Understanding Ready Mix Concrete

The Balancing Act Of Batching And Delivering Ready Mix Concrete

There are several ways to identify the phrase ready mix concrete. Others include redi mix, readymix, ready mix cement and premixed concrete. They each refer to a type of concrete that is manufactured in a batching plant and delivered to a work site using concrete cement trucks.

Ready mixed concrete is often used for lower volume projects such as urban paving. It is usually preferred over on-site concrete mixing because of the precision of the mixture and reduced work site confusion. It is a mixture of Portland cement, water, and aggregates comprising sand and gravel or crushed stone. These raw materials are purchased by weight whereas the batched concrete mixture is sold by the volume and usually expressed in cubic yards or cubic meters.

There are several advantages in using redi mix concrete to include the elimination of storage space for materials that would be required if concrete were to be mixed at the job. And, you can eliminate the labor requirement on location, plus there is less waste of materials.

Of course, there are also some problems such as lumps or segregation resulting in non-uniform concrete mix strength. Also, mixing cement at high speeds for long periods of time results in reduced air content and decreased workability.

It is recommended that once all of the components have been added at least 70 to 100 revolutions be applied to the ready mix drum at a speed of between 6 to 18 revolutions per minute.

Placement of the concrete at the job site should be within 90 minutes of the initial mixing. On-time delivery is not only vital to the success of completing a project, it is essential if the concrete producers and suppliers are to uphold a reputation for high standards.

Most ready mixed concrete is currently manufactured under computer-controlled operations and transported and placed at project sites using sophisticated equipment and methods.

There is an interesting theater played between contractors and batch plants. For instance, the contractor must order a sufficient quantity with an appropriate lead time to ensure available batch plant delivery service in an attempt to get maximum productivity from the placing crew.

On the other hand, the batch plant tries to time deliveries so all projects are served according to each contractor's needs, while simultaneously ensuring their trucks and drivers have minimum idle time. This is typically very challenging due to the nature of concrete itself, as well as the nature of the production systems.

Sometimes it makes sense to mix concrete yourself if the project is relatively small. You can mix all the individual ingredients, use pre-mixed where you simply add water, or call in the delivery cement truck with a batch of redi mix.

Pre-mixed bags are quite cost efficient and come in 60 and 90 pound sizes. However, there is a point of diminishing return where the number of bags you physically need to prepare for larger projects simply overwhelms the economy of mixing it up yourself.

It's important to remember that concrete is a perishable material. The individual components can be stored for long periods of time, but ready mix concrete has a limited life span. Once water has been added to the mix of dry materials concrete has approximately one to two hours (unless retarders are used) before the hydration process will form a gel that, if disrupted, would jeopardize the ultimate concrete durability. Consequently, fresh concrete leaves little room for variability in terms of time for delivery and placement once water has been added.

Another consideration is that different mixes may be needed in varying quantities on a single project because of different uses for the concrete such as a shear wall and a foundation slab. Not all projects require a uniquely designed mix, however. For example, standard mixes are specified by government agencies for public works projects such as sidewalk and road paving.

Accordingly, most batch plants have an on-line database with recipes for hundreds if not thousands of mixes that they can load to program their facility. This makes it easy not only to add new mixes or find those that meet an engineer’s requirements, but also to name them based on customer preferences.

The production system for ready mix concrete is governed by the plant operator’s equipment, the contractor’s placement method and their individual schedules as well as the coordination of those schedules between them. Concrete batching plants have batching capacity and delivery capacity both of which are limited.

Today’s ready-mix batch plants are fully automated and computer controlled so ingredients can be measured and mixed on demand and in virtually no time. This makes it possible to batch one truckload at a time and load one truck after another with a different mix each time when necessary.

Demand for concrete fluctuates throughout the day, week, and year. Even when the total amount of concrete for a specific placement or a project can be estimated reasonably well, the timing of the needed delivery often remains uncertain because completion of prerequisite work at the site is difficult to forecast reliably and may not be known exactly up until a day or so prior to placement.

The main challenge for cost-effectively and reliably batching and placing redi mix concrete lies in scheduling. At the time a contractor calls in an order to a batch plant, many unknowns remain. How a batch plant and the contractor handle these unknowns can be the determining factor for making or losing money.