Sand Blasting Concrete: Floors, Walls and Surface Profile

Sand blasting concrete: floors, walls and surface profile

Blasting takes concrete back to sound material and leaves it rough enough for a coating or a waterproofing membrane to bond. How rough is not a matter of opinion: it is specified against a ten-step scale, CSP 1 to CSP 10, and the work is often run wet to keep the dust down.

Flexihose sand blast hose: 175 PSI / 12 BAR working · 700 PSI / 48 BAR burst · 1/2" to 2" bore · DIN abrasion loss below 60 mm³ · Exceeds IS 5894

Why concrete is sand blasted

Concrete can be sand blasted, and on most jobs it is done for adhesion rather than for looks. The Institute of Concrete Repair’s guideline puts the goal of surface preparation as a list: remove laitance, dirt, oil, films, paint and coatings; remove contaminated or unsound concrete; open the pores; remove anything that will interfere with penetration, adhesion or bonding; and create a surface profile (ICRI Concrete Repair Bulletin, on Technical Guideline No. 310.2R-2013).

The same source is blunt about what happens when this is skipped. Poor surface preparation may lead to failure of the protective system or repair material, further repairs, added expense, loss of use of the structure, and compromised integrity of the structure. On a waterproofing job that list reads as a leak, a chase for the leak, and a second mobilisation.

In practice concrete arrives at the nozzle for four jobs. The sand blasting process is the same one used on steel, with the settings and often the media changed.

Concrete surface profile: what CSP 1 to 10 means

A concrete surface profile, written CSP, is the standard way of saying how rough a prepared concrete surface is. The scale comes from the International Concrete Repair Institute, in Technical Guideline No. 310.2R-2013, Selecting and Specifying Concrete Surface Preparation for Sealers, Coatings, Polymer Overlays, and Concrete Repair. The 2013 revision extended it to cover concrete repair, which is why a tenth profile exists: CSP 10 was added with an amplitude of greater than 0.25 in. (6 mm), alongside a new physical chip to match.

So the scale runs from CSP 1, minimal roughness, to CSP 10, very rough. The comparison is not made by eye against a photograph but against moulded chips: a set of physical replicas that are laid on the prepared surface so the profile achieved can be checked against the profile specified.

Two practical points come straight out of the guideline. First, a specification should name a range rather than a single number: ICRI’s own wording is that typically a CSP range should be used and included in the project specifications. Second, the number is not yours to choose. It comes from the manufacturer of the coating, overlay or membrane going on afterwards. The guideline’s worked example runs the other way round: a high build coating is wanted, the requirement is a profile of CSP 3 to 5, and the preparation method is then chosen to hit it.

Shot blasting concrete floors

On a flat floor of any size, shot blasting is usually the method of choice, and ICRI’s own worked example picks it for a reason worth understanding: it describes shotblasting as a contained, dust free system, capable of producing a CSP of 2 to 9. A ride-on or walk-behind machine throws steel shot at the slab, recovers it, separates the dust and reuses the shot, so it can run inside an occupied building.

Sand blasting earns its place where that machine cannot go or cannot suit: vertical and overhead surfaces, edges and corners a floor machine leaves behind, profiled or coffered soffits, small patch areas in a repair, and work where the abrasive has to be softer than steel shot. The difference between the media themselves, and what each does to the hose, is set out in our guide to shot, sand and grit blasting.

On many concrete jobs the honest answer is that both run: the shot blaster takes the open floor, and a blast hose and nozzle follow it into everything the machine could not reach.

Blasting concrete walls and exposed aggregate finishes

Walls are where blasting has no real competition. A floor machine works by gravity and containment, neither of which helps on a vertical face, so the nozzle does the work: stripping old paint and render from a facade, cleaning a retaining wall or a water tank before lining, and opening a profile on a column or a beam before a repair mortar goes on.

The appearance job is the same process taken further. Blasting away the cement paste at the surface uncovers the aggregate behind it, which is where the exposed aggregate look comes from, and how a blasted panel or a blasted band on a wall reads differently from the sawn or cast surface next to it. The depth of that cut is the same thing the CSP scale measures, so an appearance finish and a bonding profile are the same operation aimed at different acceptance criteria. Similar masking and contrast work on stone is covered in granite sand blasting.

On a wall the work is also bounded by what is behind it. Blasting removes material, and concrete over shallow cover or with corroded reinforcement close to the face is where an appearance job turns into a repair job. That belongs on the method statement, not on the day.

Wet blasting to keep the dust down

Concrete makes more dust than steel does, and on a live site that dust is the constraint that decides whether work can happen at all. Adding water to the abrasive stream is the standard answer. It is the same nozzle and the same line, with water introduced so the dust is knocked down as it is created rather than extracted afterwards.

This matters for the hose, because not every blast hose is built to run wet. The Flexihose catalogue names the duty directly: cleaning and blasting of casting metal, stone and concrete surfaces, with wet or dry sand or other abrasive material. Dry and wet are both inside what this hose is specified for. The types of blast system, wet and dry included, are compared in the sand blasting process.

There is a second reason to take the dust seriously, and it is not the abrasive. The Canadian Centre for Occupational Health and Safety lists abrasive blasting itself among the workplace activities that put workers at risk of silicosis, alongside cutting, drilling, grinding and demolishing, and it names wet methods and ventilation among the controls (CCOHS, Silicosis). In other words the exposure is attached to the operation, not only to what is in the pot, so changing the abrasive deals with one part of the problem and not all of it. What the Factories Rules require, including enclosure, ventilation and the medical surveillance that goes with this work, is in our guide to whether sand blasting is banned in India.

Preparing concrete before waterproofing

A waterproofing membrane is a coating with a lower tolerance for a bad substrate than most, because the failure shows up somewhere other than where it happened. ICRI’s guideline covers sealers, coatings, polymer overlays and concrete repair, and the preparation logic is the same across all of them: get to sound material, open the pores, and leave the profile the product asks for.

• Laitance comes off. The weak, fines rich skin at the surface is the first thing ICRI’s list names for removal, and a membrane bonded to laitance is bonded to something that is already failing.
• Unsound concrete comes out, not over. Blasting will find it. Hollow and delaminated areas sound different and are a repair, not a profile.
• The profile is the manufacturer’s number. Take the CSP range from the membrane data sheet, not from the blaster and not from the last job.
• Check it with chips, and record it. The comparison is made against the moulded CSP replicas, by area, and the result belongs on the record with the date.
• Then the conditions. Substrate moisture and ambient conditions gate the application itself; the sequence and the records for a blast and coat job are in our blasting and painting procedure.

Setting up the hose for concrete work

Concrete work is hard on a line in a way steel fabrication is not. The runs are long, because the compressor sits outside and the work is somewhere in the building. The hose gets dragged over slab edges, rebar starter bars and scaffold. And if the job is running wet, the line carries water as well as abrasive.

1. Size the bore to the nozzle, not to the drum. An undersized hose starves the nozzle and the pressure that matters is the pressure at the nozzle. The chain is worked through in our guide to sizing the compressor and nozzle.
2. Plan the run before uncoiling it. Standard coil length is 40 m, which on most sites reaches from the pot to the face without a joint. Every joint is a wear point and a place a whip check has to go.
3. Protect it where it crosses an edge. Slab edges and starter bars cut covers. A sleeve or a board at the crossing point costs nothing next to a hose.
4. Earth the assembly. Abrasive moving through a hose builds static. This hose carries a copper element that reduces static build-up, and the reason the whole line is bonded is explained in the parts of a blast line.
5. Drain and dry it after a wet job. Water left in a line with abrasive residue is how a lining gets a surprise.

Flexihose India (FLEXI HOSE), a brand of Vardhman Hydraulic Hoses Pvt. Ltd., Bhiwadi, Rajasthan, builds its sand blast hose to exceed IS 5894, with an extra thick seamless rubber lining free from mineral filler, high tenacity synthetic yarn braids and a cover resistant to abrasion, ozone and weather. It works at 175 PSI / 12 BAR with a burst of 700 PSI / 48 BAR, in bores from 1/2" to 2" (12.00 to 50.00 mm), from -40 C to +75 C, with DIN abrasion loss below 60 mm³. Standard coil length is 40 m. Full sizes are on the sand blasting hose page. It exceeds IS 5894 and is not BIS licensed to it, which is the wording to use when a contract asks. Which abrasive that lining has to survive is a separate decision, set out in choosing blasting media.

Blasting concrete wet or dry? Tell us the run

Give us the nozzle size, the abrasive, whether the job runs wet, and the length of run, and we will recommend the bore and send a quote.