In recent years, concrete has come under some very close scrutiny in terms of its environmental credentials. Concrete production actually has a low environmental impact, but it is currently manufactured in such enormous quantities that its overall footprint is very high, and world cement production is said to contribute 5-7% of annual man-made CO2 production, with China alone thought to be responsible for 3%.
Most modern concretes make use of Portland cement. The high energy demands of baking limestone (to temperatures around 3,400°F), and grinding it to a powder, are responsible for the environmental footprint of concrete.
It is thought that the production of one tonne of cement emits 800-1000kg of CO2, much of which is brought about by the chemical decomposition of the limestone itself. The cement industry is committed to reducing its environmental impact and significant R&D is being undertaken to improve energy efficiency and reduce the demand on virgin resources. Although cement itself is highly energy intensive, it is also the smallest constituent of concrete, and the most common aggregates are sand and gravel, which require minimal processing and are locally available throughout the world.
There is a current focus on the means of reducing the environmental impact of concrete, and the following are all examples:
• The simplest approach to reducing CO2 emissions is to minimise the amount of cementitious material in conventional mix designs; judicious monitoring can allow a reasonable reduction with no detriment to the finished product
• Substitution of some (or perhaps ultimately all) of the cement content with recycled, or secondary aggregate alternatives has seen beneficial end results where waste materials, such as recycled slag (from blast furnaces) and pulverised fuel ash (from coal-fired power stations), are commonly used as replacements
• Today’s precast concrete products, such as paving, cladding or furniture, are manufactured in a factory setting where strict quality controls assure extremely high standards of finish, colour and dimension with minimal waste
• The development of particularly strong concrete mixes - high, or ultra high, performance concretes (HPC/UHPC) – is leading to a lower embodied energy since less concrete mass (up to 40%) is required to do the same job
• Minimising, or entirely eliminating, the need for steel reinforcement through the inclusion of fibres in UHPC products means that they are no longer susceptible to chloride ingress (from deicing salts) and subsequent corrosion-induced deterioration; this means a longer service life and lower maintenance costs
• Embedding recycling activities throughout the entire manufacturing process: whether harvesting rainwater from the factory roof or crushing and reusing defective product to minimise waste and minimise what goes to landfill
• Face mix technology is where the more costly (and potentially scarcer) aggregates and pigments are used in the top 10mm of the product, only in the area where they are visible after installation. This enables a higher proportion of recycled content to be cast into the body of the product.
The long-term strength and durability of concrete is the most important factor, and innovation with the material does not come without risk. Quality assurance relies, to an extent, on a recipe that is as old as Ancient Egypt, and requires the balancing of long-term advantages over short-term drawbacks.
Correct specification is key. A thorough understanding of the performance requirements for a particular project or product is needed, so that the properties of a concrete mix are not over-specified. These might be related to use, environment or intended design life, and all play an important part in the decision-making process.
Concrete is an inherently sustainable material and many of its applications are constantly being enhanced through the introduction of complex developments. Simple choices, for instance the specification of stainless steel rebar, will ensure additional longevity since the concrete will not deteriorate should water or salt penetrate. This is particularly important if damage occurs and reduces the original cover over the reinforcement.
Innovative materials such as SCC and UHPC have unique physical and mechanical properties that make them particularly sustainable, since long design life and minimal maintenance mean considerably reduced life cycle costs for products.
The single most important benefit of concrete is its durability and, where design life can be as high as 100 years, its associated environmental footprint is tiny when whole-life use is taken into account. If manufacturing impact is minimised by using high-tech materials and processes, whilst designing products that will stand the test of time, then it could be said that the colour of concrete is "green".