---
title: Concrete Defects — Identification, Prevention, Repair (BM Formwork reference)
type: concept-note
status: live
created: 2026-05-05
created_by: cowork-lfcs
applies_to_job_types:
  - any-RC-cast-insitu
  - bridge
  - bridge-substructure
  - footing
  - slab
  - suspended-slab
  - wall
  - retaining-wall
  - column
  - feature-wall
related_specs:
  - "[[AS3610 Formwork for Concrete]] (Class 2 = standard civil/bridge; Class 1 = feature)"
  - "[[AS3600 Concrete Structures]]"
related_concepts:
  - "[[Formwork-Wall-Column-Slab-Technique-BM]]"
  - "[[TfNSW-bridge-FRP-subcontract-pattern]]"
  - "[[Defensible-for-variations-posture]]"
canonical_examples:
  - "[[2602 - Dec Projects - Munmorah Bridge]]" (200 RC abutments + L-walls + deck — bolt-hole + vibrator burns relevant)
  - "[[2629 - TCE - Erskine Park WWTP Slab]]" (270 mm slab — boniness on bund walls relevant)
sources:
  - BM Formwork (Ben) YouTube — "Concrete defects walkthrough" (post-pour QA review)
---

# Concrete Defects — Identification, Prevention, Repair (BM Formwork reference)

> **When to apply:** post-pour walkthrough / QA inspection / defect register entries / client conversations about cosmetic vs structural / patching scope quoting / SWMS for vibration. Wikilink in per-job CLAUDE.md Patterns section. **Pair with [[Formwork-Wall-Column-Slab-Technique-BM]]** — the build-side companion.

## TL;DR — what this pattern locks down

1. **Most "defects" are cosmetic, not structural.** Boniness, watermarks, dried-concrete dark patches, vibrator burns, paint marks — all cosmetic. Don't over-promise crispness; manage expectations early.
2. **Pinholes at the top of walls are unavoidable.** Hydraulic pressure forces air upward; you'll always have some at the top. Mitigate, don't promise zero.
3. **Boniness (honeycomb) = water leak through form joints.** Always preventable with silicone joint seal + thorough pre-pour inspection. Worth the 30 min.
4. **Vibrator burns = vibrator hit the form face.** Train the pour crew: vibrator stays off the ply.
5. **Bolt-hole crispness depends on timber stiffness.** Soft imported timber (Russian / Chinese during COVID supply gap) flexes 2-4 mm under hydraulic pressure → cone doesn't seat → ratty hole. Use bigger oversized cones to compensate, or specify hardwood for the soldiers.
6. **Long walls crack without construction joints.** Standard CJ every 6 m. Feature walls without CJ WILL crack — set client expectation up front, and budget seasonal seal-up.
7. **Hot-day pours (>35°C) leave dark patches** where concrete dried on the form face. Hose down shutters during the pour to mitigate. Day pour scheduling matters.

## Defect catalogue

### 1. Boniness / honeycomb

**What it is:** Patches where stone (aggregate) is exposed because cement paste has been washed out — typically at the bottom of walls or at form-joint locations.

**Cause:** Water leaked through a gap in the formwork (usually a ply joint, a poorly-sealed kicker, or a stop end). Water carries cement away, leaves the stone exposed.

**Prevention:**
- Silicone every form joint before pour (especially feature walls — owner sees these)
- Walk the form pre-pour with the pour crew, looking for daylight gaps from inside
- Tighten the kicker — most boniness shows at the kicker line where it's been left loose

**Repair:**
- Patcher comes in, cuts back loose paste, repacks with high-bond mortar (Renderoc or similar)
- Match-tinted if feature finish — get the patcher to do a sample first

**Pricing implication:** If feature wall, budget patching as a line item (~$80-120/m² of patched area). Don't promise crispness on hidden internal walls — overspec them and over-deliver.

### 2. Pinholes / air bubbles

**What it is:** Small black holes (3-15 mm dia) on stripped face, typically clustered near the top of walls.

**Cause:** Air bubbles in the concrete that didn't escape during vibration. At the top of walls, air migrates upward under hydraulic pressure but can't escape if vibration is incomplete.

**Why mostly at top:** Hydraulic pressure at the bottom forces all air upward. Bottom of wall almost never shows pinholes.

**Prevention:**
- Internal vibrator (poker) per pour — every 300-450 mm spacing, withdraw slowly
- **External vibration** with a small jackhammer on the OUTSIDE of the form face during pour — works air out through the form. (Industry trick — adds maybe 0.2 hrs/m² of wall area but dramatically reduces pinholes.)
- Don't overdose superplasticiser — too liquid = bubbles trap and don't migrate

**Repair:** For feature walls — patch + sand each bubble. Slow and expensive. For non-feature: leave as-is, mention in defects register as cosmetic.

### 3. Vibrator burns

**What it is:** Mark on the concrete face where the vibrator poker damaged the form ply during the pour, leaving a rough patch on the stripped concrete.

**Cause:** Vibrator hit the inside face of the form, vibrated against the ply, damaged the ply surface. Concrete then sets against the now-rough ply, picks up that texture.

**Prevention:** Train pour crew: vibrator stays in the centre of the cage, doesn't lean against the form face. Use a poker length appropriate for the wall thickness — too long a poker on a 200 mm wall = unavoidable form contact.

**Repair:** Patch sand-and-fill on stripped face. Cosmetic only.

**Pricing implication:** Common on tall narrow walls (200 thick, 3 m+). Budget 1-2 hrs of patcher time per pour for vibrator-burn cleanup on feature work.

### 4. Watermarks / streaks running down

**What it is:** Visible vertical streaks on stripped face — light or dark depending on direction.

**Cause:** Curing water trickling down the wall (client wetting down) OR rain during cure. **Cosmetic only — does not affect strength.**

**Prevention:** Wet-cure with hessian / Bituthene-style membrane rather than free-flowing water if cosmetic finish matters. Or accept and explain to client.

### 5. Construction joint cracks (long walls without CJ)

**What it is:** Horizontal or vertical hairline crack in the middle of a long wall (typical >6 m run).

**Cause:** Concrete shrinks as it cures. Without a control joint to absorb shrinkage, the wall cracks at its weakest point — usually the middle.

**Prevention — the rule:**
- **Construction joints every 6 m** in long walls is standard practice
- Feature walls (where owner doesn't want a visible CJ) — accept the crack will appear, plan for seasonal sealing

**Repair:** Hairline cracks can be sealed with epoxy injection or flexible joint sealer. Will open and close with seasons — cosmetic only on a load-bearing wall, structural concern on a tank or water-retaining wall.

**Client conversation:** Always raise this BEFORE pour on long feature walls. *"We can either put a CJ at the 6 m mark which you'll see, or we accept that there'll be a hairline crack somewhere on this wall. Your call."* Get it in writing.

### 6. Bolt-hole imperfections (ratty cone holes)

**What it is:** Bolt-hole on stripped face is fuzzy / off-round / leaking concrete — instead of a crisp circular bung mark.

**Cause:** Form face flexed during pour because timber soldiers were too soft (typically softwood imported during 2020-2022 COVID supply chain disruption — Russian / Chinese stock). Soft soldiers expanded 2-4 mm under hydraulic pressure → cone didn't seat tightly → concrete leaked around cone → ratty hole.

**Prevention:**
- **Hardwood soldiers** where possible (or laminated timber rated for formwork)
- **Oversized cones** (200 wall = 202+ cone) so the cone squashes into the form face under bolt tension and stays sealed even if the soldier flexes
- Pre-pour: tighten bolts firmly, check cones are seated flush with form face

**Repair (the standard fix):**
1. Insert a clean unused cone into the hole as a bung
2. Patch around the cone with bag dye / cement / Renderoc
3. **Pro tip:** Cut a cone in half lengthwise to use as a depth gauge → consistent hole depth across all the patches on a wall (otherwise hole depths vary and look messy)

**Pricing implication:** Bolt-hole patching = ~5-10 minutes per hole. On a 100 m² wall with bolts at 900 × 1100 grid = ~100 holes = 8-16 hrs of patching. If feature wall, this is a meaningful line item.

### 7. Dried-concrete dark patches (hot-day pour)

**What it is:** Dark coloured patches at the top of walls or at lift lines, looking like darker concrete than the surrounding face.

**Cause:** During hot-day pour (>35°C), some concrete splattered on the form face during pour, dried before the pour caught up. Subsequent concrete bonds against the dried film, leaving a shadow where the dry layer was.

**Prevention:**
- **Hose down shutters during pour** in hot weather — keep the form face wet so splatter doesn't dry
- Schedule hot-weather pours for early morning or late afternoon
- Slower pour = more chance for dried splatter; balance pour speed against hydraulic-pressure risk

**Repair:** Generally cosmetic — sandblast or acid-etch if owner insists on uniform colour. Most clients accept and call it character.

### 8. Pour run lines (lift line shadows)

**What it is:** Visible horizontal line at the top of pour run 1 / bottom of pour run 2 — typically a darker shade band 50-200 mm high.

**Cause:** First lift cured slightly before second lift was placed against it. Bond is fine (structurally OK), but colour matches imperfectly.

**Prevention:**
- Pour walls in a single continuous lift where possible
- If multi-lift required, place "fresh on fresh" — second lift while first is still wet (within 30-45 min depending on temp)
- Use a retarder admixture in lower lifts to extend working time

**Repair:** Cosmetic — acid-etch or sandblast to homogenise.

### 9. Paint marks on stripped face

**What it is:** Coloured smears or dots on the stripped face that match site marker / spray paint colours.

**Cause:** Form ply was previously used and not fully cleaned of marker / paint before reuse.

**Prevention:** Clean ply between uses. Most rubs off after first wet-curing. Worth a factory wash of 2nd-hand ply.

## Defect-rate expectations by AS3610 class

| AS3610 Class | Where used | Expected defect rate |
|---|---|---|
| **Class 1** | Feature walls, off-form architectural concrete, exposed soffits | 0-5% surface area defective; tight tolerance on bolt holes + colour |
| **Class 2** | Standard civil / bridge / commercial | 5-15% surface area defective acceptable; cosmetic patching expected; hairline cracks acceptable |
| **Class 3** | Hidden / below-grade / industrial | 15%+ acceptable; patching only if structural |

Most LFCS bridge work = **Class 2**. Most LFCS feature retainer / showroom slab work = **Class 1**. Confirm in spec; some bridges spec Class 1 for parapets visible from road.

## Pricing implications — when defects affect bid scope

1. **Feature walls (Class 1)** add ~10-15% to formwork rate for joint silicone + hardwood soldiers + careful vibration + extra patching budget
2. **Patching scope ambiguous in BOQ** — always get clarified pre-pricing. *"Make good after pour"* is open-ended; nail down whether it's just bolt holes or full Class 1 cosmetic
3. **Bond breaker spec missed** on residential slab-on-brick = post-pour cracks → callback liability. Always price bond-breaker tin in residential slabs even if BOQ silent
4. **Pour-day conditions** (hot, cold, rainy) drive defect rates — if BOQ doesn't allow weather contingency, qualify in submission

## Defect register template (for post-pour QA)

For active jobs, defect logging goes to `11 - Handover and Closeout/11a - Defects Register/_REGISTER.md` per LFCS folder spec. Each entry:

```yaml
defect_id: DEF-{job}-{seq}
date_identified: YYYY-MM-DD
identified_by: {Rocky / supervisor / client / consultant}
location: {room / wall / element}
defect_type: {boniness | pinholes | vibrator-burn | watermark | crack | bolt-hole | dried-patch | run-line | paint | other}
severity: {cosmetic | structural | spec-non-compliance}
status: {open | in-rectification | closed-acceptance | closed-rectified}
photos: [path]
client_position: {accepted | rectification-requested | spec-deviation}
LFCS-position: {Class-2-acceptable | will-rectify-cost-X | dispute-with-evidence-Y}
resolution: {description}
date_closed: YYYY-MM-DD
```

## Client conversation script — when defects appear post-pour

1. Walk the wall with the client. Identify each defect by category.
2. Classify cosmetic vs structural in plain language. *"This is a watermark — it's literally just water on the face during cure, doesn't affect anything but how it looks."*
3. Reference AS3610 class spec'd for the job: *"Spec calls for Class 2 — this is well within Class 2 acceptance."*
4. Offer rectification cost for cosmetic above-spec items. *"I can patch this back to Class 1 for $X — that's not in the contract scope but I can do it."*
5. **Document everything in defects register** — including the conversation outcome.

Don't apologise excessively. Concrete is a natural material; perfection isn't a feature unless explicitly spec'd and priced.