Many specifying authorities seek to assure the quality of concrete provided to them by choosing a prescriptive approach that narrowly defines the quality or source of raw materials, and the proportions in which they are used. This is preferred by some concrete suppliers because it relieves them of the responsibility and risk of fine tuning a mix. The negative sides is that it stifles innovation, is specific to a given locality, and may not be the most sustainable approach. The alternative is for the specification to be based on performance requirements. This means there have to be good test methods to assess the critical performance characteristics and that suppliers have to be prepared to take on additional risk along with the potential reward of reducing costs.
What is the question?
The term “mix design” means different things to different people. Implicit behind it are three different activities, which are all critical to the people involved.
One is the act of fine tuning a mix based on ongoing data. This is common in ready-mix plants where they have a constant source of materials and some freedom to tweak a mix to maintain performance through careful quality-control monitoring.
Another activity is developing new a mix using numerical models for theoretical materials that seek to optimize proportions based on assumed relationships between cost, performance, and dosage.
A third approach is finding a starting point for a trial mix based on the requirements of a specification and the availability of materials in a given location. This is the position that paving contractors find themselves in when bidding and constructing slipform pavements in different locations.
Time and effort goes into the structural design of a pavement. The alignment is optimized, support layers are argued over, and concrete thickness is analyzed in great detail. Implicit behind these activities is that the concrete delivered will be potentially strong and durable enough for its environment. It also is assumed that every load delivered will meet performance expectations.
These performance requirements often are unclear. Typically the parameters measured include strength, air content, and slump, which provide limited information about the concrete's real quality. The questions that should be asked are: What are the critical requirements? How are they attained? How can they be measured?
What is wanted from a mix and why?
FRESH (PLASTIC) PROPERTIES. Contractors worry about fresh concrete because it affects their ability to place and finish it correctly and cost effectively. They look at:
- Uniformity. Operation of a slipform paving machine is sensitive to variability from each load to load. It is desirable that every load is similar to the one before.
- Workability. Slipform paving is extremely sensitive to the workability of the mix—too wet and there is edge slump, too stiff and it is difficult to consolidate and maintain smoothness.
- Air void system. The ability of the concrete to resist freezing and thawing cycles in cold climates is strongly dependant on the quality of the air void system. Are there enough small bubbles to protect the concrete without reducing strength excessively?
- Segregation. If the mix is unstable and segregates, there is significant risk that the ability to resist abrasion from traffic is compromised.
- Bleeding. Mixes that do not bleed are likely to crack, but excessive bleeding will compromise the durability of the top surface of the pavement, leading to scaling and polishing.
- Setting. Timing of sawcutting to prevent random cracking is dependent on setting time. Variation from batch to batch will increase the risk of random cracks. Likewise, extended setting times increase the risk of plastic shrinkage cracking in hot weather.
HARDENED PROPERTIES. Pavement owners primarily are concerned about how long the concrete is going to last. In addition to plastic properties, a mix also must be durable and strong.
- Strength. Structural designs are based on the assumption that the concrete will achieve a given strength. This is not often a problem, particularly in severe climates because the efforts to make the concrete potentially durable will lead to high strengths anyway. It should be noted that high strength is not a guarantee of durability. Rapid early strength development may be desired to facilitate construction, but may reduce longer term strength.
- Cracking. Premature cracking may lead to rapid loss of serviceability. Cracking prevention is complex and requires attention paid to design detailing, materials issues, and workmanship.
- Durability. Materials can be selected and proportioned to make the mix potentially durable. However, it may still be possible for performance to be compromised through poor workmanship, such as improper curing.
COMPROMISE. Sometimes, decisions made to address one concern may compromise another. This means that a significant effort must be paid to balancing often mutually exclusive demands. An example is that permeability will be reduced, leading to improved durability, when decreasing the water-cementitious materials ratio. However, this also will make the mix more difficult to handle and increase the risk of cracking. A sound understanding of the whole system ensures good engineering decisions.