Akey component of traditional concrete mixtures is Portland cement, which is known to be a generator of carbon dioxide (CO2). Recent changes in the standard specifications for concrete for infrastructure projects in California now require that a significant portion of the cement be replaced by alternate materials which reduce the total embodied CO2. The resulting concrete mixtures have a lower impact on the environment and are more durable. Chemical admixtures are used to produce sustainable concrete, primarily to modify its plastic and hardened properties, keeping projects on schedule and maximizing the durability of the concrete while lowering the portland cement content of the mixture. Consequently, chemical admixtures have an important role in the construction of environmentally preferable, sustainable concrete structures and may help to conserve natural resources.

The California Global Warming Solutions Act was passed and signed into law in 2006. Also called ‘AB 32’ (for ‘Assembly Bill’), the bill aims to reduce carbon emissions to 1990 levels by the year 2020. This represents a roughly 25 per cent overall reduction in the amount of GHG emitted in the state.

The implementation of AB 32 triggered a number of requirements for the construction industry in California. The most important requirement for concrete is to reduce the amount of portland cement whenever possible, which has the net effect of reducing the GHG generated when any concrete structure is built, including tunnels. The amount of portland cement in concrete is typically reduced by replacing a portion of the cement with commonly available supplementary cementitious materials (SCMs) such as fly ash, slag cement, metakaolin and silica fume.

As a consequence of AB 32, Caltrans issued new requirements for concrete used in structures and pavements that require less cement and the use of SCMs. The quantity of SCMs used in concrete will depend on design parameters and inservice requirements for that concrete.

In addition, Caltrans now requires a 50 to 100 year design life for major infrastructure projects such as bridges or tunnels. For reference purposes, details about the Caltrans requirements follow at the end of this article.

Durable, sustainable concrete for tunnels – how admixtures can help
The new Caltrans requirements will increase the use of SCMs in California and promote the use of ternary blends of cementitious materials. Ternary blends constitute the use of portland cement with two other SCMs (for example, fly ash and slag cement or fly ash and silica fume). Ternary blends can provide significant reductions in permeability and greatly enhance concrete durability. Typically, they provide portland cement replacement levels of 40 to 50 per cent, but replacement levels as high as 70 per cent have been used. Invariably, chemical admixtures will play an important role in achieving the desired performance and service life of concrete. These replacement levels of portland cement require the use of a highrange water reducer (HRWR) to lower the water-cementitious material ratio, reduce concrete permeability and achieve the design strength. HRWR also increase the workability and pumpability of the concrete, which is especially important when concrete is pumped over great distances.

Integral waterproofing admixtures are another tool to lower permeability, as they act to ‘seal’ capillary channels in concrete and minimize water penetration. Any reduction in the amount of water that penetrates the concrete increases its service life and makes the concrete more sustainable. Coupled with external membranes, integral waterproofing admixtures offer an extra measure of protection against water-borne corrosive materials such as sulfates or chlorides.

Using SCMs and chemical admixtures to make concrete provides other benefits besides lowering GHG emissions. Corrosion of steel reinforcement, sulfate attack and alkali-silica reaction (ASR) are among the durability concerns that should be considered when concrete mixtures are proportioned for tunnel liners, pavements inside of tunnels and tunnel superstructure elements. Concrete in tunnels is often constantly wet in service; if the soil or groundwater contains deleterious levels of chlorides and sulfates, the use of one or more SCMs is an economical way to mitigate corrosion of reinforcement, sulfate attack and ASR of the concrete. Urabilityenhancing admixtures such as corrosion inhibitors and lithium-based admixtures can also be used to mitigate corrosion and ASR, respectively.

Steel and synthetic fiber reinforcement has been used in shotcrete tunnel liners throughout the world for at least 25 years. California is no exception—the seismic loading conditions found in most tunnels require a reinforcement system for the liner that usually includes fiber-reinforced concrete (FRC). Steel and macro synthetic fibers each add to the flexural toughness and ductility of shotcrete or cast-in-place tunnel liners, again extending the service life of the structure, especially in the event of a fault or slip. The use of fibers may also lead to a reduction in the thickness of a concrete section without sacrificing moment capacity.

Workability-retaining admixtures are another tool that can help conserve natural resources and facilitate sustainable concrete construction. Workability-retaining admixtures provide slump retention and minimize slump loss, thereby providing consistency between loads of concrete delivered to a project. As a result, workability-retaining admixtures minimize the potential for rejection of concrete at the jobsite due to inconsistent slump or air content. The consistency obtained in plastic and hardened properties also enables optimization of and a reduction in the cement content of a concrete mix that contains a workability-retaining admixture.

Quantifying the environmental footprint of constructionmaterials
Recently, owners of construction projects in California have begun to ask for “carbon accounting” from contractors, subcontractors, and material suppliers. The City of San Francisco has pioneered this approach for public works projects. There are several acknowledged methods of determining the environmental footprint of construction materials, including BASF’s own Eco-Efficiency Analysis program. The program analyzes the environmental life cycle of concrete mixtures and quantifies the economic and ecological benefits of concrete based on six environmental impact areas, including GHG.

Driven by sustainability, the requirements specified for concrete used in infrastructure projects in California, including tunnels, are now different. Today, sustainable concrete is made with significant amounts of one or more supplementary cementitious materials that replace a portion of the portland cement. Sustainable concrete has a significantly longer service life through improved durability; at the same time it has other environmental benefits such as lower carbon footprint (CO2) and overall embodied energy. Chemical admixtures improve the plastic and hardened properties of concrete and make these new sustainable concrete mixes possible.

About the authors
Tarek Khan is a Ready Mixed Segment Manager for BASF based in Sacramento, California who has 30 years of experience in concrete materials. Tarek is a civil engineer and is a Fellow of the American Concrete Institute. Reach Tarek at tarek.khan@basf.com

Lauro Lacerda is Area Leader Underground Construction for BASF based in Salt Lake City, Utah and has over 27 years of experience in mining, tunneling and underground infrastructure projects. Lauro is a registered PE in Nevada and member of SME (Society of Mining Engineers), ASCE (American Society of Civil Engineers) and ASA (American Shotcrete Association).

California’s Devil’s Slide tunnel