Sustainable Concrete Design
The two major processes in cement manufacturing contributing to embodied emissions are calcination which is reported to be responsible for approximately 60% of total direct Scope 1 emissions and clinkering, which is undertaken at approximately 1450°C and releases 90% of all indirect Scope 2 emissions. Therefore, cement reduction has become a primary focus for concrete specifications for achieving sustainability on projects and the use of supplementary cementitious materials (SCM) has been encouraged to reduce embodied emissions. This has driven the development of alternative binder materials such as geopolymers where fly ash and ground granulated (iron) blast furnace slag, or a mixture of both are combined with alkali-activators.
Reducing Embodied Emissions
With a view to reducing structure embodied energy, specifications for concrete on construction projects often call for minimum percentage replacements of SCMs to reduce Portland cement content in concrete when assessed on a per cubic metre basis. The same concrete is usually required to comply with design standards such as AS3600 covering concrete structures having a nominated 50 year design life or AS5100.5 in the case of bridge structures having a 100 year design life. Often, in addition to requirements of these standards, the concrete is required to comply with other imposed performance criteria to facilitate element structural efficiency and/or efficient construction. These standards defer to AS1379 with respect to specification and supply of concrete which then nominates standard requirements for concrete component materials such as AS3972 for General Purpose and Blended Cements and SCMs.
SCMs, most commonly fly ash and ground granulated (iron) blast furnace slag (GGBFS), have been used in concrete in Australia since the late 1950s and requirements for these are covered under AS3582.1 for fly ash and AS3582.2 for GGBFS. Their inclusion into concrete has been to enhance mechanical and durability properties of concrete with a focus on design and construction requirements, and not for cement reduction in concrete.
Challenges: Sustainability vs Design Requirements
Durability is a critical parameter for design and the technical literature often suggests use of minimum cement and binder contents to achieve design lives for concrete structures depending on imposed environmental loads. On construction projects, this goes against requirements for sustainability that are separately nominated. Further, there are significant difficulties on projects where sustainability criteria go directly against requirements for concrete design and construction that are set down to meet mechanical and durability requirements for efficient construction and asset management.
Many previous studies have reported that use of cement reduction strategies for concrete to improve sustainability in projects often leads to increases in embodied emissions in these structures.
Significant work is needed to further develop rating systems used in construction to incorporate engineering design into sustainability benefits where reinforced, pre-stressed and precast concrete is used. Current focus on cement replacement is simplistic and, in many cases, unrepresentative of construction projects. Embodied emissions can be effectively reduced through a combination of factors including cement content, volume, and steel reinforcement reduction. Future guidelines for sustainability need to consider design and construction issues in more detail in order to minimise perverse outcomes of non-optimised construction.