Recently, the Center for Construction Research and Training supported an examination of the concrete-sawing issue, and the results have appeared in the Journal of Occupational and Environmental Hygiene. Drs. Susan Shepherd and Susan Woskie, with assistance from the New England Laborers Training Center, performed an exhaustive series of tests of airborne silica exposures encountered by workers cutting reinforced concrete pipe with gas-powered portable concrete saws.
The findings are important, if not entirely surprising: using a hose or sprayer reduced airborne crystalline silica concentrations by 85% over dry cutting. That’s no small matter. For a laborer employed by a contractor specializing in “wet” utilities, “wet cutting” could be the difference between a long and healthy retirement and one plagued by silicosis and COPD.
Another finding hammers the point home. “Concrete cutting is often a short-period task and is typically assumed to be less than the PEL [OSHA Permissible Exposure Limit],” Woskie and Shepherd observe. “It is important to note that 11 tests yielded respirable crystalline silica exposures above the OSHA PEL in spite of the short sampling times.”
Volunteers from the New England Laborers Training Center were recruited to participate in a field experiment examining dust reductions through the use of water, from a hose and from a sprayer, as a dust control. In four series of tests, reinforced concrete pipe was cut under both “dry” and “wet” control conditions. Overall, the geometric mean respirable dust concentration for “dry” cutting (14.396 mg/m3) exceeded both types of water-based controls by more than tenfold.
Wet cutting reduced the respirable dust concentration by 85% compared with dry cutting when comparing tests paired by person and saw blade (n = 79 pairs). Using a respirable cyclone, a total of 178 samples were taken. Due to the high variability in dust exposure found in this and other studies of saw cutting, the data were examined for potential exposure determinants that contribute to that variability. Using mixed models, three fixed effects were statistically significant: control condition, worker experience, and location. A random effect for subject was included in the model to account for repeated measures. When each of the significant fixed effects was included with the random effect, it was apparent that inclusion of worker experience or location reduced the between-worker component of exposure variability, while inclusion of control condition (wet vs. dry) explained a large portion of the within-subject variability. Overall, the fixed effect variable for control condition explained the largest fraction of the total exposure variability.
Top Four Fatality Causes
OSHA says the top four causes of construction fatalities are falls, struck-by, caught in or between, and electrocutions. To prevent falls, wear and use personal fall-arrest equipment. Install and maintain perimeter protection. Cover and secure floor openings and label floor opening covers. Use ladders and scaffolds safely.
Experts say that heavy equipment backing over someone is still a major cause of accidents. To prevent struck-by incidents, never position yourself between moving and fixed objects. Wear high-visibility clothes near equipment/vehicles.
To prevent caught-by or in-between accidents, never enter an unprotected trench or excavation 5 feet or deeper without an adequate protective system in place. Some trenches under 5 feet deep may also need such a system. Make sure the trench or excavation is protected either by sloping, shoring, benching, or trench shield systems.
Electrocutions cause many fatalities. Look for overhead power lines when operating any equipment. Locate and identify utilities before starting work. Maintain a safe distance from power lines; learn the safe distance requirements. Do not operate portable electric tools unless they are grounded or double-insulated. Use ground-fault circuit interrupters for protection. Be alert to electrical hazards when working with ladders, scaffolds, or other platforms.
Shooting Down the Dust
According to CEO Edwin Peterson, the unit was developed specifically for large-scale mining, coal operations, and bulk material handling. “It was originally an idea of our late cofounder, Barry Brown,” Peterson explains. “He had the vision to create a mine-grade, open-pit, dust-suppression design that was extremely powerful and exceptionally reliable. Although he didn’t get to see it built, I know he’d be proud to see it become a reality.”
To boost reliability of the DB-100 Dust Boss, DCT engineered an extremely rugged and efficient design. Like all of the company’s fan-driven atomizers, the DB-100 delivers both power and dependability by using a straightforward, direct-drive fan motor. It has no drive belts, diesel engines, or complex worm gear systems that could present breakdown problems.
Despite its impressive size, the DB-100 requires minimal maintenance, with lubrication of fan motor bearings recommended every 10,000 hours, while the oscillation motor is lubricated for life. All DustBoss models are covered by a 3-year/3,000-hour warranty, and complete satisfaction guarantee.
The new design represents the culmination of two years’ development time and is based on 40 years of engineering experience in atomized mist technology. “Effective dust suppression is a complex equation,” Peterson says. “In addition to droplet size and air speed, there are dozens of variables that affect a machine’s performance. The size and precise shape of the barrel are critical, for example. A successful design must control the laminar air flow to achieve the optimal plume and distance.
“But the design really is the sum of all its parts, in that every component is specifically engineered to maximize performance. That includes the integrated fan and nozzle components, inlet valves, propellers, vanes, pumps, and filters. Failure to optimize any one of those details will detract from the unit’s effectiveness.”
The new machine’s 60-horsepower fan is fed by a manifold of 30 brass nozzles that are specifically sized and positioned for the new design. The DB-100 features simple, user-defined 359-degree oscillation, along with adjustable elevation from -7 degrees to 45 degrees. It can also be outfitted with a dosing pump to accurately meter in surfactants or tackifiers to further enhance binding of dust particles. A 10-horsepower (7.5-kW) booster pump elevates water pressure as high as 200 psi for outstanding droplet production, and the DB-100 can be set up to run water, potable or otherwise.
Because it can run unattended for long periods, the DB-100 can free up manpower and equipment by eliminating the need for water trucks or sprinklers, delivering a short payback period for many users. The versatile device is also well suited to automated control, and can be wired to operate according to a wide range of sensor inputs, including weather conditions and even specific air particulate levels.
Any DustBoss machine is available with the company’s proprietary Variable Particle Sizing technology, providing customers with a wide selection of different nozzles for suppressing a broad range of airborne solids.
