GB 50496-2018 is currently China's parent standard for mass concrete construction in industrial and civil buildings. Issued by MOHURD Announcement No. 77 of 2018, effective December 1, 2018, it replaced GB 50496-2009. Its core philosophy: "Calculate beforehand, Monitor during construction, Cure afterwards".
1. Status and Legal Effect
GB 50496-2018 remains the current national standard, applicable to mass concrete in industrial and civil building structures. Two originally mandatory provisions (4.2.2 cement inspection, 5.3.1 formwork verification) had their mandatory effect superseded by GB 55008-2021. All other technical requirements remain fully in force.
2. Definition of Mass Concrete (Chapter 2)
Definition 2.1.1: Concrete where the solid entity's minimum dimension ≥ 1 m, OR concrete expected to develop harmful cracks due to temperature changes and shrinkage from cementitious material hydration.
This dual-criteria definition is key: a hard threshold (min dimension ≥ 1 m = automatic designation) plus a soft risk-based threshold (cracking risk regardless of size).
3. General Requirements (Chapter 3) — Pre-Construction Planning
Mass concrete cannot be poured directly — it must be calculated, reviewed, and approved. A construction organization plan must include: temperature and shrinkage stress calculations, crack control measures, mix design optimization, and the temperature monitoring equipment and sensor layout plan.
Design may use 60 d or 90 d strength as acceptance basis — this can reduce cement content by 15%~25%, directly lowering adiabatic temperature rise.
| Parameter | Limit | Controls |
|---|---|---|
| Temperature rise above placement temp | ≤ 50 ℃ | Max internal temperature |
| Core-surface temperature differential | ≤ 25 ℃ | Surface cracking |
| Cooling rate | ≤ 2.0 ℃/d | Internal shrinkage stress |
| Surface-to-air temp diff (at insulation removal) | ≤ 20 ℃ | Thermal shock cracking |
4. Materials and Mix Design (Chapter 4)
Cement: Low-heat cement required. 3 d heat ≤ 250 kJ/kg, 7 d ≤ 280 kJ/kg. Aggregate: Medium sand (fineness modulus > 2.3), coarse aggregate 5.0~31.5 mm continuous grading, non-alkali-reactive. Mix design limits: Slump ≤ 180 mm, mixing water ≤ 170 kg/m³, fly ash ≤ 50% binder, slag ≤ 40% binder, combined ≤ 50% binder, water-binder ratio ≤ 0.45.
5. Construction (Chapter 5)
Layer thickness: 300~500 mm. Subsequent layer must be placed before initial set of previous layer. Secondary vibration and multiple troweling passes are recommended. This is the first national standard to include "sequence pour method" (jump-pour method) for super-long structures.
6. Temperature Monitoring and Control (Chapter 6) — The Core
Monitoring frequency: Core-surface differential, cooling rate, and ambient temperature: ≥ 4 times/day. Placement temperature: ≥ 2 times/shift. Sensor layout: At least surface, bottom, and center sensors along thickness, spacing ≤ 500 mm. Surface temperature = 50 mm inside the surface.
Sensor specifications: Error ≤ 0.3 ℃ at 25 ℃ ambient, range -30 ℃ ~ 120 ℃, insulation resistance > 500 MΩ. Sensors must survive 24 h immersion at 1 m depth before installation and must be electrically isolated from rebar.
7. Curing
Thermal + moisture curing for ≥ 14 days. Remove insulation in layers. Complete removal only when surface-to-air temperature difference < 20 ℃. No water curing when ambient < 5 ℃.
8. Five Key Implementation Milestones
- Planning (14 d before pour): Stress calculations, 60/90 d strength plan, mix verification, adiabatic test
- Materials (7 d before pour): Incoming inspection, alkali-reactivity testing, cement heat retest
- Pouring: Placement temp control, layered pouring, secondary vibration
- Monitoring (1~14 d after pour): ≥ 4 readings/day, temperature curves, dynamic insulation adjustment
- Curing (14+ d after pour): Thermal & moisture curing, gradual insulation removal
9. Three Common Pitfalls
Pitfall 1: Confusing "mass" with "large volume"
Only where min dimension ≥ 1 m or hydration heat cracking risk exists. A 30 cm pavement slab (even 10,000 m³) is NOT mass concrete.
Pitfall 2: Treating 4.2.2 and 5.3.1 as still mandatory
Their mandatory effect was superseded by GB 55008-2021. All other provisions remain fully enforceable.
Pitfall 3: Measuring only the center, ignoring surface/bottom
"Surface" means 50 mm inside — not the formwork surface. Sensors placed on formwork measure formwork temperature, not concrete surface temperature.
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