What is the slab minimum grade concrete to de-shuttering in 14 days?

Minimum Concrete Grade for Slab De-shuttering in 14 Days — Practical, Copyright-Free Guide

Short answer: there is no single universal “minimum grade” that guarantees safe de-shuttering in 14 days — but for typical reinforced floor slabs cast with ordinary Portland cement concretes, using a minimum grade of M20 (20 MPa characteristic compressive strength) combined with good curing often achieves the necessary 14-day strength to remove formwork safely, provided in-situ or companion specimen tests confirm required strength (commonly around 65–75% of design strength at 14 days). This article explains the reasoning, gives worked examples, describes testing and curing procedures, and outlines safe de-shuttering practices.

Why the question matters

Formwork removal (de-shuttering) is one of the most critical stages in concrete construction. Remove props too early and the slab may sag, crack, or fail; wait too long and the schedule suffers and costs rise. Contractors, site engineers, and clients want practical guidance: what concrete grade will be safe to remove formwork at 14 days? The correct answer depends on design strength, expected early strength gain, curing, loading conditions, and the acceptable safety margin.

Key concepts you should know

Characteristic compressive strength (f_ck)

Concrete mix grades are described by their characteristic compressive strength, e.g., M15, M20, M25, where the number indicates f_ck in MPa at 28 days (for standard mixes using OPC). A grade M20 has a characteristic strength of 20 megapascals measured at 28 days.

Early-age strength gain

Concrete gains strength with time. Typical OPC concretes will achieve a portion of their 28-day strength at earlier ages. The fraction at 7 or 14 days depends on mix design, cement type, curing temperature and humidity, and admixtures. Common practice and many codes use a strength ratio — the percentage of 28-day strength achieved at a given age — to judge safety for formwork removal.

Required strength before de-shuttering

Engineers usually require the concrete to reach a specified fraction of f_ck or an absolute compressive strength before removing formwork. A frequently used practical rule is that slabs may be de-shuttered when the concrete attains approximately 65–75% of the 28-day design strength, depending on the design and loading. Another approach is to specify a minimum absolute strength in megapascals measured on companion cubes/cylinders tested at the strip time.

Typical practical targets at 14 days

Below are typical expected strength ranges after 14 days for ordinary mixes cured properly at normal temperatures (20–27 °C). Keep in mind these are approximate and vary by many factors.

• M20 (20 MPa) — expected 14-day strength around 55–75% of 28-day strength → roughly 11–15 MPa
• M25 (25 MPa) — expected 14-day strength roughly 14–18 MPa
• M30 (30 MPa) — expected 14-day strength roughly 17–22 MPa

Note: these are general ranges. Modern mixes with higher cement content, Type II/III cements or positive admixtures (accelerators) can reach higher 14-day strengths; cold weather or poor curing reduces them.

Translating percentage to safety: a simple rule of thumb

Designers often specify that formwork supporting a member may be removed when the concrete attains a specified percentage (call it α) of f_ck. Common values for α range from 0.5 (50%) for non-load-bearing, lightly reinforced forms, up to 0.7–0.75 (70–75%) when early strength matters or when safety margins are tight. For slabs, many practitioners use α ≈ 0.7 as a conservative and practical target at 14 days.

So, if f_ck = 20 MPa (M20), 70% of that is 14.0 MPa. If companion cubes show ≥14 MPa at 14 days (with proper sampling), de-shuttering for typical slab formwork may be considered safe — again, depending on loads and construction sequence.

Worked examples

Example 1 — Residential slab, limited live load

Design: reinforced concrete slab for low-rise residential building. Grade chosen: M20 (f_ck=20 MPa). Engineer requires 70% of f_ck at formwork removal.

Calculation: 0.70 × 20 MPa = 14 MPa. If laboratory or site companion specimen tests show at least 14 MPa at 14 days and curing has been adequate, de-shuttering the slab is generally acceptable. Always ensure supports beneath the slab are not immediately removed if they carry additional structural parts.

Example 2 — Heavier use slab (commercial), prefer faster strength

Design: slab that will later support heavy finishes or early hoistings. Engineer selects M25 for extra margin. Target α = 0.75.

Calculation: 0.75 × 25 MPa = 18.75 MPa. Confirm companion samples reach ≈19 MPa at 14 days before removing formwork.

Minimum grade recommendation

Based on practical experience and typical early-age strength behaviour, a conservative recommendation for aiming to de-shutter slabs at 14 days without special accelerators or high-early strength cement is to use M20 or higher. M15 mixes may not reliably develop enough strength in 14 days under standard curing, especially in cooler climates.

Practical takeaway: if your schedule requires reliable 14-day stripping, design for at least M20 (preferably M25 if loads are higher or if you want more margin) and confirm with testing.

How to confirm strength on site

1. Companion specimens (cubes or cylinders): Cast standard cubes/cylinders from the same batch, cure them under controlled conditions, and test at 14 days. Use the measured compressive strength as the basis for stripping decisions.
2. Non-destructive testing (NDT): Tools like rebound hammers or ultrasonic pulse velocity can provide quick indications of relative strength in the field. NDT should be calibrated with lab tests; do not rely solely on NDT for a go/no-go decision.
3. Manufacturer / supplier certificates: If you are using ready-mix supplied with traceable records and strength forecasts, include those in the decision but back up with on-site sampling where possible.

Important variables that affect early strength

• Cement type and fineness: Rapid-hardening Portland cements or cements with higher early strength gain will reach the desired 14-day strengths faster than ordinary slow cements.
• Water–cement ratio: Lower w/c yields higher early strength (if workability is maintained by admixtures).
• Use of admixtures: Accelerators and high-range water reducers can speed early strength gain and improve workability.
• Temperature: Concrete cured at higher temperatures gains strength faster; cold weather slows gain (use heaters or thermal blankets if needed).
• Curing quality: Proper moist curing greatly improves early strength. Poor curing can reduce early strength dramatically.

Safe de-shuttering procedure (step-by-step)

1. Check test results: Verify 14-day companion specimen strength meets the specified fraction or absolute value required by the engineer.
2. Inspect slab surface and supports: Ensure the concrete surface appears sound, without excessive shrinkage cracking or localized distress.
3. Plan sequence: Remove facing formwork first (edges and soffits) while keeping main props if required. For large spans, remove formwork in short stretches, not all at once.
4. Retain props where needed: Even after shutter removal, keep temporary props under the slab until the structure reaches a self-supporting capacity per the construction sequence. Shoring removal is often staged over weeks.
5. Monitor deflection: After removal, check for unexpected deflection or cracking; if observed, re-prop and investigate.

Cautions and common pitfalls

1. Rely on tests, not dates: The calendar age (14 days) is only meaningful if companion specimens confirm the required strength. 14 days does not universally equal safety.

2. Design loading matters: If the slab will immediately carry heavy loads (e.g., construction equipment), do not remove shoring just because the slab is 14 days old.

3. Weather effects: Rain, cold, or rapid drying reduce early strength. Protect and cure the slab properly.

4. Different parts of the structure behave differently: Beams and columns may require longer support than flat slabs even though their formwork might appear similar.

Recordkeeping and verification

Keep a clear record of batch numbers, mix proportions, curing schedule, companion specimen test reports, and onsite NDT readings. Good documentation helps the engineer justify a safe stripping decision and is essential if any later disputes arise.

Summary checklist before de-shuttering at 14 days

• Companion cubes/cylinders show required strength (e.g., ≥70% f_ck).
• Curing has been continuous and adequate for the first 7–14 days.
• Site temperature and conditions were favourable or compensated for.
• Loading after stripping will be controlled — no heavy loads immediately.
• Engineer or qualified site staff have approved the stripping plan and sequence.

Conclusion

There is no one-size-fits-all numeric grade that guarantees safe de-shuttering at 14 days without considering mix design, curing, temperature, and evidence (tests). For ordinary reinforced slabs under standard conditions, pilots and experienced practitioners commonly use M20 as the minimum practical grade to aim for safe de-shuttering at 14 days — preferably combined with companion specimen confirmation that the concrete has reached roughly 70% of its 28-day strength. If you need faster stripping, use higher grade mixes, specialty cements, or approved accelerating admixtures and always verify with tests.