Selective Bridge Demolition

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“Do not remove a fly from your friend’s head with a hatchet.” – Ancient Chinese Proverb

Diamond Cutting Saw

Common Selective Bridge Demolition Processes

Selective demolition involves removal of part of the structure which is to be replaced. This process may take the form of cutting or mini-blasting. Cutting bridge sections is a measure of demolition in which parts of the bridge are cut out for replacement and many methods for doing so exist. Mini-blasting involves dislodging parts of the structure with small explosives. Depending on demolition requirements, wire cutting, diamond sawing, splitting, jack-hammering, abrasive water jetting, or mini-blasting may be used.

Wire cutting ([1], Figures 1 and 2) involves a wire with embedded diamonds which is directed through concrete with a pulley system. Diamond wire cutting is very precise and effective for thick sections of material which must be cleanly cut in two, in situations where hand-held demolition tools cannot reach. The wire halves the material without damaging either side.

Figure 1: Diamond Wire Cutting [1]

Figure 2: Diamond Wire Cutting Video

Diamond sawing ([1], Figures 3 and 4) is an effective method to cut thin slabs of concrete such as bridge decks without damaging the surrounding material. Diamond sawing is better suited to small slabs over wire cutting because a diamond saw can be hand-operated and does not require the set up time of a diamond wire.

An effective method to cut thin slabs

Figure 3: Diamond Saw Cutting [1]

Figure 4: Diamond Saw Cutting Video

Splitting ([2], Figures 5 and 6) is a method in which cracks are propagated in a controlled fashion to remove sections of concrete. Mechanical splitters are placed into drilled holes and when pressure is applied to them, they expand, causing the surrounding concrete to crack. Splitting is a popular method for breaking up sections into specific shapes, where the drilled holes can help direct the cracking. Once the sections of concrete have been split, the exposed reinforcement can be cut and the loose sections can be removed by crane. Splitters also exist in the form of expanding slurry compounds, pressurized water or gas, and heated carbon dioxide.

A method in which cracks are propagated in a controlled fashion

Figure 5: Mechanical Splitters [3]

Figure 6: Mechanical Splitter Video

Jack-hammering [4] involves a pneumatic drill which uses jabs powered by compressed air to break up concrete with repeated hammering of the drill bit into the material. This method is convenient because jack-hammers are common from sizes between those used on excavators (Figures 7 and to hand-held units (Figures 9 and 10). Jack-hammers are less precise than diamond cutting and are effective for breaking up material over a wide area or where precision is not an issue. In addition, jack-hammers create more vibrations than diamond cutters, and may not be applicable in locations which require low vibration.

Jack-hammers create more vibrations than diamond cutters.

Figure 7: Excavator Mounted Breaker [5]

Figure 8: Mount Breaker Video

Jack-Hammer Worker

Figure 9: Hand-Held Jack-Hammer [4]

Figure 10: Jack-Hammer Video

Abrasive water jetting ([6], Figures 10 and 11) is perfect for demolition of concrete when the reinforcing steel is not to be damaged. The water jets blow away the concrete by quickly eroding the surface, while the steel resists the jets’ effect. This method is also useful for reducing stress on the surrounding concrete, where using a saw or jackhammer would be more stressful.

The water jets blow away the concrete.

Figure 10: Abrasive Water Jetting [6]

Figure 11: Abrasive Water Jetting Video

Similar to water jetting, mini-blasting ([7], Figure 11) is an effective method to remove concrete without damaging the reinforcing steel. Mini-blasting involves drilling holes into the concrete and setting small explosive charges, which breaks up the concrete to allow for its removal. This method is effective because it may be done by hand and does not damage steel. Mini-blasting is not as accurate as cutting, and has been seen to crack concrete as far away as 50cm from the edge of the detonation.

Similar to water jetting, mini-blasting.

Figure 11: Results of Mini-Blasting [7]