P.E. Grattan-Bellew

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.

MICROSTRUCTURAL INVESTIGATION OF DETERIORATED PORTLAND CEMENT CONCRETES

Abstract The properties of materials are largely determined by their microstructure. Evaluation of the microstructure of deteriorated concrete provides an invaluable, and in many instances, the only method of determining the cause of deterioration. It is important to determine the cause of deterioration as otherwise inappropriate repair procedures, leading to poor durability of the repair, may be employed. Determining the cause of deterioration is also important because this information can be used in preparing new standards to improve the durability in future construction and/or techniques. Although concrete may appear to be a hard, impregnable, material, it is in fact, a porous material, containing relatively soluble components such as calcium hydroxide which are readily leached in concrete exposed to aggressive solutions. Thus, exposure to soft water, sea water, brine and even weak organic and inorganic acids results in dissolution of a portion of the binder which holds the concrete together. The porosity and permeability of concrete also make it susceptible to frost damage. The cement paste binder in concrete is composed of hydrated calcium aluminium silicates which dehydrate on exposure to high temperatures. This makes concrete susceptible to deterioration when exposed to fire. Sometimes, even though the concrete is essentially durable, it may crack due to expansion caused by corrosion of steel reinforcement. In other instances the concrete may contain within it the seeds of its own destruction, for example, when the concrete is made with potentially alkali reactive aggregates and contains a high concentration of alkali in the pore solution. The purpose of this paper is to illustrate some typical changes to the microstructure of concrete due to exposure to various aggressive environments, and to describe the methodology used in the investigation of deteriorated concrete. Due to space limitations, it is not possible to describe deterioration due to all possible causes. Instead, some typical examples are selected for illustration.

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.

Effect of aggregate particle size and composition on expansion of mortar bars due to delayed ettringite formation

The effect of aggregate composition and particle size on the expansion of mortar bars, due to delayed ettringite formation (DEF), was evaluated by heat-curing mortar bars made with basalt, dolostone, granite, limestone, siliceous limestone, and pure crystalline quartz. Subsequent to heat curing, the mortar bars were subjected to 3 thermal cycles to accelerate the formation of DEF. They were then stored in lime water for 59 days and length change was monitored. Only mortar bars made with quartz aggregate showed significant expansion. Expansion was found to be inversely proportional to the particle size of the quartz aggregate. The amount of ettringite formed in the mortar bars was shown to be proportional to their rate of expansion.

A study of the microstructure and hydration characteristics of tricalcium silicate in the presence of calcium chloride

The morphological and hydration characteristics of tricalcium silicate treated with 0, 2 and 5 per cent calcium chloride were followed by employing Scanning Electron Microscopy (SEM), Differential Thermal Analysis (DTA), and X-ray diffraction techniques. The water: solid ratios used were either 0.5 or 0.3. In terms of Ca(OH)2 estimation CaCl2 accelerated hydration, except with 5 per cent CaCl2 at a 0.3 w/s ratio. The maximum amount of Ca(OH)2 and calcium silicate hydrate was formed at 2 per cent CaCl2 and a w/s ratio of 0.5. Except at very early times, the rate of reaction was slower at 0.3 w/s with or without CaCl2 compared with the corresponding samples at 0.5 w/s.

Effect of phlogopite mica on alkali-aggregate expansion in concrete

The tensile strength of concrete is increased by the addition of high aspect-ratio mica reinforcement. Phlogopite mica contains about 10% K2O; soluble K+ would contribute to the alkalinity of the pore solution in concrete enhancing its potential expansivity if it is made with alkali-expansive aggregate. Electron microprobe analysis confirms that the solubility of K+ from phlogopite in cement paste, about 0.09% is the same as in mixed calcium-sodium-hydroxide solutions. The addition of 1.5% by volume of phlogopite to concrete prisms made with alkali-carbonate expansive aggregates caused a small increase in the rate of expansion of prisms made with both high and low alkali cement. It is concluded that the addition of small quantities of phlogopite as a reinforcement in concrete is not a cause for concern, although caution should be exercised in adding larger volumes to concrete made with potentially alkali-expensive aggregate.

Spatially resolved pore-size distribution of drying concrete with magnetic resonance imaging

A study of the spatially resolved water-occupied pore size distribution in a drying concrete cylinder is reported. Pore sizes are obtained from freezing point depression of pore water for a temperature range of 0 to −40 °C, assuming that the freezing point is inversely proportional to pore diameter. Single-point magnetic resonance imaging techniques were used to monitor unfrozen water content as functions of position and temperature. It was observed that freezing began at −10 °C in the cylinder center, which had the highest moisture content, and with a further temperature decrease, the freezing region gradually spread to the exposed end surfaces. The central region had a broad water-occupied pore size distribution, with pore diameters as large as 10 nm. The occupied pore sizes became progressively smaller as the moisture content decreased in proximity to the exposed surfaces.

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.

Conversion of a waste mud into a pozzolanic material

Abstract This work investigated the conversion of a waste mud from alum production into a pozzolanic material. X-ray diffraction analysis indicated that the main components in the mud were kaolinite (Al 2 O 3 .2SiO 2 .2H 2 O), quartz (SiO 2 ) and titanium oxide (TiO 2 ). After calcination at 750 o C for 5 h, kaolinite in the mud converted into metakaolinite (Al 2 O 3 .2SiO 2 ), and quartz and titanium oxide remained unchanged. The calcined mud showed very high pozzolanic reactivity. The addition of chemical activators such as Na 2 SO 4 and CaCl 2 accelerated the pozzolanic reaction between lime and calcined mud and increased the strength of lime–calcined mud very significantly.