Frequently Asked Questions About Concrete

Can it be too hot or too cold to place new concrete?

The temperature is a very key part of the curing process. On hot days when curing, the heat causes excessive moisture from the concrete, to evaporate. However on the other hand if the temperature drops too close to freezing, hydration will occur at a snails pace. If the concrete is cured below freezing, the water inside the concrete will freeze and completely ruin the internal structure of the concrete. For concrete to gain strength it is essential for the temperature to be moderate. It is extremely important that the temperature of new concrete should not be permitted to fall below 10 degrees Celsius (50 Fahrenheit) during the curing period.

What is the difference between cement and concrete?

Many people use the two terms cement and concrete interchangeably because they believe there is no difference between the two. However, cement is actually an ingredient of concrete. Concrete is essentially a blend of aggregates and paste. The aggregates consist of sand gravel and/or crushed stone; the paste is water and cement. Cement comprises from 10 to 15 percent of the concrete mix, by volume. Over many years the cement and water harden bringing the aggregates closer together, making the concrete stronger. This process is known as hydration. The process continues for a very long time, therefore concrete keeps getting stronger over many years.

What is air-entrained concrete?

Air-entrained concrete simply put, is concrete with air in the mix. The air creates many tiny air pockets. These air pockets ease internal strain and pressure on the concrete by giving water a place to expand when it freezes. Air-entrained concrete is made with air-entraining agents. The amount of air used in the mix varies but is usually between 4 percent and 8 percent of the total amount of the concrete.

What does it mean to "cure"?

Curing if done correctly greatly increases concrete strength and durability. Curing primarily means to give the hydration process a jumpstart. When concrete is in the curing process, water is available and the concrete's temperature stays within a set range. This encourages hydration and makes the concrete stronger. The duration of curing usually last for five to seven days. New concrete may be wet with sprinklers, hoses, covered with wet burlap, or curing compounds which seal in moisture.

Why does concrete crack?

Concrete's volume will slightly change after it has dried out. Typically concrete shrinks 4mm in 3 meters (1/16 of an inch in 10 feet). Contractors put joints in concrete pavements and floors to permit the concrete to crack in a uniform, straight line at the joint when the concrete shrinks and changes it's volume.

Why do concrete surfaces flake and spall?

Concrete surfaces flake and/or spall because of three main reasons. In Nova Scotia and the Maritimes we are exposed to cold weather often fluctuating just above or below freezing. This creates a problem because exposed concrete is subjected to many freezing and thawing cycles. The concrete needs to be air-entrained to have a strong resistance to flaking and scaling of the surface.

To have strong concrete, the water to cement ratio should to be as low as possible for the concrete to be durable on the surface. If there is too much water in the mix of concrete the surface and concrete will be weak and susceptible to flaking and spall damage.

The excess bleed water on the surface of the concrete should always be evaporated before continuing with the finishing of the concrete. If this excess water is worked into the concrete because the finishing has begun too soon, the concrete on the surface will have too high a water content and will be weak. It will most likely eventually flake and/or spall.

How do you protect a concrete surface from aggressive materials like acids?

Although there are numerous materials that have no effect on concrete whatsoever. There are a few materials including most acids, which can have a deteriorating effect on concrete. There are methods that improve the chemical resistance of concrete. For example using a low water-cement ratio will make the concrete stronger and less susceptible to the effects of acids. Also using suitable aggregates and air entrainment are methods of keeping concrete durable. There are numerous chemical formulations that are available to preserve concrete from a variety of weathering and climate conditions. Contact a V.J. RICE CONCRETE representative to discuss your special requirements.

How do you control the strength of concrete?

There is a simple rule of thumb for controlling the strength of concrete, and it's modifying the ratio of water to cementing materials. More cement to water typically means the strength is increased. The more water to cement and the strength is typically decreased. Concrete manufactures use this guideline to produce the proper strength concrete for certain jobs. A representative from V. J RICE CONCRETE will help you determine the strength required for your concrete job.

What does 28 -day strength mean?

All concrete will harden and gain strength as it hydrates. But the entire hydrating process takes several years to be completed. Contractors needed to test concrete to see how strong it is. But it would be pointless to wait several years to test the concrete for a job site. Therefore it was agreed that 28 days would be a period of time long enough to let a substantial amount of the hydration to take place, yet not take so long that it would be impractical to test. For that reason it has become common to test concrete for its strength after 28 days.

What is flyash and what does it do?

Flyash is a by-product of coal-fired electric generating plants. Primarily there are two classifications of flyash that are produced, according to the type of coal used. Anthracite and bituminous coal produces flyash classified as Class F. Burning lignite or subbituminous coal produces class C flyash.

Using flyash in concrete is environmentally friendly because it is a by-product, it normally would be put to waste, but it is instead used in a constructive way. While flyash offers environmental advantages, it also improves the performance and quality of concrete. Flyash affects the quality of concrete by improving workability, reducing water necessity, reducing segregation and bleeding, and lowering heat of hydration. Flyash increases strength, reduces permeability, reduces corrosion of reinforcing steel, increases sulphate resistance, and reduces alkali-aggregate reaction.

What are the most common tests for fresh concrete?

There are three common and efficient concrete tests. The three tests that are most commonly used are slump, air content, and compressive strength.

A slump test is essentially a measure of the flowability of plastic (or wet) concrete. Concrete, placed in an inverted cone, is allowed to settle as the cone is removed. The amount of settlement is measured to determine the "slump" of concrete. Too low of a slump (a "stiff mix") may indicate workability problems with the mix. When water is used to provide too high of a slump, a number of different problems with the mix may arise. These could include segregation problems, reduced strength, shrinkage problems, finishing difficulties, and increased scaling problems.

High range water reducers can increase the slump of concrete, providing necessary workability, without the bad effects of adding too much water to the mix.

The air content test measures the percent of air in the concrete. This test is very important to ensure that there will be enough air to prevent scaling and other concrete damage.

The compressive strength test is used to ensure the concrete is strong enough for the job is to be used for. To test the strength of the concrete they pour cylinders of concrete, and after a predetermined length of time measure the force required to break the concrete. According to Building Code Requirements for Reinforced Concrete, as long as no single test is 3.5 MPa below the design strength and the average of all three consecutive test is equal to or exceeds the design strength, the concrete is acceptable.

What is the correct concrete mix for my project?

The Atlantic Provinces Ready Mixed Concrete Association recommends that the following should be specified when ordering concrete:

IMPORTANT: Use DURA-MIX® for all concrete exposed to application of salts and freeze-thaw cycles. DURA-MIX® is designed to provide superior surface durability for exterior flatwork applications. DURA-MIX® maintains a minimum cement content, optimum air entrainment and proper water to cement ratio.

What is alkali-aggregate reactivity (AAR)?

AAR is a chemical reaction that takes place within aggregate particles between the alkaline solution of cement paste. Some minerals in certain rocks used in concrete aggregate trigger the reaction. There are two main forms of alkali-aggregate reactivity: alkali-carbonate reactivity, and alkali-silica reactivity. Both of these forms cause generally the same problem expansion, and cracking of concrete. For any type of alkali-aggregate reactivity to occur there must be reactive materials in the aggregates, sufficient alkali in the concrete, and an adequate amount of moisture in the concrete to support the reaction process. Therefore preventive steps can be made to eliminate, or to use the smallest amount possible of certain concrete ingredients to prevent alkali-aggregate reactions, before they take place. In Nova Scotia, class "F" flyash is typically used in the concrete mix design to reduce the potential for alkali-aggregate reactivity problem.

If you have your own questions about concrete contact

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