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Concrete Specifications for Agriculture
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ISU Extension Pub#: Pm-1589
Authors: Dwaine Bundy and Jay Harmon
Department of Agricultural and Biosystems Engineering
Iowa State University
Date: 2/95
Concrete is a mixture of Portland cement,
water, air and aggregates. Aggregates provide volume at low cost, composing 66%-78% of the concrete.
Cement and water form a paste that hardens and glues the aggregates together. The quality of concrete is directly related to the binding qualities of this cement paste.
Selection of concrete properties
Concrete is very durable and resists attack by water, animal manures, chemicals such as fertilizers and fire.
High quality concrete should be used around milk, silage, and animal manure. Portland cement concrete is a common building material with many advantages in agriculture. Concrete is versatile, strong, and economical. Most farm applications require weathering, wetting, temperature changes, and abrasive uses. The durability is usually the main reason for selecting concrete as the construction material.
The durability and strength of concrete are highly dependent on the water-cement ratio.
Sometimes fly ash which is a waste product of coal powered power plants are used to modify certain properties or as a replacement of some of the cement.
Concrete is weak in
tension. Its strength in compression depends on the proportions of the mix.
The compressive strength is 2 to 5 times that of wood. Most structural uses depend on concrete strength in compression and steel in tension.
Portland cement
A
number of variations of Portland cement have been developed for special purposes with the first three types as follows:
* TYPE I, normal Portland cement, is the general purpose type, and usually
furnished unless an alternative is specified
* TYPE II, is modified to release less heat during curing, and is therefore suitable in mass concrete.
It is moderately high in resistance to sulphates
* TYPE III, is high-early-strength; it is very finely ground and sets very rapidly. It is useful for slip-form construction and for cold
weather jobs
* TYPES I, II, and III are available as Types IA, IIA, and IIIA. These are air-entrained Portland cement formulated with a compound that releases many tiny bubbles of air
during curing. They are recommended for
agricultural applications even though they are slightly weaker than standard
cement.
Factors effecting the quality of concrete
Air-entrainment
An air-entrainment agent added to the cement produces millions of tiny bubbles in the concrete giving the concrete greater weathering resistance.
Air entrainment reduces the strength; however, increases the workability. Therefore, all concrete used for agricultural applications should be air entrained.
Maximum Aggregate Size
The maximum size of coarse aggregate that can be used depends on the sizes and shapes of concrete members and the amounts and distribution of reinforcing steel.
Generally, the maximum size should not exceed one-fifth the minimum dimension of the member, nor three-fourths the clear space between reinforcing bars or between reinforcement and forms. For unreinforced slabs on ground, the maximum size should not exceed one-third the slab thickness. Smaller sizes may be used when availability or considerations of economy require them. The larger the aggregate, the less air-entrainment is necessary.
Workability
The workability of concrete is another quality to consider.
The concrete should be placed with the stiffest consistency possible, but still be reasonable to finish. A measure of concrete workability is slump. Table I shows suggested slump ranges for various types of construction. High slump, or flowing concrete usually contains excess water which dilutes the water-cement paste which makes it weaker. Enough water is needed for full curing, but excess water leaves voids when it evaporates. If a mix is too stiff to handle well, do not add water; reject the batch or add cement and water.
Adding only one-half gal/bag of Portland cement lowers strength more than roughly 10 % (500psi).
Concrete made with excess water also experiences greater shrinkage, which typically adds to more cracking. If high slump concrete is essential to the success of the project, you can produce it by using one of the high range water reducer admixtures, also called super-plasticizer. These admixtures greatly increases the workability of the concrete for as much as 30 minutes, after which the concrete returns to its original form.
Table 1. Recommended maximum slump.
Hand consolidating includes rodding and spading.
Mechanical vibration permits stiffer mixes. Mass concrete has little or no reinforcing and is in a relatively large section; an example is a block of 2 steps and a small porch.
Slump, in.
Type of Construction Hand Mechanical
Mass Concrete 3 2
Pavements, slabs, footings,
reinforced foundation walls 4 3
Beams, columns, reinforced walls 5 4
Table II. Durability and strength for air-entrained concrete
Application Approx. Strength Water-Cement Ratio
psi lb water/lb cement
feed bunks, slats,
above ground bunkers 4500 0.44
manure tanks, parking lots
4000 0.49
feedlot floors, walls, drives, reinforced
concrete walls, beams and columns
3500 0.53
footings, foundation walls,
gravity retainer walls 3000 0.62
*Durability (weathering and chemical resistance) and strength depend primarily on
water-cement ratio.
Adding extra water rapidly lowers durablity and strength.
Placing of Concrete
A concrete slab on grade starts with a
properly prepared site. Concrete requires no special soil as long as the soil is homogeneous and provides uniform support for the slab.
A moist subgrade is especially important to prevent too rapid extraction of water from floors and similar work are being placed in hot weather. All hard and soft spots should be removed and recompacted consistent with the rest of the soil. Reinforcement rods may help bridge small soft spots below the poured concrete. Care should be taken to see that debris is removed from the spaces of the concrete.
Concrete should be placed as nearly as practicable in its final position. It should not be placed in large quantities at a given point and allowed to run or to be worked over
a long distance in the form. This practice results in segregation, because the mortar tends to flowout ahead of the coarser material. Position the truck and use movable chute to place concrete as
nearly as possible to its final position to reduce the tendency of over working.
Strike off the concrete with a vibrating screed. Some are effective to a concrete depth of
about 12 inches. Additional consolidation of concrete along bulkheads is often achieved with internal vibrators. Allowing screeds to vibrate after the concrete has been consolidated should be avoided
since this tends to bring excessive fine material to the surface.
If you don't have a screed, you can use a straight 2x4 board, working it side-to-side to work the concrete while
moving it forward. Keep a small amount of concrete ahead of the screed to prevent low spots.
Avoid frequent starting and stopping the vibrating screed because the accompanying movement removes entrained air.
Immediately after screeding, work a bullfloat over the slab at right angles to the direction of screeding. The bullfloat removes small ridges left by the screed and slightly depresses the larger aggregates.
Outdoor slabs and those with livestock traffic should not be troweled smooth.
Outdoor slabs, should normally be finished to a broom finish. Various smoothness may be desired for different ages and species of livestock. For breeding surfaces for swine, diamond shaped grooved saw cuts approximately one-quarter to three-eights deep by a one-quarter inch wide approximately four inches apart may be desirable.
Your choice of floor finishes is often the most difficult challenge for a concrete contractor.
The desired finish on floors varies even within the same type of described finish. Different types of brooms will result in different textures of floor surfaces.
Concrete Placed in Forms
Place concrete in lifts of up to 24 inches.
Place consecutive lifts while the previous lift is still plastic. Vibrate, rod, or otherwise consolidate each lift. Penetrate the previous lift about one-fourth of the depth. Immersion-type vibrators are commonly used to consolidate concrete in walls, columns beams, and slabs. Immersion spud vibrators are available in size of less than 1 inch to about 7 inch in diameter and with frequencies of 3,600 to 13,000 vpm. Vibrators consolidate concrete by pushing the coarse aggregate down and away from the points of vibration.
Form vibrators may be attached to the exterior of forms. They are especially useful for consolidating concrete in thin-walled members and where metal forms are used.
Curing
Concrete does not dry. Rather, it cures, or hardens due to the hydration reaction between the cement and water.
Curing is the process of keeping water in the concrete. You should begin the curing process as soon as the surface is hard enough to prevent damage. On hot days with low humidity and/ or windy conditions you must be sure to cover the concrete to prevent rapid evaporation of the water. Other conditions can contribute to rapid evaporation. Covering the concrete usually affords adequate protection.
Commonly used methods for curing concrete includes:
* Covering the concrete with a polyethylene sheet
* Spraying a liquid curing membrane on the concrete
* Continuously wetting the concrete with a soaker hose
Walls are usually cured by
covering then with polyethylene sheets. Exposed to typical summer temperatures, concrete will cure in three to seven days.
Cooler weather may require more time. Little curing occurs when the concrete temperatures is below 40 degrees F. Concrete must be maintained at 70 degrees for 3 days or 50 degrees for 5 days to cure properly. Do not allow it to freeze for the next 4 days to prevent damage.
Joints
There are three types of joints used in concrete construction which includes isolation, construction, and control. Isolation joints are used to separate structures that are expected to move separately (figure 1).
The movement of a floor slab usually differs from that of abutting building elements, such as walls, columns, foundations, and footings. To be effective an isolation joint must allow horizontal and
vertical differential movement. In general, an isolation joint uses one-half or three-fourths inch thick asphalt impregnated fiberboard which may be termed an expansion joint.
To isolate columns on separate footings from a floor slab, use a round or square blockout.
Construction joints are used as an aid during construction by allowing concrete to be
poured in sections of manageable size.
They also can act as contraction joints, providing stress relief by allowing horizontal movement of the slab. Construction joints are used at the edge of each strip of a large slab. The joint often consists of a tongue and groove key formed by attaching a half round or beveled 1x2 inch piece of wood to the form and then casting the next concrete strip to complete the key.
As concrete dries, it usually shrinks about 1/8 inch in 20 feet. Contraction joints also called control joints are used to control the cracking that can result from shrinkage,
temperature changes, and repeated wetting and drying.
Most contraction joints are induced. Joints are formed by cutting through the slab or wall to a depth equal to 25 % of the thickness of the slab. Commercially available plastic strips may also be used and are placed during concrete placement. The cut establishes a weakened plane that dictates the crack location if stress exceeds the tensile strength of the concrete. Use a rule-of thumb, a 4 inch slab is jointed at 8 foot intervals while a 5 inch slab is jointed at 10 foot intervals. You should not exceed a joint spacing of 15 feet.
Figure
1. Isolation joints around columns can be circular (a) or square (b). If no isolation joints are used around columns or if the corners of the isolation joint do not meet contraction joints, radial cracking (c) can occur.
Use of Synthetic fibers
Synthetic fibers are no substitute for structural (primary) reinforcement in concrete because they add little or no strength. But structural reinforcement doesn't provide its benefits until concrete hardens. Unlike structural reinforcement, synthetic fibers provide benefits while concrete is still plastic. They also enhance
some of the properties of hardened concrete. Synthetic fibers are most commonly added to concrete for slab-
on-grade construction to reduce early plastic shrinkage cracking and increase impact- and
abrasion-resistance and toughness. The fibers also can be added to precast concrete to improve resistance to handling stress,
to pumped concrete to improve cohesiveness, and to shotcrete to reduced rebound
and material waste.
Both synthetic fibers and wire mesh used as secondary reinforcement can help control cracking in cast-in-place and precast concrete. The primary difference
is when and how they work. Fibers are most beneficial
soon after concrete placement by controlling the formation of plastic shrinkage cracks. Wire mesh, on the other hand, does not prevent crack
formation. Instead, the mesh holds cracks together after they have formed.
From an economic standpoint, using fibers eliminates the costs of handling and placing wire mesh. Also, wire mesh must be
placed properly to be effective. This is not a concern with fibers, which disperse evenly throughout the concrete during mixing. And unlike wire mesh, synthetic fibers are noncorrosive and won't rust.
Joint sealing
There are several reasons to seal joints in floors:
* To keep out debris, making floors easier to clean
* To prevent entry of water, chemical, or bacteria
* To improve joint appearance
But sealing can be expensive and time-consuming, so don't
seal a joint unless you have to. In some floors, joint sealing is essential. In food-processing plants, for example, strict hygiene requirements often demand sealing of every joint.
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