Life-cycle assessment/analysis of concrete

Concrete, Mar 2004

Sustainability has become an important consideration for the construction industry. Increasing numbers of clients are analysing environmental factors in relation to proposed design, materials and construction methods. Various tools for measuring sustainability have been developed, including whole-life costing (WLC) and life-cycle assessment, also called life-cycle analysis (LCA).

Life-cycle analysis

LCA is mainly concerned with the environmental impact of materials. It takes into account the entire process, from extracting raw materials, through processing, construction, planned lifetime maintenance, demolition, disposal and recycling. LCA is a useful tool for comparing the environmental effects of concrete use with other construction materials. It also assists with the optimisation of concrete through mix design to minimise environmental impact.

LCA must be carried out in the context of a particular structure or part of a structure, known as a functional unit. This will have a specification that takes factors into account, including performance, working life, delivery, construction, maintenance and demolition. If the LCA is compared with alternative construction materials, it is essential that the functional unit for both options satisfies the same overall specification. The LCA is only valid for each individual functional unit as the specification and components may be altered, even for another relatively similar unit, thereby affecting the LCA.

Process tree

A process tree is drawn up when producing the LCA for a functional unit. This covers the 'cradle-to-grave' life-cycle of the concrete in the unit, detailing those specification items to be included. An outline process tree is shown in Figure 1. all relevant environmental inputs and outputs (materials, energy, and emissions to air, water, soil or waste) are collected for each step in the process tree. This data collection is called a life-cycle inventory (LCI).

Raw material suppliers should undertake an LCI of their products, providing the information for the first line of this process tree. This is often in the form of an environmental declaration according to ISO 14020 Environmental labelling: general principles(1). These declarations state the data collected from the LCI process in a standard format. Figure 2 shows a typical main table from an environmental declaration.

Environmental declaration

The LCI or environmental declaration information may be combined with statements from other suppliers to give a new environmental declaration for a specific concrete mix. When combined with additional data for other steps in the process tree, it will give the LCA of the functional unit.

The LCA is often presented in the form of categories, according to its effect on environmental impact, such as energy, natural resources depletion and ozone layer depletion, also known as the 'greenhouse' effect. Different LCA models may have different impact categories. Table 1 shows a typical set of impact categories. Other models try to rationalise individual impact categories into a single environmental score.

Life-cycle impact assessment

The result of this analysis is called lifecycle impact assessment (LCIA), which provides an environmental profile. Figure 3 shows a typical environmental profile for a reinforced concrete element, broken down into life-cycle phases. Individual impact values can vary by over seven orders of magnitude, so they have been standardised to 100% for each impact category for demonstration purposes. It must be remembered that some impacts will be more than one million times smaller than others. This information can be used to compare alternative construction options when combined with actual values for each impact category. For example, changing construction materials may improve some aspects of the profile, such as initial energy usage, but the overall profile may be considered worse if chemical waste and abiotic depletion is higher. Such decisions may depend on the importance of each impact category to the client.

Materials suppliers can apply the LCI and LCA results to enable targeted development or optimisation of categories with the greatest effect on the environmental profile of functional units containing their product. Consultants can combine the LCA materials with additional information related to lifetime use to produce a Whole Life Costing (WLC).

At present, there are several methods by which environmental profiles may be demonstrated. However, the basic requirements and processes of doing so are specified in ISO 14040 to 14043, all of which deal with different aspects of LCA(2"5). The LCA methods all use the same LCI information, but there are differences in the form of final analysis. Within Europe there is no single agreed method, but some more accepted methods are:

BRE Environmental(6)

* provides 13 environmental impact categories

* may be aggregated to a produce single score in UK Ecopoint products.

UKEcopoints(7)

BRE single unit measurement of environmental impact. Calculated from a defined range of LCA data.