CUTTING REINFORCED CONCRETE
The keys to penetrating reinforced concrete during rescue operations are understanding concrete as a construction material and having proficiency in the methods and techniques of concrete penetration. To understand concrete as a construction material, consider its strengths, its weaknesses, and the methods of forming it into various structural components. Methods and techniques of concrete penetration include tool selection and use and the strategy and tactics of safely cutting into concrete that is under stress either from fabrication to bear a designed load or as a result of collapse.
Reinforced concrete is a composite of Portland cement, water, aggregate (i.c., sand, stone, and/or lightweight inorganic particles), and steel. Concrete alone is strong only in compression, it is relatively weak in shear and tensile strength. Reinforced concrete derives its tensile strength from steel, which is embedded in the concrete in the form of rebar in conventionally reinforced concrete or stranded steel cables or tendons in prestressed concrete.
Inasmuch as concrete is strong only in compression and inherently weak in shear and tensile strength, it is logical to direct efforts at penetrating concrete at its weaknesses. Indiscriminate hammering on all but the thinnest concrete will be a waste of effort and could cause serious vibration impact loading to a structure that already is in trouble.
CONCRETE CUTTING STEPS
A better strategy is to take advantage of concrete’s inherent weakness in shear strength. Tactics that support this strategy involve shearing the concrete from its surface in relatively small segments. Such a procedure may involve some or all of the following steps:
- Cutting the perimeter of the area to be removed with a concrete cutting saw.
- Making additional relief cuts in an “X” or checkerboard pattern to reduce the size of the concrete that must be chiseled and increase the spaces into which the concrete can be chiseled.
- Drilling bore holes partially or completely through the concrete in a cluster at the center of the section to be removed, along the perimeter in a close stitch pattern (closely spaced holes drilled in a line, as a sewing machine would punch stitches in fabric) and throughout the section. Drilling “softens up” the concrete, which makes subsequent chiseling much easier and provides spaces into which the concrete can be chiseled or sheared. Drilling concrete with a rotary hammer prior to chiseling is especially important in confined spaces where it is difficult or impossible to maneuver a demolition hammer into different positions in an effort to vector its impact at various angles.
- Beginning chiseling with a chipping or demolition hammer within one to two inches of a relief cut or drill hole. This will yield small sections of concrete that will shear off into a void space created by the drill hole or cut. This method will have the same effect as beginning demolition at an outer edge of a slab.
- Starting the chisel bit perpendicular to the concrete, penetrating ½ to ¾ inch below the surface, and then canting the tool to direct the impact of the bit tow ard the relief cut or drill hole. This method effectively will shear the concrete into an open space. Always shear off small segments of concrete toward an open space. Continuous hammering of solid concrete at right angles is ineffective—the bit invariably will get stuck in a small crevice as you attempt to fight the concrete with compression, its only real strength.
It is critical to understand how steel reinforcing of a particular concrete component provides the strength to resist live and dead loads. When and how the steel reinforcing is severed have a profound impact on how the concrete will fail.
Conventional reinforced concrete will fail quickly when it is separated from its reinforcing bar: Cut the rebar as soon as it is exposed; the remaining concrete will chip away easier. Vibrate the rebar to break its bond with surrounding concrete with direct blows to the rebar using a demolition hammer.
Understanding how steel strands or cables support pretensioned or posttensioned concrete can make the difference between safe penetration and catastrophic collapse. As a practical matter, however, rescue workers do not have the technical background to predict how prestressed concrete will fail. Whether as a feature of structural engineering or as a consequence of failure of the building, it is very difficult to know exactly how a concrete member will react when it is cut. Urban search and rescue task forces as well as local rescue agencies must prearrange for the services of a qualified structural engineer for consultation on how to safely penetrate pretensioned or post-tensioned concrete.
TOOLS AND TECHNIQUES FOR CUTTING STEEL
Cutting rebar can be a challenge; cutting tensioned strands or cables can be suicidal unless it is done under the direct supervision of a qualified structural engineer. When it is deemed safe to cut the steel, the follow ing tools and techniques can be used:
- For all of its faults, (i.e., sparks, flames) an acetylene or other fuel gas cutting torch can be a very quick and effective tool for cutting reinforcement, especially in the hands of a skilled rescue worker with experience in burning steel. Preheating steel that is under tension will give some indication of w hat it will do w hen it is cut: As the steel is heated, it will stretch or bend in reaction to the stress it is under.
- A reciprocating saw is much safer but not as fast cutting as a torch, particularly on strands that are extremely hard so that they can withstand several thousand pounds of tension. If such a saw is indicated, select a premium quality blade with 14 to 18 teeth per inch. Be sure to provide plenty of lubrication with a soapy water solution or cutting oil and follow the manufacturer’s directions for cutting speed.
- High-speed grinders and saws equipped with abrasive disks are very effective in cutting steel. Rescuers must be prepared to deal with a shower of sparks that could be a source of ignition or injure a trapped victim.
- An exothermic torch is fast and effective but can be very dangerous when cutting rebar or cable. Rescuers must guard against a shower of sparks and as much as two feet of flame that shoots beyond the tip of the burning rod.
- Bolt cutters and cutting attachments to hydraulic extrication tools effectively can cut rebar but not without limitations. Both tools require a fairly large space to take a bite out of the steel. Cutting rebar may be delayed until sufficient concrete can be removed to get the fairly blunt cutting tips into position.
Cutting rebar larger than ⅛ inch with bolt cutters requires great physical strength and may damage the cutting tips. Capabilities in cutting large rebar and tensioned cables vary with each hydraulic cutting tool; consult the manufacturer’s specifications before cutting steel that may shatter or deform the cutting tips.
SAWING CONCRETE
Sawing concrete can be a very effective tactic. The longevity and effectiveness of a dry-cut diamond blade will be greatly enhanced if the following measures are taken:
- Do not overheat the blade. Proper cooling can be ensured by cutting no more than one inch in depth. Thicker concrete can be cut by successive “step cuts” —cutting one inch per pass of the blade. Rock the blade out of the cut every few seconds—this facilitates proper air cooling as the saw is moved backward into new material to be cut.
- The diamond particles in the blade may become blunt or polished over after cutting through rebar (sparks will tell you when you’re into the steel). It may be necessary to dress the blade by running it on an abrasive material such as asphalt or concrete block. Dressing the blade will wear away some of the matrix surrounding the diamonds, bringing fresh diamond to the surface.
- A large percentage of the dust generated by concrete sawing can be washed out of the air by applying a mist of water on the back of the blade.
- Depth of cut is limited by blade diameter: A 12-inch circular saw can cut to a depth of slightly more than four inches. A 14-inch saw can cut to a depth of a little more than five inches. Cutting through thicker concrete can be accomplished by relief cutting the perimeter of the section of concrete to be removed to the maximum depth of the saw blade. The concrete is chiseled out to the depth of the relief cut and then, allowing room for the saw, cut again to the desired depth.
VICTIM SAFETY
The location, position, and condition of a victim trapped below a floor sometimes can be ascertained by carefully drilling a hole in the slab above with a rotary hammer (make sure not to penetrate deeper than the thickness of the slab). Visual indication from a fiber optic probe or similar device inserted into the drill hole will impact where to penetrate the slab to access the victim. If possible, do not open a floor (or lean-to wall) directly above a victim — it is impossible to completely avoid hitting the victim with falling chunks of concrete and sparks. If collapse conditions give you no choice but to penetrate concrete directly above a victim, remove the concrete in one piece. Cutting and lifting out a section in one piece minimizes the amount of debris that falls on the victim.
When sawing concrete to remove a section in one piece, at least two sides should be bevel cut. This prevents the sawed section from falling in on the victim and facilitates removing the section with the assistance of pry bars.
Removing a section of concrete in one piece can be made safer and easier with rope, webbing, cable, or chain attached to the section by anchor bolts set in the concrete or run under exposed rebar. It is critical for you to support the concrete before cutting the reinforcing steel.
PRACTICE TO IMPROVE SKILLS
Rescue personnel can gain valuable experience in penetrating concrete by practicing on as many different types of assemblies as are available from local construction companies, from precast plants, or in buildings scheduled for demolition. The method by which concrete is formed into structural components determines the techniques to be used to penetrate a particular member. For exampie, cutting through thick, conventionally reinforced concrete walls or slabs requires entirely different techniques than those used to cut lightweight pretensioned members. Penetrating concrete poured on corrugated steel decking to form floors requires an altogether different technique.
Only large fire departments usually can afford to buy tools used to penetrate concrete; the infrequent and specialized use of such tools makes their purchase difficult to justify in most tight budgets. Concrete drilling, cutting, and breaking tools, however, are readily available to most fire and rescue agencies. A search of your community probably will reveal that such tools are standard equipment for local utility companies and public works departments. The availability of equipment such as demolition hammers, concrete drills, and pavement breakers should be prearranged for emergencies and periodic training to sharpen skills.
As much as practicing on concrete assemblies improves skills and techniques, it obviously cannot totally approximate actual conditions found in a building that is in trouble. Remember that you will not have an accurate and reliable picture of the forces at work on the structure as a result of collapse: The remaining structure is bearing undesigned loads in the wrong places. In reality, no one accurately can predict the behavior of concrete that is under unnatural forces it was never designed to withstand. Protect yourself by getting technical advice, and read the pattern and progression of cracks. Monitor the structure for walls and columns that move, almost imperceptibly, farther out of plumb. Pay close attention to shifting loads and strange sounds.
Concrete can be effectively penetrated with the proper tools, techniques, and guidance. Develop and sharpen skills with frequent training on various types of concrete. When you are called on to put those skills to work at an actual collapse, use them only with competent technical advice, safety, and control *
