According to the World Business Council for Sustainable Development (WBCSD), “concrete is the most widely used material on earth, apart from water, with nearly three tons used annually for each man, woman, and child.” Most structural engineers are familiar with efficient design practices for working with concrete, but there are sustainable considerations that should also be taken into account. There are at least three areas in which the characteristics of concrete can be used sustainably:
1. Exposed concrete can be used as both an interior or exterior finish, thereby reducing the additional material cost of cladding, painting, installing drop ceilings and sheeting, etc.
2. Concrete structures intrinsically have more thermal mass – a property that enables heat energy to be absorbed, stored, and later released, giving greater comfort in the both the heat of summer and the cold of winter.
3. Carefully considered mix designs can reduce the embodied carbon intrinsic to concrete, improve long-term durability, and still provide sufficient workability.
Unfortunately, the production of portland cement releases a high volume of carbon. Approximately 40% of embodied carbon is associated with powering the extremely hot furnaces needed for the transformation. Cement manufacturers are experimenting with ways to become more efficient. (4) Structural engineers can specify that cement be sourced from plants scored by the Energy Star Industrial Focus Program with an Energy Performance Indicator (EPI) above 75.
The remaining 60% of embodied carbon in cement is a result of calcination, the intrinsic chemical reaction whereby limestone is transformed into clinker, on the way to becoming cement. Therefore, the primary means of greening concrete is to reduce the amount of portland cement in the mix.
Fortunately structural engineers can have great control over the mix designs selected. Some relatively simple steps can be taken to ensure that more sustainable concrete mixes are used on your job site. These can be simplified into four rules of thumb: (5) reduce water content, (6) use complimentary cementitious materials (CCMs), (7) use the maximum aggregate size, and (8) specify proper strengths.
Reduce water content. Keep the water/binder ratio low, and although less cement is used, the same strength can be achieved. A low w/b is also good for durability. Based on the relationship of specific gravity between concrete and water, a w/b ratio greater than 0.32 is most likely to result in free water that is not bound to the binder paste. This results in unwanted voids and drying shrinkage as the free water evaporates (instead of being consumed in the chemical reaction).
High slump is often desirable for workability. (9) Fly ash and superplasticizers help improve workability without increasing water. The spherical shape of fly ash acts as a physical lubricant and thus aids in cement hydration. Water-reducers likewise increase slump, however, too much of these admixtures can cause segregation and excessive bleed water. A good rule of thumb is to limit the water-reducers or superplasticizers to 2 percent of the mass of the binders.
Use complimentary cementitious materials (CCMs). Fly ash, slag, natural pozzolans, and ultrafines can be used in lieu of portland cement. Many such materials have less embodied carbon or are recycled industrial byproducts. The basic chemical reaction between portland cement and water produces calcium hydroxide (CH). Many CCMs then react with the CH to produce calcium silicate hydrate (C-S-H), which provides a much stronger bond, particularly around the aggregates.
With regard to durability, C-S-H is known to be a much denser product and therefore less permeable. Non-cement binders also tend to reduce the heat of hydration. Although specific alkali-silica reactions are known, CCMs generally enhance both strength and durability. (10) For a more in-depth analysis of durable mixes, designers can utilize the Life-365 freesoftware from the National Ready Mix Concrete Association (NRMCA).
Use the maximum aggregate size. This reduces the surface area that the binder paste needs to cover thereby keeping the past volume lower. Normally available aggregates are stronger than the surrounding hardened paste.
Specify proper strengths. Choose target strengths at ages that realistically reflect the needs of the project. Recall the above described chemical reaction involving CSMs: C-S-H takes longer to develop and the conventional 28-day period may not be sufficient. If possible, specify 56 or even 90 day strength. This gives mix designers at the batch plant more freedom to utilize some CCMs.
The above recommendations were sourced from SustainabilityGuidelines for the Structural Engineer, Chapter 3.2 – Concrete. Current and former SEI Sustinability Committee Members influential in authoring the referenced chapter include: Helena Meryman, Sarah Vaughan, Alan Kren, and Iyad Alsamsam. This summary is by Ken Maschke, P.E., S.E., LEED A.P., associate with Thornton Tomasetti in Chicago, IL.