Phillip Frank Gower Banfill

Rheology of fresh cement and concrete

Cement-based materials are of enormous technological importance and their satisfactory performance depends on being able to transport and mould them in the freshly mixed state. This article describes the rheology of fresh cement, mortar, concrete and related products in the context of practical situations, and deals with testing and measurement, together with the main features of their behaviour. It explores the links between rheology and technology, and identifies areas where these are weak and could benefit from further experimental and computational effort.

On the viscometric examination of cement pastes

Abstract The use of flow curves and hysteresis loops in studying the rheology of cement pastes is examined critically. The shape of flow curves obtained when cement pastes are sheared in a coaxial cylinders viscometer has been studied. Hysteresis loops of three main types were found, depending on the length of time taken to complete a shear cycle. At short cycle times the downcurve is at lower stress levels than the upcurve (structural breakdown), while at long cycle times they are reversed (structural build up). At intermediate cycle times the lines cross, and double or treble loops may be obtained. The variation of loop shape with cycle time is different for different cements.

RHEOLOGICAL METHODS FOR ASSESSING THE FLOW PROPERTIES OF MORTAR AND RELATED MATERIALS

Abstract The flow properties of mortar conform to the Bingham model and mortars require two parameters, the yield stress and plastic viscosity, to be measured in order to make an unambiguous definition of their rheology. This paper describes the use of commercially available instruments to determine these two parameters. Yield stress and plastic viscosity are affected by the nature and relative proportions of cement, sand, water and admixtures in complex ways which are, however, broadly similar to the effects seen in fresh concrete. As a result the instruments are potentially useful in quality control, selection of raw materials, formulation of mixes and assessment of the suitability of mixes for particular applications.

Statistical techniques to emulate dynamic building simulations for overheating analyses in future probabilistic climates

As projections of climate change become more detailed and sophisticated, analysing the effects of these projections on, for example, building performance will become more complex. This study, as part of the Low Carbon Futures project, proposes a method for integrating the latest UK Climate Projections 2009, which are probabilistic in nature, into dynamic building simulation calculations. This methodology offers the possibility that, in an analysis of overheating in buildings, it will be viable for a building designer to assess future thermal comfort of a building in a probabilistic way, with various climate scenarios informing a risk analysis of whether that building will become unsuitable as a working/living environment. To reduce the computational requirements of such an analysis, a series of statistical manipulations and approximations are proposed that serve to reduce substantially the amount of computation that would otherwise be necessary when using such climate projections. The resulting tool, which in essence captures the behaviour of complex simulation models using linear filtering techniques and regression, is successfully validated against results obtained from building simulation software results for a domestic building case-study, including versions of the building with specific adaptation scenarios applied that might offset the predicted overheating.

Nitrite binding in cement

Pore solution has been pressed out of hardened cement pastes prepared with the addition of nitrite-containing corrosion inhibitors. The binding of nitrite was assessed by comparing the concentration in the mixing water with that in the pore solution in both the presence and absence of chlorides. The results indicate that a major controlling mechanism is the adsorption of nitrite and chloride ions on the surface of hydration products. However, there is clear competition between adsorption of nitrite and chloride on cement and as a result at the presence of chloride there is more free nitrite available to inhibit steel corrosion. Using calcium nitrite results in lower free nitrite and hydroxyl ion concentrations than when sodium nitrite is used because precipitation of calcium hydroxide is enhanced and therefore higher dosage rates are needed. All this information enables guidance for the satisfactory use of nitrite inhibitors to be produced.

RE-ALKALISATION OF CARBONATED CONCRETE - EFFECT ON CONCRETE PROPERTIES

Abstract Electrochemical re-alkalisation of concrete involves application of a direct current between an externally applied anode surrounded in a sodium carbonate electrolyte and the internal reinforcing steel. It offers a means of rehabilitating reinforced concrete in which carbonation has progressed so far that the steel is at risk of corrosion without the noise and disruption associated with mechanical patch repairs. However, potential users may be put off by the side effects of the treatment and this investigation aimed to provide information on the effects of re-alkalisation on physical and mechanical properties of concrete. Laboratory specimens exposed to high pressure carbonation were re-alkalised and the porous and strength properties compared with untreated concrete. Re-alkalised concrete is stronger, denser, less water absorbent and shows a tendency to lower levels of alkali silica expansion. Potential users may be reassured that there are no detrimental effects to the properties of the concrete as a result of electrochemical re-alkalisation.

The relationship between the sorption of organic compounds on cement and the retardation of hydration

Abstract The sorption of organic compounds from aqueous solution and from solutions in 1,4 - dioxan on to cement and related materials has been studied. The organic compounds were selected to cover a range of efficiencies in the retardation of cement hydration and the correlation between sorption and retardation have been related to theories of the retardation mechanism. The traditional theory that retarders sorb on cement surfaces and prevent the access of water for hydrolysis is erroneous. The results support the idea that nucleation and growth of calcium hydroxide is hindered by retarding admixtures.

SIMULTANEOUS RHEOLOGICAL AND KINETIC MEASUREMENTS ON CEMENT PASTES

Abstract The Strathclyde Rheometer has been modified to enable the oscillating probe to measure the viscoelastic properties of cement paste while it is hydrating in the sample holder of a conduction calorimeter. The resulting simulataneous curves showing changes in rheology and rate of heat evolution are potentially useful in giving information about the factors affecting stiffening in cementitious systems.

Tailoring a future overheating risk tool for existing building design practice in domestic and non-domestic sectors

The Low Carbon Futures Project, as part of the Adaptation and Resilience to Climate Change (ARCC) programme, has developed an overheating tool, based on probabilistic UK Climate Projections (UKCP09), to provide design advice for building adaptations in future. For dwellings this tool, initiated by a single simulation, relies on just hourly climate information to predict the internal temperature profile and for a non-domestic building, it includes internal activity profiles to account for lighting, equipment, metabolic gains and air change. To produce a tailored design tool, a qualitative investigation has been carried out to understand current building practices. This investigation shows that the two sectors take a significantly different approach to design, where dynamic building simulation is rare for domestic developments. The diversity of the non-domestic building stock poses different challenges and requires more detail to perform any overheating analysis, with dynamic building simulation playing a key role. The suitability of this tool, and the need to balance complexity and detail with usability and applicability, will be explored for the two sectors, with an approach for implementing this in the future proposed. Practical application: This paper compares current overheating analyses, as reported through correspondence with practitioners, with a suggested approach for a more detailed future overheating assessment using the latest climate projections. The required steps to bridge the gap between current and possible future design methods are explored for both the domestic and non-domestic sectors, with a prototype tool proposed that has been formulated with the aid of industry feedback. The described project is therefore able to translate complex building and climate science into an approach that is potentially useful for building practitioners.

Feasibility study of a coaxial cylinders viscometer for mortar

Abstract A coaxial cylinders viscometer for mortars containing particles up to 2 mm diameter has been constructed with the minimum of modification to the two-point workability apparatus for concrete. Mortars are characterised by the Bingham model with yield values and plastic viscosities intermediate between those of cement paste and fresh concrete. The variations in these two paramaters show similar trends to those observed in fresh concrete. Hysteresis loops in the flow curves were obtained which suggest that mortars are shear dependent. The apparatus is sufficiently sensitive to be useful in investigating the effects of properties and relative proportions of constituent materials and of different mixing procedures on the rheology of mortars.