By Gustavo Gonzalez
Dust is a byproduct of almost every precast concrete operation. It is made up of very small particles suspended in the air, usually created by material movement; grinding, cutting and drilling concrete; mold cleaning; and many other processes common to our industry. While very large dust particles settle relatively quickly, very small particles can remain in the air for long periods of time.
The accumulation of dust in the air is unhealthy and could lead to skin, eye and throat irritations, as well as to respiratory diseases. But of all the dust particles encountered in a precast plant, none is more damaging than airborne “free silica.”
When silica is inhaled into the lungs very frequently or in high concentrations, it can cause a disease called silicosis, which is an irreversible scarring of the lungs. Over time this can produce breathing problems, and in some cases even death. It has also been thought that it can increase the risk of lung cancer.
A study conducted by the University of Washington, Field Research and Consultation Group, Department of Environmental Health, focused on how often certain activities resulted in overexposure to silica. The study found the following activity percentages exceeded the American Conference of Governmental Industrial Hygienists Threshold Limit Value (TLV) of 0.05 milligrams per cubic meter for silica when they were performed for a full shift:
- Concrete surface grinding 100%
- Concrete demolition 88%
- Concrete cutting 77%
- Clean-up (sweeping) 50%
- Patching concrete 40%
The Maximum Exposure Limit (MEL) for silica dust is 0.3 milligrams per cubic meter averaged over eight hours. Therefore exposures should be reduced as low as possible and for the shortest periods of time, otherwise the use of respirators is required.
NIOSH (the National Institute for Occupational Safety and Health) recommends the following:
- Exposure up to 0.5 milligrams per cubic meter requires a respirator with a high-efficiency particulate filter.
- Exposure up to 1.25 milligrams per cubic meter requires a powdered respirator with a high-efficiency particulate filter.
The use of respirators at the workplace is regulated by OSHA 1910.134 and by Canadian Standard Association standard Z94.4-93. The services of an Industrial Hygienist are required to conduct a study of the amount of airborne silica in the plant; once the study is done, the recommendations should be adopted.
As mentioned earlier, there are many sources at the plant that contribute to dust. With aggregate, this can include deliveries, storage piles, transferring by conveyor or mechanized equipment, weigh hopper loading, and mixer or mix truck loading. Dust also comes from concrete spilled from pouring buckets; pieces of concrete chipped away while moving or loading; patching, drilling, cutting or grinding; and cement or pozzolan unloading operations. And don’t forget vehicular traffic, whether on paved or unpaved roads.
All of these activities contribute to the amount of dust present in the environment and are the significant generators of visible and invisible particulates in the air that we breathe. Not only is dust a nuisance and a health hazard for our employees, it may also create a problem with our neighboring businesses and surrounding community, which could have a negative impact with the public in general.
One indication of how important dust control is to our neighbors is the large number of agencies with the power to regulate it. Dust in the environment, or air emissions, not only falls under the jurisdiction of the Environmental Protection Agency and the Canadian Environmental Protection Agency, but states and provinces and local authorities all have their say. Those government agencies enforce regulations and/or issue the necessary permits to operate our plants under strict guidelines.
There are many ways to control or mitigate dust. While in some cases it may not be possible to eliminate dust entirely, at least we can direct our efforts to reduce as much as possible the amount of dust in the air. Most of these efforts should be in the areas of management and engineering controls, and the creation and implementation of Best Practices for plant personnel.
A dust study should be conducted to identify those areas or procedures that contribute to the creation of dust and arrive at the root cause of the problem. Based on the findings, you can then generate the required controls or procedures to reduce the amount of dust in your facility. Educate your workforce as to the importance of controlling dust by following the proper procedures and the benefits of working in a clean environment.
Housekeeping procedures should be at the top of the list, because a clean plant is generally a safe plant. Pick concrete up off the floor while it is still wet, reduce or eliminate over-pours by teaching proper pouring techniques, and enforce tighter quality control pre-pour inspections to reduce the need for product repairs and the use of dust-generating tools. Use vacuum equipment rather than brooms, and perform pressure washing whenever possible. Any concrete cutting should be done using the “wet” method.
Other procedures could involve keeping the conveyors free of spills and buildup, and material accumulation around equipment should be kept at a minimum and removed continually. Spills in the mixer discharge area, transfer cart or grizzly area and around the bins should be promptly cleaned of any buildup. Any cement left on the floor after unloading operations should be picked up immediately, and loaders should not travel with a full bucket to eliminate material spillage. Develop methods to eliminate the overflowing of cement bins, and avoid the use of compressed air to clean floors.
Moisten stockpiles of fine and coarse aggregate to eliminate dust during transfer operations, especially during the dry months of the year. This would also help in reducing the temperature of the stock during the hot summer months. However, do not wet the stockpile so much that it runs off and creates another environmental problem.
In yard operations, dust can be reduced by lowering the speed limit. On paved surfaces, mechanical broom sweepers can eliminate dust accumulation. In unpaved areas, you can achieve good results with the use of stabilizers, such as lignens and chlorides, which tend to keep the ground moist. It is recommended that you check with your local environmental authorities before using any of these products, as their use may not be authorized or could be regulated.
Equipment maintenance is another important factor, especially where a baghouse or dust collector is involved. This is perhaps the most important piece of equipment in controlling dust and air emissions but is perhaps the most overlooked. In some plants, the baghouse gets attention only when it is time to perform the annual air emissions test, and then it falls back into disrepair until the following year or until the local environmental agency investigates a complaint.
Procedures must be in place to ensure that the baghouse is shaken after each delivery of cement to the plant, otherwise it could be rendered useless.
Keep the outside of the container clean and take care of all leaks immediately. The cement collector, if any, should be emptied at regular intervals so it does not back up into the baghouse. Keep a new set of bags in stock at all times so they can be replaced at a moment’s notice. An inherent problem with most baghouse installations is that they are placed in the worst possible place, which is on top of the silo. Baghouses should be installed as close to the floor as possible in order to be easier to service and repair as needed. This also eliminates the need for employees to climb 70 or 80 feet in the air to work on them.
Form and mold maintenance and gasket replacement eliminates leaks of concrete paste on the floor. Check the buckets to make sure the gate stays closed while transferring concrete, and clean them at the end of any pouring operation, paying special attention to the pouring mechanism and gate. Inspect all areas around the mixer or mixing truck loading area to ensure all dust control equipment is functioning properly and there are no visible emissions.
A well-designed dust control plan must start at the management level by establishing and enforcing a dust control policy and creating a safety program. Plant personnel must be educated and trained in such a program, and they must follow it at all times.
Housekeeping is of the outmost importance in any precast operation. The accumulation of dirt and clutter not only projects a “we do not care” attitude, but it also contributes to lower employee morale and higher accident rates.
A dust-free plant is a healthy and therefore more productive plant with less absenteeism, less employee turnover and fewer accidents.
Gustavo Gonzalez has nearly 20 years of experience in the precast industry. He has a bachelor’s degree in Industrial Engineering Technology from Florida International University and has been employed as a plant manager for both Quikrete of Miami and Oldcastle Precast Inc. He currently serves as an instructor for the National Safety Council and the National Precast Association.