A topping that has lost bond does not announce itself. It sits there looking finished until somebody drags a chain across it on the deficiency walk and the sound changes from a tap to a drum. By then it is a warranty item, and the argument about whose scope it was has started.
Most of that argument is decided before the second truck arrives. Thickness was set on the drawings, the surface profile by whoever ran the removal equipment, the bond in the three hours before placement. This article turns those decisions into numbers you can hold a sub to.
The two toppings are not the same product
A bonded topping and an unbonded topping share a name and nothing else. Different minimum thicknesses, base-slab finishes, reinforcement and failure modes. Specifying one and building the other is the most expensive mistake on this scope.
ACI 302.1R sets the floor on both:
| Topping type | Minimum thickness | Base slab finish required |
|---|---|---|
| Bonded topping (Class 3 and Class 7) | 19 mm (3/4 in.) | Rough, open-pore, clean and textured |
| Unbonded topping (Class 3 unbonded) | 75 mm (3 in.) | Troweled, plus a bondbreaker |
| Unbonded heavy-duty top course (Class 8) | 100 mm (4 in.) | Troweled, plus a bondbreaker |
The guide’s wording on the bonded case is short: “Bonded concrete toppings should have a minimum thickness of 3/4 in. (19 mm).” On the unbonded case it is equally short: “Unbonded toppings should have a minimum thickness of 3 in. (75 mm).”
Read those rows together and the logic falls out. A bonded topping is thin because it borrows the base slab’s stiffness, so the only thing keeping it in service is the bond. An unbonded topping is thick because it is a slab in its own right, deliberately separated by plastic sheeting, roofing felt or a bond-breaking compound, carrying load on its own bending capacity.
Anything between 19 mm and 75 mm is the dangerous middle: too thick to act as a bonded skin, too thin to act as an independent slab, and it will crack on a grid nobody drew.
The base slab surface is the specification
A bonded topping fails at the interface, not in the body. ACI 302.1R asks for a base surface with “a rough, open pore finish,” free of anything that would interfere with bond — which makes laitance, curing compound, oil and any previous sealer removal items, not cleaning items.
Roughness is described in the trade by the ICRI concrete surface profile scale, CSP 1 through CSP 10, each number tied to a method — CSP 1 acid etched, CSP 2 grinding, CSP 5 medium shotblast, CSP 9 heavy scarification. Good for a subtrade scope, useless for accepting work.
Alberta Transportation’s Section 20 gives a field test that works: sandblasting immediately before placement, “sufficient to uniformly expose the fine aggregate,” with the surface free of sand, dust and contaminants. A superintendent can judge that in one pass, and it is the acceptance language to copy into your subcontract.
Two numbers from the same specification stop over-removal:
- Cold milling into the concrete is limited: “remove not more than 5 mm in depth of the concrete surface,” with overmilling at the contractor’s cost.
- Jack hammers heavier than a nominal 14 kg and chipping hammers heavier than a nominal 7 kg “shall not be used.” Heavy breakers microcrack the substrate you are about to bond to, and the bond then fails in the parent concrete rather than at the interface.
Where removal reaches steel you are into repair scope — the province removes concrete to 25 mm below exposed bars and requires supplemental reinforcement where a bar has lost 20 per cent or more of its section. That boundary is the subject of our note on partial-depth versus full-depth repair.
The three hours before the pour
This is the part that gets compressed when the schedule is tight, and the part that cannot be compressed. Alberta Transportation’s Section 20 sequence reads as a timetable:
- Soak the prepared surface continuously with clean water for a minimum of 3 hours.
- Blow off the excess water immediately before placement. Damp, not wet — standing water in a low spot becomes a weak layer.
- Apply bonding grout as a thin, even coat that “will not run or puddle in low spots.”
- Place concrete immediately behind the grout. Grout not covered within 45 minutes is rejected.
- Keep at least two grout mixers on site throughout the placement.
The grout is specified by proportion: 4 per cent silica fume, 46 per cent Type GU hydraulic cement and 50 per cent sand with a maximum 2.5 mm top size, mixed to a slurry. Not a proprietary bonding agent, and not neat cement paste.
Enforce the 45-minute clause. A grout coat that dries before the concrete reaches it is a bond breaker, and on a windy Calgary afternoon that window is shorter. Shorten the grout crew’s lead on the placing crew rather than arguing about it afterwards.
What bond strength is actually achievable
Specifications ask for bond without saying what bond is reasonable, so the number gets argued after the first pull-off test. Measured values are more modest than most specifications assume.
An International Concrete Repair Institute study of a shotcrete overlay on a cooling tower measured direct tensile bond by ASTM C1583 at 142.0 psi (0.98 MPa) on hydroblasted surfaces and 139.8 psi (0.96 MPa) on mechanically blasted surfaces — effectively the same result from two very different methods. Guillotine shear on the same cores averaged 1,016.8 psi (7.01 MPa). The same paper records the range specifications actually use: minimums of 100 to 250 psi (0.7 to 1.7 MPa), with ACI 548 recommending 250 psi (1.7 MPa) for polymer overlays. On that project the tensile results came in below the specified 250 psi — exactly the dispute this section exists to prevent.
Two cautions. That study was a shotcrete overlay on a vertical surface, not a cementitious topping on an interior slab, so treat the figures as an order of magnitude. And a bond number specified above the substrate’s own tensile capacity cannot be met by any amount of preparation — the failure plane moves into the parent concrete and the test reports the substrate, not the bond. Verify the substrate first, one use of the methods in our guide to non-destructive concrete testing.
Joints are inherited, not designed fresh
A topping does not get its own jointing scheme. ACI 302.1R is direct: “Joint spacing in the topping should be coordinated with joint spacing in the base slab,” with additional joints only where the topping thickness requires closer spacing.
On a bonded topping, every base-slab joint and every active base-slab crack telegraphs through. Saw-cut contraction joints in the base slab should penetrate at least 25 mm (1 in.), and where the topping goes down before the base joints are cut, the topping joints extend into the base slab. A joint that stops at the interface is a hinge.
On an unbonded topping the consequence is structural rather than cosmetic. ICRI’s review of plaza-deck toppings traces the sequence: deterioration “is initiated by improper concrete practices and detailing,” narrow shrinkage cracks “tend to widen and grow” under traffic, and cracked sections “will eventually debond and spall because of the lack of bond to the base structural floor.” No bond arrests the crack, so traffic finishes the job.
Curl is the other inherited problem. An unbonded topping dries from the top face only, so it curls more readily than a slab on grade, and curl at a joint turns a flatness number into a trip hazard. If the floor carries an FF/FL requirement, settle it against the topping, not the base slab — the numbers are in our guide to FF/FL flatness.
Unbonded toppings that carry traffic need real thickness
Where vehicles are involved, 75 mm is a code minimum rather than a design. For light traffic with a 3,000 lb concentrated load, ICRI’s plaza-deck analysis prefers “a minimum thickness of 3 in.” (76 mm). For heavy traffic, with concentrated loads from 8,000 to 13,200 lb, 5 in. (127 mm) “will provide a better serviceable life.” Its general advice is blunt: consider the largest possible thickness, because thicker slabs carry lower tensile stress under a wheel.
Reinforcement follows. Shrinkage and temperature steel alone “will not be adequate, particularly for applications with heavy traffic loads.” Where tensile stress exceeds the modulus of rupture, the topping needs steel designed for the traffic moments — a design item for the engineer of record, not a mesh allowance in a unit price.
One finding belongs to the waterproofing trade: where a topping sits on a membrane and protection board, stresses rise sharply once the membrane’s foundation modulus falls below roughly 500 to 700 lb/in³.
The Alberta overlay specification, read as a pour-day checklist
Section 20 is written for bridge decks, and a commercial floor is not one. But it is the most complete overlay sequence any public owner publishes in this province, and the operational clauses translate:
| Clause | Requirement |
|---|---|
| Overlay thickness | Nominal 70 mm, set from profiles at 3 m intervals |
| Concrete | Class HPC, or Class HPC with steel fibres |
| Strength | “Fully acceptable concrete shall have a 28 day minimum compressive strength of 45 MPa” |
| Discharge window | Within 70 minutes after water is introduced to the premix |
| Load size | No more than 3 m³ and no more than 85% of rated mixer capacity |
| Trucks on site | Minimum of three truck mixers per pour |
| Gap between trucks | No more than 5 minutes between discharges |
| After a stoppage | A stoppage of 60 minutes or more means resuming only after at least 12 hours |
| Between courses | A following course waits until the adjacent course is at least 72 hours old |
| Over a full-depth repair | The repair cures at least 3 days before the overlay goes on |
| Curing | Wet cure; surface drying during curing may lead to rejection |
| Finish | Positive drainage; depressions that pond water are repaired at the contractor’s cost |
The payment ladder shows how seriously the owner treats strength here: between 42 and 45 MPa the bid price drops $25/m², between 40 and 42 MPa $50/m², between 37 and 40 MPa $100/m², and below 37 MPa the work is rejected.
The three-truck and five-minute clauses are the ones contractors underestimate. A topping is a continuous operation, and a fifteen-minute gap behind a grout crew produces a cold joint in a 70 mm slab — a delamination with a start date.
Where this goes wrong on Calgary jobs
After 38 years pouring concrete in this city, we have sat through enough deficiency walks to know which three sentences precede a hollow-sounding floor.
“We ground it, it was clean.” Grinding is CSP 2 — a smooth, closed surface, the opposite of the open-pore finish a bonded topping needs.
“It’s self-levelling, it finds its own bond.” A self-levelling underlayment is a flooring product with its own primer and thickness range, and it carries no load — see self-levelling concrete. Ordering one for the other is a change order.
“We’ll strip the forms and top it the same week.” A full-depth repair cures three days before the overlay, and a following course waits 72 hours. The strength that releases the step before it is in our note on when formwork can come off.
Talk to us before the base slab is finished
The cheapest moment to settle a topping is while the base slab is still on the schedule, because the base slab’s finish is the topping’s specification — and a troweled base under a bonded topping is a removal item.
Bring us the topping type, the thickness, the traffic and the finish requirement, and we will tell you what the base slab has to leave behind. Reach our concrete repair crews in Calgary to set it up.
FAQ
What is the minimum thickness for a bonded concrete topping? ACI 302.1R sets 19 mm (3/4 in.). The topping relies on bond rather than its own bending capacity, so thickness is not what holds it down.
What is the minimum thickness for an unbonded topping? 75 mm (3 in.) for a Class 3 unbonded topping and 100 mm (4 in.) for a Class 8 heavy-duty top course. Under concentrated loads of 8,000 to 13,200 lb, ICRI’s plaza-deck work recommends 127 mm (5 in.).
What surface does the base slab need before a bonded topping? A rough, open-pore, clean and textured surface. Alberta Transportation’s field acceptance is sandblasting “sufficient to uniformly expose the fine aggregate” just before placement.
How long is the soak, and how long can the bonding grout sit? Three hours of continuous soaking, then blow off the excess. Grout not covered by concrete within 45 minutes is rejected.
What bond strength should be specified? Published specifications have cited minimums from 0.7 to 1.7 MPa, while measured direct tensile bond on a prepared substrate has come in near 1.0 MPa. Verify the substrate’s own tensile capacity first.
Do topping joints have to line up with the base slab joints? Yes, and where the topping goes down first, its joints extend into the base.
Sources
- American Concrete Institute, ACI 302.1R-04, Guide for Concrete Floor and Slab Construction (excerpt, Chapter 2 and Table 2.1): https://www.concrete.org/Portals/0/Files/PDF/3021R_04Ch3.pdf
- Alberta Transportation, Specifications for Bridge Construction, Section 20 — Deck Overlay and Concrete Rehabilitation: https://www.transportation.alberta.ca/Content/docType246/Production/10bcs20.pdf
- International Concrete Repair Institute, Bond Strength Between Shotcrete Overlay and Concrete (M. M. Sprinkel): https://www.icri.org/wp-content/uploads/2024/04/BondStrengthSprinkel.pdf
- International Concrete Repair Institute, Concrete Repair Bulletin — Unbonded Concrete Topping Slabs in Plaza Deck Systems: https://www.icri.org/wp-content/uploads/2024/04/CRBJanFeb15_Vaca-Gosain-Jime.pdf