For concrete to function in a cold climate a number of items are necessary. First is the need for the proper mix design. Water, cement and air must be used in the proper proportions to create durable concrete.
Consider this simile, your grandmother has been known for the delicious cookies she makes every Christmas. She follows her recipe to a T to achieve her desired results…that perfect melt-in-your-mouth cookie. Concrete, like your grandmother’s cookies, also requires the proper proportions of each ingredient to create the desired results. Too much water, the cookies will be flat, hard and brittle; too little water and egg, and the dough will be hard to work with.
Concrete has similar needs, and each part of the recipe must be followed to achieve the proper outcome.
So how do we know what the recipe calls for? Well that depends on what we want in the final outcome. For strength and durability, typically too much water will reduce those qualities. If we have too little air entrainment, then the concrete will be brittle and flake. If we have too much paste (flour) we will not be able to work our dough.
For each type of product used in the recipe for concrete, the most expensive part of the recipe is typically the cost of the cement. The other products, air, water and aggregate, (the chocolate chips of our mix) are relatively inexpensive. The recipe can be altered by changing or adding ingredients. For example fly ash is used as a substitute to some percentage of the cement. The fly ash is a product typically recycled from coal fired power generation plants. There is plenty of discussions on that product, beyond this information The civil or structural engineer will specify the mix recipe and the plant will provide the mix. The contractor receives the plant mix, submits it to the engineer or architect for approval, and then has it delivered. The time from the plant to the site is crucial, as is the waiting time if the contractor does not yet have the site ready for the placement of the concrete. Similar to leaving the dough out too long before the cookie cutters make the stars, Santas and trees. Engineers would call that forming the dough.
If you are in a cold climate, the number of freeze thaw cycles must be understood, and the condition of the winter shadow lines as concrete will have many wet and frozen cycles that require higher durability. The concrete mix then must be properly placed, and that placement, along with the working of the concrete, can have a good or bad impact on the final product. Similar to the way Grandma worked the dough, reworked it, then flattened it to form (shape) her cookies. Overworking the concrete mix will produce undesired results such as loss of the air or floating the fines to the top layer. Those fines will not be bound in the mix, and the concrete will spall.
In some cases chemical admixtures can be used to provide strength, air entrainment, hydration speed, workability and other desired qualities such as density. There is, as always, additional cost associated with these additions, yet the designer or contractor may need these, depending on the situation.
This link provides a mix design per ACI methods (not approved) from the http://concrete.union.edu/general.htm “ACI 211.1-91, Reapproved 2009, states: “The procedure for selection of mix proportions given below is applicable to normal weight concrete. Estimating the required batch weights for the concrete involves a sequence of logical straightforward steps. Some or all of the following specifications are required; maximum water-cement or water-cementitious material ratio, minimum cement content, air content, slump, maximum size of aggregate, strength, and admixtures.”
Hiring a consultant with a thorough understanding of concrete design and specification, as well as the ability to conduct quality assurance work will help you select the right mix for your application.
SBSA’s Solutions Before services are here to help you properly plan and execute your project. Our Solutions After services help provide you with concrete evaluations.
Written by Edward L. Fronapfel, PE and Jeff Felderman, PE.
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