Thursday, December 16, 2010

DEMOLITION OF CONCRETE STRUCTURES

This seminar paper is dealing with the demolition technology that is practiced to demolish the concrete structures in a controlled way especially in Indian conditions. The important methods practiced in this field are mentioned. The need, requirements and investigation required for demolition process are discussed.

Keywords: explosives, blasting, SCDA, CFI, water jet, hammers

1 INTRODUCTION

When the phrase "building demolition" is spoken in everyday conversation, most people visualize a giant structure exploding and crashing to the ground in a fury of dust and debris. This despite the fact that only a tiny fraction of all structural demolition projects involves explosives, or for that matter, buildings don't actually “explode” or “implode” in the process.

Demolition is the deliberate destruction of structures and materials by means of explosives, mechanical devices, fire, chemical agent etc. the demolition can be broadly divided into two by considering the nature of demolition, which are commercial demolition and military demolition.

Commercial demolition is the controlled demolition of the structure employing both mechanical means and explosives. Here the wrecker will give priority to the safety of surrounding buildings and the inhabitants. Accordingly the method of demolition is chosen. There are many methods such as cutting torch, wrecking ball or by using high power water jet, hydraulic hammers etc. When explosives are used linear shaped cyclonite or RDX are used to demolish steel framed buildings and dynamite is used to demolish reinforced concrete structures. Shaping and placing the charges requires the service of a demolition expert.

Military demolition is that kind of demolition where the main aim is to destruct more area. In military demolition, explosives are frequently used because they weigh relatively low


Under Graduate Student, Department of Civil Engineering, N.S.S College of Engineering, Palakkad-08, Kerala.

and have great destructive power. Some of the examples of such explosives are ammonium nitrate, trinitrotoluene etc.

Only commercial demolition of concrete structures is discussed here.

With the increase in demolition on congested urban sites, the construction industry has been obliged to develop more efficient demolition techniques which are quieter and less intrusive. This work deals with both the theoretical and practical aspects of the demolition of concrete and masonry buildings and the recycling of the demolished materials.

2 NEED TO DEVELOP DEMOLITION TECHNOLOGY IN INDIA

Demolition is the one area India has never bothered to concentrate upon. Developing country’s emphasis has been on construction rather than on proper demolition of unsafe and outdated structures. They develop cold feet when it comes the demolition technology.

There exist numerable old and unsafe building to be demolished to give way to new multistoried building, half lane bridges that need to be demolished to give to wide and safe bridge, traffic is increasing day by day and business on the rise thus demanding more and more buildings, offices, commercial buildings, dwelling units and a well connected and capable road transport network. All this is possible only if the old and outdated structures are removed and freed for new over to come up. The government needs to demolish the buildings which violate the building rules and regulation which ensure structural safety, health safety, fire safety and construction safety of the building. In many factories chemical compound plays an important role. Such factory buildings will become deteriorated in the long run by the action of chemicals. So the only choice is to demolish and reconstruct those structures. Because of all, the need to develop demolition technology is increasing nowadays.

In India, demolition is avoided as if it is something auspicious, involving wasteful expenditure. Another factor is the crude method of demolition. Even nowadays, chisels and hammers are using for the major part of demolition work. Running a bull-dozer into the building, or using pneumatic jack hammers are not the advanced teqnique in the demolition technology. The major problem is that, there exist no planned works to execute demolition.

India must develop demolition technology suitable to Indian condition. For this the structures are to be categorized and requirements are to be defined. The blind adoption of the technique being practiced in the foreign countries may not be constructive to India layout on a broader scale and from demolition point of view the Indian structures are divided into

(i) Tall chimneys of power plant and large sized natural draught cooling towers.

(ii) Multistoried buildings.

(iii) Heavily reinforced bridges and buildings.

(iv) Ordinary buildings, houses, bridge structures etc.

3 REQUIREMENTS OF AN IDEAL DEMOLITION PROCESS

Thinking of the requirements that the demolition technology should fulfill, the following shall form an ideal list.

(i) Quicker demolition.

(ii) Quitter demolition.

(iii) Cheaper demolition.

(iv) Environment friendly demolition.

(v) Demolition causing least disturbance in the neighborhood.

Therefore whenever a structure is to be demolished it must be seen that the method adopt in such a way that take minimum time, produce least noise, causes no pollution beyond permissible limit, does not the activity or living in the neighborhood and proves cheaper also.

4 INSPECTIONS BEFORE DEMOLITION

Physical surveys: - It essential to survey the surroundings of the structure to be demolished. The surrounding will decide whether the structure is to be toppled or made to collapse. The position of water supply lines, telephone lines and electric wires are also to be considered. It has to be ensured that the supplies to the adjacent buildings are not disturbed. Wherever explosion demolition technology are not disturbed is to be used, it is to be seen that the equipment installed in the near by buildings are not affected by the vibration produced by the explosion. If a demolition involves rise of clouds of dust, the ducts of AC System in the adjacent buildings are to be sealed against entry of dust in them. Overall approach has to be taken to cause least disturbance to the surroundings.

We have to give special importance to the reusable materials before demolition. The item removed can be re used during the construction of new development or sold to a salvaging company. This is known as deconstruction. These are done considering the economic aspects.

The construction joints of the structure to be dismantled will not be available as it may be over 40 to 50 years as since the time structure was built. Therefore core test may be done to have an idea of the concrete strength or density of reinforcement the position of the expansion joints in the building should also be assessed as it will help in planning the demolition of the building.

5 METHODS OF DEMOLITION

The following information is a summary of techniques that have been used, in one form or another, to remove concrete structures. There are many methods which are practiced, to demolish the structures. The method demolition for a particular project will depends on the economic considerations, nuisance to the public, near by structures, duration of the operation etc. By considering the methods followed in demolition technology it can be divided into 4

  1. By using chemicals.
  2. By using water-jet.
  3. By using thermal energy.
  4. By using machine.

5.1 BY USING CHEMICALS

There are two types of chemicals, one type will cause explosive demolition of the structure, where the other cause non explosive demolition. The method is commonly known as blasting where the second is commonly known as soundless chemical demolition.

5.1.1 BLASTING

FIG.1

Blasting seems to be more cost effective and is perhaps the largest used method. Explosive demolition technology has become most popular in foreign countries. High-rise chimneys and cooling towers are made to collapse through telescopic technique by placing explosives in such a manner that almost vertical collapse occurs. Under this technique large compressive stresses are produced at the base of the structure and it descends in a continuous manner like a house of cards. While the explosives cause the initial collapse further fragmentation is caused by the gravity. The explosives are used to weaken and cue the supporting members of the structure to fail, thus allowing gravity to pull the structure down over. Use of all types of explosives requires heavy security during operation time. With controlled blasting localized cutting in concrete to create small opening of about a square meter area or so is also possible. Normally the explosives are inserted in a series of bore holes, or in v shaped metal enclosures, which are easy to attach to the surface of concrete.

Blasters know how to calculate the efficiency of energy they use. By using only the amount of energy it takes to perform the job at pond, one can be assured that there isn’t enough left over to cause any ground vibration damage to properties outside the blast site. Mini blasting is a new technique, where controlled blasting is done with mild explosives. Incase of mild explosives the blasting speed is 30 m/s while that of ordinary explosives is 4000-7000 m/sate characteristics of blasting can be summarized as

· Versatile and flexible in terms of work output

· Vibration and air blast may damage surrounding structures

· Heightened safety considerations involved when compared to other demolition methods

5.1.2 NON EXPLOSIVES

5.1.2.1. SOUNDLESS CHEMICAL DEMOLITION AGENTS

FIG 2

Soundless chemical demolition agents (SCDAs) have proven to be viable substitutes for the use of explosives. SCDAs are powdery materials that will expand considerably when mixed with water. This expansion, when occurring under confinement, generates significant expansive pressures. These pressures are sufficient to break up rock and concrete when the SCDA is confined in a borehole or a series of boreholes. Experiments have been conducted with SCDAs to learn more about those variables that tend to hamper or change SCDA performance. Results show that the amount of mixing water and the ambient temperature are the most important variables in influencing the generation of SCDA expansive pressures.

The preparatory procedures involved in using SCDAs are similar to those followed in traditional blasting techniques. As with explosives, boreholes must be drilled to contain the SCDA. Beyond this, however, the similarities diminish. The SCDA must be mixed with a measured quantity of water and poured into the boreholes. It will then begin to hydrate, generating heat and crystallizing while hardening and expanding. If hydration takes place under confinement, significant expansive pressure will result. The pressures can be of sufficient magnitude that, after a period of time, they will fracture the confining material. Depending on the type of SCDA, significant expansive pressure may be generated as quickly as within 15 min., or as long as within 24 hr. The main component of SCDAs is calcium oxide, which forms calcium hydroxide when water is added; it causes a threefold increase in volume.

FIG 3

BENIFITS

They do not make noise, explode, or generate fly rock, vibration or toxic fumes. SCDAs are also safer than traditional explosives, which pose the threat of premature explosion and which may misfire, posing a significant threat after the planned explosion. Contrary to explosives, SCDAs produce their destructive forces in rock and concrete by generating significant expansive forces without generating shock waves.

While there are several advantages to using SCDAs, their relatively high cost makes explosives more cost-effective in many applications. Nonetheless, SCDAs have become a common means of breaking up boulders that have rolled onto remote mountain highways. SCDAs have also been used when excavating rock, or demolishing concrete structures or components of concrete structures near inhabited areas, natural gas lines, roadways or other areas where the use of explosives would pose a significant safety risk. SCDAs have generally been used only where blasting is prohibited, and will eliminate fly rock, noise, fumes, and vibrations associated with blasting. SCDAs have been used underwater in Seattle to remove bridge piers from docks and to avoid blasting close to shore.

STATUS

SCDAs are manufactured primarily in Japan, China, Russia, and some countries in Europe. In US, Demolition Technologies Incorporated is distributor of BRISTAR from Onoda Corp., Japan, and Daigh Company is distributor of Fract.AG from Chimica Adile, Italia.

Although the use of explosives remains the principal means by which rock and concrete structures are demolished, there has been and increased use of SCDAs over the past two decades. There is still little standardization surrounding the manufacture and use of SCDAs. In fact, there is not even a consensus as to the proper terminology by which reference should be made to SCDAs. Other terms by which SCDAs are known include soundless cracking agents, expansive agents, expansive concrete, non-explosive demolition agents, and other related variations of these terms.

BARRIERS

The high cost of SCDAs and some bad experiences in controlling the pre-splitting have caused the Corps of Engineers to limit its use of the expansive agents. Although inhibitors can be added to retard the chemical reaction and increase control over the process, the technique is still primitive, and its actual field use has been quite limited.

With expansive demolition agents involves a phenomenon known as "blow- out." This happens if the powder mix gets too hot and reacts with the water too quickly for the material to expand laterally. The result can range from a puff of smoke to a loud gunshot-like sound that can send hardened mix 30 feet into the air. Since blow-outs are unpredictable, safety procedures require workers to stay well away from the drilled holes once the mix has been poured into them. If a blow-out does occur, however, it usually has little effect on the project, since the remaining mix in the hole is usually still effective enough to crack the concrete

5.1.2.2 CONTROLLED FOAM INJECTION

FIG 4

The method utilizes the proprietary and patent protected Controlled Foam Injection (CFI) technique to fracture rock and concrete. The use of high-pressure foam as the fracturing medium completely eliminates the air blast, fly rock and toxic fume problems associated with explosive based techniques. The CFI method may be used in very close proximity to personnel, sensitive structures or equipment. The controlled breakage characteristic of the CFI method allows for rock removal to very precise dimensions with minimal damage to the remaining rock. All the process byproducts are environmentally benign and completely biodegradable.

PRINCIPLES OF THE CFI METHOD

The CFI method is based upon the use of high-pressure foam to initiate, pressurize and propagate controlled fracturing in rock and concrete. An injection barrel, incorporating a proprietary hole-bottom seal, is used to inject high-pressure foam into the bottom of a pre-drilled hole in the rock or concrete to be broken as indicated in the animation at right. The high viscosity of the foam (as compared to a gas) combined with its stored energy characteristics (as compared to a liquid) result in very controlled and efficient breakage. The foam pressures required to break rock or concrete are significantly less than required in explosive or propellant based methods. Consequently, air blast and fly rock are reduced to very benign levels, allowing the method to be applied in a continuous manner and to be used in urban and other sensitive environments.

The hardware for the CFI fracture of rock or concrete may be readily mounted on a conventional articulated boom for application to excavation and/or demolition operations. A percussive drill may be incorporated on the same boom carrying the CFI hardware so that hole drilling, indexing for injection barrel placement and breakage can be carried out in a systematic and automatic manner.

With excellent technical development progress on SBIR (Small-Business Innovation Research) programs funded be the National Science Foundation, the CFI technology is now being integrated with both Technicore Tunnel Digging Machines and conventional hydraulic excavators. These machines have been tested on small projects and are now available for dedicated commercial projects.

The CFI method is well suited for automation. The flexibility of the method (in terms of hole depth and foam pressure, quality and viscosity) allows it to be tailored to rapidly changing ground conditions or a variety of breakage conditions. The benign nature of the and fly rock of the CFI fracturing method allows drilling, breakage, mucking, ground support and haulage equipment to remain at the working point or face during rock excavation operations.

The CFI method is ideally suited for any excavation or breakage situation where conventional explosive methods are precluded, whether for environmental, legal or public concern reasons.

APPLICATIONS OF CFI BREAKAGE

The proprietary foam-based CFI method can be used in a broad variety of specialty drilling and excavation situations. The method is ideally suited for the demolition of oversized boulders encountered in both construction and mining operations.

The CFI method can be used to develop trenches for utility or pipe lines or caisson shafts for bridges and power line towers. The method can be used to excavate utility tunnels under existing streets and buildings, to provide access tunnels or shafts to existing subway systems, or to create or enlarge underground space for buildings.

In underground mining operations, the CFI method can be used for selective mining or for mine development in unstable ground. As the method imparts negligible damage to remaining rock, the need for ground support can be significantly reduced.

The CFI method is useful for demolishing or stripping concrete structures when explosive methods are precluded or where salvage and recycling of building or structure components are desirable. As the method causes no damage to remaining concrete, repairs to concrete structures may be made efficiently and economically

5.2 BY USING WATER JET

FIG 5

Hydro-demolition is characterized by water jet pressures between 12,000 and 18,000 lbs/in2, water flow rates of 25-40 gallons per minute, and a manipulator with an automatic repetitive function which controls movement of the jet. A fine jet of water under high pressure is directed at the concrete surface. The jet penetrates the surface and water is forced into cracks in the concrete, opening them up and breaking out loose material, which is flushed away. Hydro-demolition is generally used to remove poor quality concrete from finished surfaces, leaving good concrete untouched.

The combination of water jet cutting with an abrasive material makes the system more effective. While the water jet cut the concrete by itself, abrasive materials enable the jet to cut reinforcing bars, this method was developed for use in the demolition of biological concrete shield in nuclear reactors. For abrasive cutting applications, abrasive garnet is fed into the abrasive mixing chamber, which is part of the cutting head body, to produce a coherent and an extremely energetic abrasive jet stream Various abrasive materials which can be used include olivine, garnet, and corundum with a particle size of between 50 to 120 mesh (0.2 to 0.5 mm). When abrasive is required, Ingersoll-Rand provides an abrasive unit consisting primarily of an abrasive hopper, an abrasive feeder system, a pneumatically controlled on/off valve, and the abrasive cutting nozzle which contains the specialized mixing chamber.

The University of Missouri-Rolla used high-pressure water jets to cut granite into blocks, to produce a replica of Stonehenge. Twenty-foot-deep cuts have been reported in Canada. The characteristics of water jet can be briefly listed as

· Minimizes dust and eliminates dust and fire hazards.

· Can be used to cut both straight lines and contours.

· Requires the use of an abrasive and water-catching system during the cutting process.

5.3 BY USING THERMAL ENERGY

Many thermal process of concrete demolition are available, but except for melting flame they are seldom used for ordinary concrete structures.

5.3.1. OXYGEN THERMAL LANCE (FLAME-CUTTING)

Cutting concrete in tight or inaccessible locations, or when the concrete is very hard or heavily reinforced, can be accomplished with the thermal lance. The Thermit process employ burning oxygen and a 13mm to 17mm diameter pipe packed with steel wires to perforate even strongest concrete. The pipe end is heated to red-hot and pressed into the concrete surface, where oxygen is supplied at full pressure (115-200 lbs/in2). The pipe and wire core burn at a temperature of 5,400 °F or more, hot enough to melt cement sand, aggregate, and reinforcing steel. Between 2 and 6 holes are generally required per foot of cut to produce perforations up to 10 feet deep for breaking the concrete into removable slabs. The thermal lance has been used successfully in Poland for many years, although the cost is high compared to mechanical methods under normal conditions. This process has several advantages like the absence of vibration and low noise level. It is not hampered by the presence of steel plate or bars and can be used for building in narrow places. The process reportedly enables a flame to burn and cut concrete in water as well.

Concrete Coring Company performs flame-cutting services, often using a fixed torch operating at 12,000 °F, mounted on a frame and driven by a remotely-controlled motor. The major characteristics can be briefly listed as

· Excessive heat causes some deterioration of the concrete adjacent to the cutting

· Works particularly well in the presence of reinforcing steel

· Eliminates vibration and dust problems

· May create smoke and fire hazards

5.3.2 ELECTRICAL HEATING

The direct electrical heating of the concrete enables removal of concrete cover by generating a continuous crack over several reinforcing bars are electrically heated. The heating can be direct or induced .Both ends of the bars are exposed and they are equipped with electrodes. Direct current at low voltage or large current cause thermal expansion of the bars and the surrounding concrete, thus creating tensile stresses in cover concrete. Concrete cover can easily be removed by lightly hitting the portion around the crack by a hammer or chisel. The main advantage of this process is that it can easily be controlled, as electrical energy is utilized.

5.3.3 MICROWAVE

Microwave method of demolition has been developed in Japan. When concrete is irradiated by microwaves its temperature raises and the temperature gradient develops over a localized region, resulting in breaking the concrete and the stress generation due to super heated steam vapor. The heating of concrete itself occur as a result of absorption of microwave into the cement paste, aggregate and free water ,the last being the most important as its dielectric constant is 10 to 20 times larger .the harmful effects of microwaves on human beings and jamming of communication are the chief disadvantages of this methods.

5.4 BY USING MACHINES

The machines can be classified into two which are

  1. Hydraulically operated machines
  2. Mechanically operated machines

5.4.1 HYDRAULICALLY OPERATED MACHINES

5.4.1.1 BOOM-MOUNTED HYDRAULIC IMPACT HAMMERS

FIG 6

Hydraulic demolition hammers come in many sizes, and are pinned to a backhoe or excavator boom in place of the bucket. The strength of the material determines the hammer needed for the job; hard rock (granite) requires a more powerful hammer (5,000 foot-pounds [foot-lbs] or more), while softer rock (sandstone) can take a smaller hammer (500-1,500 foot-lbs). Blow-energy limitations are sometimes required to control micro cracking of remaining surfaces and to limit vibrations. Operator skill plays a large role in demolition rates.

Also, remember that while most hydraulic systems run 2000-psi pressure, the flow rate varies. For the light hammers, as little as 5 gallons per minute is required. For the heavy hammers, more than 100 gallons per minute must be supplied. Again, check the hammer and carrier manufacturers' literature to match the hammer to the vehicle. If necessary, add-on hydraulic power units are available to increase the flow rate. . Keep in mind that for a given reach, the heavier the hammer, the heavier the carrier vehicle must be. The weight of the carrier vehicle prevents overturning when the hammer is at the boom's maximum reach. Selecting a lightweight carrier decreases the boom's reach and could cause an overturning accident.

A rock cliff was excavated away from a house in Boston at a rate of 50 cubic yards per day.

5.4.1.2 HYDRAULIC SPLITTER

Hydraulic splitters apply lateral forces against the inside of holes drilled into concrete in order to break up concrete with a minimum of noise and flying debris.

About the size of a jackhammer, the tool utilizes a shaft known as a plug-and-feather assembly which, when inserted into a drilled hole and forced downward by the tool’s piston, creates the lateral forces that break the concrete. No heavy impact is utilized to form the crack, which spreads quickly and without any noise between the pre-drilled holes until the concrete is split into manageable pieces.

The typical hydraulic splitter exerts a force between about 150 and 400 tons, depending on make and model. As with most pieces of equipment of this type, the smaller models with a force capacity on the lower end of the scale are used for more lightweight work, or where the equipment must be able to be handled more easily, such as in horizontal or overhead work. Larger capacity models are capable of splitting mass concrete and hard rock.

Hydraulic splitters may be available in gas- and electrically-powered models, but most contractors prefer air-powered hydraulic pumps, as the tool’s air-compressor can also be used to power the drills used to form the holes in the concrete. Multiple hydraulic splitters can be set up to run side-by-side off the same hydraulic pump, allowing for a greater combined splitting force, helpful when breaking up particularly thick or dense concrete.

To correctly operate a splitter, the holes must be drilled straight into the concrete at the exact diameter specified for the splitter, and be deep enough to accommodate the plug when fully extended. The plug and feathers have to be kept well lubricated in order to remain in working condition.

The splitter system consists of a hydraulic pump, hoses, and one or more splitters. A hydraulically-driven wedge is inserted into predrilled boreholes to create crack planes through the full depth of a concrete wall or slab. The key to its success is drilling boreholes that are accurate in diameter, length, and straightness. Drilling 3½-inch-diameter boreholes can be expensive, and the number of splitters needed to sustain a high removal rate may not be cost effective.

DARDA rock and concrete splitters use a wedge-plug and feathers mechanisms powered by a 10,000 lbs/in2 power pack and have been used on Reclamation projects.

5.4.1.3 HYDRAULIC CONCRETE CRUSHER

Hydraulic concrete crushers are used to demolish concrete methodically and efficiently. Also called smashers, densifiers, processors, secondary crushers, and pulverizers, concrete crushers are used to reduce concrete into smaller easily manageable or recyclable pieces, as well as to separate steel reinforcement from concrete.

Interchangeable jaws in some crushers, including cracking jaws, shear jaws and pulverizing jaws are often used to work along with various types and configurations of jaw teeth in order to better fit the crusher to a particular job.

Secondary concrete crushers usually have some type of pulverizing jaws and are used on jobs where primary demolition is accomplished by hammers, crushers, blasting, ball and crane, or sawing. In this instance, the primary demolition work creates large quantities of concrete rubble which the secondary crusher further reduces, separating concrete from reinforcement.

5.4.2 MECHANICALLY OPERATED MACHINES

5.4.2.1 DIAMOND-WIRE SAW

FIG 7

Build aids offers sound solution to any technically demanding project through diamond cutting technology. In numerous construction and renovation projects, diamond cutting technology offers the comparative edge in concrete demolition and removal. Architects, engineers and general contractors agree that diamond cutting technology is cost effective and time efficient.

Rotary diamond saws have been used to make cuts in concrete up to 27 inches deep. Diamond-wire saws produce a smooth vibration-free cut through heavy reinforcement and hard aggregates, with virtually no limitation on the size or thickness of the cuts which can be made. The diamond-wire system consists of a diamond-impregnated wire made to length for each cut and a hydraulically-powered drive system. Diamond wire is routed to envelope the area to be cut (often requiring a drilled hole), then guided into a drive wheel on the power unit. The drive wheel rotates and pulls the wire through the concrete. This system allows cuts of virtually any size through stone, and through unreinforced and reinforced concrete at cutting rates which can vary from 10-50 square feet (ft2) per hour. Common wire lengths range from 50 to 70 feet, with wires longer than 400 feet unusual. A project in 1990 removed a bridge pier in 30-ton segments at a cost of about $30 per ft2. A dam rehabilitation project in New York removed over 16,000 tons of concrete in blocks weighing 60-110 tons each.

There are three basic wire types:

  • Electroplated beads with compressed steel spring spacers.
  • Impregnated beads with compressed steel spring spacers
  • Impregnated beads with injection-molded plastic spacing

There are also two main bonding systems for the diamond beads: electroplated and impregnated. Electroplating the wire involves attaching a single layer of diamond to the steel bead. The impregnated bonding system is more similar to the impregnated systems on a circular saw in which a powder metal alloy is blended with diamond, then pressed and sintered to the steel band, providing multiple layers of diamond for cutting.

STITCH-DRILLING

FIG 8

Diamond drills have been used to core holes in concrete up to 54 inches in diameter. Larger holes can be made by drilling a series of overlapping small holes in a circle or straight line, called stitch-drilling.

5.4.2.2 BALL AND CRANE METHOD

One of the oldest and most commonly used methods for building demolition, the ball and crane uses a wrecking ball weighing up to 13,500 pounds to demolish concrete and masonry structures. During the process, the ball is either dropped onto or swung into the structure that is to be demolished.

The ball and crane, however, is not suitable for all demolition applications. Some limitations:

While the concrete can be broken into rather small pieces, additional work in the form of cutting rebar may be necessary.

Only highly skilled and experienced crane operators should be used on ball and crane demolition projects -- smoothness in controlling the swing of the ball is important since missing the target may tip or overload the crane and a mild swing-back may cause the ball to hit the boom.

The size of the building that can be demolished with this method is limited by crane size and working room, including proximity to power lines. This form of demolition creates a great deal of dust, vibration and noise.

5.4.3 THE DIFFERENT TYPES OF HAMMERS

5.4.3.1 DEMOLITION HAMMERS

FIG 9

Demolition hammers are similar to rotary hammers and are used in the same way in terms of delivery hammer blows. However, demolition hammers deliver hammering action only, unlike rotary hammers, which can also be used to bore holes. What demolition hammers lack in this regard in terms of versatility, however, they make up in punch. The demolition hammer is able to deliver more powerful blows than rotary hammers, since they typically have about 35% more power. This is due to the fewer parts in a demolition hammer, and sometimes a longer piston stroke, as well. While the demolition hammer delivers fewer blows per minute than a rotary hammer, the increased strength of the tool actually makes it a quicker and more efficient means of demolishing concrete and masonry.

5.4.3.2. ROTARY HAMMERS

FIG 10

Big rotary hammers are known as either SDS-max or spline-drive hammers, depending on whether they accept SDS-max or spline-shank bits.

The versatility of the rotary hammer allows it to demolish concrete with a hammer only method, or to deliver rotary-hammer action for boring holes in concrete. This is done in the rotary hammer mode by driving twist drills and core bits, or in the hammer only mode to utilize everything from flat chisels to ground-rod drivers.

However, this versatility comes at a price, since rotary hammers have an extra drive train that rotates drill bits in the rotary-hammer mode. This siphons off energy and decreases efficiency in the hammer-only mode.

Rotary hammers use a battering ram that floats inside a cylinder and is launched and retrieved by a piston. A shock-absorbing airspace between the ram and the piston compresses and drives the ram forward as the piston advances, then sucks it back as the piston retracts.

5.4.3.3. CHIPPING HAMMER

Chipping Hammer

FIG 11

Chipping hammers are lightweight, hand-held concrete breakers that can be easily positioned to break vertical and overhead surfaces. By offering a controlled chipping action, these hammers allow operators to precisely chip away only specific areas.

The smallest chipping hammers, powered electrically, pneumatically, or hydraulically, usually weight between 5 and 30 pounds. As usual, a good indication of the power of the tools is their weight. The heavier the tool, the more powerful it is apt to be. The chipping action of this type of equipment is rapid, ranging from 900 to 3,000 blows per minute.

Because chipping hammers are most often used to break concrete on vertical and overhead surfaces, they must be light. They are maneuvered by holding a handle at the back of the tool and gripping the tool by its shaft with the other hand. Some tools have a second handle along the side. This gives operators control of the tool's weight and the ability to direct its chipping action at different angles.

By offering a controlled chipping action, these hammers allow operators to precisely chip away only specific areas.

Manufacturers are offering more options than ever on their chipping hammers, including rotating, shock-absorbing handles, and heat shields.

5.4.3.4. PAVEMENT BREAKERS

Hand-held pavement breaker

FIG 12

Hand-held pavement breakers may be thought of as a kind of heavy-duty version of the chipping hammer.

The difference is that pavement breakers are heavier, more powerful tools that are usually operated perpendicular to the ground. Because of this, a T-shaped handle is the most common design for pavement breakers. A few lightweight breakers are designed with a D-shaped handle that allows the operator to pick the breaker up and operate it horizontally.

Like chipping hammers, pavement breakers are powered hydraulically, electrically, or pneumatically. The pounding action of breakers is usually slightly slower than that of chipping hammers, ranging from 800 blows per minute to close to 2,000 blows per minute.

Pavement breakers weigh from 30 to 99 pounds. A good indication of a breaker's power is its weight.

Pavement breakers weighing between 40 and 50 pounds are particularly useful for removing concrete for partial-depth pavement repair. The weight of these tools gives them the power needed to break up a few inches of concrete without being so heavy that they demolish the entire pavement.

The pavement breakers weighing 60 pounds and more can handle medium-to-heavy demolition jobs with reinforced concrete, and are used to demolish pavements, roads, and thick concrete. One example of the type of work that the heaviest breakers do is demolition of high-strength concrete found on airport runways.

5.4.3.5 MOUNTED BREAKERS

Bobcat 1560 Breaker

FIG 13

The speed, versatility, and impact energy of mounted breakers are widely used by contractors to demolish heavily reinforced walls, slabs and decks.

Excavator-mounted breakers can have production rates of more than 1,100 cubic yards of unreinforced concrete per day. Productivity varies depending on a variety of factors including type of concrete, accessibility, and operator skill.

In addition to demolishing concrete, many hammers can be fitted with special tool shapes that allow them to drive piles and sheeting, compact soil, edge trenches, penetrate frozen ground, and cut asphalt.

Hammer impact energy is the most important selection criteria when choosing a mounted breaker for a particular job, since the hammer must be able to hit the concrete hard enough to fracture it. Only when impact energy is sufficient does impact rate (blows per minute) become a factor.

In factoring impact energy, hammer weight is important, but is sometimes confusing. Some manufacturers report the weight of the hammer with the boom bracket, surrounding housing (cradle), and working tool included. Others report just the hammer weight itself.

This is why comparing tool diameter may be the simplest and most accurate means of determining the degree of hammer impact energy. By measuring the tool diameter at the lowest point just inside the hammer and comparing it to the diameter of other tools, impact energy may be reliably measured. This is because the tool diameter is in direct proportion to the piston size.

5.4.3.6 PNEUMATIC HAMMERS

While pneumatic hammers were once considered state-of-the-art in the United States, they only account for a small portion of total hammers in use today. There are still a number of advantages in using pneumatic hammers, however. These include:

They can be mounted on lighter carriers, since their external air compressor negates the extra hydraulic demands of a hydraulic hammer

Mounting an air hammer requires only mechanical changes – no hydraulic connections, systems to service, or plumbing kits.

Pneumatic hammers work better in confined spaces than hydraulic hammers due to their high weight-to-power ratio. Pneumatic hammers are more conducive to underwater use, having few, if any, seals.

6. COMPARISON OF VARIUOS METHODS OF DEMOLITION

6.1 NUISANCE, SECURITY MEASURES AND PERFORMANCE

Nuisance.

Security measures

Performance.

Demolition

Methods.

Noise.

Vibration.

Dust fume projection

Environmental projection.

Hazard to the worker.

Execution rapidly.

cost

Remarks.

Hammers.

Hand hammers.

4

1

0

3

5

5

5

· Dust proof mask, glass, ear defenders, vibration proof gloves and safety bands are needed.

· Need a working stage.

Large hammers.

5

4

1

4

3

3

3

· Noise and dust insulation fences are required according to demand.

· Rigid working floor is required.

Wrecking ball.

4

5

4

5

5

1

1

· Prohibited to entry into the working area

· Taking care of hitting wrongly.

· Not to over turn the mounted machine.

Busters with wedges (hydraulically & mechanically operated).

2

0

0

2

1

4

4

· Noise and dusts are generated while boring other wise noise less and vibration less.

Blasting.

Mild explosive.

4

4

5

5

3

1

2

· Noise and dust occur while boring.

· Taking refuge is needed at blasting time.

· Blasting finishes immediately.

Explosive.

5

5

5

5

3

1

2

· A blast fence is needed at blasting time.

Expansive demolition agents

2

0

0

2

1

4

4

· Noise and dusts are generated while boring other wise noise less and vibration less.

· Wear protective glasses.

· Never watch a charged hole from upright position.

Abrasive water jets.

4

1

3

4

4

3

5

· Protection is needed for water jet.

· Wear ear defenders.

· No crossing a water jet.

Thermic lance.

1

0

4

2

3

5

5

· Effects of fume and fire prevention are needed.

Electrical heating of bars.

2

1

3

2

2

5

5

· Noise and dust or spray is generated during exposure of rebar and removal works. Other wise no noise and no vibration.

Microwave.

3

0

1

4

4

4

5

· Anti leakage of microwave is required.

· Prevention interfere for TV and communication facilities.

Hydraulic breakers.

2

2

3

1

3

3

4

· Taking care of the falling materials.

· Rigid working floor slab is required.

· Widely used in the urban centre.

Diamond wire saw.

4

2

0

3

3

5

5

· Crane is needed to suspend cut materials.

· Rigid working floor is required .

· Protection needed in case the saw snaps.

6.2 OPERATIONS AND APPLICATION FIELDS

Demolition

Methods.

Principle of breaking.

Mounted or accompanied machine

Need of pre work.

Size of the demolished materials.

Type construction

Column beam.

Wall slab floor slab.

Foundation.

Partial demolition.

Whole.

Application in urban cities.

Frequency.

Hammers.

Hand hammers.

Chopping of materials by repeated shock at a point.

An air compressor or hydraulic pump.

no

small

RE,C,M

O

O

O

O

N

High

Special use.

Large hammers.

Crawler or wheel type machine.

no

small

RE,C,M

O

O

O

O

O

High

Outline common.

Wrecking ball.

Repeated hitting by a steel ball.

Crawler type machine.

no

Medium.

RE,C,M

O

O

O

Low nuisance.

Rare..

Busters with wedges (hydraulically & mechanically operated).

Splitting by wedge.

High pressure pump.

yes

large

C,M

#

#

O

O

#

High

Special use.

Blasting.

Mild explosive.

Blasting.

None

yes

Medium.

RE,C,M

O

#

O

#

#

Low nuisance

Special use.

Explosive.

Blasting.

None

yes

Medium.

RE,C,M

O

#

O

#

O

Low nuisance

Special use.

Expansive demolition agents

Expansion of quick lime.

None

yes

large

C,M

#

#

O

O

#

Low nuisance

Special use.

Abrasive water jets.

Water jet containing abrasive materials.

Super high pressure pump.

no

large

RE,C,M

O

O

O

O

#

Medium.

Special use.

Thermic lance.

Melting by heat of oxidation flame of metal.

Oxygen tank and metal lance.

yes

large

RE,C,M

O

O

-

O

-

High

Special use.

Electrical heating of bars.

Cracking of concrete by heating rebar.

Transformer and frequency amplifier.

yes

large

RE

O

O

#

O

N

-

Special use.

Microwave.

Heating concrete by microwave.

Magnetron and wave guide.

no

small

RE,C,M

O

O

O

O

N

-

-.

Hydraulic breakers.

Breaking by hydraulic pressures of the jaws.

Crawler type machine.

no

small

RE,C,M

O

O

-

O

O

High

Outline common.

Diamond wire saw.

Cutting by abrasion with a diamond wire saw.

Rotating machine of wire saw.

no

Large

RE,C,M

O

#

O

O

#

High

Special use.

RE-reinforced concrete structure and steel frame reinforced concrete structure.

C-plain cements concrete structure.

M-masonry.

O-generally applicable.

# - specially applicable.

N- Not applicable.


CONCLUSION

Indian conditions are fully ripe to adopt explosive demolition technology. Dense population, haphazard growth, non existence of any bye-laws or regulations and our casual approach makes use of explosives a risky affair for us. Hydro-demolition is another area India should concentrate upon.

The telescopic collapse, triggered by the explosion technology is most suitable for tall chimneys and cooling towers. Hydro demolition suits the multistoried buildings and densely reinforced concrete structures. Diamond wire saw, thermit lances etc can employ to cut highly reinforced concrete structures. Soundless splitting agents can be used in the case of piers, bridges etc.Ordinary buildings, houses, and brick structures can be demolished by hydraulic hammers, jack hammers, impact breakers or ordinary splitters. Final choice of the technique to be adopted will however depends upon the actual site condition.


ACKNOWLEDGEMENT

I express my sincere gratitude to my seminar guide Srimati Asha Varma.P, Senior lecturer, Department of Civil Engineering, without whose valuable guidance and support the seminar would not have been a success. I would also like to thank Dr. A.K.Raji, Lecturer, Staff in charge of seminar for her assistance during my seminar.

I thank Prof. T. DIVAKARAN, Head of the Department of Civil Engineering for the good will and encouragement extended to me.

Last but not the least I thank almighty, for giving me the strength and power to complete this seminar on time.

REFERENCE

1. Barat. B.N, “ Demolition of unauthorized building in urban locality”, IE(I) Journal,

DEC I998, PP: 143-144

2. “Demolition of concrete structures”, The Indian concrete Journal, JAN 1991,

PP: 5-6

3. “Inddeco Innovative demolition for Delhi metro project”, NBM & Construction world

2003 DEC, PP: 32-33

4. Jagvir Goyal, “ Need to develop demolition technology”, NBM & Construction world

2000-JUNE, PP: 75-79

5. “Modern technology for demolition”, Civil engineering and Construction review, 1997

PP:60-61

6. WWW.IMPLOSIONWORLD.COM

7. WWW.NPS.GOV

8. WWW.OSCHEUSA.COM

9. WWW.ATLASCOPCO.COM

10. WWW.CAPPREX.COM

12 comments:

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