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Exposed Aggregate

A decorative finish where the surface paste is removed to reveal the aggregate beneath

Exposed aggregate is a decorative concrete finish where the surface paste is removed to reveal the aggregate beneath. The result is a textured, slip-resistant surface showcasing the natural color and texture of stone aggregate. It's popular for patios, pool decks, and driveways where both aesthetics and traction matter.

Why It Matters

Exposed aggregate combines beauty with function. The natural stone creates visual interest plain concrete lacks, while the texture provides excellent slip resistance. It's more durable than smooth finishes—aggregate is harder than cement paste, so wear resistance is superior. For outdoor entertaining areas, exposed aggregate offers the best combination of appearance, safety, and longevity.

The technique requires precision timing and careful execution. Remove paste too early and aggregate loosens or dislodges. Wait too late and paste won't wash off. The window is narrow—typically 4-12 hours depending on weather—making it a technique that demands attention and experience.

Technical Details

Exposed aggregate methods:

Surface retarder method (most common):

  1. Place and finish concrete normally
  2. Apply chemical retarder to surface after finishing
  3. Wait 6-18 hours (retarder slows surface setting)
  4. Wash off paste with pressure washer + scrub brush
  5. Reveal aggregate to desired exposure depth

Seeding method:

  • Broadcast decorative aggregate on fresh surface
  • Embed with float or roller
  • Proceed with retarder method
  • Allows custom aggregate selection

Abrasive blasting:

  • For hardened concrete
  • Sand or bead blasting removes paste
  • More aggressive, harder to control depth
  • Used for remedial work or special effects

Aggregate selection:

  • Pea gravel: Small, rounded, colorful
  • River rock: Smooth, natural appearance
  • Crushed stone: Angular, varied sizes
  • Recycled glass: Modern, colorful
  • Specialty aggregates: Quartz, granite, marble chips

Exposure depth:

  • Light: 1/8" aggregate exposure, subtle texture
  • Medium: 1/4" exposure, standard appearance
  • Heavy: 3/8" exposure, dramatic texture
  • Deeper exposure = more texture but potential for loose stones

Process timeline:

  1. Hour 0: Place and finish concrete
  2. Hour 1-2: Apply surface retarder
  3. Hour 6-18: Wash and expose (timing critical)
  4. Day 1-2: Neutralize retarder residue
  5. Day 2-3: Apply sealer (after full cure: 28 days better)

Common problems:

  • Paste won't wash off: Waited too long → Use stronger washing, may need light sandblasting
  • Aggregate loosens: Washed too early → Wait longer before washing
  • Uneven exposure: Inconsistent retarder application → Apply uniformly
  • Aggregate pop-outs: Weak aggregate, freeze-thaw → Use sound aggregate
  • Staining: Retarder residue → Neutralize and clean thoroughly

Finishing requirements:

  • Smooth, level surface before retarder application
  • No bleed water present
  • Timing coordinated with crew availability for washing
  • Adequate water supply and drainage for washing
  • Protection from rain until paste removed

Maintenance:

  • Reseal every 2-3 years
  • Pressure wash periodically
  • Address loose stones promptly
  • Avoid deicing salts (can cause aggregate pop-outs)

Cost comparison (per square foot installed):

  • Standard broom finish: $4-8
  • Exposed aggregate: $8-12
  • Stamped concrete: $10-15

Exposed aggregate costs more initially but requires less maintenance than smooth finishes and outlasts decorative coatings that wear or peel.

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