An integral part of the Performance-Engineered Mixtures (PEM) Program has been providing tools to contractors for proportioning mixtures with a high probability of meeting specified performance measures while reducing environmental impacts. We have traditionally accepted concrete based on measurements like strength, slump, and air. These measurements, in their current form, have very limited correlation to future performance. However, recent developments in concrete testing technologies have yielded methods that are better predictors of long-term performance.
Concrete pavement owners care most about a pavement’s long-term performance—its durability and strength. Contractors care more about its constructability—its workability, setting, and early strength that constrain when their next phase of work can begin. Both care about their bottom line… Many variations of concrete recipes exist. Mixture proportioning is the iterative process of determining how much of which ingredients available in a given locale can most economically produce the best concrete matching pavement requirements.
It is the goal of the PEM Transportation Pooled Fund—TPF-5(368): Performance-Engineered Concrete Paving Mixtures—to bring these newer technologies to state agencies and to assist states in adoption of the test methods that will help them deliver on the promise of concrete durability. The Federal Highway Administration (FHWA), 19 state departments of transportation, and 4 national associations representing the concrete paving industry have come together to fund this project. It is a coalition of federal, state, and industry leaders dedicated to maximizing pavement performance. These state agencies and contractors have adopted an approach to mixture proportioning developed by the CP Tech Center. This approach is based on the premise that given a fixed w/cm ratio and binder type, mixture performance will not improve above a certain paste content threshold. Though contrary to popular perception, research has demonstrated the validity of this premise.
The CP Tech Center approach to mixture proportioning therefore starts by:
• Seeking a combination of locally available aggregates to fit the “tarantula curve”
• Volume of space between solid particles in the resulting blend is then measured in the lab
• Next, the paste’s binder type, w/cm ratio, and air void system requirements are selected
• Specifications often directly address these parameters to assure the needed durability and strength
• Last step is adding sufficient paste (including air) to fill the voids between aggregates as well as a limited additional amount of paste to achieve required workability—While this amount of paste is best determined by preparing trial batches, experience has shown the total amount needed is typically about 1.5 times the volume of voids.
A spreadsheet developed to help practitioners apply this method of mixture proportioning has been used in a number of applications. For example, the Wisconsin DOT has permitted contractors to reduce their minimum cementitious content if they adopt this procedure, with positive results reported by both contractors and agency. Noteworthy has been the repeated finding that mixture performance using this method is not compromised—indeed in some cases it has improved, with projects’ CO₂ footprint also reduced. BELOW: Does this approach to mixture proportioning work?
Before (left) vs. after (right) mixture proportioning as described above
IMPORTANT LINKS:
PEM webpage with information, VIDEOS, and TEST METHODS on CP Tech website: https://cptechcenter.us10.list-manage.com/track/click?u=4bf55bc365dca0e5c928442da&id=2f95bccdd5&e=2945805a7f
Concrete Mixture Proportioning webpage—State of Research, Guide Manuals, Guide Specifications, Spreadsheets, MAP/Tech Briefs: https://cptechcenter.us10.list-manage.com/track/click?u=4bf55bc365dca0e5c928442da&id=5c532ef904&e=2945805a7f