Saturday 14 February 2015

The circular economy and sustainable concrete

Do engineers need to significantly change how we use materials to make what we build sustainable? I've been thinking recently about this question in the context of concrete, one of the most frequently used materials in construction. In this post I'll be exploring the two main problems with concrete: that it is extremely energy-intensive to manufacture, and it is difficult to re-use without downgrading the quality.

For example, industry statistics show that last year the UK construction industry used 15 million cubic metres (37.5 million tonnes) of ready-mix concrete and 25 million tonnes of concrete products from blocks and pre-cast walls to driven piles.  
Castleton cement works in Derbyshire
(picture by Dave Pape)

Concrete is formed from two materials: cement and aggregate. Quarrying is required for both elements (with attendant environmental and landscape impacts), but the cement also requires a chemical reaction to occur: calcium carbonate (ie limestone) is heated to a high temperature (300 degrees C) to drive off CO2 and form calcium oxide instead. Therefore making cement produces CO2 both from the fuel used for heating and from the reaction itself (although this can be somewhat recovered when used, as hardening cement absorbs CO2 to form calcium carbonate again). 

Because of this, one of the best ways to reduce the carbon impact of concrete is to reduce the cement quantities required by using other materials which also react to a lesser degree (eg PFA ash from power stations) - last year the UK used 10 million tonnes of cement and 2 million tonnes of cement-replacement, so there's still some way to go. 

The hierarchy for sustainable concrete use on site is therefore:

  1. Use as little as possible (efficient designs, thinner slabs etc).
  2. If you are redeveloping a site, aim to re-use any concrete on site. This is particularly true for foundations, where the practice of re-using piles is starting to become more common now that an agreed process for determining the load capacity and 
  3. If you cannot re-use it, you can crush it and use it as an aggregate, incorporating it within new concrete materials or using it as a granular fill: general fill material to raise site levels, or 6F2 capping material beneath floor slabs or hardstanding/road areas. In both cases, this will significantly reduce the amount of waste material you need to export and good granular fill that you need to import. 

However, this is not genuine recycling as the crushed concrete is not as useful as the original slabs or beams. If we are to close the circle of using concrete, we need to become much better at designing structures in such a way that they can be dismantled after use (for example using precast panels which can be re-used rather than cast-in-situ sections which need to be broken up). 

There is also a tendency for people to assume that use of crushed concrete aggregate is always sustainable, without a consideration of the bigger picture. In fact, as ground engineering specialists, there are two major problems which need to be discounted before crushed concrete can be used as a general or capping fill, both of which arise because concrete isn't actually inert, and crushing provides a much larger surface area for chemical reactions to occur.

The first chemical reaction to avoid arises from contact with water: crushed concrete should not be used where significant surface water infiltration or groundwater flow is expected, because the calcium carbonate and any unreacted cement (there is always some left over) are strongly alkaline and can therefore have a damaging effect on water bodies downstream (fish kills etc).

The second reaction occurs when crushed concrete is in contact with both water and sulphate-bearing materials such as colliery spoil and industrial wastes or natural materials with high sulphate levels such as Lias Clay. The concrete reacts with the sulphates and expands, causing heave of floor slabs or hardstanding areas. To avoid this, ground engineers usually test both the soil and groundwater as part of site appraisals for pH and sulphate content, and assign one of four categories (Design Sulphate classes 1 to 4) using a methodology set out by the Building Research Establishment (BRE). It is recommended that crushed concrete fill is not used where it may be in contact with soil or groundwater with Design Sulphate Level 3 or 4.

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