Dầm, sàn chuyển và bê tông khối lớn: đổ một lần hay chia đợt?Transfer beams, transfer slabs and mass concrete: one pour or several?

In high-rise buildings, a transfer structure is the storey that “carries” all the columns and walls above and redirects their loads onto a sparser column grid below (often to open up lobbies or retail podiums). Transfer beams 1.5–2.5 m deep, thousands of cubic metres of concrete and over a thousand tonnes of steel in one slab make this both a temporary-works problem (the shoring must carry a huge mass of fresh concrete) and a mass-concrete problem (heat of hydration causes cracking).

This article distils lessons from five transfer slabs built in practice, with my own perspective added.

1. Real scale: five transfer slabs we have built

All five used Ringlock scaffolding for shoring. The deeper the beams (≥ 2 m), the more likely the pour was split.

2. Why does mass concrete crack?

Under the Vietnamese standard TCXDVN 305:2004, an element counts as mass concrete when it is large enough for thermal tensile stresses from cement hydration to exceed the concrete’s tensile strength — typically when both the smallest side and the height exceed 2 m. But size is only one factor; what matters is recognising and quantifying the cracking risk.

The principle is simple: materials expand when hot and contract when cool. Volume change alone causes no stress — stress only appears when that movement is restrained.

Pour temperature depends on cement type and content · initial material temperatures · pour size · weather · formwork material and striking time · curing method. Standards (ACI 207, BS 8110, TCXDVN 305) all limit the core-to-surface temperature difference to about 20°C, and cap the peak temperature in the pour (around 70–75°C depending on the standard).

3. Four ways to reduce cracking

1. Reduce volume change — lower the heat: less cement, mineral admixtures, cooled materials.
2. Reduce restraint — sensible pour sizes and sequencing.
3. Increase the concrete’s crack resistance.
4. Distribute cracking — anti-crack reinforcement so cracks are many and fine rather than few and wide.

Materials: fresh-concrete temperature at placement no more than 25°C, ideally ≤ 20°C — chilled water or ice, shaded and sprinkled aggregates, less cement through admixtures.

Construction: pour at night, insulate truck mixers and pump lines, embed cooling pipes, and wrap the pour in insulation so the surface doesn’t cool too fast.

When is anti-crack steel needed? Side and height ≤ 1 m: not required. 1–2 m: nominal steel recommended. Over 2 m: anti-crack reinforcement must be designed, with crack-prevention measures during construction.

Temperature monitoring: every 2 hours for the first 48 hours, every 6 hours from 48 to 120 hours, then once each morning until the core drops below 45°C.

4. One pour or several?

TCXDVN 305 limits each continuous lift to 1.5 m and requires at least 4 days between lifts. With split pours there are three ways to share the load:

1. Shoring carries both pours — the first pour only carries its own weight once it has gained strength.
2. Shoring carries the first pour and part of the second — hard to assess how props and first-pour concrete share load; higher cracking risk in pour 1.
3. Shoring carries only the first pour — pour 1 must carry all of pour 2; extra flexural and shear steel, and cracking risk from early loading.

5. Shoring for transfer structures

Design workflow: propose options → preliminary design → evaluate (technical, cost, programme, subcontractor capability, client requirements) → detailed calculation → drawings → internal check and independent review. Loads usually have to be carried down through several levels of back-propping, so the number of propped floors is itself a design variable.

6. Reinforcement: a logistics problem

Project A needed about 1,200 tonnes of steel for the transfer slab, fixed in roughly 26 days. With such congestion, the fixing sequence was modelled in 3D and taught to every gang before work started.

The biggest quality risk is honeycombing because the steel is so dense. Remedies: bundle bars in pairs to leave gaps for concrete to pass, plan batches and pours carefully, and inspect the batching plant the day before the pour.

For supervisors: know the method, follow it and control every step exactly as briefed — a transfer structure leaves no room for improvising on site.