Mixing Concrete

• Concrete is a proportioned mixture of cement, aggregate, and water.

Mixing of Concrete

• Since initial set of concrete takes place one to three hours after mixing, a batch may be used any time before initial set if the mix is plastic.
• Regaging or retempering of concrete that has been left for more than half an hour is not permitted.
• Reinforced-concrete work should be mixed by machine.
• Machine-mixed concrete is more uniform in quality than hand-mixed and is generally less expensive in large volumes.
• The strength of concrete largely depends on the thoroughness of mixing.
• Continuous mixers feed materials constantly and discharge concrete in a steady stream.
• Concrete mixers are classified as drum, trough, gravity, and pneumatic mixers.
• Drum mixers are most common.
• Hand-mixing must be done on a water-tight platform.
• Cement and fine aggregate should be mixed dry until the color is uniform.
• Water and coarse aggregate should be added, and the entire mass should be turned at least three times, or until a homogeneous mixture of the required consistency is obtained.

Transporting and Placing of Concrete

• Fresh concrete should be transported from the mixer as rapidly as possible.
• Methods should permit the placing of the concrete in the forms before the initial set occurs without loss or separation of materials.
• Delivery of concrete from the mixer to the forms should be continuous and uninterrupted.
• The time of transportation should not exceed 30 minutes.
• Concrete may be transported by barrows, buggies, buckets, cableways, hoists, chutes, belts, and pipes.
• When chutes are used, the slope should not be more than 1 vertical to 2 horizontal, or less than 1 vertical to 3 horizontal.
• The delivery end of chutes should be as close as possible to the deposit point.
• Forms should be cleaned, inspected, wetted or oiled, and reinforcement properly secured before placing concrete.
• Concrete should be deposited in approximately horizontal layers in wall, column, and footing forms.
• Concrete should not be piled up in forms, which may result in separation of cement mortar from coarse aggregate.
• Concrete should never be allowed to drop freely over 5 ft. for unexposed work, or over 3 ft. for exposed work.

Shrinkage of Concrete and Temperature Changes

• Shrinkage of concrete during hardening and contraction from temperature changes causes cracks, its size based on the extent of the mass.
• These cracks can be minimized by placing reinforcement, breaking large cracks into smaller ones.
• In long, continuous concrete sections, shrinkage or contraction joints should be placed.
• Shrinkage cracks occur at joints where fresh concrete joins already set concrete; construction joints should be made on horizontal and vertical lines.

Curing of Concrete

• Concrete must be allowed to "cure" or harden after placement.
• Hardening is a slow process in which cement and water unite to form compounds that give strength and durability, continuing if temperatures are favorable and moisture is present.
• Three Factors that affect hardening are: Age or Time, Temperature and Moisture.
• Fresh concrete should be protected for about 7 days after placing to allow hardening.
• Protect against excessive vibration, loads, extreme temperatures, rapid drying, and impurities which might interfere with chemical action.
• The strength of concrete increases with age when curing conditions are favorable.
• The increase in compressive strength reaches about 60% of its maximum at 28 days and about 80% at 3 months.
• Curing maintains moisture to prevent concrete from drying too rapidly, by:
  • Covering concrete: Floors covered with wetted paper sacking, burlap, sand, or earth after concrete is hard enough to walk on.
  • Removal of forms at the prescribed time: Forms not removed until after the time specified.
  • Sprinkling or spraying beams, columns, and walls with water as soon as forms are removed.
  • Using curing compounds (admixtures).
• Different structures require different curing periods:
  • Massive footings require forms to be removed after 1 day (24 hours).
  • Massive walls require forms to be removed after 1 day (24 hrs) +1 day (24h) for every additional meter.
  • Columns with height to least diameter ratio up to 4 require forms to be removed after 2 days (48 hours).
  • Slabs with 3 to 7 ft spans should have forms removed in 5 days (120 hours). Add 1½ day (12 hours) for every additional 1 ft of span or fraction.
  • Balustrades, copings etc, which utilize steel and side forms require 1 day (24 hours).
  • See document for other structure types.

Admixtures

• Admixtures are substances added to cements, mortars, and concrete to improve or impart particular properties. They:
  • Improve workability of concrete (e.g., hydrated lime).
  • Improve durability by air entrainment.
  • Accelerate setting or hardening (e.g., calcium chloride).
  • Retard setting.
  • Improve wear resistance.
  • Impart water-repellant or waterproofing qualities (e.g., hydrated lime, KAOLINE, CELITE).
  • Impart color (e.g., MINERAL OXIDES, COLORCON, METALICHROME).
• Admixtures are grouped into those for mixing into concrete, those for mixing into mortar, and those for surface application or finish.
• Admixtures are in powder, paste, and liquid form, and are usually patented and sold under trademark names.
• Accelerators speed up setting time, develop earlier strength, and reduce the time for protection; principal ingredient is calcium chloride.
  • Maximum amount is 2 lbs per bag of cement.
  • Disadvantages: they increase the expansion and contraction of concrete, reduce resistance to sulfate attack, and increases efflorescence and corrosion of high-tension steels.
• Retarders slow down the hydration of the cement during hot weather; principal ingredients include zinc oxide, calcium lignosulfonate, and derivatives of adipic acid.
  • Disadvantages: may cause some loss of early strength and require careful control and more frequent slump tests, it also reduces the expansion and contraction of concrete.
• Air-entraining agents introduce minute air bubbles to increase resistance to freezing and thawing, increase plasticity, and reduce bleeding.
  • Addition is usually 3 to 6% of the volume of concrete.
  • Manufactured from rosin, beef tallow, stereates, and foaming agents (soap).
  • Disadvantages: Require careful control and frequent slump tests, they may also cause some loss of strength.
• Inert, finely divided powders such as powdered glass, silica sand, stone dust, and hydrated lime improve workability, used per manufacturer's directions.
  • Hydrated lime is usually in a proportion of 10 to 15% of the cement by volume.
• Waterproofing (permeability-reducing) compounds reduce the capillary attraction of voids but may decrease water absorption.
  • Manufactured from stearic acid or its compounds, mainly calcium stearate, and include asphalt emulsions.
  • Introduced in amounts of 0.1 to 4.0% of the weight of cement.
• Colored pigments give color to concrete floors.
  • Dry-cast, broadcast or dust-on: for surface coloring, dusted on, usually in two coats, after surface water has disappeared, then finished with a steel trowel.
  • Integral colors: for body coloring, incorporated in the mortar topping, mixed dry with the cement and aggregate before water is added.
  • Amount of color pigment required is not more than 10% of the cement by weight, generally 3 to 6 lbs. per bag of cement.
• Admixtures for mixing into mortar include accelerators, plasticizing agents (water-reducing agents), waterproofing agents, and color pigments.
• Surface application finishes for concrete include hardeners, color pigments, special aggregates, sealers, abrasive materials, waterproofing agents, and fillers and patchers.

Forms

• Lumber used in form construction should be partially seasoned.
  • Kiln-dried lumber tends to swell when soaked by concrete, causing bulging and distortion.
  • Green lumber dries out and shrinks if allowed to stand too long before the concrete is placed. Keep the boards thoroughly saturated with water.
• Ensure natural, well-seasoned lumber is not driven "too close" since slight swelling is desirable.
• Sizes of lumber frequently used:
  • 2-inch thick for columns, beams and girder bottoms.
  • 1-inch thick for floor panels and beam and girder sides.
  • 2x4s for struts, posts, shores, and uprights.
  • 1 or 2-inch thick for cleats.
• Use nails sparingly so that unnecessary nailing does not add labor when wrecking.
• Leave the the nail head protruding so that they may be withdrawn without injury to the lumber.
• All forms for concrete require a coating of lubricant to prevent concrete from adhering to the wood and making a rough appearance.
• Crude oil and petroline preserve forms, preferably oiled before they are set in place.
  • Do not use oil on forms against surfaces to be plastered, since it prevents adhesion; wetting is sufficient.
• The inside of forms used multiple times should be coated with approved soap or be completely wetted.
  • No soap or other material should be applied after reinforcements are in place.
• Forms should be durable, rigid, and well braced so that bulging or twisting can be reduced, and joints should be tight to prevent leakage.
• Sheathing lumber dressed at least one side and both edges even can be used for re-use application.
• Sheathing lumber dressed on all four sides should be used on smooth and true surfaces.
• Tongue-and-groove lumber will achieve tight joints between boards in floor and wall panel construction.
• Simply dressing the lumber true to edge form square of butt joints in the forms for columns, beams, and girders.
• Other form types include Plywood, Steel and Plastic.

Advantages of Concrete

• Concrete raw materials can be easily harvested.
• It has easy handling and molding of concrete into any shape.
• It is simple to transport from the place of mixing to where casting takes place.
• Concrete structure is a more economical than with steel.
• Ability to pump or spray to fill into cracks and lining of tunnels
• Reinforced concrete makes structures possible from an ordinary lintel to massive fly overs and buildings.
• The property of concrete to possess high compressive strength makes a concrete structure more economical than that of steel structure

Disadvantages of Concrete

• It requires reinforcement for tensile strength to avoid cracks.
• Expansion joints need to be added in long structures if the temperature differences are large in the area.
• Construction joints are provided to avoid shrinkage cracks and moisture expansion.
• Requires proper curing to have hydration occur, ultimately achieving the strength desired.

Proportioning

• The principles of proper proportioning are as follows:
  • Use good quality materials: Portland cement, water, and aggregate.
  • Determine the strength of the concrete using the water-cement ratio. The strength increases as the water-cement ratio decreases.
  • Determine the consistency of the mix using the slump test using as dry a mix as practicable.
  • Add correct proportions of aggregates to the cement and water as will give a mix of the desired consistency.
  • Make a mix that's workable, not harsh
• The strength of a workable concrete mix depends upon the water-cement ratio and the proportioning of the fine and coarse aggregates.
• There are several methods of proportioning concrete:
  • Proportioning by arbitrary proportions.
  • Proportioning by the water-ratio and slump test.
  • Proportioning by water-ratio, slump and fineness modulus.
  • Proportioning concrete by the arbitrary selection of the proportions is the oldest, the most commonly used, the most convenient and the least scientific method. Aggregate volumes are measured by loose volume. One sack of cement is taken as 1 cu. ft. Enough water is used to give the desired consistency.
  • Class AAA pre-stressed and post-tensioned concrete typically utilizes a 1:1:2 of cement to sand to gravel.
  • For concrete under water, retaining walls a 1:1.5:3 ration is preferrable.
  • For suspended slabs, beams, columns, arches, stairs, walls of 100mm (4") thickness a 1:2:4 is used.
  • For walls thicker than 100mm (4"), footings, steps, reinforced concrete slabs on fill, employ a 1:2.5:5 mix.
  • A class "C" concrete of 1:3:6 cement to sand to gractel is useful For concrete plant boxes, and any non-critical concrete structures.
  • Mass concrete structures use Class D with a 1:3.5:7 ration.
• Customizations are made depending on whether the water ratio and slump test is desired. The steps are to select the amount of water to be added to the cement to give the desired strength then add just enough mixed aggregate to water and cement to give the desired consistency. The cement should be in sacks with the water in gallons per sack of cement. Mixed aggregate should be listed by cu. ft. per sack. Desired ratios can be given with desired strength.
• An alternate water ration method is to test the proportions of the fine and coarse aggregate are determined by the fineness modulus method. This largest quantity of mixed aggregate may be used to produce a mix of the desired consistency.
• Coarse aggregate is most ecoonomical since it requires a lesser total surface but there should be fine aggregate to fill the voids. Voids in the combined aggregate should be at a minimum.