Quality Concrete on the Farm
Table of Contents
Concrete is used on almost every farm because of its ability to be shaped into any form, its strength and durability, and its suitability to environmentally sound practices. Processing and value-added facilities, such as composting operations, and anaerobic digestion facilities are also using concrete as an integral part of their operations.
Quality concrete provides:
Concrete quality is affected by a number of factors. This Factsheet describes the components of concrete, explains how to handle it after mixing and sets out what specifications are required for some agricultural projects.
Concrete is a mixture of cementing materials, water, fine and coarse aggregates (sand and stone), and admixtures. The concrete supplier uses these components in specific ratios to achieve the desired quality and design specifications for the given application.
When mixed, the water and cementing materials form a paste that coats the stone and sand particles and bonds them together. Through a chemical process called "hydration," the cement reacts with the water to form a gel, which grows to fill the spaces between the particles of the mix. Then the concrete hardens into a rigid rock-like mass.
The concrete must be properly placed and finished, then cured, to reach its ultimate strength and durability.
Concrete can be specified with virtually any strength, strength gain (accelerated or delayed), colour, slump or flow, and set time. When ordering concrete, tell the concrete supplier what the concrete will be used for (e.g., bunker silo wall, manure storage tank, etc.) and how the concrete will be placed in the forms (chute, wheelbarrow, pump, etc.).
Most farm applications require concrete to be strong, dense, watertight and resistant to environmental exposure, such as freeze-thaw action. Storage structures, barn floors and feeding bunks must be resistant to severe abrasion and chemical action.
Figure 1 shows a typical in-ground concrete structure used to store livestock manure.
CSA A23.1 Tables 1 and 2 define the various classes of concrete. Some of these are found in the Agricultural Concrete Quick Guide at the end of this Factsheet.
Figure 1. High-quality concrete is used to construct barns and manure storage structures.
Adding more water results in a lower-strength and less durable concrete (Figure 2).
Figure 2. Compressive strength of concrete versus its water/cementing materials ratio.
Adding as little as 2% extra water to a concrete mixture can significantly reduce the strength of the concrete as well as increase its permeability (reduced durability) and reduce its ability to resist freeze-thaw action. Since farm concrete is usually subjected to severe exposure conditions, it must be of the highest quality possible.
Concrete should be "placed" and not "poured." Do not expect concrete to flow around form-work or over the ground unless using self-consolidating concrete. See Best Practice Guidelines for Self-Consolidating Concrete on the Ready Mixed Concrete Association of Ontario (RMCAO) website. Place the concrete within 1 m of its final position in the form work to make separation of the paste from the aggregates less likely.
Consolidate or vibrate concrete placed in forms to achieve a dense product, with minimum permeability, free of air pockets (entrapped air). This procedure increases the strength and durability of the concrete, and allows excellent bonding to reinforcing steel and water stops. Do not over-vibrate the concrete as this will cause segregation (separation of the aggregate from the cement paste).
Concrete placed in slabs requires levelling (screeding), bull floating, finishing and curing. If concrete can be placed, consolidated and finished without undue effort, it is said to be "workable." While the workability of concrete is increased by the addition of water, this is not desirable unless the cementing materials content is also increased to maintain the correct water/cementing material ratio (W/CM). Instead, order concrete with increased workability without the use of water and without negatively affecting the W/CM ratio of the mix by using chemical admixtures (superplasticizers).
Curing is the process of maintaining temperature and humidity conditions so that the cement hydration reaction can take place. Proper curing of concrete has a very important influence on the final properties of the concrete. Water must be continually available to the cement particles for continuous hydration. This continuing hydration will cause the concrete to become stronger, less porous and more durable. When moisture is no longer available, or when the temperature drops below 10 °C, hydration stops.
Concrete that has moist-cured for 7 days has reached 75% of its rated strength and achieves 100% strength by the end of 180 days (Figure 3). By comparison, at 180 days, concrete cured for 3 days attains only 80% strength, and uncured concrete attains only 55% of its rated strength.
To maintain the moisture within the concrete for hydration of the cementing materials, cover the concrete with plastic or apply spray-on curing compounds. These vapour barriers (plastic or curing compounds) retain the water in the concrete by preventing evaporation. On level surfaces, add water by sprinkling or ponding on the slab, or by adding coverings such as waterproof films, damp sand, wet burlap or straw. On formed concrete, leaving the forms on helps contain moisture in the concrete.
Figure 3. Proper curing results in increased concrete strength.
To properly cure concrete, control of the ambient air temperature around the concrete placement is essential to achieve final properties. Curing temperatures can range from 10 °C 35°C; the optimum curing temperature is 15 °C. At high temperatures, apply cool water to keep the concrete cool. In cold weather, keep the concrete warm by using insulation, heated forms or heated enclosures. The heat generated by the chemical reaction of the cementing materials and water can be used if it is held in by insulation or other means.
Protect fresh concrete from freezing; expanding ice crystals within the pores of the concrete cause surface damage, cracking, heaving and structural faults. During cold weather placements, extend the concrete curing period to ensure that adequate freeze-thaw resistance exists within the concrete.
The most common causes of poor-quality concrete in farm projects are:
To meet concrete's design strength, the CSA A23.1 standard specifies that the normal curing period for concrete is a minimum of 7 days at a minimum temperature of 10 °C and as long as necessary to achieve 70% of the 28-day specified strength (MPa).
Use Table 1 as a guide for mixing concrete for farm use. The water content may vary slightly from that suggested in the table, depending on the moisture content of the sand. After a rain, the water requirement will not be as great as if the sand had been sitting through a period of hot, dry weather.
1 The combined volume is approximately two-thirds of the sum of the original bulk volumes. This mix is to be used in situations where structural capability is not critical.
All concrete on the farm exposed to freeze-thaw cycles should be air-entrained. Ordinary concrete contains about 23% entrapped air, but in large voids (>1 mm in diameter). Specifying the right concrete with air-entrainment results in millions of microscopic air bubbles being produced in the concrete (all less than 1 mm in diameter).
Using the proper concrete with air entrainment:
These tiny bubbles of entrained air act as a lubricant between aggregate particles, causing them to slide over one another easily during the placement process. These tiny bubbles then create pressure relief valves within the hardened concrete that allow any water in the pores of the concrete to expand when it freezes. When water turns from a liquid to a solid, it expands at about 9% in volume; it is this expansion that causes freeze-thaw damage in non air-entrained concrete.
Air-entrainment is required by the CSA A23.1 Concrete Standard for all applications where freeze-thaw exposure exists (see Concrete Quick Guide).
Do not use a steel trowel for finishing air-entrained concrete! A steel trowel will reduce the surface durability of the concrete and may cause delaminations and/or scaling of the concrete. Use a magnesium trowel to finish concrete. It is more rigid than the steel trowel and requires less effort to finish the concrete.
Admixtures are materials other than cement, aggregate and water that are added to concrete either before or during its mixing to alter its properties, such as workability, curing temperature range, set time or colour. Calcium chloride is used for concrete setting in cold weather. The section below describes the different types of admixtures and their roles.
Concrete Admixture Types
Accelerator Accelerates the chemical reaction between the cement and the water. Used most often in cold-weather conditions to accelerate early concrete setting. Available in both chloride- and non-chloride-based versions.
Air Entrainment Adds very small air bubbles to the mix, providing freeze-thaw protection for the concrete.
Mid-Range Water Reducer Allows for the removal of 515% water from the mix design or is used to increase the concrete workability (slump).
Retarder Slows down the chemical reaction between the cement and water. Used most often in hot weather conditions to prevent early concrete setting.
Shrinkage Reducer Reduces the shrinkage of the concrete during its early life. Often used where shrinkage cracks are to be avoided.
Superplasticizer Allows for the removal of 15% or more water from the mix design or significantly increases concrete workability (slump).
Viscosity Modifying Used in self-consolidating concrete (SCC) and tremie concrete placements to hold the concrete together while it is actively flowing into place.
Water Reducer Allows for the removal of at least 5% water from the concrete mix design.
The advantages of using ready-mixed concrete supplied by an RMCAO certified concrete facility include:
Ready-mixed concrete can also be produced as a high performance concrete (HPC) of over 50 MPa when applications require improved durability and strength. HPC has a maximum W/CM ratio of 0.40 or less and the minimum 28-day strength (MPa) recommended by an engineer (5080 MPa). Examples of HPC-recommended applications include reinforced beams, floor slats, slabs and columns over manure pits and silos, as well as applications with high chemical exposures.
The ordering of the proper ready-mixed concrete as per CSA A23.1 Table 2 takes into account the two distinct phases of the concrete:
Using concrete that conforms to all the minimum performance requirements of Ontario's Building Code ensures:
Method of placement, concrete slump (workability) and setting time are selected by the contractor when the concrete is ordered. From this information, the proper concrete mix design is selected and delivered to the project for placement and testing.
Type HS or HSb Cement Requirements Sulphate Attack
Section 184.108.40.206 of Ontario's Building Code requires that Type HS or HSb cement be used for all concrete in structures containing manure. While HS cement is not available in Ontario, the equivalent performance can be achieved using supplementary cementing materials such as slag and fly ash as specified in CSA A23.1. The blend of General Use (GU) cement and supplementary cementing materials is referred to as Type HSb cement.
Before placing concrete in these types of structures, confirm the requirements and inform the concrete supplier that the appropriate materials are being used to obtain maximum performance.
Portland Limestone Cement
One of the more recent developments in the concrete industry is the use of Portland-Limestone Cement (PLC or Contempra). The use of PLC reduces the greenhouse gas emissions associated with existing cement manufacturing by approximately 10%, while producing concrete with similar strength and durability characteristics to that achieved with existing general use (GU) cements. This new cementing material was formally recognized by Ontario's Building Code in 2011 and is now commercially available throughout Ontario. This product has been recognized in the LEED building rating system as an innovative, environmentally responsible product providing significant greenhouse gas reductions in the concrete.
Ontario's Building Code is the base regulation for all construction projects in the province of Ontario. It references the following documents that are important to agricultural projects:
Contact the Canadian Farm Builders Association (CFBA) and the Ready Mixed Concrete Association of Ontario (RMCAO) for additional information.
Concrete is used on almost every farm building project. To produce concrete structures with the needed strength, durability and integrity for long-term results, adhere to all applicable codes and regulations and follow the guidelines outlined in this Factsheet.
CSA A23.1-2009 Concrete Materials and Methods of Concrete Construction, and Tables 1, 2 and 4.
"The concrete supplier shall submit to the concrete purchaser a currently valid Certificate of Ready Mixed/Mobile Concrete Production Facilities as issued by the Ready Mixed Concrete Association of Ontario."
** This information to be used as a guideline only **
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