Old concrete structures that are withering, old and unserviceable should eventually be demolished to make room for modern structures. Keeping safety and speed as the main considerations, carrying out demolition is a challenging job, needing new techniques and tools.
Manual demolition
This technique is normally carried out by contractors using manual tools which are portable. Sledge hammer, pneumatic hammer and drills are used, supplemented by lifting tools such as cranes and tackle. This technique is considered non-engineered. The demolition contractor's survey of available documents and field conditions are the primary factors for equipment selection and removal sequence.
Engineered demolition
Engineered demolition involves the analytical evaluation of a concrete structure during all stages of demolition to verify the adequacy of its strength and stability. The deconstruction analysis is essentially a reverse process of the original construction of structure, with revised partial framing and alternate load patterns. The process should include a thorough review of the existing condition of the structure, its proximity to other structures, utilities above and below grade, the preparation of demolition sequence, the procedure, the assignment of equipment to be used, an analysis of actual and altered structural models and the stability analysis of the structure. A few techniques which are in use are described below:
Demolition ball technique: Metallic balls weighing 500 to 1,000 kg are used by suitably fitting them to cranes of adequate capacity. This is much faster than the manual method.
Pusher arm technique: Suitable pusher arms are fitted along with deflector plates to excavators to demolish the structure. The structure is reduced in height by pushing small sections at a time. This allows speedy and safe demolition.
Use of explosives: Numerous types of explosives are available with a wide range of detonators. Demolition by explosives calls for control and caution since it is ‘once only' situation for regulated orderly demolition.
Non-explosive demolition compounds: Some non-toxic inorganic powders expand when in contact with water. A number of holes are drilled along the desired line of separation and are filled to the brim with the slurry obtained by mixing the powder with water. The slow and silent bursting force developed will induce stress levels of 300 kg/sq. cm. in a few hours. This causes development of cracks along the line joining the holes. The cracks continue to expand till disruption takes place.
Pressure jetting for concrete repairs: A new series of equipment and tools has been developed wherein the destructive energy of water can be utilised for constructive purposes. Pressure jetting equipment can be used to remove cracked and spalled concrete. Although the cracked concrete can be removed by hammer and chisel, cutting back to sound concrete particularly behind reinforcing bars is rather difficult. On the contrary, it is important that cracked concrete behind the bars be removed for effective restoration. Use of power tools is awkward to handle. Pressure jetting is the only answer to such problems. The jet heads are generally small and so can easily be manipulated without affecting the steel. The action of the jet is mainly to loosen the aggregate by washing out softer mortar.
Thermic lance technique: A thermic lance holder and oxygen supply are the major components of this technique. On ignition, a flame is produced in excess of 2500 degree C, enough to melt reinforcement and concrete which aids demolition.
Microwaves technique: When microwave energy is passed into the mass, its transfer causes heat, resulting in vaporisation of the absorbed liquid. Concrete disintegrates in this process.
Concrete sawing: A new range of concrete sawing equipment has been developed to aid demolition. These basically incorporate a suitable driving mechanism and appropriately designed diamond blade. The emphasis is on increasing the speed and efficiency of the equipment coupled with reduction in noise levels.
