Current trends: concrete mix design

Jereme Montgomery, Executive Director, Nebraska Concrete and Aggregates Association

Everyone knows that Portland cement is a key ingredient in concrete. While it hasn't changed much throughout the years, concrete mix design professionals have developed the ability to manipulate their project mix designs in accordance with specific construction project requirements.

Jereme Montgomery, Executive Director of the Nebraska Concrete and Aggregates Association, discusses Portland cement and its evolution in today's concrete.

It's concrete, not cement.

Have you ever fallen on a “cement” sidewalk and skinned your knee? Then, you were pretty dusty! Portland cement is a powder that chemically reacts with water to form a hardened 'glue' that bonds rocks together called concrete. Although cement is only 10-12% of the total volume of a cubic yard of concrete, it may be the most important ingredient regarding constructability and durability of a concrete mix. Let's take a closer look at this aggregate binder.

Types of cement:

First, there are different types of cement. ASTM C150, the standard specification for Portland cement, categorizes cement into 5 different types. For this article, let's focus on a couple of categories:

Type I Cement is a general purpose cement most commonly used for general construction. People often ask, “Has cement changed over the years?” In short, the answer is “Yes.” Although the chemical composition has not dramatically changed, today's cement is ground finer than in the past. The reasoning is due to construction schedules. Owners want to move-in sooner, therefore contractors need to move faster. If the cement particle is finer, it has more surface area that reacts with the water to hydrate and harden faster. The faster the concrete gains strength, the faster contractors can strip forms or open structures to traffic.

Let’s take another type, Type III Cement, which is commonly referred to as a “high-strength" cement. Mix designs using that cement are typically used for paving repairs, allowing DOTs and municipalities to open the street within hours instead of days.

Blended cements:

Another change in cements is the use of blended cements, such as Type IP cement. The “P” refers to pozzolans. Popular examples of these pozzolans are fly ash, silica fume, and slag. Some are a byproduct of coal. Some are a byproduct of steel. Some are a byproduct of silicon. For example, one type of blended cement used in Nebraska is Type IPF cement, where 25% of the Portland cement is replaced with Fly Ash. This type of cement is required by the Nebraska Department of Roads (NDOR) for state work.

The benefits of pozzolans in concrete are threefold. First, these are materials that would normally go to the landfill and can now be used in concrete as a recycled material. Second is economic gain. Depending on where you are located, pozzolans are typically cheaper than cement. The third benefit is improved durability properties. Since pozzolans are smaller in size than cement grains, they ultimately create a denser, more durable, finished, hardened concrete mass, often referred to as particle packing. Other benefits of blended cements may include increased strength, workability and improved finishability of a given mix.

It's important to remember that these blended cements may not hydrate like a “straight” cement mixture. While they are proven to have higher ultimate strengths, it takes longer to achieve specified strengths, especially in colder climates. Some specifiers have moved to a 56-day strength requirement versus the traditional 28- day strength requirement. In the future, you will see an increase in ternary mixes, utilizing three different cementitious components in the mix.

Thousands of mix options:

While the engineer writes the spec, it is difficult for the engineer to specify constructability, placeability, or finishability. Therefore, chemical admixtures play a big role in taking a basic concrete mix and making it do things you want it to do. We use chemical admixtures to make the concrete harden in cold weather or slow down its hardening in hot weather. We use chemical admixtures to make non-pumpable mixes pumpable and also to make concrete workable with very little water. The advancement of chemical admixtures is by far the most dynamic component of concrete over the past few decades.

There used to be only a few different concrete mixes. Today, there are many, many ways to proportion a mix. Concrete placement properties can be completely customizable to the concrete project and the installer.

The clock is ticking...

Concrete is a perishable product—the mix is only produced as early as 20 minutes before it arrives on-site! Communication between the engineer, construction manager, producer and installer is critical to the success of each concrete project. A plan must be in place before the concrete mix arrives on the job. If quality control plans are executed, concrete projects usually go quite well, making concrete the world's greatest building material.

About the Author:

Jereme Montgomery has a Bachelor of Science degree in Construction Management from the University of Nebraska. He has over 18 years experience in concrete. Since 2006, he has promoted concrete and aggregate products as the Executive Director for the Nebraska Concrete & Aggregates Association. He is also the current President for the American Concrete Institute-Nebraska Chapter and current member of the Construction Industry Advisory Council for the Durham School of Architectural Engineering & Construction.

Contact:

Jereme Montgomery
Executive Director,
Nebraska Concrete & Aggregates Association
5700 Seward Av, Suite B | Lincoln, NE 68506
phone: 402-325-8414
e-mail: jereme@nebrconcagg.com website: www.nebrconcagg.com


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© 2014 L&M Construction Chemicals, Inc. | ConcreteNews Winter 2014.

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