How to Select Sealants for Expansion Joints in Concrete and Prefabricated Buildings
Quick Answer
The procurement sequence that surrounds these joints is set out in the joint-sealing procurement guide.
An expansion joint is the one place in a building where the sealant is meant to fail gracefully — by stretching — rather than hold. That inverts the usual instinct, which is to specify the strongest product available.
Three decisions set the outcome, in order:
- How much movement the joint actually sees, calculated from panel size, temperature range, shrinkage and transport.
- The joint width, which must be sized against that movement and the product’s displacement class — not chosen because 20 mm is a round number.
- Whether the joint is a seal or a structural connection. These are different products, and in prefabricated construction they are frequently confused.
Get those three right and the product choice becomes narrow. Get them wrong and no product rescues the detail.
1. Why Prefabricated Joints Are Harder Than Cast-in-Place Joints
Four movement sources act on a prefabricated joint, and they do not peak at the same time:
| Source | Timescale | Typical magnitude |
|---|---|---|
| Thermal expansion and contraction | Daily and seasonal | Large, reversible |
| Concrete shrinkage and creep | Months to years | Irreversible, one-directional |
| Transport and erection | Hours, once | Can be the largest single event |
| Foundation settlement | Years | Small but permanent |
Cast-in-place construction mostly deals with the first. Prefabricated construction adds the third, and it is the one most often omitted from the joint calculation. A modular unit that travels 300 km on a low-loader experiences racking and vibration that a joint sized only for thermal movement will not accommodate.
Where the joint sits on an external facade, facade and glazing solutions covers the adjacent details.
The other difference is substrate. Precast concrete and ALC block are porous and alkaline. Silicone adheres to them less reliably over time than MS polymer, particularly where moisture migrates through the panel — which is why the sealant choice in this category is usually an MS decision before it is a grade decision.
2. Calculate the Movement, Then Size the Joint
A joint width chosen by habit is the most common cause of premature failure in this category. The calculation is short.
Worked example. A 6 m precast panel, concrete α ≈ 10–12 × 10⁻⁶/K, surface temperature swing 50 K:
- Thermal movement ≈ 6,000 mm × 11 × 10⁻⁶ × 50 ≈ 3.3 mm, i.e. ±1.65 mm about the mean.
- Add shrinkage and erection tolerance; a realistic design movement for the joint is often 5–6 mm total.
- Required joint width = movement ÷ displacement class.
| Product class | Required minimum joint width for 6 mm movement |
|---|---|
| ±25% | ≈ 24 mm |
| ±35% | ≈ 17 mm |
| ±50% | ≈ 12 mm |
Read the table in the direction that costs money. Choosing a lower displacement class does not just reduce the sealant price — it forces a wider joint, which changes the panel module, the reveal detail and sometimes the structural layout. In early design, the cheap sealant can turn out to be the expensive decision.
The reverse also holds: on a joint that has to stay narrow for architectural reasons, a higher displacement class is not a luxury, it is the only way the detail closes.
3. Seal or Structure: the Distinction That Causes Real Failures
In the INGADA range there are MS products that look similar by name and behave oppositely:
- MS-35 and MS-35W are bonding MS adhesives — high initial grab, high shear, structural bonding. They do not carry a displacement class and are not 35-class sealants.
- MS-635 is the 35-class sealant for precast and ALC panel joints, with 100% modulus ≤0.4 MPa and elongation ≥500%.
Using a bonding adhesive in a movement joint produces a joint that resists movement until the bond line fails — often within the first heating cycle. Using a low-modulus sealant to fix a component produces a component that moves.
The same logic applies one step further: MS-55 is a structural grade (tensile strength ≥4.0 MPa, steel-to-steel shear ≥3.7 MPa, hardness 70 ± 5) intended for load-bearing connections, and is explicitly not for large-movement joints. Where a project needs both, it needs two products and two details, not one compromise.
4. Choosing the Grade
| Joint condition | Class | Reference product | Why |
|---|---|---|---|
| Standard precast panel or ALC joint | ±35% | MS-635 | Ultra-low modulus, designed for porous silicate substrates; 5,000-cycle fatigue at ±35% |
| High-movement: tall units, seismic zones, long modules | ±50% | MS-650 / SR-50L | Elongation ≥800% (MS), or ≤0.30 MPa modulus (SR) |
| Budget-constrained but movement still significant | ±25% | MS-625 | Tensile ≥1.4 MPa, elongation ≥450%, concrete adhesion ≥1.2 MPa, paintable |
Two notes that matter in review meetings:
- ±35% is the sensible default for prefabricated panel joints. Movement in this category is routinely underestimated, and the cost difference between 25 and 35 class is small relative to the cost of remediation.
- Where the joint must be painted or rendered, MS polymer is the only option in this list. Silicone cannot be overcoated.
5. Geometry Details That Decide Performance
- Width-to-depth ratio around 2:1, with the depth set by a backer rod. A joint that is deeper than it is wide will bond three-sided and tear.
- Bond breaker on the back face, always. In a cast joint this is the backer rod; in a formed reveal it may need tape.
- No sealant where a movement joint meets a rigid infill. The transition detail is where most leaks begin, because the joint moves and the infill does not.
- Tooling, not smearing. A concave tooled profile puts the tensile stress in the middle of the bead, where the material is strongest. A flat or smeared bead puts it at the bond line.
For prefabricated units, add one more: protect the joint during transport and erection. Sealant applied in the factory and then compressed or racked on a low-loader has already been loaded once before the building is occupied.
6. Specification and RFQ Checklist
- Panel size, material and joint layout, with a drawing, not a description.
- Design movement with the calculation shown — thermal, shrinkage, transport, settlement.
- Joint dimensions and the intended width-to-depth ratio.
- Substrate pair, named as a pair: precast-to-precast, precast-to-ALC, concrete-to-steel.
- Service temperature range at the joint surface.
- Whether the joint is a seal or a structural connection — stated explicitly.
- Finish requirements — painted, rendered, exposed, natural stone adjacent (staining risk).
- Compliance statement accepted for outdoor use: CE for mechanical performance, ISO 9001 quality system with ISO 11600 product standard, and RoHS.
If you are still narrowing the range itself, start from the product categories rather than from a single model.
A supplier who can answer the movement and width questions in the same reply as the price is a supplier who understands the category. One who quotes a grade without asking for the joint width has not yet engaged with the problem.
Discuss Your Expansion Joint Requirements with INGADA
If you are selecting sealants for expansion or movement joints in concrete or prefabricated construction, prepare the joint drawing, panel sizes and materials, the calculated design movement, substrate pair, joint width and depth, service temperature range, and whether the joint must remain paintable.
INGADA can review the stated parameters, confirm which class fits the calculated movement and joint geometry, and identify what still needs project-specific fatigue or adhesion evidence before the specification is issued.
Suitability depends on the calculated movement, joint geometry and substrate pair. Confirm the class against the project calculation before ordering.
