In this exclusive extract from Sustainable concrete architecture by David Bennett, we describe general approaches to embodied carbon dioxide in building materials, and the dynamic thermal response of high thermal mass buildings.
Forty years ago, ground granulated blastfurnace slag (GGBS) was virtually unheard of in the UK construction industry. It was only available from a single source at Scunthorpe, with production and sales reaching a meagre 25,000 tonnes per year. Today, the situation is very different and now nearly 50 per cent of all ready-mixed concrete supplied to sites contains GGBS. GGBS is readily available throughout Great Britain.
Pulverised fuel ash (PFA), produced from coalburning power stations, has also been used in concrete for many years. The basic properties of PFA are pozzolanic; that is, it reacts with lime to form a hardened paste. In modern times, the use of PFA in concrete was pioneered in the 1930s in the USA. PFA was first used in the UK in the 1940s, in dam construction in Scotland, following successful research work carried out by the University of Glasgow. The variability of fly ash in terms of its fineness, carbon content and lack of quality control proved problematic until the 1970s, so its uptake was spasmodic. PFA is now used in about 20 per cent of readymixed concrete consumed in the UK, where it is used as binder at 30 per cent replacement. Currently, around 500,000 tonnes of PFA per annum is used in ready-mixed and precast concrete production.
This expanding use of both PFA and GGBS in concrete has been driven by their low cost, reduced early-age temperature rise and greater resistance to alkali–silica reaction, chloride ingress and sulphate attack; and, more recently, by the significant reduction in embodied CO2 they offer.
There are reserves of both PFA and GGBS to satisfy future demand for cement replacements, but there is always the opportunity to develop new cements with low embodied CO2 using other waste materials and non-carbonate materials. The good news is that there is considerable optimism that more cements with low CO2 emissions, and even carbon-neutral concretes, will be available in the next few years. For example, ConGlassCrete, which contains recycled crushed glass – beer bottles, wine bottles, car windscreens and windows – has proven its effectiveness as a cement replacement through exhaustive and rigorous testing. RockTron, a new cement company, recovers PFA from landfill sites at a coalburning power station. Its product has just come onto the market. Pioneering new cements with great promise, based on magnesium oxide or geopolymers extracted from oil-based residues, all of which are low embodied CO2 cements, may become commercially viable in the next few years, making ‘green’ concrete very competitive on price.
The dynamic thermal response of high thermal mass buildings with exposed concrete is characterised by a slow response to changes in ambient conditions and the ability to reduce peak temperatures. This is particularly beneficial during the summer, when the concrete absorbs internal heat gains during the day, helping to prevent overheating. In addition to reducing peak internal temperatures, a high thermal mass building can also delay the temperature peak by up to six hours. In an office environment this will typically occur in the late afternoon or in the evening, after the occupants have left, when solar gains are greatly diminished and little heat is generated by occupants, equipment and lighting.
During the evening, the external air temperature drops, making night ventilation an effective means of removing accumulated heat from the concrete and so lowering the temperature in preparation for the next day. The UK variation in diurnal temperature rarely drops below 5 °C, making night cooling relatively effective.
Water can also be used to cool the slabs as an alternative, or addition, to night ventilation. Concrete's ability to absorb heat and provide a cooling effect is determined by the difference between its surface temperature and the temperature of the internal air. Consequently, the greatest cooling capacity is provided when the internal temperature peaks. Therefore, to some extent a variable internal temperature is a prerequisite in fabric energy storage (FES) systems. However, to maintain comfortable conditions and limit overheating, peak temperatures should ideally not exceed 25 °C for more than 5 per cent of the occupied period and 28 °C for not more than 1%.
Resultant temperature is an important measure of FES. It takes account of radiant and air temperatures, providing a more accurate indication of comfort than air temperature alone. The relatively stable radiant temperature provided by the thermal mass in concrete is a significant factor in maintaining comfortable conditions. It enables higher air temperatures to be tolerated than in lighter-weight buildings, which are subject to higher radiant temperatures resulting from warmer internal surfaces. Thermal mass, in the most general sense, describes the ability of any material to store heat. For a material to provide a useful level of thermal mass, a combination of three basic properties is required:
- High specific heat capacity – to maximise the heat that can be stored per kg of material
- High density – to maximise the overall mass of the material used
- Moderate thermal conductivity – so that heat conduction is roughly in synchronisation with the diurnal heat flow in and out of the building.
Heavyweight construction materials such as concrete, brick and stone all have these properties. They combine a high storage capacity with moderate thermal conductivity. This means that heat transfers between the material's surface and the interior at a rate that matches the daily heating and cooling cycle of buildings. Some materials, such as wood, have a high heat capacity, but their thermal conductivity is low, which limits the rate at which heat is absorbed. Steel can also store a lot of heat but, in contrast to wood, steel has a very high rate of thermal conductivity, which means heat is absorbed and released too quickly to create the lag effect required for the diurnal temperature cycle in buildings.
About this article
Based on real-world evidence and independent research, Sustainable Concrete Architecture provides designers with a tool to help calculate the total embodied CO2 in their building designs. Highly illustrated and detailed in scope, the book combines comprehensive technical analyses of concrete materials with useful case studies demonstrating the value of the material in low-energy, green building, and and is available to buy from RIBA Bookshops .