| | Specifications | Specification table | Specification examples
Agricultural structures often place severe conditions on concrete components. To obtain the best performing concrete for the job, it is important to know and ask for the required concrete specifications.
Concrete mix design is a complex subject. The Portland Cement Association, for example, covers this in a 200 page booklet. This leaflet summarizes specifications for concrete for most agricultural applications. The most important criteria for quality concrete are also explained.
Most concrete is provided by ready-mix. The ready-mix supplier, the owner and the contractor should have a good understanding of the correct concrete specifications.
Quality concrete will be strong, dense, water-tight and resistant to freeze-thaw action, the most destructive agent to concrete. For severe applications like hog barn floors and feed bunks, resistance to chemical action and severe abrasion is also required.
Proper placement, finishing, and curing are also critical to obtaining good quality concrete. The process begins, however, with the correct concrete mix design.
Specifications
The specification for concrete should include the following:
- strength,
- aggregate size,
- slump,
- air entrainment,
- cement type,
- special admixtures.
Water/cement ratio (w/c), and cement content are also important criteria that determine concrete properties, but are not normally specified by the buyer. A summary of these specifications follows.
Strength
Expressed as MPa, strength is primarily dependant on the water/cement ratio of a concrete mix. Typical strengths range from a low of 20 MPa, typical of basements and footings not exposed to freezing, to over 40 MPa for severe applications like barn slats and reinforced beams.
Strength is the criterion that determines most other properties of a concrete mix. The stronger the concrete, the denser, more water-tight and corrosion resistant it is. Stronger concrete also requires more cement, thus costs more. For that reason, concrete much stronger than necessary is not specified. On the other hand, concrete poorer than required will not stand up, and is a poor investment.
Water-cement ratio (W/C)
Water-cement ratio (w/c) is the mix design criterion that determines the strength and water-tightness of cured concrete. Specifying "strength" is normally all that is required; the ready-mix operator then adjusts the w/c ratio accordingly. For this reason, water should not normally be added at the job site, unless specifically planned for in the mix design.
Adding as little as 20L of water per metre of concrete will reduce strength by 15%, and reduce durability much more than this. If water must be added at the site, also add cement to balance the mix.
Concrete with a high water content has reduced strength, durability and water-tightness. It is less resistant to corrosion and freezing action, and prone to greater shrinkage and cracking.
Slump
Defines the workability of a concrete mix; the higher the slump, the "sloppier" the mix. Slumps of 50 to 125 mm are common for most farm projects. For a given strength (w/c ratio) the lowest slump results in the most economical mix design. As a general guide, specify as low a slump as practical.
Aggregate Size
Should also be specified. Usually the largest size permitted, up to 25 mm, results in the most economical mix design. Maximum size aggregate is determined by the application; slab thickness, clearance between reinforcing forms, and type of finish desired.
Table 1 summarizes the requirements for aggregate size. Related to this is the requirement for minimum concrete cover over reinforcing steel. Requirements for cover are shown in Table 2.
Air entrainment
Normally 5 to 8% by volume, greatly improves the freeze-thaw resistance, water tightness, workability, and corrosion resistance of concrete. It should be specified for all projects where these properties are required, particularly where freezing is likely.
Cement type
Normal (Type-10), high early strength (Type-30) and sulphate-resistant (Type-50), are the usual choices. Specify Type-50 cement for concrete in contact with soil high in sulphates, and Type-30 for faster curing. The latter can also be achieved by addition of an accelerator to the concrete mix.
Special admixtures
The most common are super-plasticizers and accelerators. These admixtures can improve mix properties and should be considered for some agricultural applications. Accelerators are now used in place of Type-30 cement to speed setting time for concrete where quick form removal or earlier load bearing capacity is desired.
Super-plasticizers provide higher slump (more workable) with reduced w/c ratio to maintain strength related qualities of concrete. This is particularly applicable to cisterns, tanks or slats.
Table 1. Maximum Size of Aggregates Permitted in Concrete Mixes
Application | Maximum Size of Aggregate
(use the lowest value of alternatives) |
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1/3 of slab thickness
30 mm if reinforcing present
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Reinforced structural slabs
(tank tops, etc.)
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1/3 of thickness
3/4 distance between rebar and forms
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Foundations and form work
Reinforced beams and slats
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1/5 of narrowest width of forms
3/4 of distance between reinforcing or between reinforcing and forms
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Table 2. Minimum Clear Cover for Reinforcing Steel
Application | Bar Size | Cover (mm) |
|
| All | 75 |
Exposed to weather, earth or
corrosive environment (barns)
| 20M - 50M
10M, 15M | 50
40 |
Not exposed to weather or earth:
- Slabs
- Beams and columns
|
up to 35M
All
|
20
40
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Table 3. Concrete Specifications for Agricultural Applications
Application | Strength
MPa | Slump
mm | Cement
Type | Other |
Slabs and floors, severe exposure
- Outdoor paving, slabs, feed bunks, silos
| 32 | 50 - 80 | check soil | Air: 5 - 8% |
|
| 35 | 50 - 80 | check soil
prob T-50 | Air: 5 - 8% |
Water storage structures
Manure storage tanks
| 32 | 50 - 100 | check soil | Air: 5 - 8% |
Floors indoors, high corrosion
- Livestock buildings, feed/silage floors
- Food, vegetable and dairy process plants
| 30 | 50 - 80 | T-10 | Air: 5 - 8% |
Floors indoors, not subject to corrosion
- Shops and garages
- Storage structures
| 20 - 25 | 50 - 80 | check soil | 5 - 8% air if exposed
to freezing |
Foundations and retaining walls, outdoors
| 25 | 80 | check soil | Air: 5 - 8% |
Foundations, indoors or deep
| 25 | 80 | check soil | Air optional |
Reinforced beams, structural slabs
(like tank tops, clear span reinforced floors)
| 30 - 35 | 80 - 100 | T-10 | 5 - 8% air if exposed
to freezing |
Slats in barns, feed bunks
| 35 - 40 | 100 | T-10 | Air: 5 - 8% |
Notes:
Air: means air entrainment, % by volume
Check soil: check soil and/or groundwater for sulphates
See Table 1 as a guide to maximum aggregate size
Cement types: T-10 = normal; T-30 = high early strength; T-50 = sulphate resistant
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Source: See CSA Standard A23.1 "Concrete Materials and Methods of Concrete Construction", or the Portland Cement Association for details.
Table 4. Sample Specifications for Typical Farm Applications
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Concrete Specification:
Strength: 32 MPa
Slump: 75 mm
Type: 10 (normal)
Maximum aggregate: 25 mm
Entrained air: 6%
|
For good durability, resistance to corrosion and abrasion.
Low slump, should be placed with aid of a vibrator.
Could be up to 30 mm (1/3 thickness), 25 mm most common.
Improves durability.
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Concrete Specification:
Strength: 32 MPa
Slump: 100 mm
Type: 10 (normal)
Maximum aggregate: 25 mm
Entrained air: 6%
|
For good durability, resistance to corrosion and freeze-thaw action.
Should be placed with aid of a vibrator. Consider using a plasticizer
for increased slump and less water.
May depend on rebar clearance.
Essential for outdoor exposure.
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Concrete Specification:
Strength: 25 MPa
Slump: 75 mm
Type: 10 (normal)
Maximum aggregate: 25 mm
Entrained air: 6%
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Modest strength is adequate.
Should be placed with aid of a vibrator.
Practical and economical.
Not essential, but use in case of freezing conditions sometime.
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Prepared by Engineering Services
Adapted from Canada Plan Service Leaflet 9012 |
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