Concrete Basics

Where does the concrete come from?

The Nabataea traders or Bedouins who developed a small empire in the regions of southern Syria and northern Jordan in around 6500 BC built the first concrete-like structures. They later discovered the advantages of hydraulic lime — that is, cement that hardens underwater — and by 700 BC; they were building kilns to supply mortar for the construction of rubble-wall houses, concrete floors, and underground waterproof cisterns. The Assyrians and Babylonians used clay as the bonding substance or cement. The Egyptians used lime and gypsum cement. In 1756, British engineer, John Smeaton made the first modern concrete (hydraulic cement) by adding pebbles as a coarse aggregate and mixing powered brick into the cement. In 1824, English inventor, Joseph Aspdin invented Portland cement, which has remained the dominant cement used in concrete production. Joseph Aspdin created the first true artificial cement by burning ground limestone and clay together. The burning process changed the chemical properties of the materials and Joseph Aspdin created stronger cement than what using plain crushed limestone would produce. Concrete that includes imbedded metal is called reinforced concrete. Reinforced concrete was invented (1849) by Joseph Monier, who received a patent in 1867. Joseph Monier was a Parisian gardener who made garden pots and tubs of concrete reinforced with an iron mesh. Reinforced concrete combines the tensile or bendable strength of metal and the compressional strength of concrete to withstand heavy loads. Joseph Monier exhibited his invention at the Paris Exposition of 1867. Besides his pots and tubs, Joseph Monier promoted reinforced concrete for use in railway ties, pipes, floors, arches, and bridges.

What is the difference between cement and concrete?

Although the terms cement and concrete often are used interchangeably, cement is actually an ingredient of concrete. Concrete is a mixture of aggregates and paste. The aggregates are sand and gravel or crushed stone; the paste is water and Portland cement. Cement comprises from 10 to 15 percent of the concrete mix, by volume. Through a process called hydration, the cement and water harden and bind the aggregates into a rocklike mass. This hardening process continues for years meaning that concrete gets stronger as it gets older. Portland cement is not a brand name, but the generic term for the type of cement used in virtually all concrete, just as stainless is a type of steel and sterling a type of silver. Therefore, there is no such thing as a cement sidewalk, or a cement mixer; the proper terms are concrete sidewalk and concrete mixer.

Why does all the concrete I see have lines cut into squares?

These cuts are called Control joints. These control joints allow movement (expansion and isolation) and controls shrinkage. Cracking (control or contraction) may be used to divide large areas into smaller ones to reduce the risk of uncontrolled random cracking on the surface of the concrete.
The joints in effect create a planned weaker spot in the overall mass of concrete. With any combination of movement, freezing, thawing, subgrade changes, the cracking is more controlled and unseen. If you could examine an expansion joint of older concrete and you will likely see a crack that continues from the control joint from the surface to the sub grade. Check the same control joint from an extreme cold or hot time of the year and you will likely see a noticeable size change in the crack within the joint. Many conditions play a part in random or excessive cracking.
Bridges and overpasses have large metal control joints within the concrete decking and the concrete for the same application. Without these large metal joints in bridges, the concrete would snap and fall apart.

What does reinforced concrete actually mean?

The function of steel reinforcing mesh within concrete is to hold tightly closed any cracks that may form. Where the concrete surface provides the final finish, minimizing the risk of cracking and, if cracking does occur, controlling joints are critical to the success of any floor or pavement. In the case of water-washed finishes, cracking occurs between the aggregate particles and may therefore not be apparent.

What kind of sub-base should I have?

All topsoil should be removed. A smooth, granular (stone or gravel) sub-base should be installed and compacted so that the slab has a uniform thickness. The sub-base should be properly graded so that water flows away from any structures. As a rule of thumb, the base should slope 1/4″ for every linear foot to provide proper drainage.

Why is the sub-grade so important?

The base should be evaluated before placement of concrete to minimize cracking and any excessive settlement or sliding. Instability may be the result of excessive moisture within the soil. Certain types of soils are very fine and are unstable with high moisture content. Other soils are very heavy and called fat clay, which holds excessive amounts of water and can become unstable over time. This same fat clay can become very dry and shrink in dry periods or expand in heavy moisture times. The different types of soils react differently in temperature and water changes over the weather seasons. Instability within the existing subgrade may be from excessively cracked old surfaces allowing water to be trapped below, water from pour surrounding drainage conditions over time, or even excessive in ground sprinkler water. The soil types react to adverse conditions and changes. The soil base should be firm and stable under construction equipment, i.e. no marking (rutting) from the equipment while work is in progress. Once a firm base is recognized, a layer of rock should be placed, leveled and compacted to establish a uniform and level load distribution base for anticipated uses of the concrete. A base that moves and is excessively wet and moves without much pressure on it will most certainly do the same in the short or long term under new concrete. Excessive cracking, with cracking from a control joint to an edge or another joint may be the result of a pour subgrade. Cracking and lifting of individual sections of any slab between joints also may be the result of a poor subgrade. Uncontrolled excessive water is the main enemy and likely the cause of excessive concrete cracking and failure. Once severe cracking occurs water permeates through the cracks into the subgrade, making the condition worse and replacement more costly and difficult. Most unstable subgrade conditions are improved by removing some minimal amount of additional soil in depth, then replacing the removed zone with additional rock as a new stable base. A reasonable removal would be less than 6 inches. Any probed unstable depth beyond 6 inches maybe cost excessive with no stability improvement in deeper removal.

How thick should the concrete slab be?

We recommend a 4” thickness for sidewalks and 5” to 6” for driveways, depending on the type of vehicular traffic. Aprons are generally 7″ to 8″ thick.

How long does it take concrete to cure?

28 days.
Concrete actually gets stronger as it gets older, the curing process continuing for years. The hydration process happens rapidly at first and then slows down and so the standard has become that all concretes are rated at their 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 are the properties of normal strength portland cement?

Density : 2240 – 2400 kg/m3 (140 – 150 lb/ft3)
Compressive strength : 20 – 40 MPa (3000 – 6000 psi)
Flexural strength : 3 – 5 MPa (400 – 700 psi)
Tensile strength : 2 – 5 MPa (300 – 700 psi)
Modulus of elasticity : 14000 – 41000 MPa (2 – 6 x 106 psi)
Permeability : 1 x 10-10 cm/sec
Coefficient of thermal expansion : 10-5 oC-1 (5.5 x 10-6 oF-1)
Drying shrinkage : 4 – 8 x 10-4
Drying shrinkage of reinforced concrete : 2 – 3 x 10-4
Poisson’s ratio : 0.20 – 0.21
Shear strength : 6 – 17 MPa
Specific heat capacity : 0.75 kJ/kg K (0.18 Btu/lbm oF (kcal/kg oC))

What does MPA stand for and why do you recommend 32 mpa?

The Canadian Standards (CSA) recommends 32Mpa (air entrained) as a minimum strength for exterior concretes exposed to freeze-thaw and deicing chemicals. 32 MPA converts to 4,643 psi.

How long does it take to pour concrete?

According to Canadian Standards Association standard A23.1-09, complete discharge of a batch of concrete must be completed within 90 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.

Can I pour concrete in all weather?

Extreme temperatures make it difficult for the hydration process to take place. When it is too close to freezing, the hydration slows to a standstill and the concrete will not cure and gain strength. Typically the ground should be at least 10 celsius and rising. On the other extreme, when it is very hot, too much water is lost by evaporation and care must be taken to keep the concrete wet.

For The Do-It-Yourselfer

The most obvious is the basics of placing concrete, which is an art unto itself. Proper concrete placement starts with proper sub base preparation, which often includes 3 to 6 inches of sub base material or stone that needs to be properly compacted. The next step is forming, which requires not only the proper form material, but an understanding of elevations and slope. If the proper slope is not maintained on concrete flatwork, you end up with puddling or standing water issues. As with most everything concrete, these low spots are permanent, and very difficult to repair, especially if any aesthetic value is desired. Once you get through those pre- pour necessities you have to deal with the concrete itself. Standard concrete weighs about 150 lb per cubic foot. Unless you are lucky enough to have a job where a ready mix concrete truck can pull right up to the formed area, you will be moving concrete via wheel barrow or paying someone to pump it for you. Also consider that most ready mix companies charge penalties if the truck sits at a job site for longer than 30 to 60 minutes. We can’t overlook the concrete itself. As a do it yourselfer, do you understand concrete mix designs, concrete admixtures, slump, air entrainment, and proper curing techniques? Modern concrete has become a high tech material that contains more than sand, stone, cement and water. You need to consider not only the environmental conditions the day of the pour, but the long term environmental conditions the concrete will face during its life when you chose the type of mix that will be used.

Concrete and Asphalt

Less Expensive

While concrete has always made the most sense in the long term, the rising cost of oil and enhanced refining capabilities have forced asphalt prices through the roof, making concrete the best short and long term solution for your paving needs. Asphalt is a residue left over from refining crude oil in the production of gasoline Asphalt up until the past few years was nearly ½ the price of concrete. The durability and strength decrease installation and maintenance cost in ways you can see now and down the road. In addition, the savings on surface lighting and interior cooling because of the more reflective surface will give you benefits that will not only be green, but save you some green in the long run.

More Durable

No other paving material approaches concrete’s strength and durability in standing up to heavy usage and truck traffic. Concrete lasts longer, without the need for resurfacing, patching or surface sealing.

More Attractive

Concrete’s clean look creates a good first impression and lasting sense of quality for customers, tenants and employees. Concrete can be fashioned with an array of decorative textures, shapes, patterns and colours.

More Green

Concrete is produced from abundant natural resources, reduces toxic run-off and can be easily recycled. It is also cooler in the summer which provides outdoor comfort while reducing the heat island effect. The reflectivity also reduces lighting energy need. On the road, concrete even improves fuel economy.
(Cited from Maryland Ready MixConcrete Association, Inc)

Comparing Aggregates

To compare concrete to asphalt we would need to almost double the thickness of asphalt. For example if we are comparing 4 inches of concrete to asphalt the asphalt would need to be 8 inches thick. A typical driveway with asphalt is 3.5 inches, at that thickness the longevity factor has been far reduced. The appearance factor and longevity of asphalt is 7-10 years, while concrete is a minimum of 20 years+ if installed properly. Asphalt costs can actually be twice the cost of a concrete over 20 years. If comparing initial cost of concrete vs. asphalt and looking at the long term costs, there is no real comparison, concrete wins.
Asphalt is a very flexible and brittle material, flexible when hot, brittle when cold. When asphalt is subjected to repeated freeze and thaw cycles, the expansion and contraction of the material allowing permeating water molecules to become trapped inside. Small hairline cracks will start to develop.
In the heat of the summer the black surface become extremely hot and remains hotter longer than concrete. Excessive heat of the asphalt causes vexing to occur under repeated loaded conditions such as vehicle traffic. Ruts and writing from the weight of the vehicles may appear in the heat within a short while after installation. As the freeze thaw cycles continues over time, the now possibly rutted and wrinkled asphalt becomes excessively cracked with small pieces bringing to break off. The result is complete failure and removal is necessary. Asphalt absorbs heat while concrete reflects heat. With potholes that occur in asphalt and temperatures dropping below zero celsius, the frozen water within or under the asphalt expands breaking and popping the cold brittle asphalt. Sealing asphalt every 1-3 year is an inexpensive and easy task for a home owner to do. Unfortunately this only bandages the problem. The sealing covert the cracks from season to season, keeping a small percentage of water from getting below. However the microscopic cracks continue to develop and water is actually penetrating the seal and cracks unnoticed.

Stamped Concrete

What are the decorative finishes that can be applied to concrete surfaces? Colour may be added to concrete by adding pigments-before or after concrete is place-and using white cement rather than conventional grey cement, by using chemical stains, or by exposing colourful aggregates at the surface. Textured finishes can vary from a smooth polish to the roughness of gravel. Geometric patterns can be scored, stamped, rolled, or inlaid into the concrete to resemble stone, brick or tile paving. Other interesting patterns are obtained by using divider strips (commonly redwood) to form panels of various sizes and shapes ­ rectangular, square, circular or diamond. Special techniques are available to make concrete slip-resistant and sparkling.

What are the popular types of stamps and colours?

  • Roman slate
  • Ashler slate
  • Granite
  • Wood plank
  • The most popular colours tend to be greys and earth tones. However, brick patterns are often coloured in red or russet hues.

Does it look fake?

Stamped concrete looks very realistic because most stamping mats are molded from the actual materials they are designed to replicate. To achieve natural-looking colour variations, such as you would see in real stone, stamped concrete contractors often use integral or dry-shake colour in conjunction with surface-applied colouring mediums. If anything, stamped concrete looks better than the real thing, because you won’t get weed or moss growth in between the joints, and it won’t rot or splinter (if you are looking at our wood plank stamped pictures).

What are the most popular colours and patterns being used for stamped concrete today?

Natural stone patterns, such as slate and fieldstone, are the most prevalent, with brick and cobblestone running a close second. Seamless textures that resemble natural stone, but without joint lines, are also growing in popularity. The most popular colours tend to be greys and earth tones. However, brick patterns are often coloured in red or russet hues.

Is it slippery?

Because stamped concrete is a textured surface, it is often more slip resistant than conventional concrete. However, just like natural stone, it can become slippery when wet or if a film-forming sealer has been applied. If stamped concrete will be installed in a high-traffic area, such as an entryway or pool deck, there are a number of things you can do to increase its slip resistance.

Sealing Concrete

No. It is not necessary, and frankly, a waste of money to reseal yearly. In fact, building up too much sealer on the surface of stamped concrete can cause as many problems as not having enough sealer.

How often should I seal my concrete?

It is recommended you seal the concrete the first year before winter after installation and once more the following year. Then wait 2 years and make a judgement call. If you have had light traffic on the concrete then you might be able to get away with three years.

What is bubbling of the sealer and why does it happen?

Bubbling of the sealer happens when too much is applies at one time. Also bubbling can happen if the sealer is applied on a day that is too hot. It is difficult to fix bubbling, it involves many steps of removing the sealer that bubbled, cleaning off the surface many times over and over, letting it dry and then two coats of sealer professionally done.

When is the best time to seal?

After the concrete has had a chance to cure properly (at least 28 days), a sealant can be applied to prevent moisture from getting into the slab. The key item here is that the sealant must still allow the concrete to breathe to allow moisture from the soil underneath to evaporate.

Why seal concrete?

Sealers also offer resistance to rain, sun, freezing temperatures, petroleum products, deicing salts, and debris such as leaves laying on the surface. Concrete sealers make clean up easier and also keeps the colouring looking fresh, over time.

Isn’t sealing easy?

For older concrete, cleaning by power washing with sometimes a percent mixture of acid is necessary to remove any existing stains. Sealant applied over stains or existing old sealant will result in the new stain not penetrating as it should. What is noticeable before new sealant is applied will become more visible and unsightly. Poor preparation and the incorrect sealer can actually result in yellowing, peeling or bubbling of the sealant used. Removal of incorrectly applied sealants is very difficult and time consuming, if not mostly impossible without possible damage to the actual concrete. Often it is best to contact a knowledgeable individual to evaluate the condition of the concrete surface before sealing takes place.

Why does everyone recommend two coats of sealer?

Think of painting the walls of your house, to properly cover the entire area in one coat is possible but not seamless. The first coat will penetrate and close the pours of the concrete. The second coat will cover anything that was missed, bind to the coat beneath and be stronger and longer lasting.

3 step process of sealing

Sealing stamped concrete is a 3-part process, taking approximately 2 days. First, the concrete is pressure cleaned, and washed to remove stains that normally would not come up with regular pressure washing. Second, the area is allowed to completely dry. During this time, the area must not be utilized to ensure it will remain clean for the sealing process. Third, the area is sealed using two coats of a high quality sealer.

Repairing Existing Concrete

It can be very difficult and almost impossible at times to remove a stain that has penetrated the pours of the concrete. Here are a few suggestions:

  • Stains can be removed from concrete with dry or mechanical methods, or by wet methods using chemical or water. Common dry methods include sandblasting, flame cleaning and shotblasting, grinding, scabbing, planing and scouring. Steel-wire brushes should be used with care because they can leave metal particles on the surface that later may rust and stain the concrete.
  • Wet methods involve the application of water or specific chemicals according to the nature of the stain. The chemical treatment either dissolves the staining substance so it can be blotted up from the surface of the concrete or bleaches the staining substance so it will not show.

I think salt has ruined my concrete, what can I do about it?

If the salt has eaten through the sealer and the surface of the concrete there isn’t much you can do to make it look absolutely perfect. If it is a real eyesore and you would like it to look for you then we would come and carefully cut out sections and do our very best to either match it to the existing concrete or create a new look. Give us a call for a free estimate.

How do I remove oil stains from my driveway?

Oil will only create a visual and potentially slippery hazard. Tide and a little water can be used on the oil spot to remove it. There are also degreaser products sold at the store that work great.

Is a crack repair possible?

We do not recommend addressing hairline cracks until they become 1/8″ to 1/4″. At that point, the crack can be ground out with a grinder and caulked with a self-leveling concrete caulk The caulk should be tack free within 2 hours and cures fully within one to two weeks.

Why has the surface of my concrete gone dusty? How do I fix it?

Concrete surface dusting is typically caused by finishing the concrete surface too early, while bleed water is still rising to the surface. Thus working bleed water back into the concrete weakens the concrete surface resulting in dusting of the harden concrete. Generally, repairing dusting floors is difficult, however, if the problem is not severe, the surface can be repaired by applying a chemical surface hardener. In severe cases it may be necessary to grind the floor to remove the weak surface layer and apply a bonded topping.

My concrete is sinking towards the house, why is this?

When a home is built the outside parameter of a home was excavated to the depth of the footing and then backfilled. The area that was backfilled is vulnerable to sink over-time. In this case the concrete will usually sink toward the house over-time. Once the concrete is sloping toward the house it will only continue to get worse as now the water will run toward the house. This will create more water next to the foundation softening the ground which can possibly result in a foundation sinking. The best solution in this situation is to have the concrete removed and replaced. To help prevent this from occurring again we dowel rebar every 2 feet into the foundation to lock the new concrete slab in securely.

Why does it look like my concrete sidewalk raises in the winter?

Concrete raising during the winter is due to freezing. The ground almost always contains some level of moisture, the more moisture under a concrete slab the more it will raise as the ground freezes. This is especially common when a sidewalk is at the bottom of a hill. The water runs down the hill and under the sidewalk. Effective ground preparation and the right concrete mix can significantly reduce the effects of the ground freezing, but not eliminate them completely.

What are the white stains running down my concrete?

The loss of water (or a solvent) of crystallization from a hydrated or solvated salt to the atmosphere on exposure to air. Efflorescences can occur in natural and built environments. On porous construction materials it may present a cosmetic problem only (primary efflorescence), but can sometimes indicate serious structural weakness (secondary efflorescence). Efflorescence can often be removed from using phosphoric acid. After application the acid dilution is neutralized with mild diluted detergent, and then well rinsed with water. However, if the source of the water penetration is not addressed efflorescence may reappear. Saline solutions are formed due to the presence of road salt in the winter. This saline solution is absorbed into the concrete, where it can begin to dissolve cement stone, which is of primary structural importance.

Why does concrete flake off the top sometimes?

Concrete surfaces can flake or spall for one or more of the following reasons:

  • Being subjected to freezing and thawing the concrete should be air-entrained to resist flaking and scaling of the surface. If air-entrained concrete is not used, there will be subsequent damage to the surface.
  • The water/cement ratio should be as low as possible to improve durability of the surface. Too much water in the mix will produce a weaker, less durable concrete that will contribute to early flaking and spalling of the surface.
  • The finishing operations should not begin until the water sheen on the surface is gone and excess bleed water on the surface has had a chance to evaporate. If this excess water is worked into the concrete because the finishing operations are begun too soon, the concrete on the surface will have too high a water content and will be weaker and less durable.

Why has my garage floor sank in the middle?

When a home is built the inside parameter of a garage was excavated to the depth of the footing and then backfilled. The area that was backfilled is vulnerable to sink over-time. In this case the outside parameter of the garage concrete floor sank and this will usually cause the centre of the floor to raise up.

Do I use salt on my concrete?

During new concrete’s first winter, it is very susceptible to water damage. During freezing conditions, salt melts ice and allows the water to penetrate into the mass of the concrete. When the water freezes again, it expands as much as 9%, causing the surface of the concrete to spall off in small chips. The Portland Concrete Association recommends the use of sand or cinder chips during this first season. Products containing ammonium nitrates and ammonium sulphates are especially harmful because they will actually attack the concrete chemically. Rock salt (sodium chloride) or calcium chloride will do less damage, but they can harm vegetation and corrode metal. As an alternative, use sand for traction.


When fertilizer is sprinkled on the surface of concrete and is then exposed to moisture orange dots can occur in the concrete. A highly diluted solution of muriatic acid can be used to remove the spots. Make sure to thoroughly rinse the concrete with straight water after it has been exposed to the acid.

My contractor left and didn’t finish the job, can you come and finish the project?

We certainly can and have in the past. The main issue that we will face is matching the colour and the finish of the concrete. Each contractor uses different materials from different manufacturers. Each contractor have different methods of installing and finishing concrete. We will suggest and find ways to create a professional finish.

Will I slip on the concrete?

The texture of the concrete plays a big part, which is why most walkways, pool decks, and driveways are broom finished, washed aggregate or stamped to have the grip and be more slip resistant when the concrete is wet. A grit additive can be added to the sealer in high traffic areas if requested by the homeowner or representative.

Can I resurface my existing concrete with a decorative look?

It is important to be aware that not all existing concrete can be resurfaced. The underlying base for an overlay must be sound. If your concrete is heaving, has severe cracks, is spalling due to damage from deicing salts and freeze-thaw cycles, or resting on unstable soil, resurfacing will not solve your problems. This is when total replacement will be your best option. If the existing concrete is sound, we can overlay what you have with an almost unlimited choice of colours, patterns and styles of concrete.

Why does concrete crack and how is it prevented?

Concrete, like all other materials, will slightly change in volume when it dries out. In typical concrete this change amounts to about 500 millionths. Translated into dimensions-this is about 1/16 of an inch in 10 feet. The reason that contractors put joints in concrete pavements and floors is to allow the concrete to crack in a neat, straight line at the joint when the volume of the concrete changes due to shrinkage.

Will the colour of the concrete fade?

Efflorescence, weathering, dirt and traffic can take their toll on the colour of stamped concrete. You can minimize any colour change by periodically cleaning and resealing the concrete. Even if the colour has faded due to years of neglect or lack of maintenance, it can often be restored to its original state by cleaning and resealing.

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