Setting Time Testing of Underwater Non-Dispersible Concrete: Methods, Standards, and Quality Control

The underwater non-dispersible concrete setting time test determines the initial and final setting times of anti-washout concrete using the penetration resistance method, providing essential data for planning underwater placement and controlling concrete quality. This article explains the applicable standards, sample preparation, test procedures, calculation methods, and typical acceptance criteria for the setting time test of underwater non-dispersible concrete.

Technical Overview

Underwater non-dispersible concrete, also known as anti-washout concrete, is produced by incorporating water-soluble polymer anti-washout admixtures (such as cellulose ethers or polyacrylamide-based agents) into conventional concrete mixtures. These admixtures dramatically increase the cohesion and viscosity of the fresh mixture, allowing the concrete to be placed directly through water with minimal cement leaching, aggregate segregation, or washout of fines.

Because anti-washout admixtures frequently retard cement hydration and extend the workable period of the concrete, the setting time is one of the most important quality control parameters for this material. The setting time test is performed on the mortar fraction sieved from the fresh concrete using the penetration resistance method: the initial setting time is defined as the elapsed time at which the penetration resistance reaches 3.5 MPa, and the final setting time is the elapsed time at which it reaches 28 MPa.

Accurate setting time data allow engineers to balance two competing requirements: the concrete must remain fluid long enough to flow out of a tremie pipe or pump line and self-consolidate on the riverbed or seabed, yet it must gain stiffness quickly enough to permit successive lifts, formwork schedules, and early-age strength development. The principal standards governing this test include:

• GB/T 35159, the Chinese national standard for underwater non-dispersible concrete, which specifies technical requirements and test methods for the material.
• DL/T 5117, the test code for underwater non-dispersible concrete widely applied in hydropower and water conservancy projects, which covers sampling, molding through water, and setting time determination.
• GB/T 50080, the standard for test methods of performance of ordinary fresh concrete, which defines the penetration resistance procedure for setting time.
• ASTM C403/C403M, the international reference method for time of setting of concrete mixtures by penetration resistance.

Testing Samples

Fresh Concrete Sampled in Air

A representative sample of freshly mixed underwater non-dispersible concrete is taken from the mixer, truck, or delivery line under normal air conditions. This sample type establishes the baseline setting behavior of the mixture and is used for routine quality control during production. Sampling should follow the procedures of GB/T 50080, with all portions combined and remixed before testing.

Concrete Sampled After Passage Through Water

To simulate actual underwater placement conditions, the fresh concrete may be allowed to fall freely through a column of water before sampling. This verifies that the anti-washout admixture remains effective and that water contact does not cause abnormal retardation or loss of fines. This approach reflects the through-water molding principles described in DL/T 5117 and is particularly valuable for qualifying new mix designs.

Laboratory Trial Mixes

Trial batches prepared in accredited third-party laboratories are used to evaluate different anti-washout admixture dosages, water-cement ratios, supplementary cementitious materials, and superplasticizer combinations. Setting time tests on these mixes support mix design optimization and admixture qualification before full-scale production.

Testing Items

The setting time test program for underwater non-dispersible concrete typically includes the following measured and derived items:

• Initial setting time, determined as the elapsed time from cement-water contact (or from mixing completion, as specified) until the penetration resistance reaches 3.5 MPa.
• Final setting time, determined as the elapsed time until the penetration resistance reaches 28 MPa.
• Penetration resistance development curve, plotted against elapsed time to document the hardening characteristics of the mortar.
• Setting time difference compared with a reference concrete, used to quantify the retarding effect of the anti-washout admixture in admixture qualification programs.
• Ambient temperature and mortar temperature records, since temperature strongly influences hydration kinetics and must be maintained within the specified tolerance.

Testing Methods

Specimen Preparation and Sieving

Immediately after sampling, the fresh concrete is sieved through a 4.75 mm to 5 mm sieve to extract the mortar fraction. The mortar is placed into rigid metal test containers, compacted lightly to remove entrapped air, and the surface is struck off level. Any bleed water appearing on the surface is removed before each measurement. Because anti-washout concrete is highly cohesive and viscous, careful compaction is required to ensure a uniform, dense mortar specimen. Containers are stored in a controlled environment at 20 ± 2 °C until testing.

Penetration Resistance Procedure

A penetration resistance apparatus fitted with detachable needles of various bearing areas (commonly 100 mm², 50 mm², and 20 mm² for GB/T 50080 equipment, or a comparable decreasing series for ASTM C403 apparatus) is used. At each measurement time, a needle of appropriate size is selected based on the expected resistance, and force is applied steadily until the needle penetrates 25 mm below the mortar surface. Larger needles are used for low resistance at early ages and progressively smaller needles are used as the mortar stiffens. Successive penetration points must be spaced away from the container walls and from previously tested areas so that each measurement is performed on undisturbed mortar.

Determination and Calculation of Setting Times

The penetration resistance is calculated as the force recorded at 25 mm penetration divided by the needle bearing area, expressed in megapascals. Measurements are repeated at regular intervals, typically every 30 to 60 minutes as setting is approached, so that valid data points are distributed on both sides of the 3.5 MPa and 28 MPa thresholds. The resistance values are plotted against elapsed time on a semi-logarithmic scale, and the initial and final setting times are obtained by interpolation at 3.5 MPa and 28 MPa respectively.

Reporting and Acceptance Criteria

The test report should document the concrete mixture proportions, anti-washout admixture type and dosage, sampling method (in air or through water), storage temperature, test apparatus, all penetration resistance readings, the plotted curve, and the interpolated initial and final setting times. In common engineering specifications for underwater non-dispersible concrete, acceptance criteria require an adequately long initial setting time, frequently not less than approximately 5 hours, to accommodate underwater placement operations, together with a final setting time within a controlled upper limit, commonly within about 24 hours. Exact limits should always be confirmed against the governing project specification and the applicable standard.

Applications

Setting time testing of underwater non-dispersible concrete supports a wide range of marine and hydraulic construction activities, including:

• Underwater mass placement for bridge pier foundations, pile caps, and cofferdam seals placed by tremie or pumping.
• Hydraulic structure construction and repair, including stilling basins, dam faces, sluice sills, and canal linings under limited dewatering conditions.
• Marine and offshore works such as breakwater core filling, scour protection, underwater trench backfill, and offshore wind foundation works.
• Emergency repair and sealing operations where rapid deployment through water is required and the open time of the concrete must be verified.
• Admixture qualification and mix design verification in accredited third-party laboratories prior to full-scale production.

Reliable setting time data enable contractors to schedule placement rates, sequence lifts, plan joint construction, and avoid cold joints or premature stiffening inside delivery equipment.

FAQ

Why does underwater non-dispersible concrete usually set more slowly than ordinary concrete?

The polymeric anti-washout admixtures that give the concrete its cohesion also increase the viscosity of the mixing water and adsorb on cement particles, which slows the transport of ions and slightly delays early hydration. Combined with the high-range water reducers often used in these mixtures, the result is a prolonged setting time, which the test is designed to quantify and control.

Can the Vicat needle test be used instead of the penetration resistance test?

No. The Vicat test is intended for cement paste, whereas the penetration resistance method is specifically designed for mortar sieved from concrete and is therefore the appropriate technique defined by GB/T 50080 and ASTM C403 for evaluating concrete setting behavior.

What should be done if the final setting time exceeds the specified limit?

Corrective actions include reducing the anti-washout admixture dosage (while re-verifying washout resistance), incorporating a suitable accelerating admixture, lowering the retarding component of the water reducer, reducing the proportion of slowly hydrating supplementary cementitious materials, or lowering the placement temperature. Any adjustment should be validated by a fresh setting time test on a trial mix.

Does deep-water pressure affect the measured setting time?

For most conventional placement depths, the effect of hydrostatic pressure on setting time is small compared with the effects of temperature and admixture chemistry. For very deep placements, project-specific studies or mock-up tests are recommended, and the laboratory test remains the contractual reference value.

How long does the test take to complete?

Because underwater non-dispersible concrete is deliberately retarded, measurements often continue for 24 hours or more until the penetration resistance exceeds 28 MPa. Laboratories plan test schedules accordingly and maintain the 20 ± 2 °C temperature condition throughout the entire test period.

Who can perform this test?

The test can be performed by construction site laboratories, ready-mixed concrete producers, or accredited third-party laboratories with calibrated penetration resistance apparatus and controlled temperature facilities, in accordance with the sampling and testing requirements of the applicable standards.