2.3 Concrete Admixtures, Air Entrainment & Durability Exposure Classes

2.3 Admixtures, Air Entrainment & Exposure Classes

Read the exposure classes assigned by the licensed design professional. A concrete member can carry assignments from several categories. A garage slab exposed to frequent water, freezing and deicers may have both F3 and C2 requirements. Verify the most restrictive applicable water ratio and minimum specified strength, then check each applicable air, constituent, chloride and cover requirement.

Chemical admixtures and air entrainment

ASTM C494/C494M-17 distinguishes Type A water reduction, B retardation, C acceleration, D water reduction with retardation, E water reduction with acceleration, F high-range water reduction, G high-range water reduction with retardation, and S specific performance. Its Table 1 limits water content in the qualification comparison to 95% of the control for A, D and E and 88% for F and G. The test limits describe qualification, while actual performance depends on the approved mixture and conditions. For example, a high-range water reducer may be chosen to produce the specified consistency with less mixing water. Confirm the actual approved product and admixture dosage range where specified.

An ASTM C260 air-entraining admixture promotes a system of small bubbles in the paste, providing relief as water in freezing concrete expands. Larger voids from incomplete consolidation do not provide the same protective system. The fresh air test measures total air by the specified method. Verify the result with the approved sampling location and applicable concrete aggregate type.

F exposure and combined material limits

The values below apply to reinforced normalweight concrete under ACI 318-19 Tables 19.3.1.1 and 19.3.2.1. Table 19.3.2.1 has separate F3 values for plain concrete and exempts lightweight concrete from its maximum $w/cm$ limits.

For F3 concrete, check ACI 318-19 Section and Table 26.4.2.2(b) using percentages by mass of total cementitious materials. Fly ash or other natural pozzolans conforming to C618 are limited to 25%, slag cement conforming to C989 to 50%, and silica fume conforming to C1240 to 10%. The fly ash or natural pozzolan plus silica fume combination is limited to 35%. Their sum with slag cement is limited to 50%. Include applicable constituents already present in blended cement.

For an F3 parking slab mix with 640 lb/yd³ total cementitious material, 160 lb of fly ash is 25% and 80 lb of silica fume is 12.5%. The fly ash meets its individual cap. The silica fume exceeds 10%, and their combined 37.5% exceeds 35%, even though it is under the overall 50% limit. Report both deficiencies during mixture verification.

Select the applicable air target

ACI 318-19 Table 19.3.3.1 provides target total air content for air-entrained F1 through F3 concrete. Section 19.3.3.6 permits a reduction of one percentage point at specified strengths of 5000 psi or more. Verify the approved target and apply the applicable tolerance to that value. ASTM C94/C94M-17a Section 8.2 uses ±1.5 percentage points at point of discharge from the transportation unit. ACI 318-19 Section 19.3.3.2 specifies C172 sampling and C231 or C173 air testing. Use the specified protocol and criteria for additional sampling locations.

For a 3/4-inch aggregate F3 mix with an approved 6.0% target and the stated C94 tolerance, the applicable air interval is 4.5% to 7.5%. A lower target can change the range if the project applies the reduction.

Sulfates, water and corrosion

• Sulfates. Under Table 19.3.2.1, S0 has no sulfate-specific water ratio or cement type limit. S1 uses 0.50 and 4000 psi with the applicable moderate sulfate-resistance cement requirements. S2 uses 0.45 and 4500 psi with the applicable high sulfate-resistance requirements. S3 has two options with different cementitious requirements: 0.45 and 4500 psi or 0.40 and 5000 psi. Check the corresponding ASTM cement designation, required combinations and any approved performance alternative under Section 26.4.2.2(c). For S3 Option 1, verify the applicable high-sulfate-resistance cement combined with qualifying pozzolan or slag. For Option 2, follow the listed high-sulfate-resistance cement route, including the optional ASTM C150 expansion limit when Type V is used alone. The table prohibits calcium chloride admixtures for S2 and S3.
• Water contact. W0 is dry in service. W1 has water contact without a low-permeability requirement. The table sets a 2500 psi minimum for both and no category-specific maximum $w/cm$. W2 has water contact where low permeability is required and uses 0.50 maximum $w/cm$ and 4000 psi minimum. Follow the applicable aggregate requirements in Section 26.4.2.2(d).
• Corrosion. C0 is dry or protected from moisture, C1 involves moisture without an external chloride source, and C2 involves moisture and an external chloride source. C2 adds a 0.40 maximum $w/cm$, a 5000 psi minimum and cover requirements. Maximum water-soluble chloride ion for nonprestressed concrete is 1.00%, 0.30% or 0.15% for C0, C1 or C2 respectively, by mass of total cementitious materials. For prestressed concrete the table limit is 0.06% for all three classes.

Under Table 19.3.1.1, use the more severe applicable sulfate class indicated by the soil or water concentration.

Check any separate prohibition on added calcium chloride. Section 26.4.2.2(f) prohibits calcium chloride admixtures for prestressed concrete. Section 26.4.2.2(g) applies a 0.30% water-soluble chloride limit to concrete placed on or against stay-in-place galvanized steel forms unless a stricter limit applies. A C2 assignment requires its own 0.15% limit for nonprestressed concrete and any applicable project restrictions. Under Table 19.3.2.1 note 9, the SCM mass counted for the chloride limit cannot exceed the portland cement mass. Section 26.4.2.2(e) allows a total chloride calculation based on constituent materials or the specified ASTM C1218 water-soluble test on hardened concrete at 28 to 42 days. Verify material certifications and the approved mixture before comparing air test results.