The types and amounts of excavation must be planned for equipment utilization and availability. Mixing, handling, and transporting
must be strictly followed to maintain quality control measures. As concrete is placed in its final location, various finishing
operations may be done. Initially the concrete should be screeded to the specified elevation. If a smoother surface is required,
this may be accomplished by floating with a steel trowel. Curing methods, such as sprinkling the concrete with water after
the initial set and placing plastic over the surface, provide effective moisture barriers that enhance the curing process
The types and amounts of excavation must be planned for equipment utilization and availability. Mixing, handling, and transporting must be strictly followed to maintain quality control measures. As concrete is placed in its final location, various finishing operations may be done. Initially the concrete should be screeded to the specified elevation. If a smoother surface is required, this may be accomplished by floating with a steel trowel. Curing methods, such as sprinkling the concrete with water after the initial set and placing plastic over the surface, provide effective moisture barriers that enhance the curing process
5-2. The first step in any construction procedure is to make a thorough and efficient reconnaissance of the construction site. Note possible problems in clearing and draining the site and in transporting and storing materials. Investigate the site for any unusual characteristics that can cause construction problems such as undesirable soil or rock base. You can avoid construction delays by anticipating and considering such problems beforehand.
5-3. Special consideration such as the following must be given to any planning procedures:
5-4. Most new construction takes place on undeveloped land. Therefore, the approach roads need building up to deliver materials to the site. Although these are temporary roads, construct them carefully to withstand heavy loads. Build enough lanes to permit free traffic flow to and from the construction site because the routes may become permanent roads later.
5-5. Land clearing consists of removing all trees, downed timber, brush, and other vegetation and rubbish from the site to include the remains of previous construction efforts; digging up surface boulders and other materials embedded in the ground; and disposing of all materials cleared. Heavy equipment, hand equipment, explosives, and burning by fire may all be needed to clear the site of large timbers and boulders. The methods to be used depends on--
5-6. Drainage is important in areas having high groundwater tables and for carrying off rainwater during actual construction. Use either a well-point system or mechanical pumps to withdraw surface and subsurface water from the building site.
5-7. Stake out the building site after clearing and draining the land. The batter-board layout is satisfactory in the preliminary construction phases. This method consists of placing batter boards about 2 to 6 feet outside of each corner of the site, driving nails into the boards, and extending strings between them to outline the building area.
5-8. It is important to build up and maintain stockpiles of aggregate both at the batching plant and at the crushing and screening plant.
5-9. Both aggregate and cement batching plants are essential in operations requiring large quantities of concrete. The batching-plant stockpiles prevent shortages caused by temporary production or transportation difficulties that allows the FA to reach a fairly stable and uniform moisture content and bulking factor. Large stockpiles are usually rectangular for ease in computing volumes. They are flat on top to retain gradation uniformity and to avoid segregation caused by dumping aggregate so that it runs downs a long slope; enough cement must be maintained at the cement-batching plant. The amount of concrete required by the project and the placement rate determine the size of the stockpiles. If admixtures are used, make sure that enough are on hand.
LUMBER AT CONSTRUCTION SITE
5-10. Stockpile plenty of formwork and scaffolding materials at the construction site. The size and quantity of lumber stored depends on the type of forms and/or scaffolding used.
5-11. The initial location of the aggregate, cement, and water; the aggregate quality; and the location of the work all affect where the cement batching plant is positioned. Depending on these conditions, you can operate it at the same place as the aggregate batching plant or closer to the mixer. After developing a layout, position the batching plant within crane reach of the aggregate stockpiles and astride the batch truck routes. Although the crushing and screening plant is normally located at the pit, it can be operated at the batching plant or at a separate location. A hillside location permits gravity handling of materials without excessive new construction. This may eliminate the need for cranes or conveyors if the road is good.
5-12. Plan and construct the safety facilities during site preparation. This includes overhead canopies and guardrails both to protect personnel from falling debris and to prevent anyone from falling into open excavations. Certain sites, such as those where landslides may occur, require additional safety facilities
5-13. When the building site is cleared, drained, and outlined, cut the land to the proper elevation for placing footings. Use suitable equipment for the initial excavation, but excavate to the final depths by hand. The excavation should extend beyond the exterior wall edges to allow for placing forms and applying waterproofing material. Even if you excavate too much earth, place the concrete to the actual excavation depth. Attempts to refill an excavation to the depth specified are not recommended unless an elephant-foot tamper is used to properly compact the fill, because it is difficult to compact the fill surface properly. Some type of lateral provides support for both safety and economy whenever excavation is at such a depth that the slopes become unstable. Good engineering practice dictates using shoring whenever slope stability is questionable. The type of shoring varies with the depth and size of the excavation, the physical characteristics of the soil, and the fluid pressure under saturated conditions. Sandy soils and wet earth generally require more extensive shoring than firmer soils.
5-14. Machines are a necessity for large projects requiring substantial excavation. The most suitable types of equipment include power shovels, dragline buckets, and a backhoe. When selecting equipment consider:
Due to the many variables, it is not possible to give generalized rates of excavation for various types of equipment. However, Table 5-1 gives some typical rates of excavation for specific conditions that still will vary considerably in practice.
Table 5-1. Machine excavation
5-15. Table 5-2 gives hand excavation rates which vary with soil types and excavation depth. Clear out and shape the last 6 inches of bottom excavation by hand; it is extremely difficult to excavate that closely with a machine.
Table 5-2. Hand excavation
5-16. To perform a proper analysis, you must have a working knowledge of the equipment necessary for the form work job and a good idea of how much work the form builders can turn out per unit of time. It is important to be knowledgeable of--
5-17. A carpenter of average skill can build and erect 10 square feet of wood forms per hour. This figure increases as the worker becomes more skilled in form construction. It also varies with the tools and materials available and the type of form. Some forms, such as those for stairways, require considerable physical support from underneath. Some forms take more man-hours and materials to build than simpler forms. For carpenters to move from one level to another frequently requires additional time. Therefore, increased man power support at the ground level increases efficiency
5-18. Established and well-defined concrete-mixing procedures must be followed to produce good quality finished concrete. Never become overconfident in this phase of concrete construction, whether caused by lack of competent and conscientious supervision or inattention to detail. Whoever is in charge of construction must know the concrete-mixing procedures and ensure that they are followed. The extra effort and care this requires are small in relation to the benefits in terms of strength consistency and finished appearance.
5-19. Concrete of uniform quality requires measuring the ingredients accurately for each batch within these percentages: cement, 1 percent; aggregate, 2 percent; water, 1 percent; and admixtures, 3 percent. Equipment should be capable of measuring quantities within these tolerances for the smallest batch used, as well as for larger batches. Periodically check equipment for accuracy and adjust when necessary. Check admixture dispensers daily for errors in admixture measurements--particularly over-dosages; this can cause serious problems in both fresh and hardened concrete. Always measure the following:
Figure 5-1. Measuring aggregate by weight
5-20. Although a machine generally does the mixing, hand mixing sometimes may be necessary. A clean surface is needed for this purpose, such as a clean, even, paved surface or a wood platform having tight joints to prevent paste loss like the one shown at the top of Figure 5-2 below. Moisten the surface and level the platform, spread cement over the sand, and then spread the CA over the cement as shown at the top of Figure 5-2. Use either a hoe (see the middle of Figure 5-2) or a square-pointed D-handled shovel to mix the materials. Turn the dry materials at least three times until the color of the mixture is uniform. Add water slowly while you turn the mixture again at least three times, or until you obtain the proper consistency. Although one worker can mix 1 cubic yard of concrete by hand in about 1 hour, this is not economical for batches of more than 1 cubic yard. Instead, two workers facing each other should work their way through the pile, and keep their shovels close to the platform surface while turning the materials. You can also mix in a hoe box shown at the bottom of Figure 5-2.
Figure 5-2. Hand mixing of concrete
5-21. The methods of mixing and delivering concrete ingredients and the types and sizes of equipment available vary greatly. Power-concrete mixers normally produce one batch about every 3 minutes including charging and discharging. Actual hourly output varies from 10 to 20 batches per hour. A mixer's cubic foot rating usually reflects the number of cubic feet of usable concrete that the machine mixes in one batch. Most mixers can handle a 10 percent overload. The stationary 16-cubic-feet mixer and the M919 Concrete-Mobile-Mixer unit (see Figure 5-3) are table of organization and equipment (TOE) equipment in engineer construction battalions, and are well-suited for troop construction projects.
Figure 5-3. M919 Concrete Mobile Mixer
5-22. Mixing methods include--
5-23. Mixer types include--
CENTRAL MIX PLANTS
5-24. Either stationary mixers or a battery of trailer-mounted mixers usually makes up a central-mix plant which is normally a gravity-feed operation. A clamshell bucket crane, conveyor belts, or elevators carry materials to a batching plant set high enough to discharge directly into dump trucks or other distribution equipment. Mixing time and mixing requirements do not differ much from those already discussed, but you must take special precautions to make sure that the concrete has the proper characteristics and workability upon arrival at the work site. Be especially careful to avoid segregation when using dump trucks.
SITE MIX PLANTS
5-25. The proper location of mixing equipment and materials at the site can yield large savings in time and labor. Figure 5-4 below shows a typical on-site arrangement of mixer and materials. Always locate the mixer as close to the main section of the pour as possible. On a concrete wall project move the mixer to each wall in sequence to reduce transportation distance and time.
Figure 5-4. Hand mixing of concrete
Store aggregate and sand as close to the mixer as possible, without interfering with concrete transportation. The location of the mixing water depends on the type of water supply available. If water is piped in, use a hose to carry it to a barrel near the mixer. If you are using a water truck or trailer, park it next to the mixer.
OPERATING THE 16-CUBIC FOOT MIXER
5-26. Table 5-3 below gives the physical characteristics of a typical 16-cubic-feet mixer. Normal operation of the mixer requires ten soldiers and one noncommissioned officer. The crew operates the mixer and handles the aggregate, sand, cement, and water. The noncommissioned officer, who must be competent, supervises the overall operation. The crew should produce about 10 cubic yards of concrete per hour, depending on their experience, the location of materials, and the mixer's discharge rate. You would need at least one platoon to operate an overall project like the one in Figure 5-4 above.
Table 5-3. Physical characteristics of a typical 16-cubic feet mixer
Charging the Mixer
5-27. Mixers can be charged in two ways: by hand or with a mechanical skip (see Figure 5-5 below). When using the skip, deposit the aggregate, cement, and sand (in that order) into the skip and then dump it into the mixer while water runs into the mixing drum. Place the sand on top of the pile in the skip so that you do not lose too much cement as the batch dumps into the mixer. A storage tank on top of the mixer measures the water in the drum a few seconds before the skip dumps. This discharge also washes down the mixer between batches.
Figure 5-5. Charging a 16 cubic-foot mixer
Discharging the Mixer
5-28. When the mix is ready for discharge from the mixer, move the discharge chute into place to receive the concrete from the drum. Concrete that is somewhat dry tends to cling to the top of the drum and not drop onto the chute in time. Very wet concrete may not carry up high enough on the drum to drop onto the chute. Correct these problems by adjusting the mixer speed. Increase the speed for very wet concrete and decrease the speed for dry concrete.
5-29. The mixing time starts when water runs into the dry mixture. This is normally during the first quarter of the mixing period. The minimum mixing time per batch of concrete is 1 minute unless the batch exceeds 1 cubic yard. Each additional cubic yard of concrete, or fraction thereof, requires an additional 15 seconds of mixing time.
Cleaning and Maintaining the Mixer
5-30. Clean the mixer daily, if it operates continuously, or following each period of use if it operates less than 1 day. The exterior cleaning process goes faster if you coat the outside of the mixer with form oil before you use it. Knock off all accumulated concrete on the mixer exterior and wash it down with a hose. Mixer blades that are worn or coated with hardened concrete provide less efficient mixing action. Replace badly worn blades, and do not allow hardened concrete to accumulate in the mixer drum. Clean it out whenever you shut down for more than 1 1/2 hours. To do this, place a volume of CA equal to one-half the mixer capacity in the drum and allow it to revolve for about 5 minutes. Then discharge the aggregate and flush out the drum with water. Never strike the discharge chute, drum shell, or skip to remove aggregate or hardened concrete, because concrete adheres more readily to dents and bumps. Diligence in cleaning the drum is important because hardened concrete in a mixer absorbs mixing water during subsequent batches, diminishing the effectiveness and composition of the concrete.
OPERATING THE M919 CONCRETE-MOBILE MIXER UNIT
5-31. The Concrete-Mobile-Mixer unit is a combination materials transporter and an on-site mixing plant. Table 5-4 below gives its physical characteristics and overall dimensions. The special body is mounted on a model M919 truck chassis. The unit carries enough unmixed material to produce up to 8 cubic yards of fresh concrete. Because the unit is precisely calibrated, you can produce mixes that meet or exceed both the ACI and the American Association of State Highway and Transportation Officials (AASHTO) standards for design strength. The unit operates on either an intermittent or continuous basis, although continuous operation depends on raw material availability at the site. Certain control settings for the mix operations vary from truck to truck and from site to site.
Table 5-4. M919 Concrete-Mobile-Mixer unit
5-32. When fresh concrete is left standing and not poured, it tends to stiffen before the cement can hydrate to its initial set. Such concrete is still usable if remixing makes it sufficiently plastic to be compacted in the forms. To remix a batch, carefully add a small amount of water, and remix the concrete for at least one-half of the minimum required mixing time or number of revolutions. Then test the concrete to make sure that it does not exceed the maximum allowable W/C ratio, maximum allowable slump, or maximum allowable mixing and agitating time. Do not add water randomly because this lowers concrete quality. Remixed concrete tends to harden rapidly. Any fresh concrete placed adjacent to or above remixed concrete may cause a cold joint
5-33. Concrete consistency depends on the conditions at placement. Handling and transporting methods can affect its consistency. Therefore, if placing conditions allow a stiff mix, choose equipment that can handle and transport such a mix without affecting its consistency. Carefully control each handling and transporting step to maintain concrete uniformity within a batch and from batch to batch so that the completed work is consistent throughout.
5-34. Figure 5-6 shows several right and wrong ways to handle concrete to prevent segregation of the aggregates and paste. Segregation causes honeycomb concrete or rock pockets. Segregation occurs because concrete contains aggregates of different particle sizes and specific gravities. When placed in a bucket, the coarser particles tend to settle to the bottom and the water rises to the top.
Figure 5-6. Concrete handling techniques to prevent segregation
5-35. The three main requirements for transporting concrete from the mixing plant to the job site are:
5-36. Concrete delivery equipment must be capable of handling 100 percent of the mixer capacity to meet peak demands.
5-37. Wheelbarrows or buggies (see Figure 5-7) are the most practical and economical to deliver concrete for foundations, foundation walls, or slabs on or below grade. If available, power buggies (see Figure 5-8) are best for longer runs. Both wheelbarrows and buggies require suitable runways. If possible, arrange them so that the buggies or wheelbarrows do not need to pass each other on a runway at any time. When placing concrete below or at approximate grade, set 2-inch plank runways directly on the ground to permit pouring the concrete directly into the form. View 1 in Figure 5-9 below shows a runway along a wall form that is almost level with the top of the form sheathing. To provide room for the ledger, the top wale is at 1 foot below the top of the concrete. A runway can be made economically from rough lumber consisting of 2 by 10 planks, supported by 4 by 4s, and spaced on 6- foot centers. Nail the 1 by 6 ledger on the form side to the studs on the wall form. Always brace runways securely to prevent failure. A runway for placing concrete on a floor slab also consists of 2 by 10s supported by 4 by 4s placed on 6-foot centers. Such runways are built from either the formwork or the ground. When placing concrete about 5 to 6 feet above grade, economically construct inclined runways (see view 2 of Figure 5-9) for buggy or wheelbarrow use. This will allows wheeling the concrete up inclined runways having a slope of 10:1. If a large quantity of concrete more than 5 to 6 feet is elevated, it is more economical to use elevating equipment, such as a bucket and a crane. If the difference in elevation from the runway to the bottom of the structure is large, use a hopper and chute like the ones shown in Figure 5-10 to prevent segregation. The slope of the chute should normally be 2:1 or steeper for stiff mixes.
Figure 5-7. Handling concrete by buggy
Figure 5-8. Handling concrete by power buggy
Figure 5-9. Runways for wheelbarrow or buggy use
Figure 5-10. Hopper and chute for handling concrete
5-38. Timely delivery of enough concrete on large projects requires careful planning and the selection of the right type of equipment for the purpose.
5-39. Ordinary dump trucks are not designed as concrete carriers although they are commonly used to deliver concrete on large projects in the theater of operations. Exercise care in using them because no means of preventing segregation is provided as it is on mixers or agitator trucks. Even if you use a stiff mix and an air-entraining agent to reduce segregation, keep hauling distances as short as possible, maintain slow speed, and utilize smooth roads to reduce vibration.
5-40. You can use either transit-mix or ready-mix trucks as agitator trucks to deliver premixed concrete. A transit-mix truck is a concrete truck (M919) where materials are mixed after they arrive on the site. A ready-mix truck comes from the batch plant to the site already mixed, ready for placement. The load capacity of a ready-mix trucks is 30 to 35 percent greater than transit-mix trucks, but the operating radius of the ready-mix trucks is somewhat more limited. The trucks discharge concrete either continuously or intermittently from a spout or chute that moves from side to side.
Figure 5-11. Placing concrete using a bucket and crane
Figure 5-12. Piston pump and discharge pipeline
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 subgrade or placing vapor barrier, erecting forms, and setting reinforcing steel.
Moistening the subgrade is especially important in hot weather to prevent water extraction from the concrete. Preparation includes--
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
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
5-56. The finishing process provides the desired final concrete surface. There are many ways to finish concrete surfaces depending on the effect required. Sometimes there is only a need to correct surface defects, fill bolt holes, or clean the surface. Unformed surfaces may require only screeding to proper contour and elevation, or a broomed, floated, or troweled finish may be specified.
5-57. The top surface of a floor slab, sidewalk, or pavement is rarely placed at the exact specified elevation. Screeding brings the surface to the correct elevation by striking off the excess concrete. Using a tool called a screed, which is a template having a straight lower edge to produce a flat surface or a curved lower edge to produce a curved surface, move it back and forth across the concrete using a sawing motion as shown in Figure 5-17. With each sawing motion, move the screed forward a short distance along the forms. (The screed rides on either wood or metal strips established as guides.) This forces the excess concrete built up against the screed face into the low spots. If the screed tends to tear the surface--as it may on air-entrained concrete due to its sticky nature--either reduce the rate of forward movement or cover the lower edge of the screed with metal, which will stop the tearing action in most cases. Hand-screed surfaces up to 30 feet wide, but expect diminished efficiency on surfaces more than 10 feet wide. Three workers (excluding a vibrator operator) can screed about 200 square feet of concrete per hour. Two of the workers operate the screed while the third pulls excess concrete from the front of the screed. Screed the surface a second time to remove the surge of excess concrete caused by the first screeding. This surge is when the concrete seems to grow out of the form into a convex shape.
Figure 5-17. Screeding operation
5-58. If a surface smoother than that obtained by screeding is required, work the surface sparingly using either a wood or aluminum-magnesium float, or a finishing machine. The wood float is shown in use in view 2 of Figure 5-18. Begin floating immediately after screeding while the concrete is still plastic and workable. Floating has three purposes: to embed aggregate particles just beneath the surface; to remove slight imperfections, high spots, and low spots; and to compact the concrete at the surface in preparation for other finishing operations.
Figure 5-18. Wood floats and floating operations
Do not overwork the concrete while it is still plastic or an excess of water and paste will rise to the surface. This fine material will form a thin weak layer that will scale or wear off under use. To produce a coarse texture as the final finish, float the surface a second time after it partially hardens. Use a long-handled wood float for slab construction as shown in view 3 of Figure 5-18. Use an aluminum float the same way as the wood float, to give the finished concrete a much smoother surface. To avoid cracking and dusting of the finished concrete, begin aluminum floating when the water sheen disappears from the freshly placed concrete surface. Do not use either cement or water as an aid in finishing the surface.
5-59. For a dense, smoother finish, follow floating with steel troweling (see Figure 5-19). Begin this procedure when the moisture film or water sheen disappears from the floated surface and the concrete has hardened enough to prevent fine material and water from working to the surface. Delay this operation as long as possible. Too much troweling too soon tends to produce crazing and reduces durability. However, too long a delay in troweling makes the surface hard to finish properly. Troweling should leave the surface, smooth, even, and free from marks and ripples.
Figure 5-19. Steel finishing tools and troweling operation
Avoid all wet spots if possible. When they do occur, do not resume finishing operations until the water has been absorbed, evaporated, or mopped up. When a wear resistant and durable surface is required, it is poor practice to spread dry cement on the wet surface to absorb excess water. Obtain a surface that is fine-textured, but not slippery, by a second light troweling over the surface with a circular motion immediately following the first regular troweling, keeping the trowel flat against the surface. When a hard steel-troweled finish is specified, follow the first regular troweling with a second troweling only after the concrete is hard enough that no paste adheres to the trowel, and passing the trowel over the surface produces a ringing sound. During this final troweling, tilt the trowel slightly and exert heavy pressure to compact the surface thoroughly. Hair cracks usually result from a concentration of water and fines at the surface due to overworking the concrete during finishing operations. Rapid drying or cooling aggravates such cracking. Close cracks that develop before troweling by pounding the concrete with a hand float.
5-60. Produce a nonskid surface by following the floating operation. After waiting 10-15 minutes, broom the concrete before it hardens thoroughly with a straw broom of fibers at least 4 1/2 inches long. The grooves cut by the broom should not be more than 3/16 inch deep. When severe scoring is not desirable, such as in some floors and sidewalks, produce the broomed finish using a hairbrush after troweling the surface once to a smooth finish. However, when rough scoring is specified, use a stiff broom made from either steel wire or coarse fiber. The direction of scoring when brooming should be at right angles to the direction of any traffic.
5-61. The most uniform and attractive surface requires a rubbed finish. A surface finish having a satisfactory appearance can be produced simply by using plywood or lined forms. As soon as the concrete hardens, rub the surface first with coarse carborundum stones so that the aggregate does not pull out. Allow the concrete to cure before the final rubbing with finer carborundum stones. Keep the concrete damp while rubbing. To properly cure any mortar used as an aid in this process and left on the surface, keep it damp for 1 to 2 days after it sets. Restrict the mortar layer to a minimum, because it is likely to scale off and mar the surface appearance.
5-62. A machine finish is needed when the concrete takes its initial set, but still remains in workable condition. Technical Manual (TM) 5-331D describes the operation and maintenance of machine finishers.
5-63. Set both the screeds and vibrators on the machine finisher to produce the specified surface elevation as well as a dense concrete. Generally, a sufficiently thick layer of concrete should build up ahead of the screed to fill all low spots completely. The sequence of the operation is: screed, vibrate, then screed again. If forms are in good alignment and firmly supported, and if the concrete has the correct workability, only two passes of the machine finisher are needed to produce a satisfactory surface. Adjust the second screed to carry enough concrete ahead of it so that the screed continually contacts the pavement.
5-64. Hand floating often does more harm than good; overworking can cause the wearing surface to deteriorate. Scaling is a good example. However, sometimes there may be a need to use a longitudinal float to decrease variations running lengthwise in the surface. Such a float is made from wood, 6 by 10 inches wide and 12 by 18 feet long, fitted with a handle at each end. It is operated on form-riding bridges by two workers. The float oscillates longitudinally as it moves transversely (crosswise). A 10-foot straightedge pulled from the center of the pavement to the form removes any minor surface irregularities as well as laitance. The surface should have no coating of weak mortar or scum that will later scale off. Unless the workers use considerable care as the straightedge approaches the form, it will ride up on the concrete causing a hump in the surface, especially at construction and expansion joints. When the water sheen disappears, obtain the final surface finish by dragging a clean piece of burlap along the pavement strip longitudinally. This operation, called belting, requires two workers, one on each side of the form. Be sure to round all pavement corners with an edging tool, clean out expansion joints, and prepare them for filling.
5-65. As soon as possible after removing the forms, knock off all small projections, fill tie-rod holes, and repair any honeycombed areas. Section XII in this chapter covers concrete repair in detail.
5-66. Concrete surfaces are not always uniform in color when forms are removed. If appearance is important, clean the surface using one of the methods described below.
CLEANING WITH MORTAR
5-67. Methods of cleaning with mortar includes--
5-68. Completely remove surface stains, particularly rust, by lightly sandblasting the surface. This method is more effective than washing with acid.
CLEANING WITH ACID
5-69. Use this method when surface staining is not severe. Precede acid washing by a 2-week period of moist curing. First, wet the surface and while it is still damp scrub it thoroughly using a stiff bristle brush with a 5 to 10 percent solution of muriatic acid. Remove the acid immediately and thoroughly by flushing with clean water. If possible, follow the acid washing with four more days of moist curing. When handling muriatic acid, be sure to wear goggles to protect your eyes and take precautions to prevent acid from contacting hands, arms, and clothing.
5-70. Curing keeps concrete moist. The moisture is needed for any chemical reactions.
5-71. Adding water to portland cement to form the water-cement paste that holds concrete together starts a chemical reaction that makes the paste into a bonding agent. This reaction, called hydration, produces a stone-like substance--the hardened cement paste. Both the rate and degree of hydration, and the resulting strength of the final concrete, depend on the curing process that follows placing and consolidating the plastic concrete. Hydration continues indefinitely at a decreasing rate as long as the mixture contains water and the temperature conditions are favorable. Once the water is removed, hydration ceases and cannot be restarted.
5-72. Curing is the period of time from consolidation to the point where the concrete reaches its design strength. There are numerous facts which affect the curing process.
Figure 5-20. Moist curing effect on compressive strength of Concrete
5-73. Several curing methods will keep concrete moist and, in some cases, at a favorable hydration temperature. They fall into two categories: those that supply additional moisture and those that prevent moisture loss. Table 5-5 lists several of these effective curing methods and their advantages and disadvantages.
Table 5-5. Curing methods
METHODS THAT SUPPLY ADDITIONAL MOISTURE
5-74. Both sprinkling and wet covers add moisture to the concrete surface during the early hardening or curing period. They also provide some cooling through evaporation, which is especially important in hot weather. Methods that gives additional moisture are--
Figure 5-21. Curing a wall with wet burlap sacks
METHODS THAT PREVENT MOISTURE LOSS
5-75. Moisture lost prevention methods include laying waterproof paper, plastic film, or liquid-membrane-forming compounds, and simply leaving forms in place. Moisture loss can be prevented by sealing the surface with--
Figure 5-22. Waterproof paper used for curing
5-76. Concreting in hot weather poses some special problems such as strength reduction and cracking of flat surfaces due to too-rapid drying.
5-77. Concrete that stiffens before consolidation is caused by too-rapid setting of the cement and too much absorption and evaporation of mixing water. This leads to difficulty in finishing flat surfaces. Therefore, limitations are imposed on placing concrete during hot weather and on the maximum temperature of the concrete; quality and durability suffer when concrete is mixed, placed, and cured at high temperatures. During hot weather take steps to limit concrete temperature to less than 90°F, but problems can arise even with concrete temperatures less than 90°F. The combination of hot dry weather and high winds is the most severe condition, especially when placing large exposed slabs.
EFFECTS OF HIGH CONCRETE TEMPERATURES
5-78. Three common things affect high concrete temperatures.
Figure 5-23. Concrete mix water requirements as temperature increases
Figure 5-24. Effect of high temperature on concrete compressive strength at various ages
COOLING CONCRETE MATERIALS
5-79. The most practical way to obtain a low concrete temperature is to cool the aggregate and water as much as possible before mixing. Mixing water is the easiest to cool and is also the most effective, pound for pound, in lowering concrete temperature. However, because aggregate represents 60 to 80 percent of the concrete's total weight, the concrete temperature depends primarily on the aggregate temperature. Figure 5-25 shows the effects of the mixing water and aggregate temperatures on the temperature of fresh concrete. Lower the temperature of fresh concrete by--
Figure 5-25. Mixing water temperatures required to produce concrete of required temperature
5-80. High temperatures increase the hardening rate, thereby shortening the length of time available to handle and finish the concrete. Concrete transport and placement must be completed as quickly as possible. Take extra care to avoid cold joints when placing it. Proper curing is especially important in hot weather due to the greater danger of crazing and cracking. But curing is also difficult in hot weather, because water evaporates rapidly from the concrete and the efficiency of curing compounds is reduced. Leaving forms in place is not a satisfactory way to prevent moisture loss when curing concrete in hot weather. Loosen the forms as soon as possible without damaging the concrete and cover the concrete with water. Then frequent sprinkling, use of wet burlap, or use other similar means of retaining moisture for longer periods.
5-81. Do not suspend concreting during the winter months. Take the necessary steps to protect the concrete from freezing in temperatures of 40°F or lower during placing and during the early curing period.
5-82. In your prior planning, include provisions for heating the plastic concrete and maintaining favorable temperatures after placement. The temperature of fresh concrete should not be less than that shown in lines 1, 2, and 3 of Table 5-6. Note that lower temperatures are given for heavier mass sections than thinner sections, since less heat dissipates during the hydration period. Because additional heat is lost during transporting and placing, the temperatures given for the freshly mixed concrete are higher for colder weather. To prevent freezing, the concrete's temperature should not be less than that shown in line 4 of Table 5-6 at the time of placement. To ensure durability and strength development, further thermal protection will need to be provided to make sure that subsequent concrete temperatures do not fall below the minimums shown in line 5 of Table 5-6 for the time periods given in Table 5-7. Concrete temperatures over 70°F are seldom necessary because they do not give proportionately longer protection from freezing, since the heat loss is greater. High concrete temperatures require more mixing water for the same slump and this contributes to cracking due to shrinkage.
Table 5-6. Recommended concrete temperatures for cold-weather construction
Table 5-7. Recommended duration of protection for concrete placed in cold weather (air-entrained concrete)
EFFECTS ON LOW CONCRETE TEMPERATURES
5-83. Figure 5-26, demonstrates which temperature affects the hydration rate of cement; low temperatures retard hardening and compressive strength gain. The graph shows that the strength of concrete mixed, placed, and cured at temperatures below 73°F is lower than concrete cured at 73°F during the first 28 days, but becomes higher with age and eventually overtakes the strength of the concrete cured at 73°F. Concrete placed at temperatures below 73°F must be cured longer. Remember that strength gain practically stops when the moisture required for hydration is removed. Figure 5-27 shows that the early strengths achieved by Type III or high-early-strength cement are higher than those achieved by Type I cement.
Figure 5-26. Effects of low temperature on concrete compressive strength at various ages
Figure 5-27. Relationship between early compressive strengths of portland cement types and low curing temperatures
COLD WEATHER TECHNIQUES
5-84. When heating concrete ingredients, the thaw-frozen aggregate makes proper batching easier and avoids pockets of aggregate in the concrete after placement. If aggregate is thawed in the mixer, check for too much water content. Aggregate in temperatures above freezing seldom has to be heated, but at temperatures below freezing, the FA used to produce concrete may need to be heated.
Figure 5-28. Effect of temperature of materials on temperature of fresh concrete
5-85. Concrete placed in forms or covered by insulation seldom loses enough moisture at 40oF to 55oF to impair curing. Forms distribute heat evenly and help prevent drying and overheating. Leave them in place as long as practicable. However, when using heated enclosures during the winter, moisten curing concrete to offset the drying effects. Keep the concrete at a favorable temperature until it is strong enough to withstand both low temperatures and anticipated service loads. Concrete that freezes shortly after placement is permanently damaged. If concrete freezes only once at an early age, favorable curing conditions can restore it to nearly normal, although it will neither weather as well nor be as watertight as concrete that has never frozen. Air-entrained concrete is less susceptible to freeze damage than nonair-entrained concrete. (See TM 5-349 for details of cold weather concreting.) Three methods for maintaining proper curing temperatures are described below.
5-86. Careless workers can cancel out the value of good detailing and planning by indiscriminate use of the wrecking bar. A pinch bar or other metal tool should never be placed against exposed concrete to wedge forms loose. If it is necessary to wedge between the concrete and the forms, use only wooden wedges.
5-87. Wall forms should not be removed until the concrete has thoroughly hardened, but specified curing should begin as early as possible in warm weather. Ties may be removed as early as 24 hours after casting to loosen forms slightly and permit entry of curing water between form and concrete. Ornamental molds must be left in place until they can be removed without damage to the concrete surface. In cold weather, removal of form work should be deferred or form work should be replaced with insulation blankets to avoid thermal shock and consequent crazing of the concrete surface.
5-88. When stripping forms in the vicinity of a belt course, cornice, or other projecting ornament, begin stripping some distance away from the ornament and work toward it. If there is any tendency for the forms to bind around the ornament, the pressure of the forms against projecting corners will be relieved so that there will be less chance of spalling sharp edges. Forms recessed into the concrete require special care in stripping. Wedging should be done gradually and should be accompanied by light tapping on the piece to crack it loose from the concrete. Never remove an embedded form with a single jerk. Embedded wood forms are generally left in place as long as possible so they will shrink away from the concrete. The embedded items should be separate from or loosely attached to the main form so that they will remain in place when the main form is stripped.
STRIPPING TIME BASED ON CONCRETE STRENGTH
5-89. Since early form removal is usually desirable so that forms can be reused, a reliable basis for determining the earliest proper stripping time is necessary. When forms are stripped, there must be no excessive deflection or distortion and no evidence of cracking or other damage to the concrete, due either to removal of support or to the stripping operation. Supporting forms and shores must not be removed from beams, floors, and walls until these structural units are strong enough to carry their own weight and any approved superimposed load. Such approved load should not exceed the live load for which the member was designed unless provision has been made by the engineer architect to allow for temporary construction loads.
5-90. Forms for vertical members such as columns and piers may be removed before those for beams and slabs. The strength of the concrete necessary before form work is stripped and the time required to attain it vary widely with different job conditions, and the most reliable basis is furnished by test specimens cured under job conditions. In general, forms and supports for suspended structures can be removed safely when the ratio of cylinder test compressive strength to design strength is equal to or greater than the ratio of total dead load and construction loads to total design load with a minimum of 50 percent of design compressive strength being required.
5-91. For some applications, a definite strength must be obtained; for example, 2,500 psi or two-thirds of the design strength. However, even when concrete is strong enough to show no immediate distress or deflection under a load, it is possible to damage corners and edges during stripping and for excessive creep deflections to occur. If strength tests are to be the basis for the designers, instructions to the project officer on form removal, the type of test, the method of evaluating, and the minimum strength standards should be stated clearly in specification.
5-92. The number of test specimens, as well as who should take them and perform the tests, should also be specified. Ideally, test beams and cylinders should be job cured under conditions which are similar to those for the portions of the concrete structure which the test specimens represent. (The specimens must not be confused with those cured under laboratory conditions to evaluate 28-day strength of the concrete.) The curing record including the time, temperature, and method for both the concrete structure and the test specimens, as well as the weather record, will assist both the engineer and the project officer in determining when forms can be safely stripped. It should be kept in mind that specimens which are relatively small are more quickly affected by freezing or drying conditions than concrete in the structure.
5-93. On jobs where the engineer has made no provision for approval of shore and form removal based on strength and other considerations peculiar to the job, Table 5-8 shows the minimum time forms and supports should remain in place under ordinary conditions.
Table 5-8. Recommended form stripping time
5-94. Forms are designed and constructed so that their removal does not harm the concrete. The form must be stripped carefully to avoid damaging the surface. Do not jerk the forms from the concrete after wedging at one end, or the edges will break. Withdraw all nails while stripping and immediately clean and oil all forms to be reused.
5-95. Although repairs are costly and time-consuming, field experience dictates that steps in the repair procedure cannot be omitted or performed carelessly without harming the serviceability of the repair work.
5-96. If they are not properly performed, repairs later loosen, crack at the edges, and allow water to permeate the structure. Preliminary procedures includes--
REMOVING RIDGES AND BULGES
5-97. Remove objectionable ridges and bulges by carefully chipping and rubbing the surface with a grinding stone.
PATCHING NEW CONCRETE
5-98. Before placing mortar or concrete into patch holes, keep the surrounding concrete wet for several hours. Then brush a grout made from cement and water mixed to a creamy consistency into the hole surfaces before applying the patch material. Start curing the patch as soon as possible to avoid early drying. Use damp burlap, tarpaulins, or membrane curing compounds. Patches are usually darker in color than the surrounding concrete. If appearance is important, mix some white cement into the mortar or concrete and use as a patch. Make a trial mix to determine the best proportion of white and gray cements to use.
5-99. Do not apply a single shallow layer of mortar on top of honeycombed concrete, because moisture will form in the voids and subsequent weathering will cause the mortar to spall off. Instead, chip out small defective areas, such as rock pockets or honeycomb, down to solid concrete. Cut the edges either as straight as possible at right angles to the surface, or undercut them slightly to make a key at the edge of the patch. Keep the surfaces of the resulting holes moist for several hours before applying the mortar. Fill shallow patch holes with mortar placed in layers not more than 1/2 inch thick. Make rough scratches in each layer to improve the bond with the succeeding layer, and smooth the surface of the last layer to match the adjacent surface. Allow the mortar to set as long as possible to reduce shrinkage and make a more durable patch. If absorptive form lining was used, make the patch match the adjacent surface by pressing a piece of the form lining against the fresh patch.
5-100. Patch large or deep holes with concrete held in place by forms. Reinforce such patches and dowel them to the hardened concrete as shown in Figure 5-29.
Figure 5-29. Repairing large and deep holes in new concrete
Bolt and Tie-Rod Holes
5-101. When filling bolt holes, pack small amounts of mortar into place carefully. Mix the mortar as dry as possible with just enough water to compact tightly when forced into place. Fill tie-rod holes extending through the concrete with mortar, using a pressure gun similar to an automatic grease gun.
5-102. View 1 of Figure 5-30 shows why feathered edges around a flat patch break down. First chip an area at least 1 inch deep with edges at right angles to the surface, as shown in view 2 of Figure 5-30, before filling the hole. Then screed the patch as shown in view 3 of Figure 5-30. The fresh concrete should project slightly beyond the surface of the hardened concrete. Allow it to stiffen before troweling and finishing to match the adjacent surfaces.
Figure 5-30. Patching flat surface in new concrete
5-103. When repairing old concrete, first determine how much material to remove by thoroughly inspecting the defect. Remember that it is far better to remove too much old material than not enough.
5-104. Remove all concrete of questionable quality unless nothing would be left. In this case, remove only the loose material. If the old and new concrete will join on a surface exposed to weathering or chemical attack, make sure the old concrete is perfectly sound. After removing weakened material and loose particles, thoroughly clean the cut surfaces using air or water or both. Keep the area surrounding the repair continuously wet for several hours, preferably overnight. Wetting is especially important in repairing old concrete because a good bond will not form without it.
PATCHINH OLD CONCRETE
5-105. The repair depth depends on many conditions. In large structures such as walls, piers, curbs, and slabs, the repair should be at least 6 inches deep, if possible. If the old concrete contains reinforcement bars, allow a clearance of at least 1 inch around each exposed bar. Use rectangular patches on small areas, cutting 1 to 2 inches vertically into the old concrete to eliminate thin or feathered edges. Following the wetting period, place the new concrete into the hole in layers and thoroughly tamp each layer. The patch concrete should be a low slump mixture allowed to stand for awhile to reduce shrinkage. Forms may be needed to hold the patch concrete in place. The design and construction of such forms often require a lot of cleverness, but well-designed and properly constructed forms are important in concrete repair. Reinforce deep patches to the hardened concrete. Good curing is essential. Begin curing as soon as possible to avoid early drying.
Concrete and Masonry Earthmoving Equipment Plumbing and Pipefitting Practical Elements of AutoCAD
Concrete and Masonry
Plumbing and Pipefitting
Practical Elements of AutoCAD