Choose Repair Mortar by the Repair—not the Biggest PSI Number

Choose a high-strength cement repair mortar from the repair backward. Determine whether the damage is structural, then match the material to the placement orientation, prepared depth, exposure, temperature, working window, and reopening deadline. No universal PSI or MPa threshold defines high strength, and the largest advertised 28-day value does not establish bond, durability, dimensional compatibility, or suitability for the work.

Start here: a requirement-based selection checklist

Use this six-step decision path before comparing bags:

1. Determine structural significance. Identify whether the repair affects load capacity, stability, reinforcement development, fire resistance, or another code-governed function.
2. Identify the orientation. Classify the placement as horizontal, inclined, vertical, or overhead. Do not assume that a horizontal patching mortar will hang overhead.
3. Measure the prepared depth. Record minimum, maximum, and average depth after unsound concrete has been removed—not before demolition.
4. Set the reopening deadline. Define the required return to foot traffic, vehicle traffic, heavy traffic, coating, construction loading, or another specified load.
5. Identify temperature and exposure. Consider substrate and ambient temperature, freeze-thaw cycling, deicing salts, moisture, chlorides, chemicals, and weather during curing.
6. Compare current technical data sheets on matching terms. Keep strength age, test method, temperature, curing conditions, lift thickness, traffic category, and document region visible.

Job condition	Specification to verify	Likely consequence of a mismatch
Vertical or overhead placement	Documented orientation, low-sag behavior, and maximum lift thickness	Slump, sag, debonding, or lifts that cannot hold their own weight
Deep or full-depth repair	Neat depth limit, lift limit, form requirements, aggregate-extension instructions, or repair-concrete option	Excess heat, cracking, voids, poor consolidation, or placement outside published limits
Traffic-bearing area	Explicit opening time for foot, vehicle, or heavy traffic at the expected temperature	Surface damage or loading before adequate strength develops
Freeze-thaw or salt exposure	Documented scaling, freeze-thaw, salt, or permeability performance relevant to the job	Premature scaling, saturation damage, or repeat deterioration
Hot or cold placement	Permitted ambient and substrate temperatures, plus applicable weather procedures	Short working time, slow strength gain, freezing, poor curing, or failed bond
Tight shutdown window	Application time, set time, strength by age, and service-opening time	Material sets before placement or the repair misses its reopening deadline
Thin perimeter or irregular patch	Minimum thickness and required edge geometry	Feather edges crack, dry out, or break away
Coated or waterproofed surface	Earliest coating time and documented compatibility	Trapped moisture, poor adhesion, or a delayed handoff

Do not reduce load-bearing damage, exposed or corroded reinforcement, extensive deterioration, full-depth loss, uncertain damage causes, or code-governed rehabilitation to product shopping. Those conditions warrant assessment by a qualified design professional under the applicable project and jurisdictional requirements (ACI 562-16 code and commentary).

ACI 562-16 illustrates the broader process surrounding structural concrete rehabilitation: assessment, repair design, repaired-system behavior, interface bond, durability, construction, inspection, and quality assurance. It assigns assessment and rehabilitation-design responsibilities to a licensed design professional, although the document has legal status only when adopted by the relevant jurisdiction. The cited edition does not prescribe one repair mortar or a minimum mortar compressive strength.

What “high strength” does—and does not—tell you

Published standards and product literature do not establish a universal PSI or MPa threshold for the marketing term high strength. Retail filters can place cementitious mortars in the same category as patches, adhesives, sealants, fillers, and resurfacers. Use the phrase as a shopping filter, not as a technical classification.

Several time-related properties are easy to confuse:

• High early strength describes strength developed at a stated early age.
• Ultimate or 28-day strength is a later-age test result, not an opening time.
• Pot life or application time is the workable placement window.
• Initial and final set describe stiffening; neither proves readiness for service.
• Drying or coating time controls a later finish or coating operation.
• Return to service must identify foot, vehicle, heavy traffic, construction load, or another defined load.

SikaQuick-1000 shows why those distinctions matter. Its November 2018 manufacturer sheet reports compressive strengths of 1,250 psi at three hours, 4,000 psi at one day, 5,000 psi at seven days, and 7,000 psi at 28 days under the sheet’s stated ASTM C109 laboratory conditions of 73°F and 50% relative humidity. Separately, the sheet reports opening at 73°F after four hours for foot traffic and six hours for vehicles. Those are manufacturer-reported results and instructions, not guaranteed field outcomes.

Compare compressive-strength values only when their basis is visible:

• the same testing age;
• the same unit system;
• an identified test method;
• comparable specimen preparation;
• comparable temperature and curing conditions; and
• a reliable source, preferably the current local manufacturer data sheet.

A bare retailer claim of “6,500 psi” without an age or test method cannot be ranked fairly against a manufacturer’s 28-day ASTM C109 result. Even apparently comparable laboratory values are reported, declared, or typical results. Field temperature, water measurement, mixer selection, batch size, substrate condition, placement delay, and curing can change jobsite performance.

Compressive strength is only one part of a durable repair. Selection also has to account for interface bond, shrinkage or length change, modulus and dimensional compatibility, exposure resistance, repair geometry, edge condition, and the strength and condition of the existing concrete. A very strong but poorly bonded or incompatible patch can still fail.

What ASTM C928/C928M-25 actually covers

ASTM C928/C928M-25 is the active specification for packaged, dry, rapid-hardening cementitious mortar or concrete used to repair hardened hydraulic-cement concrete pavements and structures. Its scope evaluates:

• compressive strength;
• length change;
• scaling resistance; and
• slant-shear bond strength.

Those categories are useful, but claimed compliance does not define a generic high-strength threshold. It also does not guarantee field performance, establish code approval, or demonstrate suitability for every structural repair. ASTM’s public scope does not provide the numerical acceptance limits, so those limits should not be guessed or inferred from a product’s headline PSI.

The specification distinguishes packaged repair mortar from packaged repair concrete:

Material	Aggregate retained on a 9.5 mm (3/8 in.) sieve
Packaged dry mortar	Less than 5% of total mixture mass
Packaged dry concrete	At least 5% of total mixture mass

Materials using bitumen, epoxy resin, or polymer as the principal binder are outside this cementitious specification. That does not mean every polymer-modified cement mortar is excluded. Modifying a cementitious binder with polymer is different from using polymer as the principal binder.

Use the active 2025 edition for current scope statements. ASTM C928-05 and ASTM C928/C928M-08 are historical editions, although old product literature and search results may still cite them.

Deep repairs introduce an additional scope issue. A manufacturer may direct the crew to add aggregate so that a mortar can be placed at greater depth. ASTM’s scope states that products reformulated through jobsite addition of manufacturer-recommended aggregate fall outside the specification. The extension may still be permitted by the manufacturer, but the resulting jobsite mixture should not be represented automatically as remaining within ASTM C928.

Compare products on the specifications that control the job

These examples are not rankings, and their inclusion does not establish local availability or identical specifications in every market. They show how orientation, depth, working time, reopening requirements, and source context change the decision.

Product and source	Orientation, depth, and work window	Reopening and reported strength	Test context and limitations
SikaQuick-1000, U.S. sheet dated Nov. 2018 (manufacturer data sheet)	Horizontal/full-depth candidate; 1/4–2 in. neat and 1–6 in. aggregate-extended; 0.43 ft³ neat yield per 50 lb bag	Vehicle traffic after 6 hr at 73°F; 7,000 psi at 28 days	ASTM C109; 73°F and 50% RH; typical manufacturer results may vary in the field
SikaQuick VOH, U.S. sheet dated Sept. 2020 (manufacturer data sheet)	Vertical: 1/8–3 in.; overhead: 1/8–2 in.; approximately 15 min application time	5,500 psi at 28 days; manufacturer claims ASTM C928 Type R2	ASTM C109 at 73°F and 50% RH; typical results; current local sheet required
SikaRep SA, GCC sheet dated Aug. 2018 (manufacturer data sheet)	Horizontal or vertical; 5–30 mm per layer; approximately 45 min pot life at 25°C	At least 35 N/mm²; test age is not stated in the sheet	ASTM C109 at water-to-powder ratio 0.16; no ASTM C928 or rapid-hardening claim in the sheet
Sika FastFix-1100, GCC sheet dated Apr. 2026 (manufacturer data sheet)	Horizontal or inclined repairs and bedding; 10–100 mm standard placement; approximately 20 min pot life at 20°C	Light traffic after 60 min and heavy traffic after 120 min at 20°C; approximately 20 N/mm² at 2 hr and 70 N/mm² at 28 days	EN 12190 and ASTM C109 listed; EN 1504-3 Class R4 declared; temperature instructions are inconsistent

SikaQuick-1000’s November 2018 sheet contains an overlap that requires checking before a 1-to-2-inch placement. It lists a neat placement range of 1/4 to 2 inches and an aggregate-extended range of 1 to 6 inches, while a separate instruction directs aggregate addition for repairs deeper than 1 inch. Do not resolve that overlap from the summary table. Obtain the current regional sheet and written clarification for the controlling placement rule.

For vertical and overhead placement, SikaQuick VOH is the clearest fit among these examples because its sheet expressly documents both orientations, the applicable lift limits, and a short application window. SikaRep SA offers a different tradeoff: approximately 45 minutes of pot life at 25°C, but a maximum layer thickness of 30 mm and no rapid-hardening or ASTM C928 claim in the cited sheet.

FastFix-1100 emphasizes rapid reopening for horizontal or inclined repairs and bedding. Its declared EN 1504-3 Class R4 compliance should not be equated with ASTM C928. The April 2026 sheet also contains inconsistent temperature guidance: listed ambient and substrate ranges begin at 0°C, while another instruction refers to application down to +10°C. Obtain written clarification and the latest local sheet before cold-weather use.

The practical comparison is not “Which bag has the highest PSI?” It is:

• Is the product documented for this orientation?
• Can it be placed at the actual minimum and maximum prepared depth?
• Can the crew mix, carry, place, consolidate, and finish it within the working window?
• Does it develop adequate strength by the required reopening time?
• Does it document the relevant bond, dimensional-change, and exposure performance?
• Are competing values based on the same age, method, temperature, and curing conditions?

Prepare, mix, place, and cure for the published performance

A suitable bag can still produce a poor repair when substrate preparation, water measurement, timing, or curing misses the product instructions.

Use this field checklist:

• Diagnose the cause of deterioration and correct it where possible.
• Remove unsound, loose, delaminated, or contaminated concrete.
• Continue demolition until the repair boundaries are sound.
• Form defined edges; do not feather the mortar to nothing.
• Produce the surface profile specified for the exact product.
• Remove dust, laitance, oil, curing residue, coatings, and other bond breakers.
• Condition the substrate exactly as directed.
• Confirm forms, access, batch size, labor, tools, and curing materials before mixing.
• Measure liquid accurately rather than estimating by eye.
• Mechanically mix for the specified time and speed.
• Place promptly within the published working window.
• Do not add extra water or retemper material that has started to set.
• Apply the product-specific curing method when and as required.

Saturated-surface-dry (SSD) means concrete has been pre-dampened where required, but the placement surface has no standing water. SSD is common in cementitious repair instructions, but it is not universal. Neither is a separate bonding agent. Depending on the system, instructions may call for SSD and direct placement, a scrub coat, a bonding agent, or another conditioning method.

Product requirements can differ sharply. The November 2018 SikaQuick-1000 sheet calls for sound concrete mechanically prepared to at least CSP-6 and requires moist curing. The September 2020 SikaQuick VOH sheet calls for approximately CSP-5, SSD without standing water, and curing. By contrast, the April 2026 FastFix-1100 sheet directs SSD preparation but says not to wet cure. Applying one mortar’s preparation or curing rule to another can undermine the intended performance.

Common failure risks include:

• weak concrete left at the bond line;
• inadequate surface profile;
• dust, oil, coating, or other contamination;
• standing water;
• excess mixing water;
• inaccurate liquid-to-powder proportioning;
• retempering;
• delayed placement;
• temperatures outside the permitted range;
• feather-edged geometry;
• excessive lift thickness; and
• incorrect or late curing.

Several example sheets date from 2018 or 2020. Before work begins, obtain the latest local technical data sheet and safety data sheet for the exact product, package, and region. Confirm liquid ratio, mixing time, surface profile, application temperature, curing, personal-protection requirements, and coating or reopening conditions.

Calculate repair volume and bag quantity

For a rectangular repair:

Area in square feet × average depth in feet = repair volume in cubic feet

Then:

Repair volume ÷ stated yield per bag = theoretical bag quantity

Round up to whole bags only after applying a stated allowance for expected waste and irregular geometry.

Consider a 10 ft² repair averaging 1 inch deep:

1. Convert depth to feet: 1 inch = 1/12 foot.
2. Calculate volume: 10 ft² × 1/12 ft = 0.833 ft³.
3. Using SikaQuick-1000’s reported neat yield of 0.43 ft³ per 50 lb bag: 0.833 ÷ 0.43 = 1.94 bags.
4. The theoretical minimum is therefore two 50 lb bags before waste (SikaQuick-1000 product data sheet).

To finish the procurement calculation, apply a project-specific allowance before rounding. For example, if the estimator selects an illustrative 10% allowance:

1.94 × 1.10 = 2.13 bags, rounded up to three bags

The 10% figure is an example assumption, not a manufacturer rule. The actual allowance should reflect demolition overbreak, substrate roughness, voids, spillage, material left in the mixer, multiple lifts, and irregular geometry. State the selected allowance in the estimate rather than hiding it in an unexplained rounded quantity.

Do not reuse the neat-yield calculation after aggregate extension unless the manufacturer provides the applicable extended yield. The cited SikaQuick-1000 sheet directs approximately 25 lb of compliant 3/8-inch aggregate per bag for repairs deeper than 1 inch, but aggregate type and proportion can affect handling and strength. That instruction also contributes to the previously noted overlap with the sheet’s stated neat-placement range. Obtain the current regional instructions before ordering.

Jobsite aggregate extension falls outside ASTM C928’s stated scope even when the manufacturer permits or directs it.

Final pre-purchase and pre-placement check

Before ordering material, confirm:

• structural significance and the cause of deterioration;
• horizontal, inclined, vertical, or overhead orientation;
• minimum, average, and maximum prepared depth;
• edge geometry and whether forms are required;
• moisture, freeze-thaw, salt, chloride, chemical, or coating exposure;
• expected substrate and ambient temperatures;
• workable batch size and required application time; and
• reopening deadline and exact traffic or loading category.

Check the current local technical data sheet for:

• compressive strength by age and test method;
• bond strength;
• shrinkage or length change;
• freeze-thaw or scaling resistance;
• permeability where relevant;
• neat and extended lift limits;
• yield and aggregate-extension rules;
• liquid ratio and mixing procedure;
• required substrate profile and conditioning;
• curing method;
• coating compatibility; and
• traffic-opening conditions.

At delivery, verify the bag lot, shelf life, storage condition, packaging integrity, and exact product designation. Confirm the current safety data sheet, applicable adopted codes, project specifications, and required edition of ASTM or any other named standard.

Retail labels such as “high strength” can narrow a shelf, but they are not technical standards. A filtered category may combine cementitious mortars with fillers, adhesives, sealants, and resurfacers that serve different purposes.

Stop and escalate when reinforcement is exposed or corroded, concrete loss is extensive or full-depth, the member carries load, the cause of deterioration is uncertain, or the work is governed by structural rehabilitation requirements. Those conditions require qualified assessment rather than selection by bag label alone (ACI 562-16 code and commentary). Choose from the repair backward; the biggest PSI number is not a repair design.

Can high-strength cement repair mortar be used vertically or overhead?

Yes—but only when the exact product is documented for that orientation and the prepared depth falls within its published limits. Vertical and overhead repairs require mortar formulated to resist sag and hold the permitted lift.

Do not use a horizontal product overhead merely because it develops high compressive strength. Confirm orientation, minimum and maximum thickness, batch size, application time, substrate preparation, temperature limits, and curing in the current manufacturer sheet.

Is set time the same as the time before traffic can return?

No. Set time describes stiffening. Return-to-traffic time is a separate manufacturer instruction based on strength development and stated conditions. A mortar can be set but not ready for vehicle, heavy, construction, or design loads.

Look for an explicit opening time tied to temperature and traffic type. If the sheet lists only initial or final set, do not infer a reopening time.

Should repair mortar be stronger than the existing concrete?

Not automatically. The mortar needs adequate strength for the repair design, but compatibility also matters. Bond, modulus, shrinkage, thermal and moisture movement, geometry, exposure, and substrate condition can be as important as compressive strength.

For a structural repair, required properties should come from the assessment and repair design—not from a blanket rule that the patch must always have a higher PSI than the existing concrete.

Can I add gravel to make a deep repair?

Only when the current instructions for the exact mortar permit aggregate extension and specify the aggregate type, grading, quantity, mixing procedure, and depth range. Otherwise, the repair may require multiple lifts, forms, or a packaged repair concrete designed for the depth.

Aggregate extension changes yield, handling, and potentially strength, so neat-mortar bag calculations do not apply automatically. It also places the jobsite-reformulated material outside ASTM C928’s stated scope, even when the product manufacturer recommends the extension.