Breaking Up Is Hard to Do
When planning for the excavation and removal of old, in-place concrete, two questions have to be asked. First, how much concrete is there? Second, what kind of concrete has to be removed?
Mass excavation and trenching operations typically deal with natural soil and rock. These can present their own difficulties depending on the hardness of the soil, the size of the rocks, the need to blast bedrock, etc. Concrete is a man-made material that often can be more difficult to excavate than rock formations. The wide varieties of concrete structures (pipe, slabs, footers, columns, beams, pavements, etc.) make planning for concrete excavation a complicated exercise in estimating in-place quantities. They also have one operational characteristic in common; they have to be broken up before they can be excavated. This requires time-consuming and expensive demolition work with specialized tools and equipment prior to excavation. Once removed, however, the concrete debris can be a cost-effective resource whose steel can be recycled as scrap metal and whose concrete can be further crushed into usable aggregate. Concrete that is not reused presents its own challenges for disposal in landfills specialized for construction-and-demolition (C&D) debris.
Concrete and Concrete Structures
Concrete is a mixture of Portland cement, gravel and sand aggregates, water, and various admixtures depending on the type or classification of the concrete. The main ingredient, Portland cement, is made by heating limestone with clay, then grinding the resultant clinkers with gypsum. Aggregates such as sand, gravel, slag, and ash are added to make up the bulk of the concrete’s mass and are held together by the cement. Use of cheap aggregates reduces the overall cost of the concrete.
Water is added to hydrate the Portland cement, which then hardens when the water evaporates. The admixtures are added to increase strength, reduce weight, and preserve the material characteristics and quality of the concrete during transportation. This is accomplished by entraining bubbles in the cement mix, thereby reducing the amount of water normally required in the concrete mix, slowing the setting rate during hot temperatures, increasing the fluidity and workability of the concrete, and inhibiting corrosion of the concrete’s steel reinforcement. A typical concrete mix consists of the following: 10%–15% cement, 60–75% aggregate, and 15–20% water. Entrained air bubbles may have a volume of 5% to 10%.
This steel is necessary since, though very strong when subject to compressive forces—concrete is weak under tension loads (it is only one-tenth as strong in tension as it is in compression). Concrete by itself tends to crack under loads or as the result of contraction and expansion caused by temperature changes. When any pavement slab or support beam is subject to an applied point load or spread load it will want to compress in its upper layers and tense in its lower layers as it bends under these loads. Similarly, buried pipes, footers, fountain walls, and concrete columns will experience similar reactions to applied loads. Steel, on the other hand, is very strong in tension. So steel reinforcement in the form of either a wire mesh (for concrete that will be lightly loaded) or crisscrossed steel rebars spaced and sized to handle the anticipated tension is embedded in concrete portions that are expected to go into tension.
Concrete reinforced with steel is the basic building material of modern construction. It is used to manufacture pipeline segments that are joined together to make buried sewer lines. Buried building foundations of supporting footers of concrete can extend tens of feet to hundreds of feet below the surface depending on the type and size of the building. Pavements and slabs constructed of reinforced concrete are typically found at the ground surface. Support columns, arches, and beams extend from below the ground surface to the maximum height of the structures they form. Concrete’s versatility and ability to be cast in place and shaped to purpose by forms, and capacity to be strengthened by steel reinforcement makes it the most useful of construction materials. These characteristics also make reinforced concrete difficult to break up and excavate and expensive to haul away and recycle.
Planning and Preparation
The pervasiveness of reinforced concrete makes it the most common man-made material encountered during excavation jobs performed as part of demolition operations. When planning for the excavation and removal of old, in-place concrete, two questions have to be asked. First, how much concrete is there? Second, what kind of concrete has to be removed?
Often, the amount of in-place concrete that has to be removed is easily determined from existing construction plans and as-built drawings and from site surveys. These will show the dimensions (length, width, thickness, and diameter) of in-place concrete structures as well as their location (northing, easting, elevation, depth, gradient, and extent). The plan details should also describe the type and amount of steel reinforcement that needs to be removed: plain (no steel reinforcement), mesh (shrinkage and temperature reinforcement), dowel (discontinuous rebar reinforcement placed only at the concrete joints), or continuous steel reinforced. Generally speaking, the greater the amount and extent of steel reinforcement, the harder it is to break up and remove the in-place concrete.
The volumes of other concrete structures can be more difficult to determine. Concrete can be formed into complicated shapes whose volume is not easily calculated. Even relatively simple structures, such as a multilevel parking garage, require extensive and detailed computations to determine the amount of concrete in-place. More complicated reinforced-concrete structures also contain highly variable amounts of steel reinforcement in various locations and configurations. Planning for the demolition and excavation of concrete structures must take into account the need to break up the concrete and manage the scrap steel produced during the process. The typical unit of measurement and payment for concrete demolition and excavation is the cubic yard. For simple structures, the end-average method is used to determine the amount of in-place concrete.
Breaking and Entering
Breaking up the in-place concrete is the first step to excavating and removing it. This task is usually performed by hydraulic hammer attachments on standard earthmoving equipment, as well as by the smaller, manually operated jackhammers. Both machines direct their applied cyclical forces to a hardened metal point that concentrates the force into a small area to increase applied pressures. Hydraulic hammers use rapid impact cycles and the resulting vibrations to break through the concrete.
|A Komatsu mobile crusher unit transforms large chunks of concrete debris into smaller ones.|
Hydraulic hammers should be properly matched to the equipment they are mounted on. A heavy hydraulic hammer mounted on a very long excavation arm or boom needs to be matched with a proportionally heavy vehicle chassis. Mismatched hammers and vehicles can lead to unsafe and unstable operating conditions. Hydraulic capacity and fluid requirements are also an important concern. Though most hydraulic systems running the hammers operate with pressures of 2,000 psi, the amount of working fluid can vary greatly. Small hammers require as little as 5 gallons per minute, while heavy hammers must be supplied with 100 gallons per minute. Add-on hydraulic power units can be used to increase the flow rate.
In addition to hammers, pulverizers are used to break up concrete. These applied breaking force to the concrete via a set of jaw clamps. The paired jaws consist of a movable front jaw and a static back jaw. Mounted on the end of an excavator arm, pulverizers literally bite through the concrete and are most useful for breaking flat concrete structures (pavements, decking, slabs, and walls) as well as support structures (pillars, culverts, columns, and beams) small enough to fit in the jaws. One significant advantage of pulverizers is that their crushing action efficiently separates steel reinforcement from the surrounding concrete.
Ripper attachments on dozers can also be useful in breaking up concrete pavements and slabs. There are several types of rippers: parallelogram or radial, hydraulically variable pitch or fixed or adjustable, single or multi-shank—and combinations thereof. Parallelogram rippers are relatively straight like a knife, while radial rippers are curved like a claw. Both end in a reinforced steel tip that is the business end of the ripper. The breakout or pry-out force (measured in pounds) applied at this tip is the maximum upward force generated by the lift cylinders. Conversely, the penetration force (also measured in pounds) is the downward force generated by the lift cylinders sufficient to raise the back end of the equipment off the ground and drive the tip into the surface. Depending on the size of the ripper, its digging depth can vary from 12 inches to over 7 feet. For the most part, rippers are used to attack pavements and slabs less than 12 inches in thickness having minimal steel reinforcement.
Excavation and Hauling
Extra-heavy-duty and extra-large buckets are typically used for the excavation and removal of the broken concrete debris. A specialized type of bucket, the riddle bucket (sometimes referred to as a shaker bucket) is used to separate large objects, such as bricks, rocks, or broken chunks of concrete, from finer materials, such as soil. Once a concrete structure, slab, or pavement has been broken up, it is often mixed with large quantities of soil or other materials. By using the riddle bucket to excavate out only the larger pieces of concrete debris, a contractor can greatly increase the efficiency of concrete excavation and the subsequent recycling effort. Riddle buckets are designed with large open slots that allow soil to fall out during excavation. Heavy-duty rib-backed models are suitable for rock and concrete excavation.
The trucks used to haul the concrete debris from the excavation site are similarly rugged, large, and heavy-duty. With a loose density of about 2,400 pounds per cubic yard, the truck’s ratio of maximum load (measured in tons) to its heaped capacity (measured in yards) must be at least 1.05 and preferably 1.5 tons to cubic yards. These trucks should be able to withstand the impacts of large pieces of concrete. Articulated dump trucks are preferred for onsite hauling but are less suitable for road travel. Concrete chunks falling out of the back of a poorly secured truck bed represent a serious traffic safety concern.
Crushing and Steel Removal
Once the large chunks have been excavated and hauled away from the digging site, they can be sent to a portable crusher unit that makes little chunks out of large ones. These crushing machines don’t just handle concrete; they also crush asphalt, bricks, and rocks. What they can’t accept are non-rigid materials that don’t easily break and could clog their intakes (trash, wood, organic wastes, paper, etc.). No attempt is made to remove rebar before the crushing operation; the steel goes into the crusher along with the concrete and comes out freed from its now-shattered concrete encasements. Magnets remove the now-loosened rebar for resale as scrap metal. The steel-free concrete is further processed by screening operations that separate small chunks from larger ones. Large chunks caught by the screen are fed back into the crusher for another round, and they keep going back until they have been reduced to an acceptable size.
Production rates for these crusher machines are impressive, with some of the larger portable crushers able to handle over 600 tons per hour. At an in-place density of approximately 145 pounds per cubic foot, this is equivalent to almost 8,300 cubic feet—or 550 feet of a two-lane road having a 6-inch-thick pavement surface—each hour. A mile of such a roadway could be processed each workday. Smaller mini-crushers can handle up to 150 tons per hour. The basic configuration of a concrete crusher, no matter what its throughput rate, is fairly simple: a hydraulic apron feeder to the rubble crusher, a discharge conveyor belt for carrying broken debris to a screener, and a return conveyor for chunks too large to pass through the screen. Mounted directly above the discharge belt is a magnetic separator for steel removal that is set at a height sufficient for direct discharge into waiting hauling trucks (this avoids double handling).
Primary impactors are the preferred crusher type due to their speed and versatility compared to cone crushers or jaw crushers. Since they are able to reduce incoming chunks to one-thirtieth of their original size, primary impactors usually have no need for a secondary crushing unit (unless the final product needs to be less than one inch in diameter). Looking from the outside like a strangely shaped dumpster, a primary impactor crusher is housed in heavy-duty steel with heavy-duty wear liners affixed to the inside walls. Concrete debris enters through an oversized inlet and hits a heavy-duty mono-block front apron that directs the incoming debris to a heavy-duty rotor equipped with reversible steel blow bars set in place with wedge shoes. The rapidly spinning rotor, rotating on heavy bearings, smashes the blow bars into the debris, acting as a rotating hammer. The anvil is a set of very heavy-duty impact plates whose distance from the rotor can be adjusted with a retractable housing. Caught between the fixed plate and the swinging blow bars, rock is crushed into smaller pieces that discharge through the bottom.
Disposal and Recycling
In most cases, broken and excavated concrete is disposed of in C&D landfills along with other debris (roofing shingles, wall insulation, lumber, drywall, brick and masonry, broken glass windows, etc.). Concrete, along with asphalt pavement and masonry, constitutes up to 25% of the waste mass disposed of in C&D landfills.
Typically, in-place concrete weighs approximately 145 pounds per cubic foot (or over 3,900 pounds—almost two tons—per cubic yard). This weight can vary with the type of concrete, and this variability depends on the amount and type of the aggregate, the volume of entrained air bubbles, the water content, the amount of cement utilized, and the effects of the admixtures used in the concrete. Breaking up the concrete into chunks small enough for excavation and hauling results in significant void volumes within the waste mass. The resultant jumbled pile of concrete debris and voids has a much lower density than in-place concrete.
Concrete debris has a typical density of about 2,400 pounds per cubic yard, about 60% of the concrete’s in-place density. Since the weight of the actual concrete mass has not physically changed, the reduced density is the result of air voids (which weigh nothing) taking up approximately 40% of the rubble pile. The concrete debris therefore has an increased volume of about 1.67 times that of its original in-place volume. Still, even with the reduced density, concrete debris has a unit weight up to six times that of other construction debris.
After steel has been removed (or not removed as the case may be) the concrete debris is disposed of in a C&D landfill. Unlike other waste, concrete debris cannot be physically compacted in place with typical landfill compaction equipment. For the most part, concrete is dumped onto the current working face of the C&D landfill and spread as smoothly as possible over as wide an area as possible. Though direct compaction of the concrete debris is not performed, the act of spreading does result in a modest reduction in the air voids as the concrete chunks roll over each other and fill in the gaps. The amount of effective volume reduction depends on the size and shapes of the chunks; very little volume reduction can ever be achieved with concrete debris consisting of large, flat slabs. Volume reduction as a result of spreading and subsequent settlement as additional debris is piled on top, can be as high as 20%.
To take a simple example, suppose a 6-inch-thick concrete pavement covering a factory floor area of 300 feet by 200 feet has to be excavated. The in-place volume of this material is approximately 30,000 cubic feet (or 1,111 cubic yards) and it will have an overall mass of about 4,350,000 pounds (or 2,175 tons). Once the concrete pavement is broken up and excavated, its volume has increased to approximately 1,855 square yards. After it is loaded onto dump trucks, hauled to the landfill, deposited, and spread over the work face, its disposal volume has decreased to approximately 1,550 square yards (the equivalent of almost 1 acre per foot).
Instead of being disposed of, this material can be recycled and reused. There are multiple advantages to recycling over disposal. First is the avoidance of landfill tipping fees. The national average for landfill disposal costs is approximately $30 per ton. While some C&D landfills have lower tipping fees, there is also the cost of hauling to consider. At a typical truck hauling cost of $0.25 per ton per mile, hauling one ton of concrete debris a distance of 20 miles is equivalent to one-sixth of the tipping fee upon arrival in additional costs. Second, if the concrete debris is crushed to a size small enough to allow its use as an aggregate replacement, the delivered cost of gravel and aggregates from a quarry is avoided. Depending on the hauling distance from a quarry and the amount of stone delivered per order, rock and gravel costs can vary from $5 to $15 a ton. So the true cost of dumping concrete debris compared to recycling must include the costs of hauling to a landfill and purchasing gravel, which can raise the total to about $50 per ton. Indirect advantages to recycling concrete debris include extending the life of existing landfills by not taking up air space through disposal of the debris, reducing truck emissions (especially nitrogen oxide) by not hauling debris to a landfill or aggregates from a quarry and using a material whose engineering characteristics are at least as good as those of the natural resources it replaces.
Further suppose the same 6-inch slab described above has been reinforced by minimal-sized number-3 steel rebar (0.375 inch in diameter, 0.376 pound per foot) both ways at intervals of 24 inches. That would require 150 200-foot-long and 100 300-foot-long rebars, or 60,000 linear feet of number-3 steel. This is equivalent to 22,560 pounds (or 11 tons) of potentially recoverable steel. The market price of scrap steel (or any scrap metal) is highly volatile and varies greatly from region to region.
At the time of this writing, the price of scrap steel had varied from $255 to $395 per ton over the previous 12 months. Assuming an average price of $300 per ton, the steel in this slab would have a market value of $3,300.
For the most part, recycled concrete (after the steel reinforcement has been removed by magnets from the wastestream), is used as an aggregate. Though it can be used as the sole aggregate in a new batch of concrete mix, more often it is used as a mixture with virgin aggregate. However, its primary use as an aggregate replacement is as a stable sub-base layer underlying a new concrete or bituminous-asphalt pavement surface. Placed after the soil grades have been properly compacted, the concrete-debris sub-base is placed as the lowest layer of a new roadway. The surface course is then poured over it. In any case, recycled concrete debris has to be free of contaminants, such as organic materials, wood, steel, and other C&D waste before it can be used.
Author's Bio: Daniel P. Duffy, PE, writes frequently on the topics of landfills and the environment.