound) content at different depths of the specimens while chemical testing was performed for both total and free chloride. A link was established between these recorded values and the rate of corrosion of the steel reinforcement. The chloride diffusion coefficient of un-reinforced SHCC and mortar was also determined by doing rapid chloride migration testing. Steady-state chloride penetration profiling by means of capillary and ponding 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 crack height of 60 to 80 mm in under an hour of exposure. Some other durability tests such as freeze-thaw attack, capillary water absorption and electrical resistivity of SHCC and mortar were also investigated. Finally, it was found that in cracked R/SHCC specimens in the early stage of testing, a higher change (from passive to active) in corrosion potential reading could be observed due to the electro-chemical reaction. Nevertheless for this higher potential value, no major corrosion damage can be seen in the specimen in the early stage of testing. A 25 mm cover depth was found to be an approximate threshold for chloride penetration in this specific mix of SHCC material. In addition to corrosion potential and rate readings, actual corrosion-induced pitting depth and area, mass loss and loss of tensile resistance were measured after removal of the steel from the specimens at the end of the tests. Based on the detailed experimental results obtained from this research work, empirical formulas are proposed to predict the corroded depths and loss of steel force due to pitting and mass loss. A number of recommendations are made for the corrosion rate measurement methodology 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 durability tests performed in SHCC. In the final conclusions, some approaches are suggested for future studies on durability of SHCC, which could help researchers in increasing their knowledge of SHCC properties and which may lead to the optimal use of SHCC in a sustainable way. The use of SHCC may be feasible in the protection of concrete structures from severe chlorideinduced corrosion or in delaying such corrosion.
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