Shot blasting is like power washing and sandblasting except it’s done dry and is dustless. The concrete surface is left with a sandpaper-like profile to which new coatings will readily bond. In the case of floors that already have a rough profile, and simply need to be cleaned, shot blasting has the ability to follow the contours as well as clean the pits, cracks, and joints too.
Concrete is a composite of aggregate, cement, and sand. As it’s placed, the concrete mixture is agitated so the aggregate (large stones) are pushed down and the sand (fine particles) is brought to the surface. These sand particles form the matrix which bear traffic and wears well against constant surface abrasion.
In the case of a well placed concrete slab, traffic friction is shared from sand particle to sand particle and the concrete stays intact. Concrete is designed to withstand traffic as it passes across it, but not necessarily from vertical abrasion or attack. Shot blasting exploits this design to weaken it and break away the topmost concrete layer.
Two Important Factors to Keep in Mind
The profile after shot blasting will be influenced by two factors: shot size and removal volume.
1) Shot size is basically achieved by the measurement of the steel media’s diameter based upon sieve separation. For simplicity, it’s labeled as an “S-number value.” The bigger the number, the bigger the shot size and the bigger the dent it will influence as the concrete sand is removed.
Think of a drill bit and the size opening made as it drills through a work surface. The bigger the bit, the more surface area available to remove the substrate particles per revolution and leave a bigger hole. Sizes available (expressed as “Sxxx“ and practically related to concrete shot blasting) are S280, S330, S390, S460 and S550.
2) Removal volume is the amount of the concrete’s surface that is removed and exposes that layer’s particle profile. The deeper a blast, the larger the sand particles and aggregates become and the profile thereby increases.
A shallow blast will expose the fine sand producing a concrete surface profile (CSP) of a #3-4 whereas a deeper blast will expose larger sand producing a CSP of a #4-5. Shallow blasting means less profile and deep blasting means more profile.