To pressure wash exposed aggregate, pre-treat stains, use moderate pressure with high water flow, keep a wide nozzle moving 12-18 inches above the surface, and rinse thoroughly. A 2,500-3,000 PSI machine producing about 3-4 GPM is a typical starting range, but a test patch and the concrete’s condition determine the safe setting.
Key Facts / At a Glance
Exposed aggregate can lose decorative stones when concentrated water cuts the surrounding cement paste.
A 25-degree or 40-degree nozzle is safer than a 0-degree or turbo nozzle for routine cleaning.
Organic growth usually responds better to diluted sodium hypochlorite and dwell time than to extra pressure.
A 15-inch surface cleaner generally needs about 2.5 GPM to spin effectively and clean evenly.
Allow washed concrete to dry for at least 24-48 hours before applying most sealers.
A test patch is more reliable than the pressure rating printed on a rental machine.
What Exposed Aggregate Concrete Is
Exposed aggregate concrete is a slab whose upper cement paste has been removed or controlled to reveal embedded stones such as quartz, granite, or river gravel. The visible stones are not merely sitting on top; their long-term stability depends on the bond between each stone and the surrounding cement paste.
The finish creates a more irregular cleaning surface than ordinary broom-finished concrete. Soil settles in the spaces around the stones, while algae can remain in shaded pores after the visible film has been rinsed away. A strong, narrow spray may therefore remove contamination and sound paste at the same time.
Exposed aggregate varies substantially in risk. Older slabs with dense paste may tolerate more cleaning than a recently installed slab with weak edges, open voids, or previous freeze-thaw damage. Look for loose stones, hollow-sounding areas, scaling, cracks, and sandy paste before choosing equipment.
Why Flow and Chemistry Matter
Pressure, measured in PSI, determines impact force; flow, measured in GPM, determines how much loosened soil the rinse can carry away. For aggregate, a moderate-pressure machine with 3-4 GPM often cleans more uniformly than a low-flow machine operated close to the surface.
Chemical pre-treatment reduces the force needed to remove biological films. Sodium hypochlorite oxidizes algae, mold, and mildew, while alkaline degreasers emulsify many petroleum residues. Neither product repairs weak concrete, and neither should be allowed to dry on the slab or surrounding plants.
Which Equipment Should You Use?
Choose equipment according to surface area, contamination, concrete condition, and available flow. A small electric washer can handle light dust on a patio, while a gas washer paired with a correctly sized surface cleaner is usually faster for a driveway.
| Equipment | Typical specification | Suitable area or task | Main limitation |
|---|---|---|---|
| Electric washer | 1,500-2,300 PSI, 1.2-2.0 GPM | Patios up to about 300 square feet | Slow rinsing and weak surface-cleaner performance |
| Gas washer | 2,800-4,000 PSI, 2.5-4.0 GPM | Driveways, patios, heavy soil | Excess impact can loosen stones |
| 15-inch surface cleaner | About 2.5-3.5 GPM | Flat areas of 300-1,000 square feet | Poor spinning below its minimum flow |
| 20-inch surface cleaner | About 3.5-4.0 GPM | Larger flat areas | Can leave rings if moved too slowly |
| Pump sprayer | 1-2 gallons | Applying cleaner or diluted biocide | Requires separate rinse equipment |
| Garden hose | Household supply, commonly 5-10 GPM | Pre-wetting and final plant rinse | Does not replace pressure washing |
Match Nozzles to the Surface
Use a 25-degree green tip for controlled cleaning and a 40-degree white tip when the aggregate is weathered, soft, or already losing stones. Never use a 0-degree red tip for routine aggregate cleaning. A turbo nozzle can create rapidly changing concentrated impacts and is especially risky on exposed edges.
Nozzle color is not a complete safety specification because actual impact also depends on machine pressure, flow, orifice size, spray angle, distance, and travel speed. Start farther away than expected, test a hidden area, and reduce impact if the paste becomes visibly etched.
Before You Start
Inspect, protect, and measure the work area before connecting the pressure washer. Do not pressure wash exposed aggregate that is less than 28 days old, and postpone cleaning when the slab has active scaling, crumbling paste, or stones that move under light hand pressure.
Before-start checklist
| Item | Typical requirement | Why it matters |
|---|---|---|
| Curing age | At least 28 days | Early concrete can lose paste and surface strength |
| Work time | 2-4 hours for a 600-square-foot DIY driveway | Includes setup, treatment, washing, and cleanup |
| Water supply | Continuous flow near machine demand | Prevents pump starvation and pressure fluctuation |
| Cleaner dwell | 10-15 minutes | Gives chemistry time to loosen organic soil |
| Drying before sealing | 24-48 hours of dry weather | Reduces trapped moisture and whitening |
| Protective equipment | Eye protection, gloves, long sleeves, hearing protection | Controls splash, chemical, and noise exposure |
Plan Runoff Before Cleaning
Protect nearby grass, shrubs, painted walls, vehicles, pets, and storm drains before applying chemicals. Pre-wet plants with clean water, cover sensitive fixtures, and direct dirty water toward an approved collection or drainage route rather than allowing concentrated cleaner to enter a stormwater system.
Local requirements differ. Municipal stormwater rules may prohibit detergent, oil, or chlorine discharge into a street drain. Follow the product label and local wastewater instructions, especially when removing oil, old sealer, or heavily contaminated sediment.
How to Pressure Wash Exposed Aggregate
The safest sequence is inspection, dry debris removal, pre-treatment, low-impact washing, complete rinsing, and optional post-treatment. The most important control is consistent distance and movement, because a stationary spray can damage aggregate within seconds.
Step 1: Remove Debris and Establish a Test Patch
Clear furniture, vehicles, toys, hoses, and loose decorations. Sweep or blow away gravel, sticks, and leaves, then inspect cracks, control joints, edges, drains, and steps.
Choose a 2-by-2-foot inconspicuous test patch. Wet it, apply the intended cleaner, rinse with the intended nozzle and distance, and allow it to dry before expanding the work. You have a usable setting when the paste remains intact, stones do not move, and the cleaned area matches the surrounding color without stripes.
Common mistake: starting in the center of a visible driveway. A failed test patch at the edge is easier to conceal and correct.
Step 2: Apply the Correct Pre-Treatment
Apply cleaner to a damp surface, not a hot, completely dry slab. For organic growth, a typical starting solution is 1-3% active sodium hypochlorite, with a compatible surfactant used according to its label.
Commercial pool shock may contain 10-12.5% sodium hypochlorite. A 1:3 dilution of 10% solution with water produces approximately 2.5% active solution before accounting for storage loss and measurement error. Never mix hypochlorite with acids, ammonia, or other cleaners.
For oil, apply an alkaline concrete degreaser at the label dilution, agitate with a stiff nylon brush if needed, and keep the area wet during dwell. Allow most cleaners 10-15 minutes, but do not let the product dry. Reapply lightly if evaporation begins.
| Contamination | Preferred treatment | Typical dwell or repeat action | Avoid |
|---|---|---|---|
| Algae or mildew | 1-3% active sodium hypochlorite | 10-15 minutes, then rinse | Stronger pressure as the first response |
| Soil and pollen | Neutral concrete detergent | 5-10 minutes | Dry sweeping after wetting |
| Oil or grease | Alkaline concrete degreaser | 10-20 minutes, repeat if absorbed | Acid as a substitute for degreaser |
| Rust | Labeled rust remover after testing | Product-specific, often 2-10 minutes | Leaving acid on cement paste |
| Old acrylic sealer | Dedicated sealer stripper | Product-specific, often 15-60 minutes | Trying to remove it with a narrow jet |
Common mistake: using household dish soap as the primary degreaser. A small amount of compatible surfactant can improve cling, but dish soap does not reliably dissolve deeply absorbed petroleum.
Step 3: Set Pressure, Distance, and Spray Pattern
Set the machine to a moderate starting pressure, typically 2,500-3,000 PSI at roughly 3-4 GPM, then test the surface. Keep a wand tip 12-18 inches away and hold it perpendicular to the slab rather than aiming along the stone edges.
Move in smooth, overlapping passes at a consistent walking speed. Keep the tip moving before pulling the trigger, and release the trigger before stopping. A 25-degree tip provides more cleaning force than a 40-degree tip, so use the 40-degree option when the test patch shows paste erosion or stone movement.
Success checkpoint: after one pass, soil should lift without exposed bright lines, missing stones, or a roughened cement halo.
Common mistake: swinging the wand in an arc. The distance becomes shorter at the center of the swing, creating zebra stripes and localized erosion.
Step 4: Use a Surface Cleaner on Broad Flat Areas
A surface cleaner is usually the best tool for a broad driveway because its enclosed spinning bar holds two nozzles at a nearly constant height. Pull the trigger while the attachment is moving, walk in straight overlapping lanes, and avoid leaving the machine stationary.
Match attachment width to flow. A 15-inch unit commonly performs well at 2.5-3.5 GPM, while a 20-inch unit generally needs approximately 3.5-4.0 GPM for consistent rotation. A machine that cannot maintain the manufacturer’s required flow may produce uneven cleaning and dark bands.
Overlap each lane by about 2-4 inches. At the end of a pass, lift or turn only after releasing the trigger if the manufacturer’s instructions require it. Use the wand around steps, corners, vertical edges, and narrow joints.
Common mistake: moving too slowly. Circular marks often appear when the spray bar remains over one area longer than adjacent areas.
Step 5: Rinse Slurry and Chemical Residue
Rinse from the highest section toward the planned drainage point, working in manageable sections. Use broad overlapping passes, then inspect joints and gaps where loosened soil collects between stones.
Rinse plants, walls, equipment, and nearby hardscape with clean water. Do not leave a chlorine solution or detergent film to dry on the aggregate. A final low-pressure rinse is often more useful than increasing PSI during the cleaning pass.
Step 6: Apply Optional Organic-Growth Treatment
A light post-treatment can reduce rapid return of algae on shaded, damp concrete. Apply a label-compliant diluted biocide to clean, damp aggregate, allow the specified contact time, and avoid rinsing only when the product label permits that procedure.
Post-treatment is not appropriate when runoff cannot be controlled, plants are highly sensitive, or the product label requires immediate rinsing. It also does not replace drainage, sunlight, or removal of leaves that keep the surface wet.
When Should You Seal Exposed Aggregate?
Seal exposed aggregate only after the concrete is clean, structurally sound, and dry throughout, usually after 24-48 hours of suitable weather. A breathable acrylic concrete sealer can reduce water and soil penetration, but sealing a damp slab can trap moisture and create cloudy or white areas.
Test for residual sealer by placing a few drops of water on several locations. Rapid darkening suggests absorption; slow beading may indicate an intact coating. A failed, peeling, or whitening sealer often needs chemical stripping and controlled removal rather than more pressure.
| Sealer condition | Recommended action | Typical timing | Appearance trade-off |
|---|---|---|---|
| No sealer, sound slab | Apply compatible concrete sealer | After 24-48 dry hours | Natural or wet-look finish |
| Intact acrylic sealer | Clean gently and recoat only if compatible | Manufacturer-specific | May deepen color |
| Peeling acrylic | Strip and test before recoating | Often 1-2 days plus cure | More labor and waste |
| Damp or cloudy slab | Delay sealing and improve drying | Additional 24-72 hours | Prevents trapped moisture |
| Crumbling surface | Do not seal as a repair | Repair first | Sealer cannot restore lost paste |
Always follow the sealer manufacturer’s coverage and temperature limits. Solvent-based products can intensify color and odor, while water-based products may preserve a less glossy appearance.
What Should You Do in Different Conditions?
Adjust the method for temperature, shade, contamination, and surface condition rather than using one setting everywhere. Hot sun shortens chemical dwell, cold weather slows drying, and shade often requires a longer biological-control plan.
- Hot, sunny concrete: work in small sections, pre-wet the slab, and prevent cleaner from drying.
- Cold weather: avoid freezing conditions and allow additional drying before sealing.
- Deep shade: expect algae to return unless drainage, leaves, and persistent moisture are addressed.
- Light dirt only: use neutral detergent and the least aggressive rinse that works.
- Heavy algae: increase dwell time within the product label before increasing pressure.
- Loose aggregate: stop pressure washing and use gentle brushing or professional assessment.
- Old sealer: perform a solvent or stripper test; do not assume high pressure will remove it evenly.
Pressure Washing vs. Safer Alternatives
Pressure washing is effective for sound exposed aggregate with embedded soil, but soft washing or manual scrubbing is safer when the cement paste is weak. The correct method depends on whether the contamination is biological, oily, superficial, or attached to a failed coating.
| Method | Best use | Typical equipment | Main risk |
|---|---|---|---|
| Pressure washing | Sound slab with bonded soil | 2,500-3,000 PSI, 3-4 GPM | Paste erosion and stone loss |
| Soft washing | Algae on delicate or weathered surfaces | Low-pressure pump, diluted biocide | Chemical plant damage |
| Manual scrubbing | Small stains and fragile edges | Nylon brush, detergent, hose | Labor and uneven appearance |
| Surface-cleaner washing | Large flat sound areas | 15-20-inch attachment | Rings from slow movement |
| Professional stripping | Failed acrylic sealer | Chemical stripper, recovery tools | Chemical exposure and waste |
Acid is not a general-purpose shortcut. Muriatic or other strong acids can dissolve cement paste, alter color, expose aggregate unevenly, and damage metals or plants. Use an appropriate labeled product only when the stain and surface test justify it.
Common Failures and How to Fix Them
Stones Start Popping Out
Stop immediately, move the wand to 18-24 inches, switch to a 40-degree tip, and retest. If stones continue moving, discontinue pressure washing because the bond or surrounding paste may already be compromised.
Dark Wand Stripes Remain
Re-wet the area and clean using straight, overlapping passes at a constant height. A surface cleaner usually produces a more uniform result than trying to correct a large field with an arcing wand.
Green Growth Remains
Do not respond by moving closer. Apply a compatible diluted sodium hypochlorite treatment, allow 10-15 minutes of wet dwell, and rinse gently. Persistent growth may indicate porous contamination or inadequate contact time.
Oil Still Looks Dark
Oil can penetrate below the visible surface. Repeat an alkaline degreaser treatment, use absorbent material where the product allows it, and accept that old petroleum staining may lighten rather than disappear completely.
White or Cloudy Sealer Appears
Stop adding sealer and allow the slab to dry. Moisture, incompatible coatings, excessive product, and low temperatures can all cause whitening; correction may require stripping after the coating has cured.
Aggregate Looks Bleached
Uneven chemical dwell or premature drying may have altered the appearance. Rinse thoroughly, compare dry sections, and avoid applying additional chemicals until the surface is fully evaluated.
How Much Does It Cost?
DIY cleaning commonly costs $20-$50 for chemicals and $90-$130 for a gas machine plus surface-cleaner rental. Professional cleaning commonly falls near $0.25-$0.45 per square foot, although access, oil, failed sealer, water recovery, and regional labor rates can change the quote.
| Project | DIY equipment and supplies | Typical DIY time | Typical professional price |
|---|---|---|---|
| 300-square-foot patio | $35-$90 | 1.5-3 hours | $75-$150 |
| 600-square-foot driveway | $110-$180 | 3-4 hours with a wand | $150-$300 |
| 1,000-square-foot driveway | $120-$220 | 3-5 hours with a surface cleaner | $250-$450 |
| 1,500-square-foot driveway and patio | $140-$260 | 4-6 hours | $375-$675 |
| Sealer application | $0.20-$0.60 per square foot in materials | 1-2 days including drying | $0.75-$2.00 per square foot installed |
These are typical planning ranges, not fixed market prices. A professional may finish a 600-square-foot driveway in about 30-45 minutes with commercial equipment, but setup, chemical dwell, rinsing, and cleanup still affect the total appointment.
How Often Should You Clean It?
Annual cleaning is reasonable for shaded, damp, or heavily trafficked exposed aggregate, while a sunny, well-drained surface may need only occasional cleaning every 18-36 months. Clean when organic growth or embedded soil appears, not simply because a calendar date arrives.
Regular sweeping reduces the organic debris that feeds algae. Correct drainage, trimmed vegetation, and a compatible sealer often extend the interval more effectively than repeated high-pressure washing.
FAQ
Can a 2,000 PSI Pressure Washer Clean Exposed Aggregate?
A 2,000 PSI pressure washer can clean light soil and algae when paired with suitable chemistry, a wide nozzle, and adequate flow. It may work slowly on a small patio, but low GPM can make rinsing and surface-cleaner operation inefficient. Test the surface rather than relying on the machine rating.
Should You Pressure Wash Exposed Aggregate With Hot Water?
Hot water can improve removal of grease and oily soil, but it is not required for ordinary dirt or algae. Excessive heat can soften or disturb some sealers and increases drying speed, so use the lowest temperature that matches the stain and follow equipment and chemical instructions.
Can You Use Dawn on Exposed Aggregate?
A small amount of compatible dish soap may improve surfactant cling, but dish soap is not a substitute for an alkaline concrete degreaser. Excess soap creates foam, complicates rinsing, and can carry contaminants into drainage systems. Product-label dilution and runoff control matter more than extra detergent.
How Long After Washing Can You Walk on the Surface?
People can usually walk on a properly rinsed exposed aggregate surface once it is no longer slippery, often within a few hours in warm, dry weather. Keep vehicles and furniture off the slab until it has dried fully, and follow the sealer manufacturer’s longer traffic restrictions if sealing afterward.
Is Exposed Aggregate More Difficult to Clean Than Smooth Concrete?
Exposed aggregate is more difficult to clean because its textured voids retain soil and the decorative stones can be loosened by concentrated impact. A surface cleaner, chemical pre-treatment, and controlled rinsing usually produce a more even result than a close wand pass.
When Should You Hire a Professional?
Hire a professional when stones are loose, a sealer must be stripped, oil is deeply absorbed, runoff requires recovery, or the area is large enough that inconsistent wand work could leave visible striping. A contractor should explain the planned pressure, flow, chemical treatment, test patch, and runoff controls before starting.
The Bottom Line
To pressure wash exposed aggregate successfully, use chemistry to loosen contamination and moderate mechanical force to remove it. Start with a test patch, choose a 25-degree or 40-degree nozzle or a properly matched surface cleaner, maintain 12-18 inches of distance, and stop immediately if stones or cement paste begin to move. The safest how to pressure wash exposed aggregate method preserves the bond that makes the finish durable.
