Preparing for Concrete Abrasive Blasting
The use of abrasive blasting on concrete can provide unique texturing effects as well as an effective bond for coatings. Abrasive blasting may be used to lightly clean the surface of concrete or to cut deep to expose aggregates. The practice removes any weak surface layers and opens surface voids, air pockets, and other flaws to produce a clean, strong substrate. Following basic methods for preparing concrete can lead to great abrasive blasting results.
Achieve the Right Concrete Mix for Good Abrasive Blasting
The correct mix for concrete consists of water, approximately 10 to 15% cement, and approximately 66 to 78% aggregates. Mixing water with cement produces a plastic mass; as the water chemically reacts with the cement, it hydrates and hardens that mass. The hydrating cement binds together with fine or coarse aggregate to provide the strength needed for the mixture to become concrete. Additional admixture, such as petroleum acid salts or synthetic detergents, may complement the mix of aggregate, water, and cement to accelerate or decrease the setting and hardening time for the concrete and to add air entrainment.
Working with a concrete mix provides a better surface for abrasive blasting. Air-entrained concrete contains controlled amounts of air that forms tiny bubbles throughout the concrete. Air entrainment enhances the concrete for better resistance to freezing and thawing conditions as well as scaling. Air-entrained bubbles provide space for expansion within the concrete, preventing the ruptures that leads to surface scaling. In addition, the application of air entrainment increases the workability of the concrete while decreasing slump.
Slump measures the consistency of fresh concrete, serving as one indicator of its quality and workability. Without the addition of admixtures, the amount of slump depends on the water content. The blending of air, water, cement, and aggregates with admixtures, fibers, and polymers creates a plasticized slump that places less dependence on water content.
Gap grading the concrete mix eliminates intermediate sizes of coarse aggregates, retaining more desirable larger aggregates. Coarse aggregate consists of gravel, crushed granite, crushed quartz, crushed limestone, and other types of crushed rock. Gap grading should yield a concrete mix that has lower quantities of sand, low slump, and additional cement content.
Test for Best Results
Although abrasive blasting removes many surface imperfections, it cannot remove major flaws. For example, unsealed form joints result in lines that remain visible. To seal joints, use pressure-sensitive sealing tape or a caulking compound. In addition, form joints that fit loosely and allow leakage also prevent problems when it comes to abrasive blasting. Good forming techniques prevent leakage, bulging between form supports, and uneven surfaces. Visual inspections of concrete surfaces can indicate whether abrasive blasting will produce the desired results; however, selecting a sample for blasting can disclose the presence of any problem areas and show the texture and color tone of the finished work.
Select the Right Machine and Abrasives for the Application
The proper equipment for abrasive blasting provides the best results for the specific application. When selecting equipment, consider the quantity and size of the abrasives, the desired speed, and the size of the machine. Abrasive size determines the amount of force that an abrasive, such as silica sand, exerts on the concrete surface.
A large machine may provide efficient and quick work for open surface areas such as walls or floors. In contrast, though, a larger machine will not provide the flexibility needed for confined spaces or tight corners. Center-fed abrasive blasters utilize mechanisms that flow the abrasive into the center of a centrifugal wheel, providing greater velocity. However, the center-feeding mechanism may not yield an even distribution of the abrasive. Paddle-fed machines use a mechanism that feeds a wheel from the side; while paddle-fed machines give even distribution, they throw with less velocity.
Observe Best Practices
Abrasive blasting uses compressed air to hurl abrasive granules at a surface. The force of the abrasive hitting concrete requires best practices and the use of the proper personal protection equipment (PPE). When preparing a work area, clear any objects that could become hit by stray sand and cover any exposed areas. Ensure that the work space has good ventilation; because the abrasives may contain silica, take special precautions to avoid inhaling dust. Use an approved respirator, wear safety goggles, and wear a complete coverall.
When starting an abrasive blasting job, set the machine to a lower pressure first. Position the machine nozzle eight to sixteen inches from the body and point the nozzle at the concrete surface. Increase the pressure setting for the machine after a few passes to free the concrete of dirt and other materials. Use short bursts for corners and tight areas and sweeping movements for larger areas. After completing the abrasive blasting task, use a vacuum to remove dust and debris from the surface.