Microstructural analysis of surface and interface zones in concrete

Several of OPC paste and concrete specimens, with different mix proportions, were cast against CPF and impermeable formwork (IF) and the profiles of pore structure, microhardness and scratch hardness of the cover zone were established. The chloride ingress and the depth of carbonation of the surface zone of concrete cast against CPF and IF were investigated. The main mechanisms controlling the ECR processes and the factors affecting such treatment were critically reviewed. Subsequently, as a means of restoring passivation of steel embedded in carbonated concrete, such HCP specimens were subjected to ECR. The influence of ECR on the chemistry of the pore solution and the microstructure of the surface and the steel/cement past interface zones were also studied. The main findings of this investigation were as follows: (a) The thickness of the microstructure gradient of cover concrete is significantly decreased with increasing period of water curing but is relatively unaffected by curing temperature, w/e ratio and the use of cement replacement materials. (b) The scratch hardness technique was shown to be potentially useful for characterising the microstructure and microhardness gradients of the surface zone. (c) A relationship between the microstructure gradient and mass transport properties of the surface zone was established. (d) The use of CPF resulted in a significant reduction in porosity of both the cement paste matrix and the aggregate/cement paste transition zone, and a marked improvement in the resistance of the surface zone to carbonation and the ingress of chloride ions. (e) The ECR treatment resulted in a marked densification of the pore structure and in changes to the pore solution chemistry and the cement phases of near-surface and steel/cement paste transition zones. This effect was more pronounced with current density, period of treatment and particularly with the use of sodium phosphate as an electrolyte.

Chloride resistance and durability of cement paste and concrete containing metakaolin

Metakaolin (MK), a calcined clay, was included as a partial cement replacement material, at up to 20% by weight of binder, in cement pastes and concrete, and its influence on the resistance to chloride ingress investigated. Reductions in effective chloride diffusion coefficients through hardened cement paste were obtained for binary blends and by combining OPC, MK and a second cement replacement material of pulverised fuel ash or ground granulated blast furnace slag. Steady state oxygen diffusion measurements through hardened cement pastes measured using an electrochemical cell showed that the interaction between charged species and the pore surfaces is a major factor in determining chloride diffusion rate. Rheology of the binder, particularly at high MK replacement levels, was found to have a dramatic influence on the diffusion performance of cement pastes. It was concluded that plasticising admixtures are essential for adequate dispersion of MK in cement pastes. Chloride concentration profile analysis of the concrete cylinders, exposed to sodium chloride solution for one year, was employed to obtain apparent chloride diffusion coefficients for concrete specimens. MK was found to reduce the depth of chloride penetration into concrete when compared with that of unblended mixes. Corrosion rate and corrosion potential measurements were taken on steel bars embedded in concrete exposed to a saline environment under conditions of cyclic wetting and drying. The initiation time for corrosion was found to be significantly longer for MK blended mixes than for plain OPC systems. The aggregate-paste interfacial zone of MK blended systems was investigated by steady state diffusion of chloride ions through mortar containing glass beads as model aggregate. For the model aggregate specimens tested the work confirmed the hypothesis that properties of the bulk paste are the controlling factors in ionic diffusion through mortar.

The durability of glass fibre reinforced cements made with new cementitious matrices

Widespread use of glass fibre reinforced cement (GRC) has been impeded by concerns over its durability. Three degradation mechanisms are proposed - fibre corrosion, Ca(OHh precipitation and matrix densification - although their relative importance is debated. Matrices with reduced alkalinities and Ca(OH)2 contents are being developed; the aim of this study was to investigate their hydration and interaction with alkali-resistant fibres to determine the factors controlling their long-term durability, and assess the relevancy of accelerated ageing. The matrices studied were: OPC/calcium-sulphoaluminate cement plus metakaolin (C); OPC plus metakaolin (M); blast-furnace slag cement plus a micro-silica based additive (D); and OPC (O). Accelerated ageing included hot water and cyclic regimes prior to tensile testing. Investigations included pore solution expression, XRD, DTA/TG, SEM and optical petrography. Bond strength was determined from crack spacings using microstructural parameters obtained from a unique image analysis technique. It was found that, for the new matrices - pore solution alkalinities were lower; Ca(OH)2 was absent or quickly consumed; different hydrates were formed at higher immersion temperatures; degradation under 65°C immersion was an order of magnitude slower, and no interfilamental Ca(OH)2 was observed .It was concluded that: fibre weakening caused by flaw growth was the primary degradation mechanism and was successfully modelled on stress corrosion/static fatigue principles. OPC inferiority was attributed partly to its higher alkalinity but chiefly to the growth of Ca(OH)2 aggravating the degradation; and hot water ageing although useful in model formulation and contrasting the matrices, changed the intrinsic nature of the composites rather than simply accelerating the degradation mechanisms.

Metakaolin as a cement extender

The effect of 10% and 20% replacement metakaolin on a number of aspects of hydration chemistry and service performance of ordinary Portland cement pastes has been investigated. The analysis of expressed pore solutions has revealed that metakaolin-blended specimen pastes possess enhanced chloride binding capacities and reduced pore solution pH values when compared with their unblended counterparts. The implications of the observed changes in pore solution chemistry with respect to chloride induced reinforcement corrosion and the reduction in expansion associated with the alkali aggregate reaction are discussed. Differential thermal analysis, mercury intrusion porosimetry, and nuclear magnetic resonance spectroscopy have been employed in the analysis of the solid phase. It is suggested that hydrated gehlenite (a product of pozzolanic reaction) is operative in the removal and solid state binding of chloride ions from the pore solution of metakaolin-blended pastes. Diffusion coefficients obtained in a non-steady state chloride ion diffusion investigation have indicated that cement pastes containing 10% and 20% replacement metakaolin exhibit superior resistance to the penetration of chloride ions in comparison with those of plain OPC of the same water:cement ratio. The chloride induced corrosion behaviour of cement paste samples, of water:cement ratio 0.4, containing 0% , 10%, and 20% replacement metakaolin, has been monitored using the linear polarization technique. No significant corrosion of embedded mild steel was observed over a 200 day period.

Electrochemical chloride extraction to halt corrosion of reinforcement

This thesis presents results of experiments designed to study the effect of applying electrochemical chloride extraction (ECE) to a range of different hardened cement pastes. Rectangular prism specimens of hydrated cement paste containing sodium chloride at different concentrations were subjected to electrolysis between the embedded steel cathodes and external anodes of activated titanium mesh. The cathodic current density used was in the range of 1 to 5 A/m2 with treatment periods of 4 to 12 weeks. After treatment, the specimens were cut into sections which were subjected to pore-solution expression and analysis in order to determine changes in the distribution of free and total ionic species. The effect of the ECE treatment on the physical and microstructural properties of the cements was studied by using microhardness and MIP techniques. XRD was employed to look at the possibility of ettringite redistribution as a result of the accumulation of soluble sulphate ions in the cement matrix near the cathode during ECE. Remigration of chloride which remains after the ECE treatment and distribution of other ions were studied by analysing specimens which had been stored for several months, after undergoing ECE treatment. The potentials of the steel cathodes were also monitored over the period to detect any changes in their corrosion state. The main findings of this research were as follows: 1, ECE, as applied in this investigation, was capable of removing both free and bound chloride. The removal process occurred relatively quickly and an equilibrium between free and bound chlorides in the specimens was maintained throughout. At the same time, alkali concentrations in the pore solution near the steel cathode increased. The soluble sulphate ionic concentration near the cathode also increased due to the local increase in the pH of the pore solution. 2, ECE caused some changes in physical and microstructural of the cement matrix. However these changes were minimal and in the case of microhardness, the results were highly scattered. Ettringite in the bulk material well away from the cathode was found not to increase significantly with the increase in charge passed.3, Remigration of chloride and other ionic species occurred slowly after cessation of ECE with a resultant gradual increase in the Cl-/OH- ratio around the steel.4, The removal of chloride from blended cements was slower than that from OPC.

Pore structure and diffusional properties of hardened cement pastes

This study has investigated the inclusion of pulverised fuel ash (PFA) and blast furnace slag (BFS) into hardened cement pastes (HCP) in retarding the ingress of chloride ions and oxygen molecules from the external environment. The influence of environmental factors such as drying and carbonation on the pore structure and diffusional properties of OPC, OPC/30%PFA and OPC/65%BFS hardened pastes was investigated. Specimens were desorbed from a saturated surface dry condition to a near constant weight at 65% relative humidity (RH) while others were simultaneously exposed to a 65% RH atmosphere and a carbon dioxide atmosphere of up to 5% by volume until there were fully carbonated. The presence of the interfacial zone at the cement paste-aggregate interface was critically reviewed and identified. The influence of the interfacial zone on porosity and chloride ingress in assumed periodic composites of glass bead mortars was also studied. The investigations have demonstrated the following: (a) The use of fly ash and slag in blended cement pastes has resulted in a marked reduction in capillary porosity and rate of chloride ingress. (b) The ratio of oxygen to chloride diffusion coefficients increased from values close to 1 in permeable pastes, to values of around 15 in low-permeability blended fly ash and slag pastes. This supports the view that the diffusion of chloride ions is retarded by the surface charge of the hydrated cement gel in low-permeability pastes. (c) Compared with plain OPC pastes, the carbonation of blended fly ash and slag pastes resulted in a marked increase in the coarse capillary porosity and a corresponding increase in the oxygen and chloride diffusion rates.

1.14Tb/s DP-QPSK WDM polarization-diverse optical phase conjugation

Optical phase conjugation (OPC) of a polarization-multiplexed comb of 10x114Gb/s DP-QPSK signals has been demonstrated for the first time, occupying a spectral bandwidth of >1THz (~9nm). The nonlinear element employed for the OPC was highly nonlinear fiber (HNLF) optimized for the suppression of stimulated Brillouin scattering (SBS) and configured in a bi-directional loop offering polarization diversity. Pump power (each way about the loop) and input signal power to the OPC subsystem were optimized at 29.7dBm and + 3dBm respectively producing a Q2 penalty of ≤0.9dB over all conjugate wavelengths, polarizations and output OSNR (up to 20dB).

Optical-phase conjugation nonlinearity compensation in Flexi-Grid optical networks

Optical-phase conjugation nonlinearity compensation (OPC-NLC) in optical networks is evaluated using a built-in tool including self-channel and crosstalk channel interference effects. Though significant improvements are observed, a further refined launch power policy is required to fully take advantage of OPC-NLC capability.

Transmission performance improvement using random DFB laser based Raman amplification and bidirectional second-order pumping

We demonstrate that a distributed Raman amplification scheme based on random distributed feedback (DFB) fiber laser enables bidirectional second-order Raman pumping without increasing relative intensity noise (RIN) of the signal. This extends the reach of 10 × 116 Gb/s DP-QPSK WDM transmission up to 7915 km, compared with conventional Raman amplification schemes. Moreover, this scheme gives the longest maximum transmission distance among all the Raman amplification schemes presented in this paper, whilst maintaining relatively uniform and symmetric signal power distribution, and is also adjustable in order to be highly compatible with different nonlinearity compensation techniques, including mid-link optical phase conjugation (OPC) and nonlinear Fourier transform (NFT).

Signal power asymmetry optimisation for optical phase conjugation using random DFB laser Raman amplification

We numerically optimise in-span signal power asymmetry in advanced Raman amplification schemes, reaching 3% over 62 km SMF, and evaluate its impact on the performance of systems using mid-link OPC using 7 × 15 16QAM Nyquist-spaced WDM-PDM. © 2015 OSA.

Signal power asymmetry optimisation for optical phase conjugation using Raman amplification

We numerically optimise in-span signal power asymmetry in different advanced Raman amplification schemes, achieving a 3% asymmetry over 62 km SMF using random DFB Raman laser amplifier. We then evaluate the impact of such asymmetry on the performance of systems using mid-link OPC by simulating transmission of 7 x 15 Gbaud 16QAM Nyquist-spaced WDM-PDM signals. (C) 2015 Optical Society of America

Raman fibre laser based amplification in coherent transmission systems

The thesis presents a detailed study of different Raman fibre laser (RFL) based amplification techniques and their applications in long-haul/unrepeatered coherent transmission systems. RFL based amplifications techniques were characterised from different aspects, including signal/noise power distributions, relative intensity noise (RIN), mode structures of induced Raman fibre lasers, and so on. It was found for the first time that RFL based amplification techniques could be divided into three categories in terms of the fibre laser regime, which were Fabry-Perot fibre laser with two FBGs, weak Fabry-Perot fibre laser with one FBG and very low reflection near the input, and random distributed feedback (DFB) fibre laser with one FBG. It was also found that lowering the reflection near the input could mitigate the RIN of the signal significantly, thanks to the reduced efficiency of the Stokes shift from the FW-propagated pump. In order to evaluate the transmission performance, different RFL based amplifiers were evaluated and optimised in long-haul coherent transmission systems. The results showed that Fabry-Perot fibre laser based amplifier with two FBGs gave >4.15 dB Q factor penalty using symmetrical bidirectional pumping, as the RIN of the signal was increased significantly. However, random distributed feedback fibre laser based amplifier with one FBG could mitigate the RIN of the signal, which enabled the use of bidirectional second order pumping and consequently give the best transmission performance up to 7915 km. Furthermore, using random DFB fibre laser based amplifier was proved to be effective to combat the nonlinear impairment, and the maximum reach was enhanced by >28% in mid-link single/dual band optical phase conjugator (OPC) transmission systems. In addition, unrepeatered transmission over >350 km fibre length using RFL based amplification technique were presented experimentally using DP-QPSK and DP-16QAM transmitter.