Dry sandblast cleaning is a relatively new method of cleaning newly built masonry, although the system has been used for many years in masonry restoration work.
Many architect/engineers prefer sandblast cleaning over conventional wet (acid) cleaning because of possible adverse acid reactions with certain types of brick. Other designers are reluctant to permit sandblast cleaning for fear the blasting will erode the face of the brick and mortar joints.
Sandblast operators can be compared with other construction tradesmen: some are artisans and others are incompetent. However, with a qualified operator, proper specifications and good job inspection, sandblast cleaning is as good as any other system and is sometimes superior in many ways.
Sandblast Cleaning Equipment
Basically, sandblast cleaning involves the following equipment: portable air compressor, blasting tank, blasting nozzle, operators' protective clothing, and hood.
Air pressure delivered by the compressor to blasting tank may range from 40 lbs. to 100 lbs. per square inch. Blasting tank is charged with the specified abrasive material and pressurized to force the mixture of abrasive material and air into blasting hose and to the nozzle.
Blasting pattern is determined by nozzle size, type, and air pressure. Speed of cleaning is determined by type of abrasive used, nozzle size, type, air pressure, nozzle-to-wall distance, and condition of the surface to be cleaned.
Material Used in Cleaning
Abrasive material used in brick cleaning is usually sand, quartz, or granite and must be clean and finely graded.
Sandblast cleaning material should conform to one of two particle size graduations outlined in the specifications below.
Type "A" gradation is to be used when the masonry is very lightly soiled or when only a very light or fine texturing of the brickwork is permitted.
Type "B" gradation is used for cleaning heavy mortar stains from brickwork and where medium texturing of the masonry is permitted.
Sandblast Cleaning Uses
Sandblast cleaning may be used for cleaning burned, non-glazed, smooth, or textured brick. Included in this category are reds, buffs, whites, grays, chocolates, and more.
Lightly sanded, coated, slurry, or sandbox brick should not be cleaned by sandblasting, unless cleaning cannot be accomplished by any other method, as the brick face can be permanently damaged.
Handmade or reclaimed brick may also be permanently disfigured by sandblasting.
As a further precaution, approval of the brick manufacturer must be obtained before permitting sandblast cleaning.
How to Sandblast Clean Brick
Wait for the mortar to harden. Brickwork should be completely dry and at least seven days old, preferably 14 days.
Remove all large mortar particles with hand tools before blasting. Use a wooden paddle, the rough edge of a brick, or a metal scrape hoe. Chisels may be used if necessary to remove hardened mortar or concrete. This "pre-cleaning" is a very important part of sandblast cleaning. A sandblast operator would irreparably damage a wall if large droppings are left for him to remove by blasting.
Provide adequate protection for all non-masonry surfaces adjacent to work areas, Use plastic sheeting and duct tape to protect windows, doors, etc. If possible, painting and caulking should be done after the sandblast operation is completed.
When all surfaces are prepared and protected, the operator can begin a first test cleaning.
The operator should clean a small area with the nozzle first close to a wall, and then at varying distances from the wall, trying to select a working distance that will give the best cleaning job with the least damage to brick and mortar work.
A job superintendent and architectural inspector should be present at this time to confirm the acceptable practice. Approved areas should be marked and identified as an acceptable standard for the entire job.
Specifications for Sandblast Cleaning
This section includes cleaning of newly constructed clay masonry with dry abrasive material forced by compressed air from tank through hose and nozzle.
Cleaning material must be dust-free and abrasive. Hardness should be approximately 6 on Mohs' Scale. Material size shall conform to one of the two categories listed below according to acceptable finish of masonry surface.
Type "A" (Fine Texturing)
Typical Screen Analysis
|U.S. Sieve Size
The following material is acceptable for "fine texture" sandblasting: Blast Sand Size No. 120 furnished by KMG Minerals, Inc., Kings Mountain, NC.
Type "B" (Medium texturing) For concrete work and extremely difficult masonry cleaning jobs.
Typical Screen Analysis
|U.S. Sieve Size
The following material is acceptable for "medium texturing" sandblasting: Blast Sand No. 55 furnished by KMG Minerals, Inc., Kings Mountain, NC. Local materials may be used when dried and screened to meet required size and hardness and when determined to be free of grease or other impurities.
The air compressor must be capable of producing pressure between 60 pounds and 100 pounds per square inch at the machine and should have a minimum air flow capacity of 125 cubic feet per minute.
Nozzle inside orifice or bore size may vary from 3/16" diameter to 5/16" diameter.
Sandblast machine (or tank) must be equipped with controls to regulate flow of abrasive materials to nozzle, and shall be capable of supplying sand at a minimum rate of 300 pounds per hour.
Warning: The operator must wear O.S.H.A.- approved hood and protective clothing.