Some governments create maximum SCM limits for various uses of concrete through their construction specifications. This can be a limiting factor when trying to create lower carbon mixes, sometimes called “lean” mixes, which have higher SCM percentages. Use of fly ash and slag are often capped at percentages that are lower that what the literature suggests is feasible, and also in some cases at percentages that are lower than caps put in place by the state agencies, as is the case shown in the table below for the Seattle Department of Transportation (SDOT) and the Washington State Department of Transportation (WSDOT) (Tracy, 2014; Sound Transit, 2014, A.6). Even with these lower-than-necessary limits, most projects are replacing cement at well below the allowable maximum. The table below shows the results of a survey of over 15 different SDOT and WSDOT projects. As the table below makes clear, actual cement replacements are in most cases not reaching allowable maximums.
A report prepared for Sound Transit Authority identified 4 primary reasons that the actual SCM percentages achieved in the table above fall below the allowable maximums:
- Concern around schedule delays due to the longer set times for high SCM concrete
- A lack of compelling financial incentives that would offset the negative impact to the schedule
- Lack of priority for public agencies and lack of encouragement for contractors to achieve the max allowances already in specifications
- Lack of minimum SCM allows mixes to be in compliance w/o any SCM content. When minimums do exist, however, there is little incentive to ever exceed the minimum which stifles further advancement.