Application of life cycle assessment to compare the environmental footprint of concrete methods

Abstract

Industrialization of the construction process being a current trend can considerably reduce the on-site labor intensity and shorten the construction time. Prefabrication of construction components is one of the most commonly used methods to facilitate the industrialization in the sector. Since the mid-1980s, the Hong Kong Housing Authority (HKHA) has begun to introduce prefabricated concrete components into public housing projects. While the provision of public housing is of paramount importance in Hong Kong as approximately 30% of the population live in public rental housing (PRH) flats and 20% reside in home ownership scheme (HOS) flats, prefabricated concrete components can replace some 17% or 65% in extremely cases of in-situ concrete casting by volume.

Due to high labor cost and a lack of land in Hong Kong, prefabricated concrete units are mostly manufactured in mainland China and transported to the construction sites in Hong Kong. This leads to a more complicated logistic system when compared to the conventional cast-in-situ concrete method. The transportation of heavy prefabricated concrete units can generate pollutants and consequently compound the environmental burden. Nevertheless, using the prefabrication approach can help minimize on-site wastage.

In this study, the environmental footprint of prefabricated and cast-in-situ concrete is compared using a life cycle assessment (LCA) approach. A software program known as the Environmental Modeling of Construction (EMoC) has been developed based upon the concept of LCA. EMoC considers the ‘cradle-to-site’ processes from raw material extraction, manufacturing of components, transportation to on-site construction processes. More importantly, the on-site activities related to prefabricated and cast-in-situ concrete can be evaluated separately. To facilitate comparison, the life cycle inventory of concrete has been collected from a local prefabrication yard and concrete batching plant.

The comparison is based on the unit of 1 m3 of unreinforced prefabricated concrete and the equivalent amount of cast-in-situ concrete. Through this study, it is found that the overall environmental footprint of prefabricated concrete as represented by the endpoint single score is 36 which is indeed much better than the score of 57 for cast-in-situ concrete. While cast-in-situ concrete contributes more to the toxicity, eutrophication, land occupation, and resource depletion than prefabricated concrete, the carbon footprint of prefabricated concrete is only 6% less than that of cast-in-situ concrete. The analysis reveals that timber formwork is the primary contributor towards a larger environmental footprint of cast-in-situ concrete. The results show that prefabricated concrete can outweigh conventional cast-in-situ concrete from an environmental point of view due to a dramatic saving in the amount of temporary materials.