FAQ’s

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.

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.

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.

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.

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.

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.

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.

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))

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.

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.

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.

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.