Exposed aggregate pavers or a poured slab

A paver arrives with its finish already made, while a poured slab gets its finish on your property. Exposed aggregate means concrete with the stones in the mix left showing at the surface. Unilock, a paver maker that sells in Ontario, says on its data sheet that "Series pavers feature an exposed aggregate surface composed of natural granite and quartz." Its page for Toronto homeowners lists Series among its patio pavers.

A poured slab starts as plain concrete. Toronto Concrete Repairs, which sells the work, says that for exposed aggregate "the surface cement is washed back to reveal the stone underneath". The site's guide to exposed aggregate driveways covers that finish's cost and winter wear.

One method slows the hardening of the top layer and then washes it away. Euclid Chemical, a Cleveland maker of concrete products, publishes a best-practice sheet for its EZ Expose retarder, a liquid sprayed on fresh concrete to delay the set of the surface. The sheet says to spray it "typically around the time you'd broom-finish an exterior slab". A broom finish is the ridged texture left by dragging a broom across wet concrete.

The same sheet shows how much rests on timing:

• "EZ EXPOSE is a retarder, not a 'stopper.' The concrete will set if left too long."
• Removal can come within a few hours or as late as 24 hours, and the sheet says temperature and humidity affect when.
• The owner should approve a sample panel made with the actual mix, colour and etch level before full-scale work.

The mix matters too. Nice Guy Concrete, a GTA seller, says professionals use "32 MPa with 5-7% air entrainment for Ontario's freeze-thaw cycles". MPa (megapascals) measures concrete strength, and air entrainment means tiny bubbles added to the mix on purpose.

They come out of a factory in fixed sizes and go down unit by unit. Unilock's Series data sheet lists shapes from small cobbles up to 36 by 36 inches, from 60 mm to 150 mm thick. It says the pavers meet ASTM C936 and the slabs meet ASTM C1782, product standards from the standards body ASTM International.

The sheet lists a minimum compressive strength of 8,000 PSI (pounds per square inch) and water absorption under 5%. It adds that "Colors may vary by plant of manufacture" and that the final choice "should be done with full size samples and mock-ups". These are one maker's figures for one product line.

Unilock's Basic Tech Guide says pavers are laid on a 1 inch bed of coarse sand, or of 1/8 inch open-graded chip, over the compacted base. It calls edge restraints, the plastic, metal or concrete edging that stops pavers shifting sideways, "a critical element". A plate compactor then settles the pavers before joint material goes in.

Both rest on compacted gravel, but the published depths differ. Unilock's Basic Tech Guide lists a gravel base of 6 to 8 inches (15 to 20 cm) for patios and walkways and 10 to 14 inches (25 to 35 cm) for driveways. It adds that clay soil needs deeper digging than sandy soil.

Toronto Concrete Repairs describes its poured driveways differently. It says it builds on 4 to 6 inches of compacted granular base and pours 6 inches of concrete for a residential driveway. It sets control joints, the planned grooves that let a slab move, every 8 to 12 feet.

Ask each installer to put the base depth in writing. The patio planning guide covers permits and drainage for a ground-level patio.

The two options overlap on price, and the paver side has a gap. Every figure below comes from a company that sells the work, which is how most prices in this trade get published. None of the price pages checked for this guide prices an exposed aggregate paver, installed, as its own line.

Interlock, in the site's stamped concrete and interlock comparison, means modular pavers you can lift and relay. Ultrascape warns that material-only prices "do not reflect real-world installation costs", so those rows leave out the base, the labour and the edging. Toronto Concrete Repairs, which sells the work, now lists exposed aggregate on its driveway page only as priced above its broom finish, which it puts at about $30 per square foot removed and replaced. The concrete patio cost guide lists every poured patio row with its source.

Pavers can be lifted and laid again, while a poured slab stays one piece. Khan Scapes, which sells the work, publishes "$8 to $15 per square foot" to re-level sunken pavers. It prices a full relay, which removes the pavers and re-compacts the base, at "$15 to $25 per square foot". On joints, it says to re-sand "Every 2 to 3 years if you use salt."

Unilock's Series product page says "Series has a deep texture which may trap sand particles" and tells installers to blow the surface clean before the joint sand is stabilized. Its Basic Tech Guide says that if you seal pavers, "you should plan to reseal every 3-5 years".

A poured slab has no sand joints to keep up. Euclid's sheet recommends a penetrating sealer for lighter etches and an acrylic sealer for deeper ones. A patch on a poured slab has to match the stone in the original mix, and the repair, resurface or replace guide covers poured repairs. None of the pages checked compares how long the 2 options last, so this guide makes no lifespan claim.

The index lists installers, not makers. None of the Google Maps business labels on its records names a paver, slab or precast maker or dealer. The only supplier label is "Retaining wall supplier", on 1 record. One company name contains the word precast, and Google labels that company a concrete contractor.

The site's crawler also stored text from company websites: titles, descriptions, headings and a page excerpt. A search of that stored text found:

• 57 records that mention exposed aggregate;
• 134 records that mention interlock, pavers or paving stone;
• 23 records that mention both.

Of the 57 that mention exposed aggregate, 47 carry Google's Concrete contractor label and 34 sit in the Concrete Patios & Walkways category.

The site tags records with themes their stored Google reviews repeat, and the theme for interlock or paving stone work appears on 65 records. Brand names rarely appear: the stored text of 1 record names Permacon, and none names Unilock or Techo-Bloc. The index cannot tell you whose pavers a company lays, and its methodology page says the score's own-website part "is not a check of anybody's paperwork".

Ask to see the surface before you commit, then put the same questions to every installer.

For pavers:

1. Which maker and product line is in the quote, and can I see a full-size sample?
2. How deep is the gravel base, and how does that compare with the maker's published figure?
3. What edging and joint material go in, and how often will the joints need sand if I salt?

For a pour:

1. Will you make a sample panel with the actual mix and stone before the job?
2. Which stone is in the mix, and is it named in the written quote?
3. What strength and air content does the mix have, and where do the control joints go?

If you are undecided, start with the 23 records that mention both surfaces. Ask them to price both on the same footprint and base, so the finish is the only change. The concrete against interlock comparison covers the wider driveway trade-offs.