Five Sack Concrete Mix

Concrete – when cured it normally looks gray and is hard. Not much more to know about it… right?

Bag ConcreteWrong. All concrete is not created equal. Modern concrete mixtures often contain many additives, some of them used to decrease the selling price of the pre-mix, others for strength or curing characteristics.

Back in the “olden days”, concrete was ordered from ready-mix suppliers by specifying the number of 94 pound bags of Portland cement which would be added per yard of concrete. Terms like five sack or six sack were very familiar to all involved.

As more cement is added per yard of pre-mix concrete, the compressive strength of the concrete is increased. Compressive strength is measured in PSI (pounds per square inch) which cured concrete can withstand before failure. The psi of a concrete mix is termed in a 28 day strength test. In other words a 3000 psi mix, tested by a lab facility 28 days after being poured, must not fail below 3000 psi of force.

More compressive strength may provide stronger concrete, but is not always the best solution for a particular use. Often contractors will try to “one up” the competition by promoting the use of higher strength concrete than their competitors. Many do not realize high amounts of Portland cement, especially in combination with over hydration (too much water), can result in shrinkage cracks in finished concrete.

On a sidebar – when I was a general contractor, clients would often ask what the guarantee was on concrete slabs in their new pole buildings. My response was, “I will guarantee it will crack”.

What is the best mixture to use? For concrete footings and encasements around the columns of pole buildings, a 3,000 psi mix is usually more than sufficient.

With interior flatwork, like the concrete slab inside a pole building, which is not exposed to freeze and thaw cycles, a 3,000 psi mix without air added will be fine in the great majority of cases.

Outdoor pours, especially when dealing with freezing and thawing cycles, prove to be more of a challenge. To avoid or minimize spalling, Increasing the concrete mixture to 4,000 psi is often recommended. Adding “air” to the mix, as well as using a low slump, helps to avoid spalling.

An experienced local redi-mix company can help with selection of the correct mixtures for any application, so don’t be afraid of utilizing them as a resource.

4 thoughts on “Five Sack Concrete Mix

    1. Matt ~ Thank you very much, quite a compliment coming from a concrete expert! When we expand to the U.K., hope to have our clients there pouring your concrete in our buildings.

    1. Mobile transit concrete mixers are a mix-and-match selection of engine, truck frame and rotating mixer. The mixer is similar, though larger in scale, than the smaller ones found on construction sites. A large motor, separate from the engine, rotates the drum on the truck body, and a series of blades or a screw powered by the same motor keeps the aggregate, water and cement in constant motion. This keeps the premixed concrete from setting, though the clock is often ticking to get the load to the construction site, road section or parking lot. Most cement manufacturers suggest keeping the time between mixing and pouring to 90 minutes at most. It’s even better to get it to the site in less than an hour.

      As technology has changed, so has the basic mixer design. While many transit mixers still have rotating drums, most don’t simply pick up a load of wet cement and transport it. The few which still do head mostly to road sites where it’s possible to pour the mixture immediately.

      Most transit mixers have a separate water tank in the truck. The spinning drum keeps the dry ingredients, aggregate and cement mixing during most of the trip. When the driver is within a few miles of the site, water is added to create concrete fresh for delivery.

      This is considered “batch” delivery of ready-mixed concrete, mixing ingredients off site and trucking them where they’re needed. Advances in technology have made it possible to mix concrete at the job site, though transit mixers are still the workhorse of the field.

      Volumetric and metered mixers are becoming more common. Both types are essentially on-site custom concrete plants. Separate holding tanks of aggregate, cement and water are contained in one truck with a computer hooked to augers and pumps. At the site, the customer can order a specific type of concrete (there are more than a dozen) which can be mixed by the truck.

      Before the concrete is pumped or poured, a number of simpler machines act in concert to keep the concrete from setting, and even mix the concrete at the site. Some of the older portions of the fleet, like early mixers, used paddles to stir the concrete and keep it from “settling out,” or separating into its component pieces. This technology has been largely replaced by the use of augers and fins. Inside a traditional batch mixer is a concentric series of fins with a slight corkscrew pattern. The direction of the drum’s spin squishes the wet concrete into the back of the mixer. When the mixer arrives at the site, the driver reverses the direction of the machine to push it out of the mixer to the chute. From there, gravity does the rest.

      Volumetric mixers use augers to move concrete. These are similar to the blades in the batch mixer but smaller. Inside the mixer, an operator feeds data into the mixer and several augers feed aggregate and cement together. Water is added to the mix and larger augers blend the components.


Leave a Reply

Your email address will not be published. Required fields are marked *