Concrete Mix Proportions

Concrete mix proportions are so essential, yet so often overlooked.

Ever see someone randomly shoveling rocks and cement into a wheelbarrow and then blasting it all with the garden hose? I call that the "guess, shovel and saturate method."

That all too common method results in an inconsistent mix of horrid quality (but at least it gives some laughs to better informed passers-by).

Thankfully, you are going to be way better off because you are investing some time to understand how to optimize your mix properly.

Preparing on the front-end to optimize your mix design will save you lots of time going forward.

Focus on utilizing correct concrete mix proportions so that your concrete will achieve a higher yield strength. In order to design an effective mix, it's essential to understand concrete mix proportions as well as the individual raw materials that are blended to create concrete.

At it's most basic level, concrete is just a blend of coarse and fine rocks (aggregates), cement, and water.

Admixtures (i.e. superplasticizers, fly ash, water reducers, air entrainers) are often added to the basic mix to alter select characteristics (i.e. workability, strength) of the concrete.

Before designing a concrete mix, you should begin with the end application in mind.

There is no need to design a 20,000 psi (pound per square inch)mix for a backyard slab.

Over-designing your mix can blow your project budget and make things way more complicated than they need to be.

The best way to assure an optimal mix is to proportion your concrete mix following ACI (American Concrete Institute) Standards. Here are the American Concrete Institute steps to follow to achieve effective concrete mix proportions:

1. Choose Slump- When choosing the slump for your mix, you are prescribing the workability of your finished mix.

2. Select Maximum Aggregate Size- The aggregate size used in your mix will affect both the workability and overall strength of your concrete.

3. Estimate Mixing Water and Air Content- Consult this table to learn the optimal mixing water and air content quantities required to achieve your desired slump.

4. Determine Water-to-Cement (w/c) Ratio- The water-to-cement ratio has much to do with the yield strength and durability of your mix. Typically, a lower proportion of water-to-cement will result in higher durability.

5. Determine Cement Content- The proper cement content for your mix is figured by the above steps of selecting mixing water content and the water-to-cement ratio.

An often used practice for smaller concrete projects to estimate cement content based on a sack system (i.e. "5 sack mix" or "10 sack mix").

This practice assumes that a particular mix will require "X" number of 94lb cement bags for the needed mix.

If you are content to complete your project based on guesstimates, the "sack method" might be ideal for you. There are countless websites that will show you how that method works.

However, you will create stronger and more consistent concrete mixes using proven and absolute ACI methods.

6. Figure Coarse Aggregate Content- The coarse aggregate content of your mix will affect the overall workability of the mixture. Nominal sizes for coarse aggregate range between 0.375 inches and 2 inches.

For DIY projects such as slabs, footings, stairs, etc., utilize coarse aggregate between 0.375" and 1". Sizes above 1" are typicaly used in heavy-grade pavements or large load-bearing structures.

7. Figure Fine Aggregate Content- The fine aggregate content fills the remaining volume of the concrete mix after volumes of mixing water, portland cement, and air have been decided upon. For most DIY purposes, sand will typically be the fine aggregate used.

8. Adjust for Aggregate Moisture- The final step in concrete mix proportioning is to adjust for aggregate moisture. The aggregate in your mix will always either absorb added mixing water or be pre-saturated and alter the overall water content of your mix.

It seems like a small thing, but the hydrous state of the aggregates can significantly affect the composition and workability of the mix.

Therefore, you should tactically adjust your moisture content. I will be glad to help you.

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