CONCRETE 411

What is concrete?
What is the difference between cement and concrete?
What is air entrainment?
What are the keys to a successful project?
What are some of the key things to look for during concrete placement?
Can the weather affect concrete placement?
What steps do I take when pouring concrete in cold weather?
What steps do I take when pouring concrete in hot weather?
What is curing and why is it so important?
What are the key factors to proper curing?
What can I do to prevent moisture loss?
What can I do to supply supplemental moisture?
I completed my concrete project, and something isn't right. What went wrong?


What is concrete?

In its simplest form, concrete is a mixture of paste (cement & water) and aggregates (sand & stone). The paste coats the surface of the fine (sand) and coarse (stone) aggregates and binds them together into a rock-like mass known as concrete. In addition to paste and aggregates, concrete may also contain highly specialized chemical admixtures that enhance the specific properties of concrete. Chemical admixtures are used to improve both the workability and/or the durability of concrete.


Within this process lies the key to a remarkable trait of concrete: it is plastic and can be molded or formed into any shape when newly mixed, and is strong and durable when hardened. These qualities explain why concrete is a primary building material throughout the world that is used to construct everything from skyscrapers, bridges, highways and dams to sidewalks, curbs, patios and houses.

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What is the difference between cement and concrete?

While the terms are sometimes used interchangeably, concrete and cement are not the same. Cement, a fine gray powder, is a key ingredient in concrete production. Cement typically comprises 10 to 12 percent of the entire mix. When cement is mixed with water, sand and gravel, it turns into concrete, the rock-like substance associated with sidewalks, roadways, building foundations, and more.

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What is air entrainment?

All concrete exposed to freeze-thaw action must be air entrained using chemical admixtures. Proper air entrainment involves the placement of very tiny air bubbles (≤1mm in diameter) within the concrete at a very tight spacing. These bubbles act to reduce the stresses created within the concrete when water expands during its transformation to ice during cold weather.

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What are the keys to a successful project?

The key to achieving a strong, durable concrete for your projects are as follows:
Utilizing high quality raw materials

  • Utilizing high quality raw minerals
  • Ordering the proper concrete for the application
  • Skillful placement and finishing
  • Following proper curing practices and protecting the concrete

The first step in any concrete project is utilizing high quality raw materials. All members of the Ready Mixed Concrete Association of Ontario (RMCAO) utilize only raw materials that meet CSA standards for concrete production.

The second step is being aware of the minimum concrete specifications required for your concrete application. In Ontario, these requirements are dictated by the Ontario Building Code and CSA A23-1 “Concrete Materials and Methods of Concrete Construction." You should be aware of these minimum requirements and you should ask your contractor what they propose to use for your project. If they suggest anything less that these standards, or if they don't know what the minimum requirements are, you should find another contractor.

The third step in the process is hiring a quality contractor to complete the work. You should use a contractor that has been trained to an industry certification standard such as the ACI Flatwork Finishers Certification Course and ask for a list of previous projects and references. Make sure to visit projects that have gone through at least two winters and look for any signs of deterioration.

The final and most important step is to ensure that your contractor follows proper curing practices and cures the concrete effectively. Seven days of curing is recommended for all exterior flatwork and three days of curing is required for all other concrete. Failure to complete this step properly will negate all of the good work done during the previous steps. Care must also be taken to maintain and protect your concrete investment. Avoid the use of de-icing chemicals during the first 12 months and consider the application of concrete sealers to further protect the concrete.

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What are some of the key things to look for during concrete placement?

  • Has the sub grade been properly prepared and compacted? While the minimum aggregate thicknesses vary depending on the application, you should see free draining aggregate that is properly compacted as a sub-base. If the tires of the concrete truck are leaving large ruts in the sub-base, then the material has not been properly compacted or it hasn't been placed in sufficient thickness.
  • Ensure that the area is properly drained and sloped to prevent water build-up. Flowing water and frost heaving are the most common reasons for early deterioration. Ensure that both the concrete and the sub-base are properly drained.
  • Ordering of the proper concrete. Ask to see the concrete delivery tickets from the ready mix truck to verify that the proper concrete has been ordered.
  • Place the concrete within 120 minutes of batching. The standard "shelf life" of concrete is 120 minutes. This means that we have 2 hours from the time that we first mix the concrete until we must place the concrete. The time batched is always included on the concrete delivery ticket. Note: Chemical admixtures (set-retarders) can be used to extend this 120 minute placement time.
  • Do not allow the addition of water to the concrete after the concrete placement has started. Retempering of concrete may be allowed once the concrete truck arrives on site to address minor slump variations but once the concrete placement proceeds past the 10% portion of the load, additional water should not be added. Water addition leads to higher concrete permeability and lower strengths. Slump enhancement with chemical admixture is acceptable after the 10% discharge point and will not adversely affect the concrete quality.
  • Has the contractor prepared a control joint layout to prevent uncontrolled cracking? All concrete shrinks as it cures and proper control, isolation and construction joints must be used to prevent unsightly concrete cracking.
  • How will the concrete be properly cured? What curing method will be utilized to protect the concrete and how long will the protection be applied? Discuss with the contractor prior to concrete placement to ensure the proposed methods and times meet the Ontario Building Code requirements for your application.

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Can the weather affect concrete placement?

Yes! Weather conditions can have a dramatic effect on both the setting time and concrete placing, finishing and protection systems that must be followed for proper concrete placement.

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What steps do I take when pouring concrete in cold weather?

Cold weather concreting conditions are typically defined as when the air is ≤5°C or when there is a probability that the temperature may fall below 5°C within 24 hours of placing the concrete.

Because the hydration process is a chemical reaction, it is strongly affected by ambient air and subgrade/formwork temperatures. At low temperatures, concrete gains strength and sets very slowly and must be adequately protected from freezing and thawing. Concrete that is allowed to freeze while in its plastic state can have its potential strength reduced by more then 50% and its durability properties will be dramatically reduced. Concrete must achieve at least 3.5 MPa before it is frozen and should obtain at least 20 MPa before it is exposed to multiple freeze/thaw cycles.

General procedures for cold weather concreting include:

  • Removing all ice and snow from the subgrade or formwork.
  • Supplying the necessary supplemental heat required to ensure that forms, sub grades, and reinforcing steel is maintained at a minimum of 5°C well prior to the concrete placement.
  • Ordering concrete with a temperature between 10°C - 25°C.
  • Concrete should be ordered using the lowest practical water slump since this will reduce bleeding and setting times. Chemical admixture can still be used to improve the workability of the concrete.
  • Chemical admixtures and mix design modifications can be used to offset the slower setting times and strength gain of concrete during cold weather conditions. Considerations should be given to ordering concrete that will obtain higher early strengths.
  • Concrete temperature must be maintained at a minimum of 10°C for the full curing period.
  • The surface of the concrete should not be allowed to dry out while it is still plastic since this may cause plastic shrinkage cracking. The longer set times encountered during cold weather combined with the effects of hot dry air from heaters being blown along the top surface of the concrete significantly increase this risk.
  • Wet curing methods are typically not recommended during cold weather conditions since the concrete will not have a sufficient time period to air dry before the first freeze/thaw cycle.
  • The possibility of thermal cracking must be considered when the heating supplied during the curing period is going to be suspended.


Caution regarding the use of portable gas fired heaters:

Plastic concrete exposed to a carbon dioxide source (CO2) during the concrete placing, finishing and curing period will develop a soft, chalky, carbonated surface, known as dusting. Carbon dioxide is an odourless and colourless gas that is heavier than air and is produced by all forms of combustion. Typical sources include open flame heaters (stacks must be vented outside), and internal combustion engines (e.g. on trucks, power trowels, concrete buggies, etc.). Precautions must therefore be taken to properly vent the placement area.

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What steps do I take when pouring concrete in hot weather?

Hot weather concreting conditions typically include high ambient air temperature (≥28°C), low relative humidity conditions, high wind speeds and solar radiation or heat gain.
These conditions can result in the following challenges for the concrete contractor:

  • Increased concrete water demand
  • Accelerated concrete slump loss
  • Increased rate of setting leading to placing and finishing difficulties
  • Increased tendency for plastic shrinkage cracking
  • Increased concrete temperature resulting in lower ultimate strength
  • Increased potential for thermal cracking

Increased potential for thermal cracking
The first step that must be taken is to identify when hot weather concreting conditions may apply and modify the normal concrete placing and finishing procedures accordingly.

Possible steps that may be taken include:

Preparation
ACI recommendations regarding the pre-wetting of the subgrade have recently changed so that this procedure is not typically recommended. The only exception is during hot weather conditions where plastic shrinkage cracking may be an issue. The subgrade should be pre-wetted and forms and reinforcing steel should be dampened prior to concrete placing (there should be no standing water). The purpose of these actions is to prevent the absorption of water from the concrete into the subgrade.

Ordering
Inform the ready mixed concrete producer of your placing schedule and whether a chemical retarder will be required. For exposed flatwork the use of retarding admixtures or supplementary cementing materials should be discussed with the concrete producer. In extreme cases, the concrete temperature may also be lowered by using chilled water, ice or liquid nitrogen.

Slump
A concrete consistency (slump) which allows for rapid placement and consolidation should be considered. Chemical admixtures such as super-plasticizers can dramatically improve the concrete slump and reduce placement times.

Placing
After the concrete is properly mixed, ensure that it is discharged as soon as possible. Consider the use of large crews to accelerate placement times.

Finishing
In cases where protection against rapid evaporation of water from the concrete surface is a concern, consider the use of one or more of the following actions:

  • Erect sunshades and wind breaks
  • Cover the surface with white polyethylene sheets
  • Apply fog spray
  • Place and finish at night or early morning
  • Apply temporary evaporation retarder after the screening operation

Curing
Curing should be started as soon as the concrete has set enough to avoid any surface damage. Concrete should be cured for at least 7 consecutive days after placing. Ensure that the concrete is kept moist throughout the curing process.

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What is curing and why is it so important?

Curing is defined as "maintenance of a satisfactory moisture content and temperature in the concrete for a period of time immediately following placing and finishing so that the desired properties may develop." Early curing is critical when the concrete will be exposed to harsh Canadian weather conditions since it dramatically affects the permeability and durability of the concrete. In some instances curing must be initiated even before the finishing operations are complete to provide the necessary concrete properties.

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What are the key factors to proper curing?

Since the strength and durability properties of concrete are set by the chemical reactions of the various components during the hydration process, there are three key factors to proper curing.

  1. Moisture - Having sufficient moisture to ensure the hydration process continues
  2. Temperature - Maintaining a sufficient temperature (≥10°C) to ensure that the chemical reaction continues
  3. Time - Maintaining both the moisture and temperature requirements for a minimum period of time (3-7 days) to ensure that the durability properties fully develop. Curing needs to be initiated as soon as the finishing operations are complete and the surface will not be damaged by the curing operation.

Other notes to keep in mind when curing concrete:

  • Alternating cycles of wetting and drying during the curing process is extremely harmful to the concrete surface and may result in surface crazing and cracking. This should be avoided at all cost.
  • A 28 day air drying period is recommended immediately following the 28 day curing period to provide the necessary freeze/thaw resistance for the concrete. Curing methods that result in fully saturated concrete, which will be exposed to freeze/thaw cycles once the curing period is over, may result in premature deterioration of the concrete (even if the concrete is properly air entrained.)
  • Concrete with low water/cement ratios (≤0.40) may not have sufficient free moisture in the mix to allow for the use of "moisture loss prevention" curing methods. This situation should be reviewed prior to the start of the project.

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What can I do to prevent moisture loss?

Curing Compounds

  • Form a membrane over the top surface of the concrete preventing moisture loss
  • Must be applied at the manufacturer's suggested application rate
  • Should be applied in two applications with the second being at the right angles to the first to ensure uniform coverage
  • Should be applied as soon as the concrete surface is finished and when there is no free water on the surface
  • Curing compounds can affect the "bond" of some floor coverings
  • Confirm that this curing method is suitable for the final floor covering application

Plastic Sheeting

  • Ensure that the plastic sheeting covers 100% of the concrete surface and that it is adequately sealed at the edges to prevent moisture loss
  • Select the appropriate colour (white, black, or clear) of the plastic based upon the ambient air conditions
  • If uniform colour is a requirement for the project, ensure that the plastic is not placed directly on the concrete surface
  • Ensure that plastic sheeting is not damaged by subsequent construction activities during the curing period

Leaving Formwork in Place

  • This system is most effective for vertical elements (walls, columns, beams, etc.). Care must be taken to also protect the top surface of the concrete appropriately
  • "Breaking" or "releasing" the formwork dramatically reduces the effectiveness of this curing method since air flow is now possible between the concrete and the formwork
  • If uniform colour is an issue, then a uniform curing time and temperature must also be maintained and form removal scheduled accordingly

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What can I do to supply supplemental moisture?

Water Ponding

  • Flooding of the concrete surface to provide both moisture and a uniform curing temperature
  • Curing water should not be more than 12°C cooler than the concrete temperature to avoid the possibility of thermal cracking
  • The water must cover the entire concrete surface

Water Sprinkling

  • Spraying water over the concrete surface. The entire surface must be wet for this method to be effective
  • The concrete surface must have sufficient strength to avoid damaging the surface
  • Excess water will run off the concrete and must be drained away
  • This protection method can be adversely affected by high winds which prevent proper curing on the "upwind" side

Wet Burlap

  • Pre-soaked burlap is applied to the concrete surface and is covered with plastic to prevent moisture loss or water is reapplied as necessary to prevent the material from drying out
  • Burlap should be rinsed prior to its first use to avoid possible staining
  • Materials utilizing both geo textile fabric and plastic top coatings can be reused throughout the project

Wet Sand

  • Wet loose material such as sand can be used to cure concrete slabs and footings
  • The sand thickness must be sufficient to prevent moisture loss at the concrete surface or the sand may be wetted throughout the curing period

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I completed my concrete project, and something isn't right. What went wrong?

There are a number of different factors that can affect the look and integrity of your concrete, beginning with subgrade preparation to finishing method. The Ready Mixed Association of Ontario has compiled a list of common problems that occur with concrete, complete with possible causes and helpful tips.


"Concrete Construction Troubleshooting Tips" is available for you to review.