We see concrete everywhere in our day-to-day including sidewalks, driveways, walls, and nearly every man-made structure new or renovated. To answer some common questions people may have when it comes to usage, proper management, and other considerations with concrete, we have compiled the knowledge and insight from our experts at Bilt: Jeff Gould (President), Travis Murphy (Construction Manager), and Muhammad Ali Hassan (Junior Project Manager).
Concrete is made up for four main ingredients: Sand (Fine Aggregate), Gravel (Coarse Aggregate), Cement, and Water. They are bonded together with a fluid cement (cement paste) that hardens (cures) over time. It is used as a foundational base for nearly all construction structures due to its high stability and bonding ability.
Weather Considerations for Concrete
“Rain, snow, and other weather patterns primarily affect the pouring and curing process. For instance, during pouring, typically between +7 and 12 degrees Celsius provides the ideal temperature for a smooth pour. If it becomes too cold, say -30 degrees Celsius, the concrete can get compromised and cracked.” – Jeff Gould, President
During the curing process, concrete produces a lot of heat. When the temperature is too high, this can slow down the process. Similarly, if the temperature is too high, then the curing process is also slower and moisture from the concrete will freeze rather than harden. When combined with the heat generated from curing, it can cause a “shock” like placing a cold glass in a hot oven.
In hotter parts of the world, you can expect to see workers spraying hot water on concrete as it cures. This process is known as “hydration” to induce evaporation and generate heat. Hot water spraying continuously allows the curing process to slow down and stretch it to the ideal 28 days it takes to sufficiently harden. Keeping a careful watch on the weather during a project timeline is crucial for the pouring and curing phase, but weather itself after concrete has cured typically is not affected.
The Truth About Cracks
Cracks are an inevitability when it comes to concrete work. After all, the constant pressure and accumulated years of weight will eventually satisfy general laws of physics. But not all cracks require the same amount of attention.
Spider cracks, for instance, usually should not be a cause for concern. They are quite common as the first signs of wear and seen nearly everywhere. They are generally more cosmetic than structurally compromised. When the crack becomes larger (wider), then an investigation would be warranted. If a crack becomes the width of a dime (approximately 1 centimeter), then that would be a cause for concern. For renovation projects, these are common places to be addressed. If some cracks become larger issues, then demolition may be an option.
Similar to a hot air balloon, heat causes concrete to expand. The force from the expansion will deliver a force in all directions to push anything in its way (for example, a brick all or adjacent slab).
“Expansion joints are used as a point of separation (or isolation), between other static surfaces. Typically made of a compressible material like asphalt, rubber, or lumber, expansion joints must act as shock absorbers to relieve the stress that expansion puts on concrete and prevent cracking.” – Muhammad Ali Hassan, Junior Project Manager
If cracks are appearing immediately after pouring or right after curing, it may show signs that it was not done properly; otherwise, cracks are completely normal.
How Long Does It Take For Concrete To Set?
Concrete should ideally take 28 days to properly set by normal standards. Since there is heat generated from the curing process, the temperature where concrete is setting needs to have the right balance with the temperature of the environment to maintain a proper curing pace. Crystals are formed during concrete that helps strengthen concrete. Curing too quickly and fewer crystals are formed. That is why 28 days is the most ideal time as it allows for these crystals to form. For renovation projects, it is imperative that the concrete is perfect for many years to come.
Outdoor vs. Indoor Concrete
As we already understand, temperature and weather elements for outdoor concrete requires more consideration. In winter, a specific type of mix is required to properly cure in harsh, colder temperatures.
In contrast, indoor concrete work differs due to its controlled environment. In these controlled settings, creating a strong slab of concrete can occur much easier. For example, let us explore the precast steps for exterior concrete steps for houses. These are manufactured in controlled indoor environments to strengthen. When they arrive on site for installation, they are already incredibly strong and very unlikely to see cracks during precast steps.
When it comes to understanding concrete and its strength, it is also important to understand tensile strength and compressive strength; concrete has low tensile strength but high compressive strength. Tensile strength in concrete is extremely weak especially when it is just a slab of concrete such as a driveway or garage pad. That is why we need rebar to help give it the needed strength as steel is high in tensile strength. On a horizontal surface, the tension on the plane of concrete would have many weak points that requires compensation from steel rebar. In other instances where concrete is structured vertically such as pillars or upright cylinders, they are naturally strong because of the compressive strength (vertical force of bearing the load). Without the needed strength, particularly tensile, concrete is fragile and brittle. But in terms of compressive strength such as a vertical cylinder or pillar, it is one of the strongest elements there is and is why it is commonly used in constructing multi-story buildings.
Ultimately, the more controlled and protected the process, the better the quality of the concrete. For exterior pours while the concrete is curing, concrete should be protected and insulated even if it means covering it with a tarp. Fortunately, working around harsh weather conditions such as extreme cold, does not mean that concrete pouring is impossible. Other procedures to consider such as ratio mix and added mixtures to the concrete can balance out the rigors caused by colder temperatures.
The Right Amount of Rebar
“I have seen guys out there lying down too little rebar and the concrete just doesn’t have the adequate strength. But the I’ve also seen guys out there who lay down too much rebar. This is also a problem because we need to have some flex as well; the earth below our feet is always moving and shifting so too much rebar can cause the concrete to be too stiff.” – Travis Murphy, Construction Manager
As we have already mentioned the issues caused by inappropriate temperatures, another consideration to avoid structural issues is choosing the right amount of rebar. With tensile strength in concrete we know how brittle it is and that just a slab of concrete laying flat on a surface could easily break. When combined with steel rebar, this provides the compressive strength required for a pad to be walked on. Too little rebar would not provide adequate strength and become susceptible to collapse. Conversely, too much rebar can cause issues as a bit of flexibility is required to account for the constant shifts and movements from earth. When concrete is too stiff from excessive rebar and combined with natural forces from earth, it will snap and crack under pressure.
“An interesting fact: if you ever head East to Toronto and you find a viewing spot of the CN tower from a good distance you can try and line your eyes to the C Tower and a pole—or even line your pointer fingers vertical in front of your eyes against the CN Tower. On a windy day you can actually see that the CN tower will sway left and right. In fact, the tower can actually move six feet on either of its sides; again, this was constructed purposely for flex and that it won’t snap against too much pressure.” – Travis Murphy, Construction Manager