Pablo J. Prado

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.

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.

Spatially resolved relaxometry and pore size distribution by single-point MRI methods: porous media calorimetry

A single-point magnetic resonance imaging study of water freezing in saturated and non-saturated porous materials is presented. Relevant considerations about relaxation time mapping in short relaxation time systems ( of tens to hundreds of microseconds) are discussed and results in concrete and mortar samples under cooling conditions are shown. A spatially resolved estimation of the occupied pore size distribution is achieved in specimens presenting short relaxation times. To the best of our knowledge, this is the first report on the effective determination of spatially resolved pore size distribution of intact concrete materials. Evolution of one-dimensional images as a function of temperature is qualitatively compared with a series of differential scanning calorimetry experiments for cement pastes conditioned under controlled humidity. No supercooling effect is observed in a water saturated concrete single-point imaging experiment.

Concrete thawing studied by single-point ramped imaging

A series of two-dimensional images of proton distribution in a hardened concrete sample has been obtained during the thawing process (from -50 degrees C up to 11 degrees C). The SPRITE sequence is optimal for this study given the characteristic short relaxation times of water in this porous media (T2* < 200 micros and T1 < 3.6 ms). The relaxation parameters of the sample were determined in order to optimize the time efficiency of the sequence, permitting a 4-scan 64 x 64 acquisition in under 3 min. The image acquisition is fast on the time scale of the temperature evolution of the specimen. The frozen water distribution is quantified through a position based study of the image contrast. A multiple point acquisition method is presented and the signal sensitivity improvement is discussed.

Concrete/mortar water phase transition studied by single-point MRI methods

A series of magnetic resonance imaging (MRI) water density and T2* profiles in hardened concrete and mortar samples has been obtained during freezing conditions (-50 degrees C < T < 11 degrees C). The single-point ramped imaging with T1 enhancement (SPRITE) sequence is optimal for this study given the characteristic short relaxation times of water in this porous media (T2* < 200 microseconds and T1 < 3.6 ms). The frozen and evaporable water distribution was quantified through a position based study of the profile magnitude. Submillimetric resolution of proton-density and T2*-relaxation parameters as a function of temperature has been achieved.

Uniaxial pressure effects in the cubic phase of

Chlorine nuclear quadrupole resonance (NQR) measurements of the effect of the application of uniaxial pressure applied along the (100) and (111) directions in single crystals of are reported at 78.0 K. The free induction decay times decrease with pressure due to the additional strains introduced. The corresponding frequency domain spectra indicate that the inhomogeneous broadening of the NQR signals is dominated by point defects, but that the nature, number and/or distribution of these defects is different in the two crystals. The spin - spin relaxation times are dependent on the crystal orientation and are independent of pressure. The measured -results agree with calculated values. Spin - lattice relaxation data further illustrate the difference between the two crystals. It is described by a single exponential for the (111) crystal, but by a double exponential for the (100) crystal. The latter behaviour indicates the presence of dynamic clusters some 30 K above the temperature of the phase transition. The application of pressure is seen to hinder their formation.

Electronic population effect in halogen nuclear spin - lattice relaxation in praseodymium trihalide compounds

Nuclear quadrupole resonance determinations of the spin - lattice relaxation rates of the , and nuclei in the praseodymium trihalide crystals and are reported. Data are presented in the temperature range 7 - 297 K. They are shown to be dominated by magnetic dipole interactions between the halogen nuclear spins and the paramagnetic ions. The relaxation rates display unusual temperature dependences. This behaviour is a consequence of the crystalline electric field splitting of the ground-state multiplet of the configuration into six states, and the resultant depopulation of the ground paramagnetic state as the temperature is raised. The qualitatively different temperature variation for the two compounds is a result of the different energy splittings of the multiplet levels in the two instances. The analysis utilizes an electron-spin correlation time described by a temperature-independent term, the characteristic time for spin exchange between neighbouring ions, plus a temperature-dependent exponential term.