The concrete industry favors the word “sustainable” instead of “green.” Green has come to be defined by examining what a product is made of, the amount of energy consumed and exhaust gases put into the atmosphere, how far materials must be transported to a jobsite, insulation values, and whether ingredients are renewable.
The concept of sustainability focuses more on the useful life cycle of a product or structure, and energy efficiency over time. Concrete can perform well in these settings. Sustainability comes into sharp focus whenever specifiers require concrete to have a service life of more than 20 years, and in some instances as long as 100 years as is the case with a growing number of bridge projects.
Some owners or specifiers want both green and sustainable attributes for their projects, especially if the goal is a LEED certification award. This aspiration can put these two concepts at opposition of one another, necessitating the need to determine whether being green is more valuable than being sustainable.
Pozzolans, such as fly ash, slag, and silica fume, are green because they are byproducts from other industries; fly ash from the burning of coal at electrical power producing plants, slag from steel mills, and silica fume from the production of silicon and ferrosilicon alloys (silica fume is collected from the smoke of an electric arc furnace using quartz, coal, and wood chip ingredients.)
Almost everyone in the concrete industry knows that replacing a percentage of the portland cement with fly ash in a concrete mix can make it greener, and sometimes even more sustainable. Less popular as a cement replacement, though typically less available, is slag. Some big box owners allow the addition of slag instead of fly ash because it produces a more light-reflective surface than other pozzolans. Both fly ash and slag reduce the heat of hydration temperatures of cement so mix specialists often replace cement with both materials for mass concrete placements; mass concrete is regarded as placements more than 3 feet thick. Pozzolans produce 25% to 50% less early heat of hydration than cement does—even more when Class F with very low calcium oxide contents is used. It’s very common in the northeastern states.
Replacing 20% of the portland cement in a mix with fly ash is popular today—a few applications replace as much as 85%. Because it’s a byproduct from the burning of coal, it’s considered energy neutral. On the other hand, the production of cement is energy intensive, resulting in the production of approximately 1 ton of carbon dioxide for every ton of cement produced. Making concrete by replacing some portland cement with fly ash consumes less energy to produce and therefore less carbon dioxide (CO2), qualifying for LEED points.
Pros and cons
The two most common classes of fly ash are Class C and Class F. Class C fly ash is produced by burning lignite or sub-bituminous coal. Class F commonly is used in parts of the country where aggregates are prone to alkali silica reaction (ASR).
Class F fly ash is produced from the burning of anthracite and bituminous coal. Similar to Type C ash, it produces even, slower initial setting times with higher overall compression strength compared to all-portland cement mixes. This can represent a problem for floor contractors, however, because increased setting times translate into lost dollars when conditions are cooler.
Concrete mixes with significant cement replacements of fly ash are stickier, which can cause problems for contractors during the finishing process because sticky concrete increases the chances for blisters and other finishing problems to develop. In the eastern states, Type F especially makes stickier concrete with longer delayed setting times. In the west, it has more calcium oxide content, resulting in additional decreased setting time. However, when a mix is “harsh” because of manufactured sand, very coarse sand, or a very low cementitious material content, fly ash can help solve problems.
In the summertime, when both ambient and concrete temperatures increase, replacing some portland cement with fly ash delays concrete setting times. Contractors often switch to these mixes so they can manage placing and finishing processes more efficiently. However, delayed setting time also means there is more time for moisture loss due to evaporation, adding to a contractor’s problems.
It’s been observed recently that it’s harder to achieve a high-gloss finish with diamond polishing equipment when fly ash is added to floor mixes, partially due to slower surface-setting characteristics. Another observation is that many polished floors with fly ash replacements don’t hold their polish as long as all-portland cement mixes and durability is affected.
Many Class F fly ashes do help to mitigate the affects of ASR, so there are times when specifiers require them in mixes for that reason. In these situations, the problem isn’t trying to make concrete green, it’s about creating a mix that can survive under local conditions. Even owners who typically don’t allow fly ash in their floor mixes will specify appropriate fly ash when ASR is a threat.
Fly ash delays initial setting time, an increasing problem when specifiers try to increase the greenness of their floor mixes by replacing increased amounts of portland cement with fly ash. This makes floor construction especially difficult under cold-weather conditions. In the past, adding calcium chloride has been frowned upon as an accelerator but big box owners are recognizing its benefits. Setting curves can be improved by adding calcium chloride up to 2% by weight of cementitious material without significant side effects, so long as steel reinforcing isn’t involved. It’s a low-cost alternative compared to using nonchloride accelerators.
Constructing a sustainable floor
Sustainability is primarily concerned with the useful life of a structure and its parts, or when the energy efficiency of a structure becomes important. To design for sustainability, concrete contractors must have the freedom to make adjustments related to construction conditions. For instance, if an owner is interested in attaining LEED points through the use of fly ash in a floor mix, concrete contractors must be able to initiate mix design changes that make successful installations possible when winter closes in, regardless of the green status of the floor—sustainability being more important than green in this instance.
Sustainable floor construction starts with good subgrade control. A suitable compacted soil support system in the top 12 inches must be able to support the load carried by a floor—concrete can’t perform alone. There is also evidence to suggest that a less compacted top thin layer of subgrade can cushion a greater area of concrete, making more intimate contact with the soil support system, reducing cracking as a slab curls. This is because less of the panel is held in the air.
Uniform moisture content that isn’t excessive under a slab also is very important. Membrane vapor/gas retarders placed directly under the concrete are increasingly being used to improve the long-term sustainability of concrete floors. They limit the amount of moisture that passes through a slab and make it possible to install moisture sensitive surface finishes on top of them. They also limit radon gas accumulation inside buildings—an increasing concern because radon gas is the second leading cause of lung cancer.
The best floors exhibit the least amount of curling. Concrete mix design ingredients influence how much floor panels will curl. Mixes that resist curling best use well-graded aggregate distributions with larger top-sized aggregates, ideally 1½ inches or larger. This allows mix designers to reduce the amount of cementitious paste because less is needed to coat aggregates. Less paste means less water demand and that translates to less shrinkage and curling. Using well-graded aggregates as large as possible also provides internal restraint to shrinkage and curling; external restraint is usually bad, but internal restraint is good.
Big box owners are increasingly specifying densely hard-troweled floors in the belief they are more abrasion resistant and add to the floor’s service life. There are other sustainable rewards too. Troweled surfaces become vapor retarders that reduce the need for curing (see “Self-Curing Warehouse Floors?” in the August 2010 issue of Concrete Construction). With very few pores on the surface, dirt can’t penetrate as well into surfaces, making them easier to keep clean.
Big box retail facility owners know that customers like glossy floors. These floors allow for reduced lighting, which result in savings on electricity each month. Thus the final step before contractors turn their work over to owners often involves improving the gloss reading of hard-troweled surfaces. This is measured by metering systems that read gloss value. Most owners wanting an overall gloss value of 40 or more.
There is no such thing as a maintenance-free floor. The most common failure occurs at control and construction joints—repairs that can be very expensive. For that reason, some designers design mixes and floors with fewer joints. One way to do this is by adding dosages of macropolymeric fibers to concrete mixes in the range of 71/2 pounds per cubic yard. It’s possible to attain a 60-foot or more joint spacing this way—fewer joints mean less maintenance and more trouble-free use. Long joint spacing also can be accomplished with shrinkage-compensating concrete, post-tensioned concrete floors, and steel fiber mixes.
Finally, when construction is completed, a useful life cycle depends on proper maintenance. Dirt on a floor becomes an abrasive as foot traffic works it back and forth on the surface, causing a reduction in gloss value. Contractors need to prepare owners ahead of time for the need to perform regular maintenance with machines and products that enhance gloss without damaging the surface finish.
Sustainability versus green
The concrete industry is marketing the concept of sustainability as opposed to green. Green is associated with LEED points and LEED classifications. LEED points are often the goal for project owners, architects, and engineers. But attaining LEED points doesn’t necessarily mean that projects also meet sustainable goals, and there can be disparity between the values represented by these points. Hard won LEED points, achieved by replacing portland cement with pozzolans, can sometimes be achieved by other simple means such as installing a bike rack outside a retail facility. Designing for sustainability keeps the values and benefits of concrete in sharp focus.