It’s normal to have questions about concrete materials, installation, applications, specifications and more. Below, you’ll find answers to some of the most common concrete-related questions provided by our in-house experts!
What is concrete?

Concrete is the second most used substance in the world, after water, and it is the most widely used building material on earth.
Its worldwide consumption is twice that of steel, wood, plastics and aluminum combined. This fundamental building material can be used
for various applications (footings, foundation wall, slab floors or roofs, sidewalk and curbs, caissons, bridges, etc.).It’s delivered to the job site in a plastic, mouldable, unfinished state On job sites it is poured and finished into any shape.” , with “On job sites produced concrete can be placed and moulded into any shape desired. After it hardens it forms a man-made rock. It’s a combination of materials… blended with specific proportions to give it certain properties

Concrete Properties

In its hardened state concrete has the following properties:

  • Strong, durable
  • Non-flammable & fire resistant
  • Good insulator, sound barrier
  • Abrasion-resistant
  • Won’t corrode or decay
  • Resists insect infestations
  • Requires very little maintenance
What goes into a concrete mix?

1) Cement
A powder which hardens (sets) and binds to other materials

2) Aggregate
Fine (sand) and coarse (gravel/limestone) materials

3) Water
We use both recycled, and municipal water

4) Admixtures
Today’s concrete utilizes various chemicals to achieve certain functions
Water Reducers, Super Plasticizers, Air Entrainers, Retarders, Accelerators, Corrosion Inhibitors, etc

5) Additives
Colour, Fibres, Waterproofers, Ice, etc.

What is the composition of a concrete mix?

Composition by Mass:

  • Aggregates (Coarse + Fine) ~ 70-75%
  • Cement ~10-20%
  • Water ~5%
  • Air Content ~5%
What is concrete strength?
  • Compressive strength, measured in Megapascals (Mpa), is the measure of what pressure concrete can withstand before cracking. (Note: 1 Mpa = 145 PSI)
  • When a customer requests a certain concrete strength (i.e. 32 Mpa), the concrete must reach a minimum of 32 Mpa at 28 days from when it was poured. Typically, 75% of its design strength is attained at 7 days
  • Concrete strength requirement is often dependent on the concrete application. Strengths can vary from 0.4 Mpa (unshrinkable fill mixes), to the more common 32 Mpa mixes (sidewalks, driveways), to high performance mixes (50 Mpa +) for large footing applications
  • Concrete strength is influenced by several factors: quantity & type of cement, water/cement ratio, admixtures, temperature, curing, etc.
  • Note that while concrete can attain great compressive strengths, it is very weak under tension (tensile strength ~10% of compressive strength)
What is slump?
  • Slump is common measure of concrete consistency or “wetness”. It is a good indicator of the concrete workability (ease with which concrete could be placed & consolidated)
  • It is measured by using a standard slump cone and expressed in units of mm or inches
  • The slump ordered is dependent on the concrete application. The most common slump is 80mm (3 inches) often used for sidewalks, driveways, patios, walkways, etc. However, drier mixes (20-60mm) may be used for curbs & stairs and wetter mixes (150-200mm) may be used for walls or caissons
Why do I need air in concrete?
  • Air entrainment is always used in exterior concrete, susceptible to freezing & thawing during the Winter months
  • A liquid air entrainment admixture added to concrete at the correct dosage (5-8% measured air for 20mm stone) can enhance concrete’s ability to withstand surface deterioration with repeated freeze-thaw cycles, particularly in the presence of de-icing chemicals
  • The “soapy” air admixture added to concrete helps stabilize the many air bubbles created during concrete mixing. The resulting microscopic air voids formed in hardened concrete are interconnected with tiny channels and together act as relief valves when water freezes, reducing the surface pressure and preventing cracking or scaling
Why should I not add excessive water to concrete mix?
  • For most concrete applications, the addition of excessive water to the concrete mix in its plastic, mouldable state (yielding > 130mm slump) will adversely affect concrete strength and durability
  • Not only can excess water weaken the concrete strength, but it can contribute to greater shrinkage cracking, scaling or dusting on the concrete surface
  • CeMix can offer Super Plasticizers (High Range Water Reducers) to assist our customers with more flowable slumps (up to 220mm) while not affecting concrete strength or durability
Why is rapid surface evaporation protection important?
  • Various environmental conditions can accelerate the surface evaporation moisture loss during finishing operations. This can create difficulties with finishing, potential spraying of water on the surface and can result in surface defects, plastic shrinkage cracks and the compromising of concrete durability
  • Rapid Surface Evaporation (RSE) is most prevalent during the Spring & Autumn seasons when the concrete surface has the greatest exposure to high winds, low relative humidity, direct sunlight, and high ambient temperatures
  • The following protective measures can be used to minimize the effects of RSE:
    • Place concrete during the early morning, late afternoon or shady parts of the day
    • Pre-wet subgrade or the substrate prior to placing concrete; especially important during the summer months where ambient temperatures exceed 25C
    • Apply windbreaks
    • Apply fog misting above the concrete surface or apply liquid surface evaporation reducer directly on the concrete surface
Why, when & how to install proper concrete control joints?
Why? Installing properly placed control joints (i.e. expansion joints or saw cuts) protect…

    • Concrete shrinkage as it dries
    • Sub-grade settlement
    • Temperature & moisture changes in concrete
    • Application of loads on concrete surface
    • Restraint of concrete movement during either expansion or contraction

When?… Expansion joints should be installed along the perimeter of rigid, adjacent walls, columns, stairs, etc. prior to pouring concrete. Saw cutting should completed as soon as the concrete can be cut without causing reveling. Depending on the season, saw cuts should be made as early as 4 hours or as late as 12 hours after finishing.

How? The basic rules for joint layout are as follows:

    • Space joints (in feet) no more than 2-3 times the slab thickness (in inches) (A 4” slab should have joints 8-12 feet apart)
    • The panels created by these joints should be as square as possible. The length/width ratio of the panels should never exceed 1.5
    • Joint depths should be at least ¼ the depth of the slab
    • Control joints should be located at all “re-entrant” corners (corners with angles greater than 90 degrees)
    • “T” intersections of control joints should be avoided since the random cracks will tend to continue though into the next slab
Why is concrete curing so important?
  • After concrete is placed and finished, curing concrete properly is the most important step to ensuring great concrete strength and durability
  • Curing is defined as the “maintenance of a satisfactory moisture content & temperature in the concrete for a period immediately following the placing and finishing of concrete”
  • Curing of concrete can be completed by 2 methods:
    • Preventing the loss of moisture from the concrete (curing compounds, plastic sheeting, leaving forms in place)
    • Keeping the exposed surface continuously wet (water misting, wet burlap)
  • It is recommended that temperature and moisture requirements be maintained for a period of up to 7 days while in concrete’s hardened state
Winter concreting: precautions to take
  • One faces cold weather concreting conditions when the ambient temperature falls below 5°C. During these conditions there is a risk of concrete being retarded and potentially freezing. This may result in concrete strengths dropping as low as 50% or other quality issues may arise (scaling or cracking)

What CeMix does to protect against concrete freezing?

    • The use of hot water, steamed sand and truck heat exchangers help maintain our targeted concrete temperatures between 18-24°C
    • Offer our customers value-added solutions:
      • Non-Chloride Accelerators (1%, 2% or 3%): Dosage is dependent on ambient temperatures. Will normalize set time (ie. reduce finishing time) and prevent freezing
      • Accelerated Mixes (24, 48 & 72 hours): Designed to reach early stripping or loading strengths faster by achieving 75% of strength at the given age

What you can do to protect against concrete freezing?

    • Remove all snow/ice and ensure subgrade is sufficiently thawed out
    • Avoid using excessive water in mix design (avoid using a slump >110mm) as this will retard concrete set times. Consider using a Super Plasticizer as an alternative when pouring at high slumps is essential
    • After placement and finishing of concrete, cover with an insulating blanket
    • Apply additional heat to concrete when there is a risk of excessive heat loss
When can de-icers be applied to concrete?
  • Do not apply any de-icers until the concrete has gone through its first Winter season. During this time sand alone should be applied for traction and slip resistance
  • New concrete containing air entrainment to resist the effects of freeze-thaw cycles during the Winter months may exhibit surface scaling if chemical de-icers are applied too soon after construction
  • After the first year, light to moderate amounts of rock salt or Sodium Chloride (table salt) may be applied to concrete surface
  • Avoid the use of harsher de-icing salts such as Calcium Chloride, Magnesium Chloride or other Ammonium based de-icers, as they can be very damaging to the concrete surface