2.1 Concrete Mix Proportions, Water-Cementitious Ratio & Submittal Review

2.1 Concrete Mix Design Submittals & Proportion Verification

The approved mixture designation connects the delivery to the member, specified strength and exposure requirements. Start with the approved drawings, specifications and submittal for that member. For example, a mixture approved for interior columns can have the same specified strength as a garage slab mixture and still lack the specified air system or satisfy a different water limit.

Review the mixture documentation

ACI 318-19 Sections 26.4.2 and 26.4.3 address mixture requirements and proportioning. Check specified strength and age, assigned exposure classes, maximum $w/cm$, nominal maximum coarse aggregate size and any specified air, slump, density or construction-stage strength. Compare the proposed constituent materials and sources to the evidence used to establish proportions. Check applicable limits on cementitious materials and chloride content. Section 26.4.3.1 requires each proposed mixture to satisfy the construction documents for its use.

Model IBC Table 1705.3 item 5 assigns periodic verification of the required mixture. A truck ticket with the wrong mixture ID is a discrepancy even when the slump looks typical. Compare the ticket with the approved designation, report the discrepancy to the contractor and follow the approved inspection procedure before documenting the disposition. Use the submittal process for substitutions.

Specified strength and required average strength

The specified compressive strength $f'_c$ is the design and acceptance reference at the designated age. The proportioning target $f'_{cr}$ is higher to allow for variability in production. ACI 318-19 Section 26.4.3.1(b) permits ACI 301 Article 4.2.3 or an alternative acceptable to the licensed design professional that provides at least the same probability of satisfying the standard-cured acceptance requirements. Under the ACI 301 route, ACI 318-19 limits the age of the supporting strength test records to 24 months.

Under ACI 301-20 Section 4.2.3.3(a), use the larger of the two expressions in the applicable row, with standard deviation $s_s$ from qualifying records under Section 4.2.3.2 and the adjustment factor $k$.

For 15, 20, 25 and at least 30 qualifying tests, $k$ is 1.16, 1.08, 1.03 and 1.00 respectively. The table permits linear interpolation. Check the record requirements before using a sample standard deviation. When qualifying deviation data are unavailable, Table 4.2.3.3(b) uses $f'_c+1000$ psi below 3000 psi, $f'_c+1200$ psi from 3000 through 5000 psi and $1.10f'_c+700$ psi above 5000 psi. Documentation under Section 4.2.3.4 may include qualifying field records or trial mixtures. The fallback proportioning target still requires evidence that the proposed proportions can attain it.

For a mixture with $f'_c=4000$ psi and 32 qualifying tests giving $s_s=420$ psi, $k=1.00$. The two calculations give 4562.8 and 4478.6 psi. Use about 4563 psi for proportioning. Evaluate production test results separately under ACI 318-19 Section 26.12.3.1, using the three-test average and the permitted individual test shortfall.

Account for mixing water and aggregate moisture

Under ACI 318-19, $w/cm$ is the mass of mixing water divided by the total cementitious mass, including applicable supplementary materials. ASTM C94/C94M-17a Section 9.3 includes plant batch water, ice, free moisture on aggregates, retained mixer wash water and permitted added water. It also includes water from admixtures when that water increases $w/cm$ by more than 0.01. Absorbed aggregate water is excluded from mixing water.

Use the moisture basis given. With total moisture $M$ and absorption $A$ expressed as fractions of oven-dry aggregate mass, the moist batch mass is $W_{\mathrm{OD}}(1+M)$, the saturated surface-dry mass is $W_{\mathrm{OD}}(1+A)$ and the free-water adjustment is $W_{\mathrm{OD}}(M-A)$. A negative adjustment indicates aggregate below SSD. For an SSD design weight, divide by $(1+A)$ first to find the oven-dry weight.

Worked example. A one-cubic-yard design has 600 lb of cementitious material and a maximum $w/cm$ of 0.45. Coarse aggregate is specified at 1850 lb SSD, with 1.8% total moisture and 0.8% absorption. Fine aggregate is specified at 1250 lb SSD, with 4.5% moisture and 1.5% absorption. The metered water is 220 lb and assume no other mixing water inputs.

1. Coarse aggregate contributes $(1850/1.008)(0.018-0.008)=18.35$ lb.
2. Fine aggregate contributes $(1250/1.015)(0.045-0.015)=36.95$ lb.
3. Mixing water is approximately 275.30 lb and $w/cm=275.30/600\approx0.459$.
4. The limit permits $0.45(600)=270$ lb. The measured inputs exceed that amount by about 5.30 lb. For a future batch with the same aggregate moisture and no other water inputs, meter at most about 214.70 lb. Report the existing batch discrepancy for disposition.

Check cementitious materials and aggregate size

ASTM C150/C150M-17 distinguishes Type I general-use portland cement, Type II for moderate sulfate resistance, Type II(MH) for moderate heat of hydration and moderate sulfate resistance, Type III for high early strength, Type IV for low heat and Type V for high sulfate resistance. Where specified, identify ASTM C618 fly ash or natural pozzolan, ASTM C989 slag cement and ASTM C1240 silica fume by the approved type and source. Proportioning and durability checks apply to the approved combination. An SCM label alone does not establish the required exposure performance.

Use ASTM C33/C33M for aggregate requirements where applicable. In C33/C33M-16 Section 6, the standard fine aggregate grading includes a fineness modulus from 2.3 to 3.1 and a maximum 0.20 variation from the base fineness modulus for continuing shipments from a source. The section also allows qualifying performance evidence for fine aggregate outside its grading requirements. Fineness modulus describes relative coarseness from cumulative percentages retained on the specified sieves. It does not replace the sieve-by-sieve check.

Under ACI 318-19 Section 26.4.2.1(a)(5), compare one-fifth of the narrowest dimension between form sides, one-third of slab depth where applicable and three-fourths of the minimum specified clear spacing between the applicable reinforcing bars, wires, bundles, tendons or ducts. The smallest applicable value sets the limit unless the licensed design professional approves an exception under the code provision for workability and consolidation. For a formed grade beam with a 14-inch width and 2-inch minimum specified clear bar spacing, the limits are 2.8 and 1.5 inches. The beam therefore uses a nominal maximum aggregate size no greater than 1.5 inches absent an approved exception. The slab-depth criterion is inapplicable.