Theodore W. Bremner

Magnetic Resonance Imaging and Moisture Content Profiles of Drying Concrete

The spatial distribution of moisture in concrete, along with the role this moisture plays in various modes of deterioration, is of fundamental importance to the understanding of concrete behaviour. In this paper a new magnetic resonance imaging technique is utilized for the first time to obtain drying profiles of concrete with sub-millimetre resolution. This technique permits observation of the drying mechanisms, as well as the effects of water-cement ratio and moist curing time on drying behaviour.

Spatial resolution enhancement of a Brillouin-distributed sensor using a novel signal processing method

It is known that the ultimate spatial resolution for a Brillouin-based sensor is limited by the lifetime of the phonons in the fiber that mediate the Brillouin loss process. At optical pulse widths less than 10 ns (corresponding to one meter spatial resolution) the Brillouin line width is considerably broadened, causing a severe penalty in resolving the Brillouin frequency shift. Around 5 ns the Brillouin line width is too broad to allow an accurate frequency determination. The fiber optics group at the University of New Brunswick, Canada, has recently developed an automated system for strain measurements in a distributed sensing system that uses a novel signal processing technique to measure strain at resolutions finer than the Brillouin line width limit. Strain has been resolved to 20 /spl mu//spl epsiv/ at 500 mm and to 40 /spl mu//spl epsiv/ at 250 mm.

Moisture transport in initially fully saturated concrete during drying

Moisture content changes during drying were investigated in the present work. Particular emphasis was placed on the initial stage of drying of saturated concrete, where moisture contents are high. For this stage of drying, experimental data are lacking, and no comprehensive theory exists to describe it.The present investigation was performed experimentally and numerically for drying of cylinders with one exposed end, made of normal weight and lightweight concrete with varying water to cement ratio (w/c). The gravimetric technique was employed to obtain the spatial distribution of moisture content. The experimental results obtained indicate that drying of concrete becomes diffusion controlled when the average moisture content decreases below 70 to 80% of the initial saturation. Typical drying rates are in the order of magnitude of 0.18 kg/day/m2 and 0.02 kg/day/m2 for the first and the second stage of drying, respectively.The lightweight concrete cylinders as compared to those made of normal weight concrete exhibited higher levels of moisture content throughout the process. At high w/c ratios, the moisture profiles for both types of cylinders, as expected, show steeper changes with time. Large, constant drying rates were observed both experimentally and numerically in the beginning of the drying.The numerical model developed is based on a generalized mathematical formulation for mass and heat transfer in porous media, and its predictions are in agreement with the experimental data within the uncertainty range of the input data.

Accelerated testing of plain and epoxy-coated reinforcement in simulated seawater and chloride solutions

Abstract To simulate marine and bridge deck environments, plain and epoxy-coated reinforcing steel bars were cast in concrete slabs and exposed in the laboratory to synthetic seawater and 3% sodium chloride solution. They were monitored on a regular basis using the linear polarization and open-circuit potential techniques. The concrete slabs had a water to cement ratio of 0.60 and the steel reinforcement had 20 mm concrete cover. The epoxy-coated bars were with no damage, and 1% and 2% damage to the coating. A 2-year monitoring program indicated that the corrosion current density was negligible for epoxy-coated bars with no damage to the coating regardless of the exposure conditions and that undamaged epoxy-coated bars provided excellent performance in preventing corrosion activity in reinforced concrete structures subjected to a chloride environment. Similarly, rebar with damaged epoxy coating gave no evidence that sufficient rust had accumulated at the steel/concrete interface to cause the concrete cover to crack.

Structural monitoring by use of a Brillouin distributed sensor

The testing of a fiber-optic distributed-strain sensor attached to a simple structural member is reported. A Brillouin scattering-based sensor system was used to measure both tensile and compressive strains along the length of a cantilever beam subjected to various loads. The sensing fiber was attached to the beam in such a way that some sections experienced uniform strain, whereas others were subjected to a nonuniform strain distribution. A spatial resolution of 0.4 m was used, and a measurement precision of approximately +/-50 microepsilon was achieved.

Strain measurement in a concrete beam by use of the Brillouin-scattering-based distributed fiber sensor with single-mode fibers embedded in glass fiber reinforced polymer rods and bonded to steel reinforcing bars

The strain measurement of a 1.65-m reinforced concrete beam by use of a distributed fiber strain sensor with a 50-cm spatial resolution and 5-cm readout resolution is reported. The strain-measurement accuracy is +/-15 microepsilon (microm/m) according to the system calibration in the laboratory environment with non-uniform-distributed strain and +/-5 microepsilon with uniform strain distribution. The strain distribution has been measured for one-point and two-point loading patterns for optical fibers embedded in pultruded glass fiber reinforced polymer (GFRP) rods and those bonded to steel reinforcing bars. In the one-point loading case, the strain deviations are +/-7 and +/-15 microepsilon for fibers embedded in the GFRP rods and fibers bonded to steel reinforcing bars, respectively, whereas the strain deviation is +/-20 microepsilon for the two-point loading case.

Determination of chloride diffusion coefficient and gas permeability of concrete and their relationship

Chloride diffusion coefficient and gas permeability of concrete were experimentally determined. The relationship between them is discussed. Chloride diffusion coefficient was determined using saturated concrete by an accelerated electrical testing method. The chloride diffusion coefficient was found to be controlled by the water-to-cement ratio with about 2.2 times higher chloride diffusion coefficient for concrete with the water-to-cement ratio of 0.6 as compared with that of 0.4 water-to-cement ratio. Gas permeability was greatly influenced by the degree of saturation of the concrete. A correlation was found between chloride diffusion coefficient and gas permeability. Lightweight aggregate concrete was tested and compared with normal weight concrete with respect to chloride diffusion and gas permeability.

Natural marine exposure results for reinforced concrete slabs with corrosion inhibitors

Reinforced concrete slabs were cast with a concrete cover of 20 mm. The water-to-cement ratio was 0.40, and the concrete slabs were either uncracked or precracked. A simulated crack 0.2 or 0.4 mm wide was formed transverse to the axis of the reinforcing bar during the casting. Two types of commercial corrosion inhibitors were added to concrete mixtures for corrosion protection. Slabs were placed about 1 m below high tide at the Treat Island, Maine, USA, natural marine exposure site. The specimens were visually inspected and the corrosion rates were measured annually using the linear polarization technique. Some of the concrete slabs were broken open after 12 months of exposure and corrosion damage was evaluated. Water-soluble chloride content analysis was performed at this time. After three years of exposure, it was found, that both corrosion inhibitors were effective in reducing the corrosion rate for uncracked concrete slabs, but relatively ineffective in preventing localised corrosion of reinforcing steel in the crack area for precracked concrete slabs.

The influence of shrinkage-cracking on the drying behaviour of White Portland cement using Single-Point Imaging (SPI)

The removal of water from pores in hardened cement paste smaller than 50 nm results in cracking of the cement matrix due to the tensile stresses induced by drying shrinkage. Cracks in the matrix fundamentally alter the permeability of the material, and therefore directly affect the drying behaviour. Using Single-Point Imaging (SPI), we obtain one-dimensional moisture profiles of hydrated White Portland cement cylinders as a function of drying time. The drying behaviour of White Portland cement, is distinctly different from the drying behaviour of related concrete materials containing aggregates.

Concrete Freeze/Thaw as Studied by Magnetic Resonance Imaging

Abstract A recently developed magnetic resonance imaging (MRI) technique is used to study the concrete freeze/thaw process. Ice formation is spatially resolved in a nondestructive manner as changes in the MRI signal intensity are observed. The phase transition temperatures are in agreement with published differential scanning calorimetry thermograms. The concrete samples were air dried for varying times. The imaging of both saturated and non-saturated specimens demonstrates the ability to monitor non-adsorbed water in a range of pore sizes. The freeze/thaw thermodynamic behaviour was found to depend on water content and sample history.