Sooner or later, on one of your jobs, a big green monster is likely to show up—a green building or sustainable design project that uses a high-volume fly ash (HVFA) mix design. In high-volume mix designs, the ash replacement is more than usual, and in many cases, exceeds a 50% weight replacement for cement. So, technically, there is a higher than normal volume of ash within the mix design, and because of the specific gravity difference between cement (3.15) and a standard fly ash (2.54 to 2.65), the volume of total cementitious material can increase.
When faced with a job using or specifying HVFA, the contractor or producer inevitably asks, “Why exactly are we using this?” The primary motivation comes from the U.S. Green Building Council's (www.usgbc.org) LEED Certification program. A few years ago, someone asked how green the different environmentally friendly construction methods really were, so the USGBC developed a method for measuring just that and established a series of voluntary metrics that an owner can use when designing and constructing a new project.
One of those metrics used to evaluate the “greenness” of a project, is to sum up of all the post-industrial waste used in the project. (The USGBC defines post-industrial waste as the output from a process that has not been used as part of a consumer product, that is sold, traded, or exchanged under commercial terms as feedstock for another industrial process, and that would have otherwise been land filled, incinerated, or somehow disposed of as a waste.) Fly ash fits the definition of post-industrial waste. The more fly ash that is used in the concrete, the higher the percentage of total post-industrial waste on the project. Because of the sheer volume of concrete used on a project, a small increase in fly ash replacement can translate into a big impact on the total recycled content. If “greenness” is important, then there is good reason to use as much fly ash as possible within a design (and in the overall project) to maximize the recycled content.
Initial set and strength
Of course this discussion so far has been largely academic. As a contractor, you are much more interested in questions such as, what does the concrete look like? How does it perform? Finish? And most importantly, what does it cost?
From a technical standpoint, two very explicit requirements factor into the mix design.
- What is the initial set time?
- What strength is expected at what day?
On too many projects the set time requirement is implicit. But when it comes to HVFA, the contractor who will be placing the concrete must make the set time an explicit requirement. Most commercial contractors expect set times between four and six hours and assume that the concrete supplier knows this. But when HVFA is involved, it is critical during the pre-construction meeting to make sure everyone involved understands that there is an explicit requirement for concrete set time. The contractor's expectations must be made very clear, and the concrete supplier clearly has a design target when developing the HVFA design.
These requirements must also take into account a variety of weather conditions. If the job lasts long enough to go through several temperature swings and ranges, the ambient and concrete temperatures are going to dramatically impact the set time. That is why it is so important to give a clear target and performance expectation to the concrete supplier. The contractor must make it clear that he expects a consistent set time, regardless of the ambient temperature conditions.
While there is ample experiential evidence to suggest that increasing the fly ash content of a mix design will also increase the set time, this does not always have to be the case. In fact, with advances in admixture technology and proper proportioning, HVFA mix designs can have characteristics identical to typical concrete. Looking at the results in Figure 1, any contractor might see those results as typical of a commercial concrete. However, if I tell you that those results were for concrete that used a 51% ash replacement, the results are almost unbelievable—especially for mixes B and C. In fact, they begin to challenge what we thought could not be done with HVFA. Figure 1 also includes the strength results. For all three mix designs, the 28-day design strength far exceeds most commercial requirements.
While set time and strength gain are specified as explicit performance requirements, there are several implicit requirements as well, most importantly finishing quality. HVFA concrete produces minimal, if any, bleed water. In formed concrete this is not really an issue. For large flat-work sections and those receiving a hard trowel finish, though, bleed water is essential to the finishing operation. If there is no bleed water, the contractor faces some challenges. First, with no water on the surface of the concrete, almost immediately after the strike-off operation moisture from the surface begins to evaporate. The moisture that is evaporating is not excess water on the surface but rather is the water in the paste that is needed to hydrate the cement and fly ash. This means that the paste on the surface of the concrete will begin to dry out and look as if it is forming a crust, leading to rapid plastic shrinkage.
This kind of mix could be thought of as being like chocolate pudding. You know that chocolate pudding is taken from the stove, put in small glass bowls, and then placed in the fridge to gel and stiffen. In many cases, the cook decides to cover the bowls with an airtight plastic cover. But what happens if they don't? The surface layer of the pudding begins to stiffen (even though the rest remains fluid) and cracks and tears start to form. That is exactly the same principle with HVFA. If an evaporation retarder is not used (much like not using the plastic cover) the top surface will begin to stiffen and tear due to rapid evaporation.