SECTION VII. PLACEMENT
IMPORTANCE OF PROPER PROCEDURES
5-41. You cannot obtain the full value of well-designed concrete without using proper placing and curing procedures. Good concrete placing and compacting techniques produce a tight bond between the paste and the CA and fills the forms completely, both contributes to the full strength and best appearance.
5-42. Preparation prior to concrete placement includes compacting, moistening sub-grade or placing vapor barrier, erecting forms, and setting reinforcing steel.
Moistening the sub-grade is especially important in hot weather to prevent water extraction from the concrete. Preparation includes--
Preparing rock surfaces. Cutting rocks out will make the surfaces either vertical or horizontal, rather than sloping. The rock surfaces should be roughened and thoroughly cleaned. Use stiff brooms, water jets, high-pressure air, or wet sandblasting. Remove all water from the depressions and coat the rock surfaces with a 3/4-inch-thick layer of mortar. Make the mortar with only FA using the same W/C ratio as the concrete. The mortar should have a 6-inch slump. Finally, work the mortar into the rock surfaces using stiff brushes.
Moistening clay sub-grades. Moisten a sub-grade composed of clay or other fine-grained soils to a depth of 6 inches to help cure the concrete. Sprinkling the soil with water intermittently will saturate it without making it muddy. Clean the surface of debris and dry loose material before placing the concrete.
Preparing gravel and sand sub-grades. Cover a sub-grade consisting of gravel, sand, or other loose material with tar paper or burlap before placing concrete. Compacted sand is not covered, but be sure it is moist before placement to prevent water absorption from the concrete. Lap tar paper edges not less than 1 inch and staple them together. Join burlap edges by sewing them together with wire. Sprinkle the burlap with water before placement.
Preparing forms. Just before placement, check the forms for both tightness and cleanliness. Check the bracing to make sure the forms will not move during placing. Make sure that the forms are coated with a suitable form oil or coating material so that the concrete will not stick to them. Remember, in an emergency, moisten the forms with water to prevent concrete from sticking. Forms exposed to the sun for some time dry out and the joints tend to open up. Saturating such forms with water helps to close the joints.
Planning placement. The joint between fresh and hardened concrete is never as strong as a continuous slab. Ideally, all the concrete required in a project is placed at once in a monolithic placement. If subsequent placements are needed between placements, see Figure 4-2. The size of each day's placement is planned based on where construction joints can be placed.
Depositing fresh concrete on hardened concrete. To obtain a good bond and a watertight joint when depositing new concrete on hardened concrete, make sure that the hardened concrete is nearly level, is clean and moist, and that some aggregate particles are partially exposed. If the surface of the hardened concrete is covered by a soft layer of mortar or Laitance (a weak material consisting mainly of lime), remove it. Wet sand-blasting and washing is the best way to prepare old surfaces if the sand deposit can easily be removed Always moisten hardened concrete before placing new concrete; saturate dried-out concrete for several hours. Never leave pools of water on the old surface when depositing fresh concrete on it.
5-43. The principles of proper concrete placement include:
5-44. When constructing walls, beams, or girders, place the first batches of each layer at the ends of the section, then proceed toward the center to prevent water from collecting at the form ends and corners. For walls, stop off the inside form at the construction level. Overfill the form to about 2 inches and remove the excess just before the concrete sets to make sure of a rough, clean surface. Before placing the next lift of concrete, deposit a 1/2-to 1-inch thick layer of sand cement mortar. Make the mortar with the same water content as the concrete, with a slump of about 6 inches to prevent stone pockets this will help produce a watertight joint. View 1 of Figure 5-13 shows the proper way to place concrete in the lower portion of high wall forms. Note the different kinds of drop chutes that can place concrete through port openings into the lower portion of the wall. Space the port openings at about 10-foot intervals up the wall. Show how to place concrete in the upper portion of the wall. (View 2 of Figure 5-13.) When placing walls, be sure to remove the spreaders as you fill the forms.
Figure 5-13. Concrete placing technique
5-45. When constructing slabs, place the concrete at the far end of the slab first, and then place subsequent batches against previously placed concrete, as shown in view 3 of Figure 5-13. Do not place the concrete in separate piles, level the piles and work them together, or deposit the concrete in big piles and then move it horizontally to its final position. These practices would result in segregation.
PLACING CONCRETE ON SLOPES
5-46.View 4 of Figure 5-13 shows how to place concrete on slopes. Always deposit the concrete at the bottom of the slope first, then proceed up the slope placing each new batch against the previous one. When consolidated, the weight of the new concrete increases the compacting of the previously placed concrete.
5-47. Concrete consolidation eliminates rock pockets and air bubbles.
5-48. Except for concrete placed underwater, compact or consolidate all concrete after placement. Consolidation brings enough fine material both to the surface and against the forms to produce the desired finish. Mechanical vibrators are best for consolidating concrete, such hand tools such as spades, puddling sticks, or tampers, may also be used. Any compacting device must reach the bottom of the form and be small enough to pass between reinforcing bars. The process involves carefully working around all reinforcing steel with the compacting device to ensure proper embedding of reinforcing steel in the concrete. Be careful not to displace the reinforcing steel because the strength of the concrete member depends on proper reinforcement location.
5-49. Mechanical vibrators consolidate concrete by pushing the CA downward away from the point of vibration. (See Figure 5-14 below.) Vibrators allow placement of mixtures that are too stiff to place any other way, such as those having only a 1- or 2-inch slump. Stiff mixtures are more economical because they require less cement and present fewer segregation or excessive bleeding problems. However, do not use a mix so stiff that it requires too much labor to place it. The vibrators available in engineer construction battalions are called internal vibrators, because the vibrating element is inserted into the concrete. An external vibrator is applied to the form and is powered by either an electric motor, gasoline engine, or compressed air. When using an internal vibrator, insert it at about 18-inch intervals into air-entrained concrete for 5 to 10 seconds and into nonair-entrained concrete for 10 to 15 seconds. The exact time period that you should leave a vibrator in the concrete depends on its slump. Overlap the vibrated areas at each insertion. Whenever possible, lower the vibrator into the concrete vertically and allow it to descend by gravity. The vibrator should not only pass through the layer just placed, but penetrate several inches into the layer underneath to make sure a good bond exists between the layers. Vibration does not normally damage the lower layers, as long as the concrete disturbed in these lower layers becomes plastic under the vibratory action. The concrete will be consolidated enough when a thin line of mortar appears along the form near the vibrator, the CA disappears into the concrete, or the paste just appears near the vibrator head. Withdraw the vibrator vertically at about the same gravity rate that it descended. Some hand spading or puddling should accompany all vibration. Do not vibrate mixes that you can consolidate easily by spading, because segregation may occur. Vibrated concrete has a slump of 5 or 6 inches. Use vibrators to move concrete any distance in the form.
Figure 5-14. Using a Vibrator to consolidate concrete
5-50. Manual consolidation methods require spades, puddling sticks, or various types of tampers. To consolidate concrete by spading insert the spade downward along the inside surface of the forms, as shown in Figure 5-15 below, through the layer just placed into the layer underneath several inches. Continue spading or puddling until the CA disappears into the concrete.
Figure 5-15. Consolidation by spading and a spading tool
PLACING CONCRETE UNDERWATER
5-51. Whenever placing concrete underwater, make sure that the work is done under experienced supervision and that certain precautions are taken.
5-52. For best results, do not place concrete in water whose temperature is below 45°F, nor in flowing water whose velocity is greater than 10 feet per minute, even though sacked concrete can be placed in water having greater velocities. If concrete is placed in water temperature below 45°F, make sure the concrete temperature is above 60°F when deposited, but never above 80°F. If the velocity of flowing water is greater than 10 feet per minute, make sure that the cofferdams or forms are tight enough to reduce the velocity through the space to be concreted to less than 10 feet per minute. Do not pump water either while placing concrete or for 24 hours thereafter. Attempting to remove water during this time may cause water to mix inadvertently with concrete thus reducing strength.
5-53. You can place concrete underwater using several methods. The most common is with a tremie device shown in Figure 5-16 below. A tremie is a long, funnel-shaped pipe into which you feed the concrete. The pipe must be long enough to reach from a working platform above the water level to the lowest point underwater at which you must place concrete. The discharge end of the pipe is usually equipped with a gate that you can close to fill the tremie before inserting it into the water. You can open the gate from above the water level at the proper time. When filled and lowered into position, the discharge end remains continually buried in newly placed concrete. By keeping the tremie constantly filled with concrete, you can exclude both air and water from the pipe. To start placement, lift the tremie a little bit--slowly--to permit the concrete to begin to flow out, taking care not to lose the seal at the discharge end. If the seal is lost, you must raise the tremie, close the gate, fill the tremie, and lower it into position again. Do not move the tremie horizontally through deposited concrete. When you must move it, lift it carefully out of the newly placed concrete and move it slowly to the new position, keeping the top surface of the new concrete as level as possible. Use several tremies to deposit concrete over a large area, spaced on 20- to 25-foot centers. Be sure to supply concrete to all tremies at a uniform rate with no interruptions. Pumping concrete directly from the mixer is the best way to do this. Suspend large tremies from a crane boom and easily raise and lower them with the boom. Concrete placed with a tremie should have a slump of about 6 inches and a cement content of seven sacks per cubic yard of concrete. About 50 percent of the total aggregate should be sand, and the maximum CA size should range from 1 1/2 to 2 inches.
Figure 5-16. Placing concrete underwater with a tremie
CONCRETE BUCKET METHOD
5-54. Concrete can be placed at a considerable depth below the water surface using open-top buckets having a drop bottom. The concrete that is placed this way can be slightly stiffer than that placed by tremie, but should still contain seven sacks of cement per cubic yard. First, fill the bucket completely and cover the top with a canvas flap attached to one side of the bucket only. Then lower the bucket slowly into the water without displacing the canvas. Do not discharge the concrete before reaching the placement surface. Make soundings frequently to keep the top surface level.
5-55. In an emergency, place concrete underwater using partially filled sacks arranged in header and stretcher courses that interlock the entire mass together. Cement from one sack seeps into adjacent sacks and thus bonds them together. First, fill jute sacks of about 1-cubic foot capacity about two-thirds full. Then lower them into the water, placing the header courses so that the sack lengths are at right angles to the stretcher course sacks. Do not attempt compaction because the less concrete underwater is disturbed after placement, the better the concrete product.
|David L. Heiserman, Editor||
Copyright © SweetHaven
Revised: June 06, 2015