TxDOT Item 421: Hydraulic Cement Concrete Mix, Testing, Curing

TxDOT Item 421, Hydraulic Cement Concrete, is the specification that governs every load of concrete placed on a Texas Department of Transportation project, from the aggregate source to the cylinder break that decides whether the pour stays or comes out. The 2024 edition applies to pavements, bridges, culverts, retaining walls, and virtually every other concrete element built with state funds.¹ It sets the material sources you can draw from, the strength class each element must meet, the mix design method, the discharge time limits, the tests run at the point of placement, and the acceptance and pay-adjustment rules that follow.

Approved Materials and Sources

Materials have to come from prequalified sources listed on the TxDOT website. Aggregates specifically must come from sources listed in the Concrete Rated Source Quality Catalog (CRSQC).¹ A non-listed source can be used only if it is sampled, tested, and approved before batching, and TxDOT allows itself 30 calendar days for that process.²

Each material has its own departmental spec:

Hydraulic cement: DMS-4600
Fly ash and related coal ash products: DMS-4610
Slag cement: DMS-4620
Silica fume: DMS-4630
Natural pozzolans: DMS-4635¹

A few hard prohibitions govern admixtures. Calcium chloride-containing admixtures are banned from all TxDOT concrete, and Type C, E, F, and G admixtures cannot be used in Class S bridge deck concrete. Recycled crushed concrete is allowed as coarse or fine aggregate in Class A, B, E, and P concrete, but cannot exceed 20% of the total fine aggregate when used on the fine side. White hydraulic cement projects require light-colored aggregates and prohibit supplementary cementitious materials entirely.¹

Concrete Classes and Strength Requirements

The plans dictate which class of concrete is used in each element, and the class fixes the minimum design strength and the maximum water-to-cementitious-material (w/cm) ratio. From Table 8 of the 2024 specification:¹

Class A, 3,000 psi, 0.60 w/cm. Curbs, gutters, sidewalks, driveways, backup walls, anchors, and non-reinforced drilled shafts.
Class B, 2,000 psi, 0.60 w/cm. Lower-strength applications with minimal structural demand.
Class C, 3,600 psi, 0.45 w/cm. Drilled shafts, bridge substructure, traffic rail, culverts (except top slabs of direct-traffic culverts), headwalls, wing walls, inlets, manholes, and traffic barriers.
Class S, 4,000 psi, 0.45 w/cm. Bridge slabs, top slabs of direct-traffic culverts, and approach slabs.
Class P, 0.50 w/cm. Concrete pavement, with strength governed by Item 360.
Class H, 0.45 w/cm. Precast and post-tensioned members, with design strength on the plans.
Class CO, 4,600 psi, 0.40 w/cm. Bridge deck concrete overlays.
Class LMC, 4,000 psi, 0.40 w/cm. Latex-modified concrete overlays.
Class SS, 3,600 psi, 0.45 w/cm. Specialty structural applications.

Design strength must be reached within 56 days. Classes F, H, K, and HES take their strengths from the plans rather than the table. A higher-strength class with an equal or lower w/cm ratio may be substituted for the specified class when the Engineer approves.¹

Mix Design Rules

Mix designs are developed under ACI 211, Tex-470-A, or another approved procedure. The 2024 specification requires the absolute volume method unless the Engineer approves an alternative, and cement replacement uses the equivalent weight method. The mix cannot exceed the maximum w/cm ratio for its class, and total cementitious material is capped at 700 pounds per cubic yard unless otherwise specified.¹

Supplementary Cementitious Materials

Item 421 offers three SCM replacement options:

Option 1. Up to 50% replacement with coal ash or natural pozzolan, and up to 70% for mass concrete placements.
Option 2. 35–50% replacement with slag cement.
Option 3. 35–50% combination of coal ash, slag, natural pozzolan, or silica fume, with silica fume capped at 10%.¹

For Class A, E, and P concrete with cementitious content at or below 520 pounds per cubic yard, any coal ash or natural pozzolan on the Material Producer List may be used at 20–50% replacement. When high-performance concrete is required and less than 20% of cement is replaced with SCMs, the contractor must provide a certified test report from a licensed professional engineer showing permeability below 1,500 coulombs under ASTM C1202, tested after either 56 days of moisture curing at 73°F or 7 days at 73°F followed by 21 days at 100°F.¹

Air Entrainment

When the plans call for air-entrained concrete, the target air content is 4.0% for Class P and 5.5% for all other classes, with acceptance based on a tolerance of ±1.5% from the target. If air content exceeds the target by more than 1.5% but less than 3.0%, the concrete may still be accepted based on strength tests. For mix designs with specified strengths above 5,000 psi, a 1% reduction in target entrained air is permitted.¹

Delivery and Discharge Time Limits

Mixers and agitators must produce a uniform mass meeting at least five of the six requirements in Tex-472-A, and any equipment that fails the uniformity test is pulled off the job. Topping new concrete onto returned concrete in the truck is prohibited.³

Delivery time is controlled by the temperature of the fresh concrete. Table 14 sets the maximum time from batching to the start of discharge:¹

90°F and above. 45 minutes without a retarder, 75 minutes with a Type B or D admixture.
75°F to 89°F. 60 minutes without a retarder, 90 minutes with a Type B or D admixture.
Below 75°F. 90 minutes without a retarder, 120 minutes with a Type B or D admixture.

Concrete that has not begun discharge within those windows is rejected. On a hot day, the 45-minute limit from plant to pour site is a serious operational constraint, and route planning matters as much as the mix.

Field Testing at the Point of Placement

Three tests run at the pour: slump, air content, and casting of strength specimens.

Slump is measured under Tex-415-A.⁴ Concrete below the minimum slump after all withheld water has been added is rejected unless the Engineer allows it. Concrete over the maximum slump may still be used at the contractor’s option, but the Engineer will cast and evaluate strength specimens to confirm performance.¹

Air content is measured under Tex-414-A, which references ASTM C173 for the volumetric method.⁵ Loads outside the ±1.5% tolerance go to the Engineer for an acceptance decision. Fresh concrete showing segregation or excessive bleeding is rejected regardless of other test results.¹

Compressive strength is verified through cylindrical specimens made and cured under Tex-447-A.⁶ Specimens must be delivered to curing facilities and removed from molds within 24 to 48 hours, then placed in curing tanks, and later crushed to check the class-specific design strength within the 56-day window.¹

The maturity method under Tex-426-A is allowed when specified or permitted by the Engineer. Temperature sensors embedded in the concrete estimate strength development in real time. Any change in mix ingredients or proportions requires a new strength-maturity relationship, and personnel using the method must complete a TxDOT-recognized training program.³

Acceptance, Pay Adjustment, and Removal

Acceptance sits in Article 421.5. A placement is accepted at full price when the average of all test results meets the required design strength and no individual test falls below 85% of that strength.⁸

The department must decide to reject or apply a pay adjustment within 56 days after placement.³ Contractors who read silence as acceptance can be caught by a late deduction.

Placement and Curing

Item 421 does not prescribe placement and curing procedures itself. It directs contractors to place, finish, and cure concrete in conformance with the pertinent Items, which means Item 420 for structures and Item 360 for pavement.¹

TxDOT recognizes three curing methods: form curing, water curing with wet mats or spray, and membrane curing with a compound applied at 180 square feet per gallon per coat when water curing is not practical.⁹ For structures, the first coat of curing compound must go on within 45 minutes after the evaporation retardant is applied. For pavements, the first coat must be applied within 10 minutes after texturing, with a second coat within 30 minutes.

Who Can Run the Tests

Testing personnel need certifications recognized by the department. The industry standard is the ACI Concrete Field Testing Technician–Grade I certification, which covers seven basic fresh-concrete tests including temperature, slump, air content by pressure and volumetric methods, density, and specimen preparation.¹⁰ Personnel using the maturity method need separate TxDOT-recognized training on top of the standard field testing credential.

1
Texas Department of Transportation. 2024 Standard Specifications – Item 421 – Hydraulic Cement Concrete
2
Texas Department of Transportation. Concrete Rated Source Quality Catalog
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Texas Department of Transportation. Item 421 – Hydraulic Cement Concrete
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Texas Department of Transportation. Tex-415-A – Slump of Hydraulic Cement Concrete
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Texas Department of Transportation. Tex-414-A – Air Content of Freshly Mixed Concrete
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Texas Department of Transportation. Tex-447-A – Making and Curing Concrete Test Specimens
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8
Texas Department of Transportation. Item 421 – Hydraulic Cement Concrete
When test results fall short but a structural review finds the concrete adequate to remain in service, TxDOT applies a pay adjustment rather than requiring removal. The formula uses the ratio of actual average strength to specified strength to calculate a reduced unit price for the entire placement in question, and the further below spec the concrete tests, the steeper the reduction. If the structural review determines the concrete is not adequate, the Engineer defines the limits of what must be removed, and the contractor bears the cost.7Texas Department of Transportation. Item 421 – Hydraulic Cement Concrete
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Texas Department of Transportation. Concrete Curing
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American Concrete Institute. Concrete Field Testing Technician – Grade I