Methods and Equipment

Methods and EquipmentEquipment for Moving or Handling ConcreteEquipment should be carefully selected to move concrete from its point of production to point of delivery and into place in the finished structure. The slump, sand content, maximum size of aggregate, or concrete mix should not be governed by the equipment; rather the equipment should be capable of expeditiously handling, moving, and discharging concrete of such slump, sand content, maximum aggregate size, or mix proportions considered otherwise suitable and which can be placed by vibration or other suitable placing methods.Chutes - are simple and expenditious way of transferring or moving concrete to a lower elevation. Chutes must have sufficient slope so that concrete at the specified slump will readily move down them by gravity.

Wheelbarrows, Concrete Buggies or Motorized carts -transport concrete at short distances. These vehicles may be used on small jobs on level ground.

Buckets - is aheavy equipmentmachine used inconstructionto move aside or load materials such asasphalt, demolition debris,dirt,snow,feed,gravel,logs, rawminerals, recycled material,rock,sand,woodchips, etc. into or onto another type of machinery (such as adump truck,conveyor belt, feed-hopper, orrailroad car).

Conveyor -used to transfer concrete horizontally and modest distances vertically. Conveyors are particularly useful in areasof limited room , such as in tunnels, but widely used on large areas such as floor slabs and bridge decks.

Concrete Pumps - most commonly used to move concrete from the delivery truck to the place where it is used. Ther are two types of concrete pumps, the first type of concrete pump is attached to a truck. or longer units are on semi-trailers. It is known as a boom concrete pump because it uses aremote-controlledarticulatingroboticarm (called aboom) to place concrete accurately. The second main type of concrete pump is either mounted on a truck or placed on a trailer, and it is commonly referred to as aline pumpor trailer-mounted concrete pump. This pump requiressteelorflexibleconcrete placinghosesto be manually attached to the outlet of the machine BOOM CONCRETE PUMP

TRAILER-MOUNTED CONCRETE PUMP Placing Concrete in FormsBefore any concrete is placed the entire placing programme consisting of equipment, layout, proposed procedures and methods is planned and no concrete is placed until formwork is inspected and found suitable for placement.Equipment for conveying concrete should be of such size and design as to ensure a practically continuous flow of concrete during depositing without segregation of materials considering the size of the job and placement location.Concrete is placed in its final position before the cement reaches its initial set and concrete is compacted in its final position within 30 minutes of leaving the mixer and once compacted it should not be disturbed.In all cases the concrete is deposited as nearly as practicable directly in its final position and should not be re-handled or caused to flow in a manner which may cause segregation, loss of materials, displacement of reinforcement, shuttering or embedded inserts or impair its strength.For locations where direct placement is not possible and in narrow forms suitable drop and Elephant Trunks to confine the movement of concrete is provided. Special care is taken where concrete is dropped from a height especially if reinforcement is in the way particularly in columns and thin walls.Concrete should be placed in the shuttering by shovels or other methods and should not be dropped from a height more than one metre or handle in a manner which will cause segregation.Concreting once started should be continuous until the pour is completed. Concrete should be placed in successive horizontal layers of uniform thickness ranging from 150 mm to 900 mm. These should be placed as rapidly as practicable to prevent the formation of cold joints or planes of weakness between each succeeding layers within the pour. Internal Vibrator or Immersion VibratorsIt essentially consists of a steel tube (with one end closed and rounded) having an eccentric vibrating element inside it. This steel tube called poker is connected to an electric motor or a diesel engine through a flexible tube. They are available in size varying from 40 to 100 mm diameter. The frequency of vibration varies upto 15000 rpm. However a range between 3000 to 6000 rpm is suggested as a desirable minimum with an acceleration of 4g to 10g The period of vibration required may be of the order of 30 seconds to 2 minute. The concrete should be placed in layers not more than 600mm high.

External or Shutter VibratorsThese vibrators are clamped rigidly to the form work at the pre-determined points so th These vibrators can compact up to 450mm from the face but have to be moved from one place to another as concrete progresses. These vibrators operate at a frequency of 3000 to 9000 rpm at an acceleration of 4g. at the form and concrete are vibrated.

Surface VibratorsThese are placed directly on the concrete mass. These best suited for compaction of shallow elements and should not be used when the depth of concrete to be vibrated is more than 250 mm. Very dry mixes can be most effectively compacted with surface vibrators. The surface vibrators commonly used are pan vibrators and vibrating screeds. The main application of this type of vibrator is in the compaction of small slabs, not exceeding 150 mm in thickness, and patching and repair work of pavement slabs. The operating frequency is about 4000 rpm at an acceleration of 4g to 9g.

Consolidating Concrete with Vibration Most engineers would say as water-cement ratio increases strength decreases. Unless concrete is properly consolidated, voids reduce strength regardless of the water-cement ratio. Right after its placed, concrete contains as much as 20% entrapped air. At a constant water- cement ratio, each percent of air decreases compressive strength by about 3% to 5%. Consolidating the concrete, usually by vibration, increases concrete strength by driving out entrapped air. It also improves bond strength and decreases concrete permeability.

Consolidating Concrete without VibrationWhen vibration is not used concrete can be adequately placed but with more physical effort. The concrete will need to be more workable for hand-placing methods with more slump and perhaps a higher sand content, although excessive high slumps and sand contents are not necessary and are undesirable as they result in lower-quality concrete with greater potential for shrinkage, lower strength and less durability. Simply dumping concrete into a form from a ready-mixed concrete truck without some means being taken to consolidate it,even with high-slump concrete, cannot achieve a satisfactory job.Preparation For Placing Concrete

Foundation Preparation Excavation rocks should be excavated to sound material and be completely exposed and rock surfaces should be normal to the direction of load.Controlled blasting, supervised by specialist in this field, can minimize rock problems caused by careless blasting and result in much better construction. Good practices, such as closer and better spacing of drill holes, controlled powder charges and delays, can result in closer control of rock excavation to required line and grade and less damage to the rock.Overexcavation may be corrected by backfill with suitable earth material adequately compacted or controlled low-strength material.Drainage such drains are usually designed as part as part of the job and are provided for in the plans and specifications. Drains may be constructed by placing the concrete or clay pipe on a clean gravel or crushed-stone blanket in a trench or depression in the foundation, backfilling, around it with the same clean gravel or crush stone (3/4 X 3/8 in. or 1 X in. (20 to 10 mm or 40 to 20 mm)). Usually the concrete is covered with burlap to prevent mortar from infiltrating the gravel when concrete is place over it, and it may be covered with a thin layer (1 to 2 in) (25 to 50 mm) of in (20 mm)) maximum size aggregate concrete to protect it from the hazards of construction if some time will elapse before the concrete it place over it. Construction-Joint PreparationWhen fresh concrete is placed on or adjacent to previously placed concrete, whether old or new concrete , and where a bond between the two surfaces is required, it is essential that the surface of the previously placed concrete be clean and properly prepared. The surface should be free of laitance, carbonation, scum, dirt, oil, grease, paint, curing compound, and loose or disintegrated concrete and should be slightly roughened. Several methods of cleanup are available, depending on the size of areas to be cleaned, age of the concrete, skills of the workers, and availability of equipment.

The Mortar LayerA rock surface or a construction joint in previously placed concrete should be properly prepare to receive the concrete placement as discussed previously. Research by the Corps of engineers indicates that it may not make much difference whether or not a layer of mortar broomed onto a properly prepared surface of either structural or mass concrete prior to placed concrete upon the surface. This research does indicate that a thin mortar either flowed or broomed onto the surface is superior to a thick mortar. In some case superior joints were obtained without mortar. Also dry joints appeared to give superior results to wet joints.

FORMWORKSFORMWORK- is the term given to either temporary or permanent support moulds into which concrete or similar material are poured.

TYPES OF FORMWORKFoundation formworkWall formworkCeiling formworkBeam formworkColumn formwork

FORMWORKForm PreparationThe design and construction of forms are discussed in Sec. 22.2.2 and in Refs. 6 through 8.Details of form construction are covered in Sec. 25.2.2, but certain details are important to those concerned with preparation for placing concrete. Particular care must be taken with forms for concrete that will permanently exposed to view or in which special architectural effects are required.

A good formwork should satisfy the following requirements:It should be strong enough to withstand all types of dead and live loads.It should be rigidly constructed and efficiently propped and braced both horizontally and vertically, so as to retain its shape.The joints in the formwork should be tight against leakage of cement grout.Construction of formwork should permit removal of various parts in desired sequences without damage to the concrete.The material of the formwork should be cheap, easily available and should be suitable for reuse.The formwork should be set accurately to the desired line and levels should have plane surface.It should be as light as possible.The material of the formwork should not warp or get distorted when exposed to the elements.It should rest on firm base.

WOOD FORMWORK

STEEL FORMWORKSteel forms compared with timber formwork:

Steel forms are stronger, durable and have longer life than timber formwork and their reuses are more in number.Steel forms can be installed and dismantled with greater ease and speed.The quality of exposed concrete surface by using steel forms is good and such surfaces need no further treatment.Steel formwork does not absorb moisture from concrete.Steel formwork does not shrink or warp.

Use of Forms Safety ProvisionProvision should be made, particularly on wall forms, for safe working conditions for the placing crew. Ladders, working platforms, hand railing, kick boards, and other safety necessities should be provided, as required by all safety codes.

Form Coating dorms must be treated with a form oil or other coating material to prevent adhesion to the concrete. Many form oils and parting compounds, such as plastic, lacquer, or shellac, are available. The form coating should be formulated for the particular usage and material to which it is to be applied and should protect the form from water, strip readily from the concrete, not interfere with subsequent curing, painting, or other surface treatments, or stain or cause softening of the surface. And form oil or coating must be applied before the forms are erected.Cleaning with FormsAll dirt, sawdust, shavings, tie wire, loose nails, and other debris should be removed from within the forms before concrete placing is started. Provision should be made in wall or similar forms, particularly those for thin sections, for washing or blowing out all such debris, by leaving a panel or the bottom of the form loose or otherwise providing an opening at the bottom or end of the form for washout. Sometimes nails and wire clippings can be picked up by a magnet on a pole. The ends of reinforcing steel tie wires should not rest against the form where they will cause unsightly rust stains.

Concrete PlacingThe placing inspectors and contractors form watcher should carefully watch the forms during the concrete placing for any signs of difficulties. Should any develop, placing should be immediately stopped or slowed as conditions warrant so that the conditions can be inspected and corrected.Form Removal

Factors to be considered in determining when forms are to be removed are the effect of the form-removal operations in damaging the concrete, the structural strength or deflection of the concrete, curing and protection, finishing requirements, and requirements for reuse of the forms. Forms may be removed from a few minutes to days after the concrete is placed.Forms should be removed using tools and equipment that will not damage either the concrete or the forms if the forms are to be reused or salvaged.Strength of the concrete and structural requirements determine the form-stripping time for arches, beams, girders and similar load carrying structural members.Order and method of removing formwork:

The sequence of orders and method of removal of formwork are as follows:Shuttering forming the vertical faces of walls, beams and column sides should be removed first as they bear no load but only retain the concrete.Shuttering forming soffit of slabs should be removed next.Shuttering forming soffit of beams, girders or other heavily loaded shuttering should be removed in the end.

Reuse of FormsForms should be designed to permit easy removal and reuse. Forms should be carefully remove, cleaned, repaired, handled and stored so that they are not damaged.Reinforcement Reinforcing steel, commonly called resteel or rebars, is placed in concrete to reinforce the concrete structure adequately so that it will support expected loads.Concrete is weak in tension, and most reinforcing is use to provide tensile reinforcement.Steel is also used to provide compressive reinforcement, in columns and in arches, for example, and to resist shrinkage and temperature stresses.

ReinforcementReinforcing should be accurately placed.Inaccuracies can reduce the strength of the structural unit containing the steel or, if the steel is placed in the wrong place (the wrong side of a cantilever beam, for example), might lead to failure of the structure.It is particularly important that reinforcing steel be properly placed in areas subject to earthquakes, tornadoes, and hurricanes.Investigation of structural failures following such disasters shows many violation of specification and good practice in placing the reinforcing steel and making adequate connections between structural elements. This cause or contribute to the failure. Also indicated is a lack of competent inspections. Spacing of BarsDesign and detailing should provide for proper spacing of the reinforcing steel in accord with the ACI 318 Bldg. code or other requirements.This limits the closeness of the bars to each other in various situation.This conditions have been provided for the reinforcing-steel detailer. Sometime bars are bundled: for instance, up to four bars may be placed together. Sometimes temporary removal of moving of bars, particularly in heavily reinforced match, may be done to facilitate placement of the concrete.SplicesSplices are frequently necessary because of manufacturing, fabrication, handling, or transportation considerations. Splices should be located as shown on the design or detailing drawings. Splices should occur at location at minimum stresses in the structure.Splices may be made by lapping the bars a distance determined by the bar size, grade of steel, and concrete strength as given in the ACI 318 Building Code or by the designer.Cover Concrete protects reinforcing steel from corrosion and serves as fire-proofing. Adequate cover or embedment of the steel in the concrete to provide this protection is specified in the ACI 318 Bldg. code.

The following minimum concrete cover for cast=in=place concrete is given. Cover for present and prestressed concrete may vary from this, consult the ACI 318 Bldg. Code.

Bar Supports, Spacers and TiesBars should be supported, anchored and tied to hold them in place before and during concrete placing.Supports, anchors or ties should not permit subsequent entrance of water or moist air into the hardened concrete which could initiate corrosion.In footing, mats of bars can be supported by precast concreted blocks made for this purpose or chairs. Cleaning ReinforcementReinforcement at the time of placing concrete should be free of mud, oil or other foreign matter that will reduce or prevent bond. Mill scale or a light coating of rust tightly bonded to the steel is not objectionable.Loose, flaky, scaly rust which would affect the bond, should be removed by scrapping, shock treatment (dropping, hammering, vibration), wire brushing or possibly sandblasting.Grease or oil should be removed by a propane torch, using care not to overheat the bars, or washed off with kerosene or gasoline with the adequate safety precaution for using such materials.When oiling forms, oil may get onto the bars. If the form oil evaporates or is a very thin coating, it will do harm. If it is a thick coating, it should be removed.Reporters:

BARANGAY, JAY-ARGALLA, JOHN ZENDRESBACUD, PATRICK ARCHIETUMBAGA, ROSVIN JOHNALMAREZ, DARNELL JAKE