Exposed Aggregate Concrete

Why this matters

Exposed aggregate is a decorative finish where the surface paste is removed to expose the embedded stone. The result is a textured, attractive surface that hides minor cracks (which are common in concrete) and provides excellent slip resistance. It's a popular choice for driveways, patios, walkways, and pool decks. The technique requires specific timing and method; the wrong approach produces uneven exposure or surface damage. A contractor who can execute exposed aggregate consistently captures the premium pricing (a modest per sq ft vs a modest for standard) at high margin.

What exposed aggregate concrete is

Standard concrete has a cement paste surface; the stone aggregate is hidden below. Exposed aggregate intentionally washes the paste from the surface, revealing the stone. The exposed stone provides:

• Visual interest (color, texture from the stone)
• Slip resistance
• Hides minor surface imperfections
• Hides hairline cracks
• Long visual life (the stone color is permanent)

Methods to expose aggregate

Method 1: Surface retarder + wash

The most common method:

1. Pour standard concrete
2. Finish with bull float
3. Apply surface retarder (delays cement cure at surface)
4. Cover with plastic
5. Wait (typically overnight)
6. Wash and brush the surface to expose aggregate

Method 2: Seeded aggregate

For larger or special aggregate:

1. Pour concrete
2. Bull float
3. Broadcast aggregate stones onto the surface
4. Tamp lightly to seat
5. Cover
6. Wash off cement paste later

Method 3: Sandblasting

Less common for residential:

• Cure concrete fully
• Sandblast the surface
• Removes some paste
• Exposes aggregate

Aggregate choice

The aggregate visible in the finish depends on what's in the concrete:

Standard residential: typically uses local river gravel for cost. Custom aggregate is special order at additional cost.

Procedure for typical exposed aggregate driveway

Step 1: Plan and preparation

• Determine aggregate exposure level (light, medium, heavy)
• Verify aggregate selection
• Plan for the appropriate cement / aggregate mix

Step 2: Pour standard concrete

• Use 4,000 psi or higher mix
• 4 inch thickness typical for residential
• Standard placement and screed

Step 3: Bull float

• Smooth the surface
• Don't over-float (don't drive aggregate too deep)

Step 4: Apply surface retarder

• Use sprayer or brush
• Apply per manufacturer (typically uniform coverage)
• Allow to penetrate (typically 30 to 90 seconds)

Brands: Stamp Master Wax-Off, Dayton Superior Set Retarder, others.

Step 5: Cover with plastic

• Plastic prevents premature surface dry
• Allows retarder to work
• Overnight typical

Step 6: Remove plastic and wash

The next morning (or per manufacturer timing):

• Remove plastic
• Spray with water at low pressure (garden hose; not pressure washer)
• Wash and brush with stiff broom
• Aggregate becomes visible

Step 7: Continue washing

• Wash until paste is removed
• Stop when aggregate is exposed to desired depth
• Different aggregate exposures:
  • Light: aggregate visible; mostly paste still
  • Medium: aggregate prominent; paste between
  • Heavy: aggregate dominates; minimal paste

Step 8: Allow to cure

• Continue normal cure
• Avoid heavy traffic for 7 days

Step 9: Seal (typically required)

• Apply sealer to enhance color and protect
• See sealer types article
• Typical: acrylic sealer

Timing critical

The timing window for the wash is narrow:

• Too early: wash removes too much; surface damaged
• Too late: paste is too hard; can't remove

Indicators:

• Light: try washing one small area; verify aggregate exposure
• Then proceed if appropriate

The window is typically 12 to 24 hours after pour.

Exposure level options

Light exposure

• Surface texture
• Some aggregate visible
• Subtle decorative effect
• Good for budget-conscious customers

Medium exposure

• Most popular
• Aggregate clearly visible
• Good balance of decoration and durability

Heavy exposure

• Aggregate dominates
• Premium decorative effect
• Most "pebbled" appearance

Common applications

• Driveways (slip resistance plus decorative)
• Patios (decorative + comfortable underfoot when bare)
• Walkways (durable, attractive)
• Pool decks (slip resistance is huge benefit)
• Step treads (anti-slip)

Maintenance

Exposed aggregate maintenance:

• Annual cleaning (pressure wash or hose + scrub)
• Sealing every 2 to 5 years
• Address any cracks promptly
• Avoid de-icing salt (damages exposed aggregate)

Common exposed aggregate mistakes

Wrong timing

Wash too early: removes too much; surface damage Wash too late: aggregate not exposed

Inadequate retarder application

• Spotty exposure
• Some areas paste removed, others not

Pressure washing too hard

• Damages the surface
• Removes aggregate

Not sealing

• Premature wear
• Aggregate works loose over years

Wrong concrete mix for the application

• Some mixes don't have appropriate aggregate
• Specify mix for exposed aggregate work

Skipping pre-pour planning

• No discussion with concrete supplier
• Wrong aggregate in the mix

Variations

Salt finish (a different technique)

• Coarse salt broadcast on fresh concrete
• Brushed off after cure
• Creates pitted texture
• Less decorative than true exposed aggregate

Seeded with specialty aggregate

• Standard concrete pour
• Specialty aggregate broadcast on surface
• Provides unique appearance

Sandblasted exposed aggregate

• Cured concrete sandblasted
• Less common for residential
• More expensive

Customer expectations

• "The texture you see is the aggregate stones embedded in the concrete"
• "Color depends on the aggregate (and any added pigment)"
• "Slip resistance is much better than smooth concrete"
• "Sealing every 2 to 5 years recommended"
• "Easier to walk barefoot than rough concrete"

When NOT to use exposed aggregate

• Indoor applications where smooth surface is needed
• Areas where chairs or furniture sit (texture damages furniture legs)
• Wheelchair / mobility access areas (texture creates obstacles)
• Customer wants smooth, modern appearance

References

• ACI 332 (Residential Concrete Construction).
• Decorative Concrete Council literature.
• Manufacturer literature: Dayton Superior, Bomanite, Brickform.
• Manuall internal: Concrete Contractor Residential Flatwork Pour, Concrete Contractor Decorative Concrete Techniques.