A concrete plan
In a world of finite resources, efficiency is important for ready-mixed concrete. Chris Clear, of the Mineral Products Association, takes a look at the recently formulated Resource Efficiency Action Plan for this construction material.
Ready-mixed concrete is a factory-produced construction material, locally supplied and transported as a flowable material ready for placing and compacting into any desired shape and size on construction sites. As the exact amount required can be ordered and delivered, ready-mixed concrete is a naturally resource-efficient construction material. However, there are a number of issues requiring attention, which led to the development of the Resource Efficiency Action Plan (REAP).
The plan was developed with the input of a stakeholder group consisting of a range of professionals drawn from the supply chain, including material suppliers, ready-mixed concrete producers, logistics suppliers, designers and main contractors, as well as demolition and recycling contractors.
Concrete’s inherent durability has given it an historic advantage and it is difficult to envisage any type of permanent construction works that does not include a significant proportion of concrete. Currently, concrete produces very little waste from the point of manufacture to the point of delivery – only 0.44kg per tonne of ready-mixed concrete delivered. In addition, because of the extensive network of batching and mixing plants, the average delivery distance is only 12km.
Of course, this is too simplistic a view. Like all construction materials, ready-mixed concrete is not used in isolation – it forms only part of the construction works. Its entire life cycle, along with its partner materials, requires assessment of resource efficiency on a scientific basis. The basis for this was established within a framework set out by a European Standard. The factors affecting the resource efficiency of ready-mixed concrete go well beyond the percentage of waste and delivery distance, and a number of the major aspects are reviewed.
Recipe for success
The essential ingredients for concrete are:
- coarse aggregates, such as river or marine gravels, crushed rocks
- fine aggregates, such as river or marine sand, crushed rock fines
- cement, such as Portland cement, or combinations of Portland cement with additions such as a ground granulated blastfurnace slag (GGBS) or flyash
- chemical admixtures, such as water reducing or plasticising admixtures
A typical concrete comprises one part cement, two parts sand, four parts course aggregate, with a free water/cement ratio of around 0.55. In terms of proportions by mass per m3, this is 1,900kg of coarse and fine aggregate, 310kg of cement and 170kg of water. Up to three litres of chemical admixture is generally used to reduce water content by up to 20 litres or increase workability with a lower water saving. As nearly 80% of concrete’s mass is attributable to the aggregates, it is important that these are sourced as close as possible to the ready-mixed concrete plant to minimise transportation impacts.
The ready-mixed concrete industry is accustomed to ensuring its mixing and batching facilities are based within an aggregate quarry, wharf or rail depot, wherever possible. Proximity to raw materials has to be balanced with the proximity of the potential market, and for large metropolitan areas it is inevitable that materials have to be imported from outside the region. For this reason, marine aggregates are often supplied into those metropolitan areas where dredgers can discharge directly, such as London, Southampton, Bristol, Liverpool and Hull.
In terms of embodied carbon, it is the cement that carries most of the impact. For UK cement, the average impact is about 850kg of CO2 emitted (CO2e) per tonne, which can be reduced if larger than average amounts of GGBS or flyash are incorporated into the cement. The figure for aggregates is 3–10kg of CO2e per tonne and for water no more than 1kg CO2e per m3. For the typical cubic metre of concrete, the embodied carbon impact attributable to the materials is around 275kg CO2e, where 95% is attributable to the cement. It is easy to see that, for resource-efficient concrete, the ready-mixed concrete supplier will minimise the cement content required to achieve the specified fresh and hardened properties with the required factor of safety.
Typically, concrete density is 2,380kg/m3 and, as the weight of a goods vehicle with four axles is limited to 32 tonnes on the UK road network, the volume that can be carried by a standard ready-mixed concrete truck is no more than 8m3. Larger articulated ready-mixed concrete trucks with five axles that can carry up to 12m3 are available but are rarely commissioned, because of the congested nature of most urban construction sites in the UK, where access is limited.
Part of the delivery process is to ensure that the delivery chute and rear of the truck is clean and free of any particles of aggregate that could drop off when the vehicle returns to its batching plant. This necessarily means that the chute has to be washed down with a minimum of water, which is normally carried out on the construction site as instructed by the personnel. Wet cement is alkaline, with a pH of around 12, so appropriate precautions must be made for storage and disposal.
To be useful, ready-mixed concrete must be formed into a functional unit or a construction element. For many foundations, it is possible to cast concrete directly against the ground, where this is beneficial in obviating the need for formwork. Conversely, if the ground is poor there may be considerable overbreak or a need for interim support, in which case the use of formwork and temporary supports may minimise resource use. The most resource-efficient method is simply to discharge concrete where it needs to be, but where this is not possible, pumping is a very efficient method of transporting concrete once on site.
The predominant feature of concrete construction is durability and robustness, and these attributes are useful in minimising resource use simply on the basis that the longevity of high-quality concrete structures mean the impact is spread over a long life. Unfortunately, concrete being used inappropriately, not being maintained or no longer deemed fashionable may lead to premature decommissioning. A significant part of the REAP is to raise awareness and promote best design and construction practice.
Eventually, when the building has completed its working or service life, its crushed concrete is a readily usable material in new construction and, as such, is generally recovered separately from other demolition streams. While still usable, any significant contamination with masonry generally means that the crushed concrete is downgraded to recycled aggregate or fill, where it is commercially less useful. Although the impacts generated by the demolition are attributable to the building, any re-used, recovered or recycled materials are normally considered outside the system boundary for building lifecycle assessment. As demolition contractors are becoming increasingly aware, segregation of the waste stream is the most important aspect and is incorporated into the REAP.
Many challenges and potential actions were identified by the stakeholder group, encompassing all aspects of the supply chain. As the plan is put into action it will bring about measurable improvements in resource efficiency for ready-mixed concrete.
Download the Ready-Mixed Concrete Resource Efficiency Action Plan at www.brmca.org.uk/sustainability