Contents
- How to use this guide
- Shrinkage basics: what SRAs change
- Where SRAs fit vs other crack-control tools
- Key selection factors
- Dosage, addition, and batching notes
- Compatibility: PCE, air, SCMs, fibers, curing
- Recommended tests and acceptance criteria
- Troubleshooting: symptoms → likely causes
- Procurement specs & acceptance checks (COA/SDS)
- RFQ notes
- FAQ
How to use this guide
This is a practical decision aid for readymix producers, precast plants, contractors, and procurement/EHS teams. Use it to align on: target performance (shrinkage reduction and crack-risk reduction), acceptable trade-offs (air/strength/set), and how you will verify performance (standard tests + job-site KPIs).
Important: SRAs reduce drying shrinkage potential, but they do not replace good curing, joint layout, and restraint management. If job-site practices are weak, SRAs cannot “guarantee” crack-free slabs.
Shrinkage basics: what SRAs change
Concrete cracking risk is driven by restraint + tensile stress development versus tensile capacity. Shrinkage contributes by generating tensile stresses as moisture leaves the system and capillary stresses develop.
What SRAs typically do well
- Reduce drying shrinkage potential (project- and mix-dependent).
- Lower capillary stress during drying (mechanism depends on chemistry).
- Improve crack-risk margin in restrained slabs and toppings when used with proper curing/joints.
What SRAs do not “solve” alone
- Plastic shrinkage cracking from wind/heat/rapid evaporation (needs curing and finishing control).
- Thermal cracking in mass pours (needs thermal management + mix design).
- Bad joint layout or uncontrolled restraint.
Where SRAs fit vs other crack-control tools
| Tool | Main benefit | Best use case | Trade-offs / notes |
|---|---|---|---|
| SRA | Reduces drying shrinkage driving force | Slabs-on-grade, toppings, restrained elements, flatwork with crack sensitivity | Verify air/set/strength impacts; validate with trials |
| Fibers (micro/macro) | Controls crack widths, improves toughness | Plastic shrinkage, early age cracking, reduced rebar in some designs | Finishability and dosage selection matter |
| Internal curing (e.g., SAP/LWA) | Supports hydration, reduces autogenous shrinkage | Low w/c mixes, high-performance concrete | Design and QC complexity |
| Shrinkage-compensating systems | Expansive reaction offsets shrinkage | Specialty slabs and restrained applications | Requires careful design, curing, and testing |
| Curing + joint design | Controls moisture loss and restraint cracking | Every project | Highest ROI; must be executed |
Key selection factors
When specifying SRAs, start with the job reality: exposure, slab geometry, joint spacing, finishing method, and curing plan. Then select SRA grade and dosage to meet performance without unacceptable side effects.
Project drivers
- Crack sensitivity (architectural slabs, coatings, watertightness)
- Restraint level (subgrade friction, reinforcement, geometry)
- Climate (wind/heat, indoor drying conditions)
- Schedule (early strength, saw-cut timing)
Mix design variables
- w/c ratio and paste volume
- SCMs (fly ash, slag, silica fume)
- Aggregate grading and moisture control
- Air entrainment requirement (freeze-thaw)
Operational constraints
- Batch plant dosing accuracy
- Packaging (drum/IBC/bulk)
- Storage temperature and shelf life
- Documentation and compliance needs
Dosage, addition, and batching notes
SRAs are commonly supplied as liquids and dosed per cementitious content. Actual dosage depends on target shrinkage reduction and the specific formulation.
Practical dosing guidance (how to talk about it in an RFQ)
- Target: define the shrinkage reduction objective (e.g., “reduce drying shrinkage vs control mix”) and the test method/timepoint.
- Dosage window: request a supplier-recommended range for your cementitious content and performance goal.
- Addition sequence: confirm best practice with your plant (some SRAs interact with air and PCE timing).
- Trial approach: begin with a baseline dosage and adjust based on measured shrinkage + air/set/strength results.
Commercial tip: SRAs are usually costed on cost per m³ (or per ton), not just price per kg. Compare suppliers on delivered cost at the dosage needed to hit your shrinkage target, including any offsetting changes (air entrainer, PCE demand).
Compatibility: PCE, air, SCMs, fibers, curing
Compatibility is the biggest “make or break” factor in field success. Validate with representative materials and job-site placing/finishing conditions.
| System element | What can happen | What to do |
|---|---|---|
| PCE superplasticizers | Demand shift; slump retention changes | Run trial mixes at expected temperature; confirm set and finishing window |
| Air-entraining agents | Air content drift; bubble stability changes | Measure air consistently; adjust AEA dosage; confirm spacing factor if required by spec |
| Set control | Set acceleration/retardation depending on chemistry and dosage | Check initial/final set, finishing timing, saw-cut window |
| Strength (early/28d) | Possible early strength reduction at higher dosages | Check compressive strength at required ages; balance with curing and PCE optimization |
| SCMs (slag/fly ash/silica fume) | Workability and set changes can amplify | Use actual project binder blend in trials; verify finishability |
| Fibers | Finishability interaction; surface appearance impact | Trial with the same fiber type and finishing method; watch plastic shrinkage risk |
| Curing compounds / wet curing | Curing dominates crack performance | Maintain strong curing plan; do not reduce curing because SRA is used |
Recommended tests and acceptance criteria
Specify tests that match your risk: restrained cracking, drying shrinkage, and construction practicality. Align your acceptance criteria with the project spec and local practice.
Commonly used evaluation set
- Drying shrinkage: compare SRA mix vs control at defined ages and curing conditions.
- Restrained ring cracking: evaluate crack time/width tendencies under restraint.
- Fresh concrete: slump/slump retention, air content, unit weight, temperature.
- Set and finishing window: initial/final set + job-site observations.
- Compressive strength: early age and design age requirements.
Procurement-ready performance statement
A strong RFQ can say: “Supply SRA suitable for our binder blend and PCE system, with documented shrinkage reduction vs control in our trial protocol, with no unacceptable impact on air content, set, finishability, or specified strengths.”
Troubleshooting: symptoms → likely causes
| Symptom | Likely causes | First checks |
|---|---|---|
| Air content out of spec | SRA interaction with AEA/PCE; dosing sequence changes | Verify dosing accuracy, sequence, and temperature; retune AEA in trials |
| Finishing window feels different | Set time shift, evaporation conditions, paste rheology change | Measure set times; review weather/evaporation control; confirm water additions were controlled |
| Early strength lower than expected | High dosage; curing deficiency; binder/SCM effects | Check curing, temperature history, and dosage; evaluate a slightly lower dose or mix optimization |
| Cracking still occurs | Restraint/jointing/curing issues dominate | Audit curing plan and joint spacing/timing; evaluate plastic shrinkage controls and restraint conditions |
Procurement specs & acceptance checks (COA/SDS)
Ask for the information receiving can verify and operations can run reliably. Consistency matters for air and set performance.
| Category | What to request | What to verify on receipt |
|---|---|---|
| Identity & traceability | Product name/grade, manufacturer, batch/lot, production date | Labels match PO and COA; traceability preserved |
| Quality (COA) | Appearance, density, pH (if applicable), viscosity range, active/solids (if applicable) | COA within agreed limits; no phase separation or freeze damage |
| Performance references | Recommended dosage range and compatibility notes (PCE/AEA/SCMs) | Trial plan aligns with your binder blend and job conditions |
| Safety | Current SDS, PPE guidance, storage and spill guidance | SDS revision current; EHS review complete |
| Logistics | Packaging (drum/IBC/bulk), shelf life, storage temperature limits, lead time | Storage conditions available; dosing equipment compatible |
RFQ notes (what to include)
- Application: slab-on-grade, topping, precast, watertight concrete, architectural finish, etc.
- Binder system: cement type, SCM %, total cementitious (kg/m³), w/c ratio.
- Admixture package: PCE type, AEA requirement, retarders/accelerators, fibers (type/dose).
- Target performance: shrinkage reduction goal and evaluation method/timepoint; crack-risk concerns.
- Constraints: strength requirements (early/28d), air content limits, set/finish window expectations.
- Volumes & packaging: monthly demand, drum/IBC/bulk preference, destination country/incoterms.
- Documentation: COA, SDS, batch traceability, and any project-specific compliance needs.
FAQ
When is SRA a strong choice?
SRAs are often a strong option when drying shrinkage is a key contributor to cracking risk—such as large restrained slabs, toppings, overlays, or where coatings/watertightness require fewer microcracks—provided curing and jointing are executed well.
Can we reduce joints or curing because we use SRA?
Generally, no. SRAs can improve the shrinkage margin, but joint design and curing are fundamental controls. Any changes to joint spacing/timing should be engineered and validated with trials and field performance history.
What’s the most common reason SRAs “disappoint” in the field?
Inconsistent job-site practices (curing delays, high evaporation, uncontrolled water additions) and compatibility issues (air/set shifts). A controlled trial with the exact binder and admixture package is the fastest way to de-risk.
Educational content only. Always follow site EHS rules and supplier SDS/TDS for safe handling and use. Performance depends on materials, curing, and restraint conditions. Share your binder blend and project constraints for a tailored shortlist.