What You Should Know About Concrete and Concrete Installation

Concrete is a good material to choose for driveways, walkways, front entrances and backyard patios in the GTA. Contrary to what some homeowners may think, concrete is a durable and low-maintenance option that works great even in colder climates like Ontario, provided the concrete contractor follows proper steps and practices during its installation. Here are the five main things that homeowners should know about concrete.

What you need to know about concrete mixtures

Concrete is made up of a mixture of 4 essential components.

  • Cement
  • Coarse Aggregate (i.e. gravel)
  • Fine Aggregate (i.e. sand)
  • Water

The aggregates combined makes up for 60% – 75% of the mixture while the rest is a combination of cement, water and possibly fly ash and/or chemical admixtures. The fine particles of sand fills in the void spaces between the coarse aggregates and cement acts as a glue to hold them together. Water, when mixed with cement, begins the hydration process (the hardening of concrete) and also binds the aggregate together. The water to cement ratio is the most critical factor to the overall quality of the concrete. Because too much water in the mixture will reduce the concrete’s compressive strength. And too little water will make the concrete unworkable. The careful balance of water to concrete ratio is required to make the perfect concrete – strong and workable. By adding just 1 gallon (3.785 L) of water to 1 cubic yard (9 sf) of concrete, it can decrease compressive strength 150 – 200 psi (PSI = pounds of load per square inch), increase shrinkage and waste 1 quarter bag of cement.

What you need to know about concrete finishing

It takes someone with experience to know when the optimal time is to begin concrete finishing. Just by the absence of bleed water and a successful footprint test is not enough to determine if the concrete is ready for this step.  Premature finishing and over finishing may result in surface defects such as blisters, dusting, crazing or delamination of the concrete.

Premature finishing: mixing of bleed water into the top surface of the concrete or prematurely sealing the surface.

Over finishing: can significantly reduce or even destroy the air-entrainment (which increase the durability of the concrete, especially in climates subject to freeze-thaw) along the surface of the slab.

What you need to know about concrete curing

Once concrete is poured and finished, curing occurs. Curing concrete is the process of stopping freshly poured concrete from drying out too quickly by maintaining moisture levels and temperature. The longer the curing or hydration time, the stronger the concrete. This should range between 3 to 14 days depending on the specific application. A curing compound can be applied. This compound acts as a blanket and forms a thin membrane over the concrete which prevents water from evaporating quickly. General curing timelines for a driveway installation is as follows:

First 48 hours: keep off concrete.

After 48 hours: concrete is strong enough to be “walked” on without causing any damages.  However, refrain from allowing bikes, skateboards or skidding/twisting of shoes on the concrete.

After 7 days: concrete is strong enough to hold a normal household vehicle with passengers, but avoid driving near edges and dragging sharp objects across the driveway.

After 30 days: concrete is at its maximum hardness and strength.

What you need to know about reinforced concrete

Reinforced concrete is when wire mesh or steel bars are embedded into the concrete to carry the tension forces.  These reinforced steel bars are also known as ‘rebar’. Concrete’s compression strength is very strong, however, it is weak in tensile strength. For example, if a concrete beam is poured without using rebar, the middle of the beam will carry a lot of tension. With the help of rebar, this tension stress will be absorbed, making the concrete/rebar combination much stronger.

With the natural occurrence of temperature changes and moisture cycles, concrete will not stand the test of time and will eventually crack, even with rebar. Structural reinforcements do not prevent cracking but will hold the crack faces together.

To avoid random cracking in your concrete landscaping, control joints should be placed. Control joints, to define simply, are cuts made on a slab of concrete so you can “plan” where the “crack” will occur. These cuts allow for movements caused by the natural weather cycles. And since slabs of concrete greater than 120 sf and 4 inches thick will produce a crack, control joints should be cut within these parameters.

What you need to know about concrete durability

Corrosion of metals, freeze-thaw cycles, chemical attacks and alkali-aggregate reaction are all causes for concrete to deteriorate. The main culprit is the exposure to moisture. It is commonly mistaken that concrete is impermeable but in reality, liquid can still pass through. A good quality and correctly consolidated 4 inches or more concrete slab is impermeable to the passage of water from the ground, however, it is not impermeable to the slow passage of water vapor.

The key to avoiding concrete to deteriorate is to make the concrete less permeable. This can be achieved by lowering water to cement ratio, uniform aggregate gradation and density and apply admixtures (superplasticizer or silica fume) and vapor retarder. By reducing the concrete’s permeability and adequate air void system, it will increase the concrete’s durability.

As well, lifting of slab edges at the joints and cracks occur because of shrinkage. When the top and bottom of the slab have different moisture level and temperature variance, it results in greater shrinkage on the top surface hence creating the curling of slabs.

For a more comprehensive explanation of concrete construction and its myths, see video below:

Visit our concrete articles and resources page for additional information on concrete and concrete landscaping. To find out how much concrete driveway installations cost in Ontario, click here.

Choosing the right concrete contractor plays an important role in the longevity of your concrete driveway, walkway or backyard patio. Experienced concrete contractors will know the importance of having the right depth for the foundation, the right concrete mixture to use in Ontario, the distance of which each rebar should be placed, what concrete strength to use to prevent cracking of concrete, and many other factors. Contact us if you have any questions about using concrete for your home, or if you’d like a  no-obligation consultation for your landscaping project.