Buckets, cranes, carts and wheelbarrows

Transcription

1 Buckets, cranes, carts and wheelbarrows For jobs ranging from precise placement of architectural concrete to mass placement in dams, this equipment frequently offers the greatest flexibility at the lowest cost BY WILLIAM C. PANARESE, SENIOR CONCRETE TECHNOLOGIST PORTLAND CEMENT ASSOCIATION BUCKETS Tra n s p o rting and handling conc rete in buckets is often considere d the most flexible method for moving concrete from the point of delive ry at the job site to the point of placement in the form s. Buckets are made in differe n t shapes and in sizes va rying fro m one-third cubic yard up to 12 cubic y a rds for different applications. Some buckets have re c t a n g u l a r c ross sections but most are circ u l a r. The concrete is released by opening a gate that forms the bottom of the bucket. For massive work the buckets often have vertical or very steep sides with gates that open to the full a rea of the bottom. Howe ve r, for most types of work, buckets having the lower part of the sides sloping to a smaller gate are usually preferred for better control of concrete d i s c h a rg e. On the ave rage job, buckets with gates that can be re g u- lated to control the flow of concrete and closed after only part of the c o n c rete has been deposited are preferred. Some gates are operated manually while others are powe red by 1. General purpose buckets can be used for handling concrete in a wide variety of situations. pneumatic or hyd raulic means. Airactuated gates are operated by an air supply hose that must be connected at the placing site or by a re c h a rgeable compressed air tank built into the bucket. The gates for buckets should be grout-tight, particularly if material is to be tra n s- p o rted for some distance by tru c k, boat or rail car. Buckets for general use Se ve ral kinds of concrete buckets a re available and the choice depends on the size of aggregate in the c o n c re t e, consistency of the conc re t e, and the capacity of the equipment used to lift the bucket. L i g h t weight buckets are made with small gates for use with light c ra n e s. They can handle two- to sixinch slump concrete with less than t h ree-inch maximum size aggreg a t e. Capacities range from onet h i rd cubic yard up to about two cubic yard s. St a n d a rd or general purpose buckets (Photo 1) have larger disc h a rge gates and are built to handle n o rmal slump concretes with up to t h ree-inch maximum size aggreg a t e. Si zes range from one-third cubic yard up to four cubic yard s. The gates of lightweight and standard buckets are usually of the self-closing, double clamshell type and are c o n t rolled manually by levers for hand- or rope-pull opening. Se l f - closing and nonjamming features in the gate are important to allow partial load discharge at the will of the o p e ra t o r. Heavy construction Heavy duty or low slump buckets handle one- to three-inch slump mixes with aggregate sizes up to six or eight inches. They are used mainly for dam construction and other mass concrete work as illustrated in Photo 2. Si zes range from one up to 12 cubic yard s. These buckets have steep side slopes which feed to extra l a rge gate openings. Gate contro l can be either manual or powe re d but the large size buckets are usually air actuated. Gate closure should be automatic when the air supply is cut off. Special buckets and attachments A number of special types of buckets are made. The layd ow n bucket shown in Photo 3 perm i t s l ow height loading from the tru c k mixers or other sourc e s. The bucket is loaded in the hori zontal or layd own position and shifts to the ve r- tical position on lifting. Layd ow n buckets are available in all we i g h t s to handle normal slump as well as l ow slump concre t e. Si zes ra n g e f rom one-half cubic yard up to eight cubic yard s. W h e re payload/weight ratio is a c ritical factor in concrete bucket selection for lifting by helicopter, light cra n e, or some tower cra n e s f e a t h e rweight buckets are ava i l a b l e in sizes up to two cubic yard s. These buckets are constructed from part s

2 made of aluminum alloy, magnesium and steel and weigh only about one-third to one-half of what their all-steel counterparts we i g h. Special buckets also are made for use on fork lift tru c k s, for lowe ri n g c o n c rete into deep caissons, and for placing concrete under water. Und e rwater buckets must be cove re d to pre vent loss of concrete by washing while the bucket is being lowe red through the water. Di s c h a rge of c o n c rete can take place automatically upon contact with the bottom or manually with a gate line fro m 2. Heavy duty low-slump concrete usually have air-actuated gates. This four-cubic-yard bucket is handled with a power operated crane hook that is completely controlled from the crane cab. a b ove. Most standard or heavy duty buckets can be modified for placing c o n c rete under water. A va riety of bucket attachments can be used for better control of disc h a rge into wall forms or other narrow form s. There is an accord i o n type of rubber collection hopper that can be fixed directly to the gate of a bucket. When the bucket is set d own for reloading, this accessory folds under the bucket. A suspended steel subhopper is available with sections of elephant trunk or dro p chute that permit controlled placing into deep form s. A pedestal supp o rt allows the subhopper to remain at the f o rm w o rk while the bucket is being re l o a d e d. These attachments pre ve n t waste from the l a rge bucket gate o p e n i n g s, assist in avoiding segregation, and p e rmit more u n i versal use of c o n c rete buckets on all phases of a job. Handling buckets by crane 3. Laydown buckets are especially useful when they must be filled from truck mixers and low filling height is required. Buckets may be handled and t ra n s p o rted in many ways. A c rane swinging a bucket can be an efficient means of handling conc rete when job conditions allow f ree access and m ove m e n t, when crane capacities are not exceeded and when the placing area is open for ve rt i c a l handling of concre t e. Small and l a rge quantities of concrete can be handled with equal efficiency if t h e re is adequate lifting capacity, reach, and mobility of the cra n e. To a void loading delays when charg i n g buckets from truck mixers, it is a common practice to use two or t h ree buckets. CRANES Of the many types of cranes of widely va rying chara c t e ristics and capacities used for handling bucke t s, the most common are the tre a d - mounted crawlers that must be t ra n s p o rted over the road by tra i l e r and the truck-mounted units that a re self propelled. These cranes util i ze a boom that may be anywhere up to 450 feet in length. Ca p a c i t i e s range from two and one-half to 175 tons and more. Howe ve r, a cra n e s capacity may be misleading since this is normally the lifting capability with the boom positioned nearly ve rtical. For example, a 110-ton c rane with a 250-foot boom has a capacity of only one ton at a ra d i u s of 220 feet. Ou t riggers are re q u i re d to re a l i ze the rated capacities of many cra n e s. Tower cranes Co n ventional cranes opera t i n g on the ground must keep a distance away from the face of a stru c t u re in o rder to accommodate the boom

3 a n g l e. Thus they have difficulty reaching across a stru c t u re to interior locations. This difficulty is especially serious on congested and narrow sites. The problem can be s o l ved by using another type of c ra n e, the tower cra n e. T h e re are seve ral basic designs of t ower cra n e s, the more popular being the counter balanced hori zo n t a l T-boom type illustrated in Photo 4. The load is hoisted by cable supp o rted from a trolley that tra vels radially on the boom which in turn rotates 360 degre e s. As long as the boom clears the working area and s u r rounding obstru c t i o n s, loads can be picked up and deposited with pinpoint accuracy anywhere within the radius and capacity of the particular crane; this may range up to 30 tons at short radii and three tons at a 200-foot ra d i u s. T-boom tower cranes may operate from either a static foundation or a track mounted carri a g e. They can stand free as high as 200 feet without bracing and may be extended seve ral hundred additional feet when braced against a stru c t u re or when guyed. In smaller capacities these cranes are available as truck or c rawler mounted units. Self-climbing T-boom towe r c ranes have re vo l u t i o n i zed the placing of concrete in high stru c t u re s. They take a minimum of space and can climb up with the building. When the building provides supp o rt, the capacity of the crane may be limited by the stru c t u ral stre n g t h of the building fra m e. The cra n e climbs with the aid of built-in hyd raulic jacks. In addition to lifting concrete in b u c k e t s, tower cranes can efficiently handle most other materials on a j o b. The electric power by which they work ensures silent opera t i o n s, which can be an important enviro n- mental factor in populated are a s. The higher a stru c t u re the more e x p e n s i ve the handling of concre t e b e c o m e s. For this reason extra attention must be paid to achieving maximum efficiency when there is a need to move concrete above the first story level. Co r rect choice of 4. Tower cranes are versatile and efficient for handling concrete buckets, especially in situations where loads must be lifted up and over the construction to interior locations. 5. A materials hoist tower should always be considered when studying methods of handling concrete for medium and high-rise structures. equipment becomes especially imp o rt a n t. Lift trucks Fo rk-lift or high-lift loaders can be used to elevate concrete two or t h ree stori e s. The maximum lift with c u r rent models is about 40 feet. With lifts of this height a rigid and stable frame and chassis are essential. Models with tilting upper decks a re pre f e rable since they permit safe and level concrete handling on une ven ground. These loaders have 6. Costly overhead cableway systems are frequently used to transport buckets where vast quantities of concrete are required for constructing large mass concrete projects such as dams. 7. On some mass concrete projects, buckets are filled at a central batch plant and transported a short distance by trucks or rail transfer cars before lifting to the forms by crane or cableway. the advantage of being quick and m a n e u ve rable so that concrete can be discharged at a particular leve l close to where it is needed and thus m i n i m i ze rehandling. The ve r s a t i l i- ty of the fork lift has been incre a s e d g reatly by a wide range of accessory attachments for handling concre t e. Both storage hoppers and buckets a re available that have capacities of up to 20 cubic feet. They can be manually operated at dumping level or controlled from the lift operat o r s seat.

4 Tower hoists Tower hoists (Photo 5) are used e x t e n s i vely in building concre t e s t ru c t u res because of their good c o n c rete delive ry ra t e, ease of ere c- tion and minimum space re q u i rem e n t s. When considering concre t e handling methods for medium- and h i g h - rise work, the tower hoist should always be considered. A tower system for the ave rage job consists of an elevator bucket, a movable receiving hopper set at the leve l w h e re concrete is to be placed, and, of course, the tower itself and the hoisting equipment. Some systems may also include a personnel elevat o r, one or more jib booms, and a receiving hopper at ground leve l. From the top hopper concrete is t ra n s f e r red by hand or power cart s, belt conve yo r s, or other methods for d i s t ribution into the form s. Sl u m p s of three to six inches can be handled most effectively with this method. Tower hoists are of two basic t y p e s. With one the concrete bucket is hoisted within the well of the tower fra m e. With the other the bucket is external to the fra m e w o rk. Double- and tri p l e - well towers are ava i l- able for large jobs. All types must be rigidly founded and braced, part i c u- larly on sites exposed to high winds. Ex t ra tower height above the placing l e vel must be allowed to accommodate the hoisting pulleys and to all ow headroom for bucket and hopper discharg e. Hopper and bucket capacities range up to about two cubic yard s. When a job is to be supplied by a t ower hoist a properly engineere d 8. Large flying crane helicopter delivers four-cubic-yard bucket to bridge pier from shore several miles distant. A barge-mounted hopper received the load and feeds a belt conveyor that moves the concrete into the pier form. system should be used. Simple pulley arrangements with makeshift wood scaffolding built on the site cannot be justified except for handling small quantities of concre t e. En g i n e e red towers are best made of tubular steel elements or lightweight stru c t u ral scaffolding that is quickly assembled. Cableways, helicopters, boats and barges O ve rhead cableways (Photo 6) and trestle cranes are used almost e xc l u s i vely to handle mass concre t e on large dam pro j e c t s. Eq u i p m e n t i n vestment is substantial and can be justified only for jobs invo l v i n g vast quantities of concre t e. Wi t h such equipment heavy duty low - slump concrete buckets with larg e capacities can be tra n s p o rted at high speeds to the exact point of placement without further handling. Buckets are commonly hauled a short distance from a cent ral mixing plant to lifting position on trucks or ra i l road (transfer) cars, with two or more buckets on each hauling unit (Photo 7). On occasion helicopters may be used to tra n s p o rt concrete in buckets to inaccessible areas such as b ridge piers and remote mountain or water-bound sites. Fe a t h e rwe i g h t buckets are best for use with small h e l i c o p t e r s, but copters have tra n s- p o rted four- c u b i c - y a rd steel buckets of a special design that minim i zes weight. The bucket usually is slung below the helicopter on a single cable as shown in Photo 8. Se ve r- al buckets are normally used so that one is always loaded and ready for p i c k u p. One or more lines should be fastened to the bucket so that workmen may steady the bucket while the helicopter hovers over the form. An additional line attached to the bucket gate-release handle may be used to discharge the concre t e. Boats and barges are used sometimes to tra n s p o rt concrete in buckets (or truck mixers) when the cons t ruction site is water-bound. This method is useful for small quantities of concre t e, when setting up a floating concrete plant is undesira b l e, or where there is no room for s t o ra g e, handling and mixing of c o n c rete materials adjacent to a wat e r-bound site. The method is slow and re l a t i vely expensive compare d to standard ones. CARTS AND WHEELBARROWS Hand pushed carts (also called buggies) va ry in size from about six to eight cubic feet capacity. A man can move an ave rage of about thre e to five cubic yards per hour with one of these units, a much larger output than with the wheelbarrow. Ha n d c a rts are most efficient when handling moderate quantities of conc rete over small hori zontal dist a n c e s. Suggested maximum distance is the same as for a wheelb a r row, about 200 feet. Ru n w a y s should be level, smooth, and ri g i d, and cart wheels should have pneumatic tires as shown in Photo 9. Na r- row cart s, about 30 inches wide, are easy to maneuver through narrow d o o rways and on narrow ru n w a y s. The narrow carts also make it easier to discharge concrete into collection hoppers. Because of their size, weight and good maneuve ra b i l i t y, hand carts are most effective in handling concrete in difficult situations. The positive tipping discharge of the hand cart enables it to be used for c o n c rete of any slump. Howe ve r, c a rts with rocker attachments are p re f e r red when handling low - s l u m p mixes as the rockers permit steeper d u m p i n g.

5 11. Hand carts (buggies) transport concrete short distances for a highrise building project. 9. Wheelbarrows, preferably with pneumatic tires, are useful on small jobs where wheeling distance is short and quantity of concrete is small. 10. Power driven carts (power buggies) are made in two basic types: walking and riding. The walking type is illustrated here. Power carts The power dri ven cart (powe r buggy) has many of the va l u a b l e c h a ra c t e ristics of the hand cart. In addition, it is capable of handling l a rger quantities of concrete than the hand cart and can operate ove r longer running distances at good speeds even on uphill gra d e s, re s u l t- ing in an excellent rate of delive ry of c o n c rete to the form s. The initial investment for a power cart is conside rably higher than that for a hand c a rt, and to be effective a power cart re q u i res a skilled opera t o r. In some cases the weight of a power cart may be a disadvantage compared to the hand cart. The ave rage power cart ranges in capacity from nine to 12 cubic feet but units as large as one cubic yard are ava i l a b l e. Power cart s a re made in two basic types. In one the operator walks behind the unit ( Photo 10) and the hourly placing capacity is limited to walking s p e e d s. In the other more efficient type the operator ri d e s, either standing or seated; this perm i t s g reater speeds and greater per hour c a p a c i t y. The longer the haul, the g reater the economy of riding type c a rts over wheelbarrow s, hand c a rt s, and walking type power cart s. A suggested maximum distance is 1,000 feet. The ave rage power cart can move 15 to 20 cubic yards of concrete per hour over a 600-foot hauling dist a n c e. Howe ve r, with the larg e s t u n i t s, placing rates much higher than this are not uncommon. Gradients present no serious pro b l e m s to properly designed power cart s. Some units can handle a 35 perc e n t g ra d e. Runways should be smooth and firm and at least five feet wide to allow turn a round area and sideways dumping. They should also be arranged to allow free uninterrupted flow of tra f f i c. Loads should be cove red if weather conditions a re bad. Wheelbarrows W h e e l b a r row s, used mainly to m ove small quantities of concre t e s h o rt distances, are especially useful in areas inaccessible to other equipment. Labor costs per unit vo l u m e of concrete for this method of handling are high, although the method remains popular for small ow n e r- placed jobs. The capacity of a w h e e l b a r row ranges from about two and one-half to three and one-half cubic feet and one man with a w h e e l b a r row can place up to one or one and one-half cubic yards per h o u r. Suggested maximum ru n n i n g distance is 200 feet. Wheelbarrow s with pneumatic tire s, square noses, and ro c k e r-type knees as illustra t e d in Photo 11 are pre f e rable for handling concre t e. They minimize compaction of concrete by jolting, help in discharging heavy loads, and permit easier dumping with less s p i l l a g e. Runways should be as smooth as possible and the ends of planks forming runways should butt rather than ove r l a p. P a rt III of this series will appear in the October 1972 issue of CONCRETE CONSTRUCTION. It will present information on conveyors. P U B L I C AT I O N# C Copyright 1972, The Aberdeen Gro u p All rights re s e r v e d