The concepts of “sustainability” and “sustainable development” are receiving considerable attention as the causes of global warming and climate change are debated. The World Commission on Environment and Development has defined sustainable development as “meet[ing] the needs of the present without compromising the ability of future generations to meet their own needs” (WCED 1987).
A major focus of this concept involves ensuring that sustainable practices in pavements go hand in hand with economic success. This is indeed true of concrete pavements. Particularly because of its long life, concrete is an economical, costeffective pavement solution that consumes minimal materials, energy, and other resources for construction, maintenance, and rehabilitation activities over its lifetime.
A sustainable pavement is one that achieves its specific engineering goals, while, on a broader scale, (1) meets basic human needs, (2) uses resources effectively, and (3) preserves/restores surrounding ecosystems. Sustainability is context sensitive and thus the approach taken is not universal, but rather unique for each pavement application. Furthermore, a “sustainable pavement” as defined here is not yet fully achievable. Today it is an aspirational goal to be worked towards, and ultimately achieved at some point in the future as sustainability best practices continue to evolve.
Concrete pavements suffer from a perception that they contribute a considerable amount of carbon dioxide (CO2) to the atmosphere due to the use of portland cement that binds the aggregates together. Cement industry is one of the primary producers of carbon dioxide. The CO2 emission from the concrete production is directly proportional to the cement content used in the concrete mix; 900 kg of CO2 are emitted for the fabrication of every ton of cement, accounting for 88% of the emissions associated with the average concrete mix. It is very vital to identify a substitution for cement or reduce the use of portland cement in concrete mixes to make a more cost effective and environmental friendly concrete.
Use of Portland cement can be reduced through partial replacement of cement with supplementary cementitious materials. Besides the environmental benefits, supplementary cementitious materials can enhance concrete properties when used in appropriate quantities. For example, they can improve workability of the mixture, decrease concrete permeability, improve durability, and enhance strength.
Supplementary Cementitious Materials (SCMs)
Supplementary cementitious materials are byproducts from other industries that beneficially react with portland cement to enhance the performance of concrete. The effective use of SCMs reduces not only the amount of portland cement required but also the need to dispose of what otherwise would be industrial waste. The two most commonly used SCMs in paving concrete are fly ash (byproduct of coal burning) and slag cement (byproduct of iron production).
SCMs, when blended with portland cement, chemically and physically complement the hydration of portland cement, often resulting in more durable concrete. Hydration of portland cement produces calcium silicate hydrate (C-S-H) that provides the backbone of strength and impermeability of the cementitious system. But portland cement hydration also produces calcium hydroxide (CaOH), a crystalline material. Calcium hydroxide contributes little to strength or impermeability. Most SCMs contain glassy silica and aluminate phases that react with the calcium hydroxide to form more C-S-H and/or calcium silica aluminate hydrate (C-S-A-H), enhancing the long-term performance of the concrete mixture.
Using SCMs in concrete pavement has several environmental benefits. First, recovering industrial byproducts avoids the use of virgin materials needed for cement manufacturing. Additionally, beneficial utilization reduces the amount disposed in landfills. More importantly, however, are the greenhouse gas and energy reductions achievable by using SCMs to replace a portion of portland cement. CO2 and energy savings are related to the percentage of SCM used in the concrete mixture design.
Fly ash is a byproduct of burning pulverized coal for the generation of electrical power. The rock embedded in the coal melts in the furnace and is carried up the stack in the flue gases. As it rapidly cools, small glassy spheres are formed that are collected before the flue gases are emitted to the air. Because of the small size, glassy form, and chemical composition of the ash, it dissolves and reacts with the cement paste to contribute to the performance of the mixture.
The use of fly ash in concrete pavement mix helps to place the concrete at a lower slump value without altering the required workability. The mix requires less handwork for the contractor and a better surface finish. A properly proportioned quantity of fly ash can provide required compressive strength for concrete pavement. Substitution of 25% of cementitious material with fly ash can give equivalent compressive strength in all ages. The use of fly ash in concrete also results in denser concrete with greater ultimate strength and durability. Fly ash provides good-quality hydration products, which reduces the amount of non-hydrated cement in the concrete. It reduces the amount of air voids, making the concrete more dense and impermeable.
Slag cement is an industrial co-product from the smelting of iron in a blast furnace in which molten slag is quenched using water to form a glassy sand-like material containing amorphous oxides of calcium, aluminum, magnesium and iron. It is subsequently ground to a fineness that is similar to that of portland cement. It is slowly reactive in the presence of water or more vigorously when activated in water in the presence of calcium hydroxide, which is present in the pore solution of hydrating portland cement.
Slag cement is an attractive SCM for a number of reasons. For one, the typical dosage of slag cement is usually in the range of 25 to 35 percent of the total cementitious materials for paving concrete, although it can be used in even higher amounts. Furthermore, slag cement creates very light colored concrete that some find aesthetically pleasing and can produce a higher albedo concrete pavement that may help reduce the urban heat island effect. In addition, concrete permeability and chloride ion ingress are reduced when slag cement is used, and slag cement can be used to effectively mitigate alkali-silica reactivity (ASR) and sulfate attack.