Phil Purnell

A static fatigue model for the durability of glass fibre reinforced cement

This paper presents a model for predicting service lives for glass-fibre reinforced cement (grc) components using hot-water accelerated ageing. It improves on previous models, being derived from consideration of a specific proposed micro-mechanical strength loss mechanism based on static fatigue principles and can be applied from time = 0. The model fitted well to all available strength vs. time data pertaining to various grc formulations. The activation energies thus derived for the strength loss process (80–90 kJ mol−1) were consistent with those derived previously and those proposed for general glass dissolution mechanisms. Updated acceleration factors for predicting service lives for grc are advanced. The model was also applied to grc made with modified cement matrices. For metakaolin modified matrices, the activation energy appeared similar to that for OPC-grc, thus the use of similar acceleration factors appears justified. There is some evidence that calcium sulphoaluminate modified grc degrades according to a different activation energy. More data are required for modified matrix grcs if the model is to be applied thereto with confidence.

Preliminary investigations into the supercritical carbonation of cement pastes

Interactions between supercritical carbon dioxide (scCO2) and hydrated cement pastes, of various water/cement ratio, have been investigated. The carbonation process was greatly accelerated in the scCO2 compared to that in natural or CO2 enriched environments. The nature of the reactions was dependent on the amount of water present in the paste. Thus carbonation of samples dried prior to treatment resulted in the reaction of all the unhydrated C3S and C2S, but little conversion of calcium hydroxide to calcium carbonate. In contrast, carbonation of samples containing moisture resulted in the conversion of most of the calcium hydroxide whilst the amounts of C3S and C2S reacted increased as the water/cement ratio increased. During the carbonation treatment, the pore structure of the cement pastes was altered and substantial reductions in porosity were achieved. The process may be used to improve the durability of glass fibre reinforced cement by lowering the alkalinity and calcium hydroxide content of the matrix.

Accelerated ageing characteristics of glass-fibre reinforced cement made with new cementitious matrices

Glass-fibre reinforced cement composites have been made using new types of cement matrices and subjected to hot water immersion ageing regimes. The matrices were characterised by pore solution expression and analysis, XRD and DTA/TGA. Composite specimens were subjected to direct tensile tests. The new matrices displayed substantially reduced pore solution alkalinity and calcium hydroxide precipitation when compared with ordinary Portland cement (OPC), thus providing a less aggressive environment for the fibres. Mechanical tests showed that the rate of degradation of composites made from the new matrices, when aged at 65°C in water, was an order of magnitude lower than that for OPC matrix composites. The validity of this type of accelerated test is discussed.

Microstructural observations in new matrix glass fibre reinforced cement

Glass fibre reinforced cement (GRC) durability is generally thought to be governed by weakening of the reinforcement by the alkaline matrix and/or precipitation of hydration products, particularly calcium hydroxide (CH), within and around the strands. Previous microstructural investigations of GRC have exclusively used scanning electron microscopy (SEM). This paper is concerned with the application of thin-section petrography (TSP) to GRC. TSP allows identification of CH and other phases associated with the strands and unlike SEM, sample preparation causes minimal interfacial disturbance. Three matrices were studied; OPC, OPC plus metakaolin and OPC plus calcium sulphoaluminate cement. It was found that the degree of composite degradation was unrelated to the amount of CH or other hydration products precipitated within the strands or at the interface. No significant loss of fibre section was observed in degraded composites. It is postulated that gradual enlargement of pre-existing fibre flaws causes strength loss in GRC.

Managing Critical Materials with a Technology-Specific Stocks and Flows Model

The transition to low carbon infrastructure systems required to meet climate change mitigation targets will involve an unprecedented roll-out of technologies reliant upon materials not previously widespread in infrastructure. Many of these materials (including lithium and rare earth metals) are at risk of supply disruption. To ensure the future sustainability and resilience of infrastructure, circular economy policies must be crafted to manage these critical materials effectively. These policies can only be effective if supported by an understanding of the material demands of infrastructure transition and what reuse and recycling options are possible given the future availability of end-of-life stocks. This Article presents a novel, enhanced stocks and flows model for the dynamic assessment of material demands resulting from infrastructure transitions. By including a hierarchical, nested description of infrastructure technologies, their components, and the materials they contain, this model can be used to quantify the effectiveness of recovery at both a technology remanufacturing and reuse level and a material recycling level. The model’s potential is demonstrated on a case study on the roll-out of electric vehicles in the UK forecast by UK Department of Energy and Climate Change scenarios. The results suggest policy action should be taken to ensure Li-ion battery recycling infrastructure is in place by 2025 and NdFeB motor magnets should be designed for reuse. This could result in a reduction in primary demand for lithium of 40% and neodymium of 70%.

Material nature versus structural nurture: the embodied carbon of fundamental structural elements.

The construction industry is under considerable legislative pressure to reduce its CO(2) emissions. The current focus is on operational CO(2) emissions, but as these are compulsorily reduced, the embodied CO(2) of structural components, overwhelmingly attributable to the material from which they are manufactured, will become of greater interest. Choice of structural materials for minimal embodied CO(2) is currently based either on subjective narrative arguments, or values of embodied CO(2) per unit volume or mass. Here we show that such arguments are invalid. We found that structural design parameters (dimensions, section choice, and load capacity) for fundamental structural components (simple beams and columns) are at least as important as material choice with regard to their effect on embodied CO(2) per unit load capacity per unit dimension, which can vary over several decades within and between material choices. This result demonstrates that relying on apparently objective analyses based on embodied CO(2) per unit volume or mass will not lead to minimum carbon solutions; a formal definition of the correct functional unit for embodied CO(2) must be used. In short, there is no such thing as a green structural material.

The application of time-frequency analysis to the air-coupled ultrasonic testing of concrete

Air-coupled ultrasound has been used for the nondestructive evaluation of concrete, using broad bandwidth electrostatic transducers and chirp excitation. This paper investigates the benefits of using time-frequency analysis in such situations, for both waveform retrieval and imaging in the presence of low signal levels. The use of the short-term Fourier transform, the wavelet transform, and the Wigner-Ville distribution all are considered, in which accurate tracking of the ultrasonic chirp signals is demonstrated. The Hough transform then is applied as a filter. An image of a steel reinforcement bar in concrete has been produced to illustrate this approach.

3D Printing of cement composites

Abstract Abstract The aims of this study were to investigate the feasibility of generating three-dimensional (3D) structures directly in rapid hardening Portland cement (RHPC) using 3D printing (3DP) technology. 3DP is an additive layer manufacturing (ALM) process that generates parts directly from CAD in a layer-wise manner. Three-dimensional structures were successfully printed using a polyvinylalcohol/RHPC ratio of 3:97 w/w, with print resolutions of better than 1 mm. The test components demonstrated the manufacture of features, including off-axis holes, overhangs/undercuts, etc. that would not be manufacturable using simple mould tools. Samples hardened by 1 day post-build immersion in water at RT offered modulus of rupture (MOR) values of up to 0·8±0·1 MPa, and, after 26 days immersion in water at RT, offered MOR values of 2·2±0·2 MPa, similar to bassanite based materials more typically used in 3DP (1-3 MPa). Post-curing by water immersion restructured the structure, removing the layering typical of ALM processes, and infilling porosity.

Super-critical carbonation of glass-fibre reinforced cement. Part 1: mechanical testing and chemical analysis

Abstract The deterioration of glass-fibre reinforced cement (GRC) arises to a substantial extent from the alkalinity and calcium hydroxide content of the matrix. Carbonation of the matrix significantly lowers both factors, but under normal circumstances the reaction proceeds too slowly to be of practical use in improving durability. If carbonation is effected using supercritical carbon dioxide the reaction can be completed within hours rather than years, rendering it potentially attractive as a treatment for enhancing the durability of GRC. The efficacy of such treatment is dependent on the moisture content of the samples prior to treatment. GRC samples thus treated, with various moisture contents, were mechanically tested before and after a period of accelerated ageing. The supercritical carbonation treatment significantly increased the design strength and toughness of the GRC and greatly increased the fibre–matrix bond. Retention of toughness and degree of carbonation were both correlated with pre-treatment moisture content while initial property enhancements were not.

Interpretation of climatic temperature variations for accelerated ageing models

Using materials in new applications (in particular the built environment) requires that durability predictions be made. Often, the only way this can be done within economically favourable timescales is to use accelerated ageing tests. Extrapolating these tests to in-service temperatures requires that some average of the climatic temperature be used. The normally published mean temperature for a location does not in general adequately represent the time-temperature envelope and its use in calculations based on Arrhenius type relationships may lead to serious underestimation of degradation rates. This paper shows how an equivalent temperature, T*, can be calculated from the numerical integration of a single annual temperature cycle, accounting for both daily and monthly temperature variations. The difference between the average temperature and T* is shown to be strongly correlated to the daily and monthly temperature ranges, to be significant for most locations and sometimes as high as 10°C.