Our frequently asked questions (FAQs) about concrete cover a wide range of topics, from basic facts and terminology, to common techniques and methods, to national engineering standards.
You can also contact our knowledgeable and experienced staff who can provide detailed information and answers suited to your specific concrete needs.
Click on any of the topics below for frequently asked questions and answers.
What is the difference between cement and concrete?
Although the terms cement and concrete are often used interchangeably, they are actually different materials. Concrete is the hardened, rock-like mass that we are all familiar with, and is a mixture of coarse and fine aggregates (gravel and sand), and paste.
Cement, or more correctly Portland cement, is the active binder in the paste that “glues” the aggregate particles together to give 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 years after the concrete is placed, meaning that concrete gets stronger as it ages.
Why does concrete crack?
All concrete shrinks as it dries and loses moisture, and this shrinkage commonly results in cracks. The amount of shrinkage is typically 1/16th of an inch over a 10 foot span (5mm over a 10m length). Contractors will put joints into floors and pavement to encourage the concrete to crack in a controlled straight line as the volume changes from shrinkage.
How are different strengths of concrete produced?
With modern computerized batch plants, we are capable of closely controlling the amount of sand, coarse aggregate, cement, and water added into a batch of concrete. This gives us the ability to consistently produce a nearly infinite variety of concrete mix designs with varying properties.
The easiest way to produce higher strength concrete is to add more cement to the mix. By adding more cement, we increase the amount of “paste” binding the aggregates together which increases the strength of the hardened mass.
As we adjust the mix design, the water-to-cement ratio (w/c) has the greatest influence on the final concrete properties. As the w/c ratio increases (more water in the batch), the strength (as well as every other desirable property) of the concrete will be negatively affected. Similarly, reducing the water added during mixing will increase the final strength of the batch.
We also stock a variety of admixtures (chemical additives) that are mixed in to a batch of concrete to change the concrete’s properties.
How does concrete harden?
Concrete hardens through a chemical process called hydration. Cement particles react with water to form new stable compounds that coat and bind the aggregate particles in the mix.
What is “28-day” strength?
Concrete hardens and gains strength as it hydrates. The hydration process proceeds very rapidly initially, but will continue at a slower and slower rate for years after placement. Since it is not practical to wait years to determine the ultimate strength of the concrete, a 28-day period was chosen by engineering authorities as a suitable length of time for curing before strength testing of samples. In general, when we refer to a concrete’s strength, we are referring to its 28-day strength.
What is “MPa”?
MPa is an abbreviation for “megapascal,” and is the metric unit of measure for the compressive strength of concrete. In the U.S., concrete strength is measured in 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 in either the plastic (unhardened) or hardened state. We stock a range of admixtures that can be added at a customer’s discretion to adjust the concrete to best suit the customer needs based on the application or the climate at the project. Admixtures that we carry include hardeners, colourants, and waterproofers. Please contact us to inquire about admixtures available.
What does “placing” concrete mean?
Placing is the process of distributing and consolidating the concrete from the mixer truck into the forms, moulds, or excavations as required at the job site. Concrete is typically poured into forms using a concrete pumper unit, a conveyor unit, or wheelbarrows, or can be poured directly from the mixer chute depending on the job. Placers will use rakes and screeds (long, straight boards) to move the concrete around within the form to create the correct elevations and profiles for the final product.
What does “finishing” concrete mean?
Finishing is the process of working the exposed surfaces of the concrete after placement to create the desired final appearance. This may include trowelling to create a flat and smooth surface, trowelling in joints to control cracking, brushing for skid resistance, edging, colouring, and stamping for an attractive architectural appearance, wash-off for an exposed aggregate finish, or other techniques.
Placement and finishing have a very significant impact on the final quality of the concrete. In particular, proper finishing can make or break any job, so you should take care to ensure that your placer/finisher is qualified for the work to be done.
Why is it important to place concrete in a certain timeframe?
According to Canadian Standards Association standard A23.1-09, complete discharge of a batch of concrete must be completed within 120 minutes after initial mixing unless otherwise agreed by the owner and supplier. In practice, the workability period of a load is impacted by both type of cement used and by ambient climate, and can be modified by set-retarding or accelerating admixtures.
We stock both accelerator and retardant admixtures to enable delivery to remote locations and in both hot- and cool-weather conditions without any loss in final product strength or durability.
Can I add water to the mix at the jobsite?
In past times, water was commonly added to batches of concrete at the jobsite to increase its slump for greater workability and/or ease in finishing. However, adding water also negatively impacts both the strength and durability of the hardened concrete.
Canadian Standards Association standard A23.1-09 places strict limits on the addition of water to a batch of concrete at the jobsite. We batch our concrete with the correct amount of water at the plant to meet the water/cement ratio required for the mix design in order to meet 28-day strength. We do not recommend adding any water to the mix at the jobsite to improve workability unless directed by an appropriately trained expert. We will require acknowledgement and sign-off by the owner or general contractor that any warrantee is voided before water is added at their direction.
If you want greater slump or workability, all our mixers carry plasticizer admixture that can be added at the job to improve workability without any loss in final properties in the hardened state.
Why can cold weather be a problem for concrete projects?
Concrete needs to set and cure at a suitable internal temperature to achieve its maximum material properties. As the temperatures approaches 0°C, hydration slows and stops. While the hydration process does generate some heat by itself, it is not enough to permit proper curing when the ambient temperature approaches freezing.
In the interior of B.C., winters are too cold to enable concrete production using standard methods for a large part of the year. Winter Services allow us to produce concrete that meets all properties and can be successfully placed and cured in ambient temperatures down to -20°C. As a result, we can produce and deliver concrete year-round and in almost all climatic conditions.
What special provisions are needed at the jobsite for cold weather concreting?
Cold weather concreting requires extra provisions at the pour site. Freshly poured concrete needs to be kept at temperatures between 5°C and 30°C during its initial curing period to achieve maximum strength. During cold weather conditions, the contractor will commonly need to install hoarding and heating around the formwork to protect the concrete while it cures.
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 A23.1-09/A23.2-09 are the reference standards in Canada for concrete production and construction. These standards are developed and maintained by the Canadian Standards Association (CSA), which is an independent, not-for-profit standards development and testing organization serving consumers, government, industry, and business in Canada and in the global marketplace.
CSA A23.1-09 (Concrete Materials and Methods of Concrete Construction) provides requirements for input materials and methods of production for concrete used in buildings and structures to meet the National Building Code. A23.2-09 (Test Methods and Standard Practices for Concrete) comprehensively describes all principal test methods for hardened and freshly mixed concrete and for concrete input materials.
Together, these standards give us a methodology to ensure that a quality product is produced and validated when delivered.
Air entrainment is a modification to the concrete that dramatically improves its durability throughout freeze/thaw cycles. Air-entrained concrete has billions of microscopic bubbles of air distributed through the mix that relieve internal pressure in the concrete by providing cells for water to expand into during freezing.
Air entrained concrete is produced by adding a carefully controlled amount of air-entraining admixture to a load of concrete during batching. Typically, air entrainment is 4% to 7%, but this can be changed depending on the requirements or conditions of the job.
Note that air entrainment refers only to the microscopic air bubbles created by the admixture, and not the visible voids often seen in cast-in-place concrete after the forms are removed. Large-sized or numerous voids are a result of poor consolidation during placement, and result in reduced strength and durability of the concrete.
When concrete is poured into a form, it will retain large air pockets or voids that significantly reduce its strength when cured. Consolidation is a process used during placement that eliminates or reduces the size and number of these voids by mechanical agitation. Consolidation is typically completed by rodding the concrete with a pole, or by vibration with a concrete vibrator.
Curing is the process of controlling the conditions to which the concrete is exposed immediately after placement in order to maximize hydration. The curing period is considered to be the first 5 to 7 days after placement when most of the hydration reactions occur.
Curing is one the most important factors in determining the final properties of concrete. Proper curing conditions will dramatically improve the final strength and durability of a pour by allowing the cement to fully hydrate. For optimal curing, the concrete temperature must be kept between 5°C and 30°C, and the surfaces of the concrete must be kept fully moist.
Ready-mixed concrete is a perishable product. It has a limited lifespan after mixing before it must be poured and allowed to harden. Plastic (unhardened) concrete that is too old at the time of discharge is difficult to work with during placing and finishing, and may not achieve full properties after hardening.