Salmon Arm Ready Mix Ltd. | Frequently Asked Questions

Read about the difference between concrete and cement, the concrete production process and other bits of useful information.

What is the difference between cement and concrete?

Although the terms 'cement' and 'concrete' are often interchanged, they are actually different materials. Concrete is the hardened, rock-like mass that we are familiar with, and is a mixture of coarse and fine aggregates (gravel and sand), and paste (cement).

Cement, is the active ingredient that creates the binder for the sand and rock, which gives concrete its strength. Concrete hardens through a chemical process called hydration where the cement combines with the water in the mix. The hydration process actually continues for many years after the concrete is placed, meaning that concrete gets stronger as it ages.

Why does concrete crack?

All concrete shrinks as it sets and loses moisture, and this shrinkage commonly results in cracks. The amount of shrinkage is typically 1/16” of an inch over a 10 ft. (5 mm over 10 m) span. Contractors will deliberately put joints into floors and pavement to encourage the concrete to crack in a controlled line as the volume changes. Keeping the cement-to-water ratio as low as possible (workable) will also help minimize cracking.

How are different strengths of concrete produced?

With modern technology and computerized batch systems, concrete batch plants can very closely measure all ingredients that go into a load of concrete. By simply adding or subtracting certain materials, the strength can be easily altered. To increase strength, the easiest method is to simply add more cement powder, creating more paste to bind the aggregates. Keeping a correct cement-to-water ratio strongly influences the ultimate strength of concrete. By adding too much water, the ratio becomes too lopsided and the strength will rapidly drop.

How does concrete harden?

Concrete hardens or sets through a chemical process called hydration.

What is MPa?

MPa is the abbreviation for “megapascals,” and is the metric measure for the compressive strength of concrete. MPa is the equivalent to the US standard of PSI, or pounds per square inch.

What are admixtures?

Admixtures are chemical additives that are mixed in to a batch of concrete to change its properties either during its early state or hardened state. Many products are available, such as water reducers, which are essential in every load, as well as accelerators, retarders, viscosity modifiers, air entrainment, and many more.

What does “placing” concrete mean?

Placing concrete is the process of “distributing” and consolidating the concrete into forms, excavations, or flat slab works. Concrete is typically poured at different slumps (wetness) depending on its application. Placers will use different tools like rakes, hand trowels, and straight boards to move the concrete, creating a surface as flat or properly-elevated as possible.

What engineering standards govern the production and use of concrete?

Concrete is produced and used as an engineering material according to specific engineering standards that govern testing of the input material, process equipment, and final product.

CSA A23.1 1-09 and A23.2-09 are the reference standards in concrete production and construction. CSA A23.1 1-09 provides requirements for input materials and methods of production for concrete used in buildings and structures.