Anti-washout admixtures for concrete are water-soluble polymers or formulated viscosity modifiers used to keep cement paste and fine particles from dispersing when fresh concrete meets water. In addition, they increase cohesion and water retention, so a flowable mixture can remain more stable during underwater placement.
When we speak of underwater concreting, we have to mention anti-washout admixtures. Engineers also use them as viscosity-modifying admixtures in grout, shotcrete, pumped concrete, and some self-compacting concrete mixtures. Therefore, the purpose is similar, but the required performance and dosage are not automatically the same.
What Does an Anti-Washout Admixture Do?
Underwater concrete must be workable enough to move through a tremie or pump line and flow into the form. At the same time, the paste must not separate when it contacts the surrounding water. Therefore, this is the balance the admixture helps to achieve.
So, yes, designers often use a high-range water reducer to provide flowability without adding excess water. In contrast, the anti-washout admixture provides cohesion and washout resistance. Accordingly, do not select either product alone because their interaction can change flow, air content, setting time, pump pressure, and strength.
Mechanism of Anti-Washout Admixtures
Polymeric anti-washout admixtures are highly hydrophilic. They bind part of the mixing water and increase the viscosity of the liquid between the solid particles. Depending on the polymer and its concentration, several mechanisms may act together:
- Water binding: the polymer holds water within its molecular structure, reducing the amount that can move freely and carry cement particles away.
- Bridging flocculation: a polymer chain adsorbs onto more than one cement particle and links them into a more cohesive network.
- Polymer association and entanglement: long chains interact with one another and increase apparent viscosity, especially when the concrete is at rest.
- Thixotropic response: the concrete can become easier to move under mixing or pumping shear, then build structure again when the movement stops.
As a result, the mixture is not simply “thicker concrete.” For example, a suitable mixture can be fluid while moving and still have enough cohesion to resist segregation and washout after placement.
Types of Anti-Washout and Viscosity-Modifying Materials
There are several ways to classify these materials. A practical starting point is to separate natural, semi-synthetic, and synthetic polymers, then check the actual product chemistry and performance data. In addition, fine mineral materials can improve cohesion, but they should not be treated as interchangeable with a proprietary polymeric anti-washout admixture.
| Material group | Examples | Main action | Practical check |
|---|---|---|---|
| Natural biopolymers | Welan gum, diutan gum, xanthan gum, guar gum, alginates | Increase water-phase viscosity and may form a particle-binding network | Flow retention, air, set, biological stability, and compatibility with the selected water reducer |
| Semi-synthetic polymers | Cellulose ethers, modified starches, modified guar derivatives | Water binding, thickening, and polymer entanglement | Slump or flow loss, mixing time, retardation, and pumpability |
| Synthetic polymers | Polyacrylamides, polyethylene oxide, polyvinyl alcohol, selected acrylic polymers | Thickening, adsorption, bridging, or association depending on chemistry | Dosage sensitivity, batching sequence, strength, and compatibility |
| Fine mineral or colloidal systems | Silica fume, colloidal silica, bentonite, other fine powders in formulated systems | Increase surface area, paste volume, water retention, or particle packing | Water demand, shrinkage, heat, finishing, pump pressure, and whether the product is actually qualified as an AWA |
Role During Underwater Placement
Use the placement method to protect fresh concrete from direct water contact as far as practical. In tremie placement, the pipe starts full and its lower end remains embedded in the fresh concrete. Therefore, new concrete enters the protected mass instead of falling through open water. Pump placement follows the same idea: keep the discharge end immersed when possible, keep the line watertight, and maintain continuous flow.
An anti-washout admixture adds a margin of stability, particularly where the concrete must spread laterally in a thin section or where crews cannot avoid limited water contact. However, it does not correct a broken tremie seal, leaking pipe joints, interrupted supply, excessive free fall, or strong uncontrolled current.
How Washout Resistance Is Tested
The current product-qualification standard is ASTM C1882/C1882M-24e1, Standard Specification for Anti-washout Admixtures for Concrete. It qualifies a material as an anti-washout admixture; ASTM states directly that the results are not intended to determine dosage for a specific application. The older U.S. Army Corps of Engineers CRD-C 661-06 specification is still named on many product data sheets and provides the established drop-through-water procedure described below.
In the CRD-C 661 washout test, a perforated container holds a fresh-concrete sample with an initial mass of 2,000 ± 20 g. Next, the technician drops the container through a 1,700 ± 5 mm water column, recovers it, drains it, and weighs it. The technician repeats the sequence three times on the same sample.
Mi = mass before the first drop; Mf = mass remaining after the specified drop.
Report the results as D1, D2, and D3: cumulative mass loss after each drop. However, CRD-C 661 notes that the method’s precision has not been determined and its bias cannot be determined because no standard reference material is available. For this reason, a project specification should define its own acceptance plan rather than relying on a single number without context.
Qualification and Project Acceptance
| Check | What ASTM C1882/C1882M-24e1 does | What the project still must decide |
|---|---|---|
| Washout, D3 | Requires reference concrete with at least 10% washout and at least a 50% reduction for the test concrete | The maximum permitted mass loss for the actual work, water conditions, and environmental controls |
| Setting time | Requires the result to be recorded; it does not impose one universal limit | Maximum or minimum set time needed for lift continuity, finishing, and form pressure |
| Air content | Limits the increase over the reference concrete to 3 percentage points | Target air content and air-void requirements for the exposure |
| Compressive strength | Requires at least 90% of reference strength at 7 and 28 days | The specified strength, curing condition, additional test ages, and acceptance criteria |
| Slump or flow | Product qualification does not replace the project’s placing-consistency requirement | Required slump, slump flow, flow retention time, segregation resistance, and placing method |
Effects on the Concrete Mixture
| Change | Useful effect | Possible tradeoff | What to verify |
|---|---|---|---|
| Higher cohesion and plastic viscosity | Less bleeding, segregation, and loss of cement paste | Sticky handling, lower slump or flow, higher mixing and pumping demand | Flow retention, pump pressure, discharge behavior, and surface finish |
| Use with HRWR | Restores flow without simply adding water | Competitive adsorption or sequence effects can change rheology | Product compatibility, addition order, remixing time, and delayed dosing |
| Interaction with air entrainment | Required air can still be produced with a compatible system | Air content and air-void quality may change | Fresh air, unit weight, strength, and durability tests when exposure requires them |
| Water retention and hydration effects | Less dilution and a more intact paste underwater | Some products retard set or change early strength | Setting time and strength at the ages important to construction |
| More fines or supplementary materials | Can improve packing, cohesion, and pumpability | May change water demand, heat, shrinkage, and admixture demand | Full mixture performance, not one ingredient in isolation |
For example, the original article warned about mixer and pump demand. That warning remains useful, but a fixed percentage increase is not reliable across all polymers, dosages, and mixtures. Therefore, measure the actual behavior in the trial batch and placement trial.
Selecting and Accepting an Anti-Washout System
- Define the placing condition. Record water velocity and agitation, placement depth, fall height, flow distance, reinforcement congestion, concrete volume, lift geometry, and the tremie or pump arrangement.
- Select the product as part of an admixture system. Review the current data sheet, CRD-C 661-06 or other specified qualification evidence, chloride information when relevant, storage, mixing time, addition order, and known incompatibilities.
- Use project materials in trial batches. Keep the proposed cementitious materials, aggregates, water, air entrainer, HRWR, batching sequence, temperature, and expected holding time.
- Test both movement and stability. Check slump or slump flow over time, segregation, bleeding, washout mass loss, air, unit weight, setting time, pumpability, and strength. Add durability tests required by the exposure.
- Make a representative placement trial when risk is high. Confirm the discharge method, flow distance, line pressure, continuity, surface laitance, volume reconciliation, and inspection plan before the production pour.
Benefits and Limitations
Overall, the main benefit is better integrity of concrete placed in water: less paste loss, lower turbidity, reduced segregation and bleeding, and a better chance of achieving the intended in-place strength and bond. In addition, the same rheology control can improve the robustness of selected SCC, grout, shotcrete, and pumped mixtures.
However, the limitations are just as important. Too much viscosity can reduce flow, trap air, increase pumping demand, and make the mixture sticky. In addition, some products can retard setting or change air content and strength. Water movement can still erode exposed fresh concrete, and an anti-washout admixture cannot repair a poor placement plan.
Look at the data sheet of the product you are going to use and understand it clearly. Then test the whole mixture. If in doubt, do trial batches.
References
- ASTM C1882/C1882M-24e1, Standard Specification for Anti-washout Admixtures for Concrete.
- U.S. Army Corps of Engineers, CRD-C 661-06, Specification for Antiwashout Admixtures for Concrete.
- USACE EM 1110-2-2002, Evaluation and Repair of Concrete Structures.
- FHWA-NHI-10-029, Underwater Bridge Repair, Rehabilitation, and Countermeasures.
- EFNARC/EFCA Guidelines for Viscosity Modifying Admixtures for Concrete.
- Comparative study of anionic and nonionic anti-washout admixtures, Buildings, 2024.
- ASTM C494/C494M-24, Standard Specification for Chemical Admixtures for Concrete.