Specifying Supplementary Cementing Materials in Alberta: The Percentages That Are a Window, Not a Ceiling

Bryan Regular
October 28, 2026

A cement replacement limit is almost always read as a maximum. Put in less and you are safe; put in more and you are over. That reading is correct for ordinary concrete classes and wrong for the class where supplementary cementing materials are not optional, and the difference is the difference between a mix that is accepted and one that is rejected for being too conservative.

Alberta’s public bridge specification is unusually explicit about this. For most classes it publishes a single fly ash ceiling. For the high-performance class it publishes three ranges and a minimum cement content, and a mix that lands under any of those floors is as far outside the specification as one that lands over the ceiling.

What an SCM limit actually constrains

An SCM limit is a proportion of cementing materials by mass, not of the mix and not of the Portland cement. That denominator is the first place a clause gets misread.

Two consequences follow for anyone pricing from a spec line:

A percentage band and a minimum cement content are different constraints that interact. Satisfying the band says nothing about whether you have enough Portland cement; satisfying a minimum cement content says nothing about whether the SCM proportion is legal.
Changing the total cementing content to chase strength moves the SCM mass needed to stay inside the same percentage. The percentage is not a quantity you can carry over from a previous mix submittal.

The practical effect is that SCM clauses are proportioning constraints, not shopping instructions, and they have to be checked against the mix as proportioned rather than against the ingredient list.

Where the public numbers are, and where they are not

This is worth stating plainly, because it determines what any Canadian specification can tell you.

CSA A23.1 is the governing standard, and its cementing-material tables are not public. Public specifications routinely point at them rather than reprint them. Nova Scotia’s cast-in-place high performance concrete specification is a clean example: it names the permitted materials — Class F fly ash, Type SF silica fume, and Type S ground granulated blast furnace slag — and refers to CSA A23.1:19 tables for cementing material types, without publishing a single replacement percentage or a total SCM limit. If you only read specifications of that shape, you would conclude no Canadian authority publishes numbers.

Alberta’s bridge specification does publish them. Section 4 of Alberta Transportation’s Specifications for Bridge Construction carries both the ordinary-class fly ash cap and the full composition requirements for the high-performance class. That makes it the most useful openly available Canadian reference on this question, with one caveat handled below.

Nova Scotia’s spec is still useful here for a different reason. It shows that silica fume batched as a separate component must be batched in whole packages, rounded up to the next full package as the batch volume requires. That is a real constraint on small loads, and it is the kind of clause that makes a theoretical percentage impossible to hit exactly on a 2 m³ delivery.

The fly ash ceiling on ordinary classes

For the ordinary classes, Alberta’s specification gives one number. The notes to the class-of-concrete table state that fly ash shall not exceed 30% by mass of cementing materials, with high-performance concrete governed separately by clause 4.4.2.

Two things in that clause are easy to miss.

It is a one-sided limit. There is no floor. A mix with no fly ash at all satisfies it.

Slag is not addressed. The specification does not mention ground granulated blast furnace slag in these clauses at all, and it sets no silica fume percentage for the non-high-performance classes — only material quality requirements for silica fume itself, including a minimum silicon dioxide content of 85%, a maximum loss on ignition of 10%, and a maximum sulphur trioxide content of 1%. Absence of a percentage is not permission to use any proportion; it means the governing limit lives in the project specification or in CSA A23.1, not in this clause.

There is also a permission rather than a limit worth knowing: the same note states that fly ash may be used in concrete mixes where the aggregate is assessed to be potentially alkali-silica reactive. That is a durability tool rather than a ceiling, and it is the clause that connects SCM selection to aggregate assessment rather than to strength.

Why the high-performance class turns the ceiling into a band

For Class HPC, Alberta’s clause 4.4.2 does something the ordinary classes do not: it requires the SCMs and then brackets them on both sides.

Constraint	Published requirement
SCMs required	Silica fume and fly ash must both be included
Minimum cement content, excluding SCMs	335 kg/m³
Silica fume plus fly ash, combined	17% to 20% of cementing materials by mass
Silica fume alone	6% to 8% of cementing materials by mass
Fly ash alone	11% to 15% of cementing materials by mass

Read the three ranges together and the arithmetic is tighter than any one line suggests. The individual ranges would permit a combined total as low as 17% and as high as 23%, but the combined range caps it at 20%. A submittal at 8% silica fume and 14% fly ash satisfies both individual ranges and fails the combined one. That is the single most common way a mix inside every line item is still outside the specification.

The minimum cement content is the other half of the trap. It is specified excluding SCMs, so increasing SCM content to improve durability cannot be paid for by reducing Portland cement below 335 kg/m³. The floor on cement and the band on SCMs push in opposite directions, and the total cementing content has to rise to satisfy both.

What the band does to a mix already proportioned

A one-sided ceiling tolerates the normal variability of a batching operation in one direction. A two-sided band does not tolerate it in either.

That matters because the quantity actually delivered is subject to batching tolerance, moisture correction on the aggregate, and — where silica fume is a separately batched component — package rounding. A target placed at the edge of a band leaves no room for any of it. A target placed at the centre of each band leaves room on both sides.

Three implications for a submittal:

Propose mid-band, not edge-of-band. A silica fume target of 7% and a fly ash target of 12% sits comfortably inside all three constraints with a combined 19%. Targets of 6% and 11% satisfy the letter of the clause and fail on any negative deviation.
Check the combined constraint last and treat it as governing. It is the one that is not visible from either individual line.
Re-check the percentages whenever cementing content changes. A strength revision that raises total cementing material changes the mass of silica fume and fly ash required to hold the same proportion.

When a specification names a percentage and a standard at once

The most common defect we see in an SCM clause is not a wrong number. It is two sources of truth in the same paragraph: a project percentage written out longhand, plus a reference to a standard that sets its own limits for the exposure class in question.

When that happens, there is no general rule that resolves it, and averaging the two is not an answer. The project specification has to say which governs. A clause that specifies a percentage and also invokes a standard, without stating precedence, is a clause the supplier cannot price without a question, and the question is cheaper at tender than at the plant on pour morning.

One further caution on editions. The percentages above are quoted from Section 4 of Alberta’s Specifications for Bridge Construction as published in the openly available section file. The 2017 Standard Specifications volume retains the same clause structure — a class-of-concrete table at 4.4.1, Class HPC at 4.4.2, and supplementary cementing materials at 4.2.2 — but we could not extract its Section 4 body text to confirm the figures line by line. Cite the number and the clause number, never the table number, and confirm against the edition named in your Contract Documents, which is what governs.

Reading an SCM clause before you price the mix

Identify the denominator. Is the percentage of cementing materials, of Portland cement, or of total mix mass?
Identify whether each limit is a ceiling, a floor, or a band. Only bands constrain you from below.
Find the minimum cement content, if any, and check whether it excludes SCMs.
Add the individual ranges and compare the sum against any combined range. Note which one binds.
Check whether the aggregate has been assessed for alkali-silica reactivity, because that may change which SCM is appropriate rather than how much.
Confirm whether silica fume is batched separately, and whether package rounding is compatible with your load size.
Settle precedence between any project percentage and any invoked standard, in writing, before the submittal.

Where this article stops

This article covers published composition limits. It does not cover the concrete temperature window at discharge or the seasonal placement restrictions that accompany these classes, which are their own subject; the mix submittal and review process, or who carries responsibility for the mix under each specification alternative; the relative environmental merits of fly ash and slag; or the strength-gain behaviour of high-replacement mixes at early ages. It also does not reproduce the exposure-class tables, which decide the durability requirement your SCM selection has to serve.

Questions a spec writer asks

Is 30% fly ash the Alberta limit? It is the limit stated in the notes to the class-of-concrete table in Section 4 of Alberta’s bridge specification, for the classes that table covers, and high-performance concrete is expressly excluded from it and governed by clause 4.4.2 instead. Treat it as that specification’s number for those classes, not as a province-wide rule.

Does the combined range or the individual range govern a Class HPC mix? Both apply, and the combined range is the binding one, because the individual ranges permit totals the combined range does not.

Can I reduce Portland cement to make room for more SCM? Not below the published minimum cement content of 335 kg/m³, which is specified excluding SCMs.

Why does a public specification name materials without naming percentages? Because the percentages sit in CSA A23.1, which is not published openly. A spec that names Class F fly ash, Type SF silica fume and Type S slag and then refers to the standard’s tables is complete as drafted; the numbers are simply elsewhere.

Send us the clause with the project details

The fastest way to find out whether a mix can hold a band is to put the clause in front of the people who will batch it. If your specification carries SCM percentages, a minimum cement content, or a combined limit, send the clause along with the job.

Send us your project details — pour date, mix type, PSI, volume — and our team will get back to you within 1 business hour with a quote and delivery slot. For the related reading, see custom-mixed concrete in Calgary, how aggregate assessment drives SCM choice in alkali-aggregate reaction in Alberta concrete, the material comparison in fly ash vs slag cement, and the durability requirements behind it all in the CSA A23.1:24 exposure classes.