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Concrete contractors dream about placing perfect concrete for precisely positioned columns, beams, and slabs. But sometimes things aren’t quite perfect or plans change and it’s necessary to demolish concrete. In many cases, a 60-pound breaker or a smaller chipping hammer are the answer, but there are alternatives that should be considered, especially when noise, dust, or vibration are important issues. Here are three techniques to consider: expansive grouts, microblasting, and hydrodemolition.
Drill a line of holes, pour in some grout, wait a few hours, and the concrete or rock cracks exactly where it was supposed to crack. Sounds too easy to be true, but it is almost that simple. Expansive grouts have improved in recent years and deserve a place in every concrete contractor’s bag of tricks. Three reliable expansive grouts are Bustar, Dexpan, and Ecobust.
To use this material requires drilling a series of 1 1/2-inch-diameter holes to 80% of the concrete thickness. The holes should be located about 8 inches apart in reinforced concrete. The holes are cleaned with compressed air then the slurry is mixed and poured into the holes. Within two to eight hours, depending on temperature and relative humidity, the grout will create up to 20,000 psi of expansive force and expand up to four times in volume.
“Demolition jobs don’t have to be filled with noise, fumes, and dust,” says Ecobust’s president David McNamara. Expansive grout, he says, is ideal in places where heavy equipment or blasting would not be allowed. And without the concussive effects of mechanical demolition, the adjacent concrete is not cracked and rebar can be exposed without damage.
“It saves a lot of back strain for workers,” says Richard Azevedo, sales manager with Dexpan. “In places where vibration or noise is an issue, this material is very effective.”
The material comes in various formulations related to the temperature at which it will be used. “Our hottest mix is used from 23°F to 50°F,” says Azevedo, “but it can be used in even colder conditions, it will just take longer.” At higher temperatures—especially if the concrete is in the sun—the greater danger is that the material will set too fast and blow out of the holes. In these conditions, the best approach is to cover the holes with a tarp or damp straw.
Drilling the correct hole pattern is essential to getting the desired effect. Holes are drilled no farther than 12 inches apart. Some holes may be drilled and left empty to allow needed free space for the expansion to occur. Empty holes also are used to stop a crack at the end of a pattern.
Although this may be a fairly safe demolition technique, there are still some safety issues to consider. The material itself is not toxic but is highly alkaline, like portland cement, so skin and eye protection is necessary. Never look into the holes after they are filled, because blowouts of the material can occur, usually in hot weather.
Dexpan costs under $100 for a 44-pound bag that will fill 35 lineal feet of 1 1/2-inch holes
Breaking concrete from the outside requires overcoming its compressive strength. If, however, it can be broken from within, it’s only necessary to overcome the tensile strength which is about 15% of the compressive strength. Dynamite is an option for imploding a building, but according to the National Demolition Association, implosions account for only about 1% of demolition work and dynamite is difficult to control and often is not an option.
The Ezebreak Micro-Blaster System uses the same concept as dynamite but in a smaller, more controlled manner. Two small cartridges of smokeless powder are placed into a 5/16-inch diameter by 10-inch-deep hole that has been cleaned out with air to remove all dust that could interfere with the firing pin. A firing mechanism is placed in the hole and tubes are laid out to 25 feet away. The charge is initiated using compressed air or a CO2 cartridge.
The blast is large enough to break thick concrete but results in little flying debris, which can be controlled by placing carpet over the blast area. “The head and firing pins are very tough tool steel,” says Ezebreak owner and inventor Carroll Bassett. “With 1000 systems in use, we’ve never replaced a head.” The only thing that seems to wear, according to Bassett, is the air hoses.
One recent project where a Micro-Blaster was used was in a library wall that had previously been a bank. The problem, to cut a doorway through the 20-inch-thick bank vault wall, was solved with a three-head system. “Before the contractor approached us for advice on the project, he had determined that the cost of the project including wire sawing and removal of the large chunk of concrete would be $10,000 and require the library to close for two weeks,” says Bassett. “Instead we used the Micro-Blaster System, chipping hammers, and a concrete saw to cut a kerf around the perimeter of the opening to limit the extent of cracking. Working time including cleanup was four days and the cost to the library was half of the original estimate. A further benefit was that the library, although a bit noisier than usual, remained open during work, making our librarian very happy.”
The Micro-Blaster is simple enough that a blasting license is not needed. Training is accomplished in a couple of hours. Safety would seem to be an important issue, but Bassett says that with 600,000 cartridges sold there has been only one minor safety incident. The only way to trigger the blast is remotely using the compressed air or CO2.
Cutting concrete with water seems unlikely and yet it has proven to be very effective, especially for surface preparation and scarification. Hydrodemolition is done with various devices, from handheld guns to robotic equipment. The water is directed at the concrete at 20,000 psi and 10 to 15 gallons per minute (for handheld guns). It can cut clean lines or remove the surface as deeply as desired.
Because there is no vibration or impact, the surrounding concrete is not prone to bruising (microcracking) as it is with surface preparation using mechanical devices. The surface ends up looking like rough exposed aggregate, ready for new concrete and capable of achieving high bond strength. Rebar is not damaged or cut (at these pressures) and any corrosion on the bars is removed.
Taking advantage of what Rampart Hydro Services calls “coincidental and beneficial removal,” loose or slightly damaged concrete near the removal area that might well be missed by mechanical demolition also is removed by the high pressure water. Any loose or delaminated concrete is automatically reduced to small 1/2-inch pieces. Where hidden delamination existed, the hydrodemolition will cut deeper with no “collateral damage” to adjacent sound concrete.
An important issue with hydrodemolition is water. “Hydrodemolition is not recommended in remote locations with no water source,” says Richard Scruggs, product manager with Jetstream, a hydrodemolition equipment manufacturer. In those cases, water would need to be trucked into the site—which is possible but will add cost. Handheld guns may use as little as 10 gallons per minute (gpm) while larger machines can use as much as 60 gpm. Concrete is removed with water only—no abrasives are added.
Disposing of the water is also an important issue. “The environmental regulations on the slurry are all over the place,” says Scruggs. “Some regions don’t regulate the slurry at all while some require complete segregation and conditioning of the water.” Check with local environmental officials when considering hydrodemolition to determine how to handle the slurry.
Training to safely and effectively use hydrodemolition equipment usually is provided by the manufacturer. Jetstream provides a one-day class for simple work or a three-day blaster training course for more sophisticated equipment. “Training provides a good foundation,” says Scruggs, “but it takes experience to know how to balance the pressures and flow rates.” The robotics systems actually are simpler to operate because the machine removes a lot of the guess work.
Prices for hydrodemolition systems can range from as low priced as $2000 for the “shotgun” systems up to $300,000 for the full robotic devices.
Demolition work by any method raises safety risks. Although expansive grouts, microblasting, and hydrodemolition are reasonably safe techniques, each has their own dangers and often are accompanied by drilling, sawcuts, and chipping. Every contractor needs to have a demolition section in their safety program. A couple of good resources for this are the National Demolition Association (NDA) and the Concrete Sawing & Drilling Association (CSDA).
The NDA, for example, has a series of safety talks including number XLVII on prestressed and post-tensioned concrete. This short primer has some great advice on the need to recognize post-tensioned structures, because there is a significant amount of energy stored in the cables that must not be released without warning. Not only can tendons turn into missiles but a sudden release can lead to progressive collapse of a structure. “Demolition of these types of members requires specific knowledge of the types of tendons and the exact placement of the tendons,” advises the safety talk. For more, visit the NDA at www.demolitionassociation.com.
The CSDA also has an extensive safety manual and a series of tool box talks on safety issues related to cutting and drilling. Here, for example, is an abbreviated version of CSDA’s safety rules for chain saws:
1. Operators must read and understand the manufacturer’s operating manual and safety requirements. Always wear personal protective equipment, including, but not limited to, hard hat, hearing protection, safety glasses, and safety footwear.
2. NEVER operate a diamond chain saw with the side cover missing or broken.
3. NEVER insert a diamond chain into a slot that is narrower than the chain segments, as rapid pushback might occur.
4. NEVER install or run the chain backward. The bumpers should lead the segments into the cut. The chain should flow away from the operator on the top of the bar and return to the operator on the bottom of the bar.
5. NEVER run a diamond chain saw upside down. Concrete debris can fly back into the operator’s face.
6. Be aware of what is on the backside of a cut.
7. Always turn the saw off before performing any maintenance.
8. Always maintain secure footing when operating a diamond chain saw. Housekeeping in the work area is important for operator safety.
9. Operators should always use both hands on the saw and keep their arms close to their body.