Voids Under a Congested Transfer Beam Bar Layer
A 1200 mm deep × 800 mm wide transfer beam spans 11 m clear and is reinforced with three layers of eight #36 bars at the bottom, #13 stirrups at 100 mm, and 40 mm cover; clear gaps between bars are about 60 mm and between layers about 55 mm. The approved mix uses 20 mm coarse aggregate, 140 mm slump, w/c 0.42 and 30% slag, pumped through a 125 mm line at 1.6 m³/min. The crew discharged from the top of the form, a free fall of up to 2.2 m, then vibrated with a 50 mm poker at roughly 700 mm spacing, 8–10 s per insertion. After stripping, the beam-column joint and the lower half of the beam show voids 10–40 mm deep on the vertical faces, and a continuous void runs under the bottom bar layer. The concrete between the voids is dense and well bonded, with no bleed-water channel or aggregate pocket visible at the top of the pour. A 100 mm core through the soffit shows 120 mm of void beneath the first bar layer.
1. Diagnose the cause of the voids, distinguishing between segregation, insufficient consolidation, and obstruction by reinforcement.
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Rule out segregation. Segregation leaves coarse aggregate at the bottom of the lift, laitance and a weak, porous matrix, plus a bleed channel up the bar or the form face. Here the concrete between the voids is dense and the top of the pour shows no bleed channel, so the mix held together.
Rule out under-vibration as a global cause. Global under-vibration produces honeycombing over large areas, not discrete voids beneath one bar layer.
Most probable cause: obstruction and arching. Three layers of #36 bars with 55–60 mm clear spacing act as a screen. Concrete flowing from above arches over the top mat, and the grout that drains through cannot fill the enclosed space beneath; the void is therefore a placement artefact tied to the bar geometry and the drop point. The free fall of 2.2 m worsens the situation by separating mortar from coarse aggregate at impact.
Secondary contributor. A 140 mm slump mix with 30% slag is stiff enough that it does not flow laterally under the mat. Vibration at 700 mm spacing gives an influence radius of roughly 200–300 mm in congested steel, leaving zones that are never energised; the bottom layer cannot be reached from the top at all.
2. Recommend changes to the mix, placement sequence and vibration procedure, with the reason for each.
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Mix. The 20 mm aggregate is acceptable against a 60 mm clear gap (limiting size is about 3/4 of the gap), so aggregate size is not the fault. The fault is mobility. Increase slump at point of discharge to 180–200 mm with a superplasticizer rather than water, holding w/c at 0.42; the slag already gives a longer workable window, which suits a multi-layer pour. Add a viscosity-modifying or higher-fines balance only if the mix bleeds.
Placement. Limit free fall to about 1.5 m, or use a tremie pipe or elephant trunk into the congested zone. Place in layers of 400–500 mm so each layer is vibrated before the one below sets; do not try to fill 1200 mm in one pass.
Vibration. Use a 25–40 mm head in the congested zone, insertions at 300–400 mm spacing, penetrating 100–150 mm into the layer below, each held until the surface just turns glossy (typically 10–20 s) - over-vibration is the lesser risk here. For the bottom layer, vibrate from the side through form windows.
3. The pump operator asks to add 15 L/m³ of water to get the mix through the line. State your decision and the controls you would impose.
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Decision: refuse. Adding 15 kg/m³ of water raises w/c from 0.42 to about 0.46 at 380 kg/m³ cement, costing roughly 3–5 MPa of strength and, more importantly here, raising permeability in a member already at risk of voids. It also changes the approved mix without a design check.
Permitted alternative. Add superplasticizer at the point of discharge, within the dose limits on the product data sheet and within the specified slump life of the mix. Remix at mixing speed for the specified number of revolutions so the admixture is distributed, then re-test slump.
Controls to impose.
- Slump and temperature at the point of discharge, not at the plant.
- Reject concrete that has exceeded its specified time since batching, or that is above the maximum placing temperature.
- If superplasticizer cannot restore the slump, do not force the concrete through the line; return it.
- Record every admixture addition and keep the batch ticket with the pour record.
Underlying principle. The problem in this beam is placement and mobility, not water content. Watering the mix treats the symptom and creates a durability defect in the same member.