In-depth survey report: control technology for crystalline silica exposures in construction-effect of exhaust flow rate upon-respirable dust emissions for tuck pointing operations-preliminary evaluation-ventilated tool for brick cutting at international masonry institute, Cascade, MD, CT-247-18*.
As brick buildings age, mortar deteriorates and needs to be replaced. Before replacing the mortar, the existing mortar is removed to a depth of 0.5 to .75 inches. Typically, a 4-inch diameter grinder, operated at 10,000-12,000 rpm, is used to remove mortar. Mortar removal causes exposures to respirable crystalline silica concentrations as high as 10 mg/m3. A tool resembling a router can also be used to remove mortar. For four different shrouds, the effect of exhaust flow rate upon respirable dust emissions was experimentally evaluated. To conduct this testing, a small brick wall was built and enclosed in a hall-shaped, ventilated test chamber. The grinder was mounted on a mechanical trolley which moved the grinder horizontally down the wall at a constant velocity of approximately 1 m/min and the mortar was removed at a fixed cut depth of 0.5 or 0.75 inches. A vacuum cleaner equipped with high efficiency filters (99.9% at 0.3 microm) exhausted air from the shrouds to a location outside the enclosure. The vacuum cleaner's exhaust air flow was varied by controlling the voltage applied to the vacuum cleaner. An air flow rate of 2794 cubic feet per minute was drawn through the test chamber and past mixing baffles and into an exhaust duct. A time-of-flight aerosol spectrometer was used to measure the respirable dust concentration in the duct. Dust emissions per volume of mortar removed were plotted as a function of the exhaust flow rate. For uncontrolled grinding, respirable dust emissions were about 20 mg/cm3 of mortar removed. As flow rates increased, respirable dust emissions were reduced to under 0.2 mg of respirable dust per cm3 of mortar removed. For the 4-inch diameter grinding wheel and the router, 80 and 35 cubic feet per minute (cfm), respectively, were the minimum exhaust volumes which reduced respirable dust emissions to under 0.2 mg/cm3 of mortar removed. Further flow rate increases did not provide useful emission reduction. In addition, the facility used to evaluating dust control measures for tuck pointing was used to conduct a preliminary study of a dust control for the dry cutting of bricks. Generally, brick cutting is done wet. However, some contracts specify that bricks are to be cut dry. Dry cutting is specified due to concerns related to brick staining and reduced adhesive strength between the brick and the mortar. An Equipment Development Company (EDCO) masonry saw was used for the wet cutting of bricks and it was equipped with two exhaust take-offs. One exhaust take-off was located below the brick in an exhaust channel. The second exhaust take-off removed air from the blade guard located above the brick and spindle for blade. Vacuum cleaners were used to provide exhaust volume to both exhaust ports. The EDCO masonry saw was set in the enclosure and bricks were cut with and without ventilation. With no ventilation, the in-duct respirable dust concentration was 13 mg/m3. When 93 cfm was exhausted from the exhaust channel below the brick and 113 cfm was exhausted from the blade guard, the in-duct respirable dust concentration was 0.05 mg/m3. Because the lower exhaust channel had a cross-sectional area of about 1 inch square, the static pressure loss was greater than 30 inches of water. This exhaust channel needs to be redesigned to operate at a transport velocity of 4500 feet per minute (fpm) and enlarged to reduce the pressure loss. The results presented here for the 4" grinder and 10" inch brick saw indicate that a minimum exhaust flow rate of at least 20 cfm/inch of blade or grinding wheel diameter is needed to control the dust emissions. This is in agreement with the ACGIH recommendation that 25 cfm/inch of blade or grinder diameter are needed. This ACGIH recommendation should be used as the specification for choosing exhaust flow rates. The ACGIH recommendation involves a prudent amount of over design which compensates for equipment deterioration with age and unknown factors that influence the capture of dust.