Honeycombing and Spalling in Concrete: the Difference, the Causes, and When They Are Serious

Honeycombing is a concrete workmanship defect that shows right after the formwork is removed: voids and exposed gravel on the surface, caused by poor compaction or the mix water leaking out. Spalling is the rusting of the reinforcing steel inside the concrete, which shows over time as rust stains, cracks and flaking until pieces of concrete fall off. The two are directly linked: honeycombing leaves the steel exposed to moisture, and over time moisture starts the spalling.

In this guide we explain the difference between the two, the causes of each, the signs to look for in your villa, when you need a structural assessment, and how the Fahison team helps you catch them at the right time.

The difference between honeycombing and spalling

• Timing: honeycombing shows right after the formwork is removed; spalling shows after months or years.
• Cause: honeycombing is a mistake in the pour itself; spalling is the result of moisture and salts reaching the steel.
• Appearance: honeycombing is voids and exposed gravel; spalling is rust stains, long cracks, flaking and falling pieces of concrete.
• Best time to find it: honeycombing before the plaster; spalling at the first sign of rust or flaking.

What is honeycombing in concrete?

Honeycombing is voids and visible gravel on the concrete surface, looking like a honeycomb. It usually shows at the bottom of columns, under and along the sides of beams, on the sides of footings, and where beams meet columns. It is serious because it weakens the concrete at that spot and reduces the concrete cover that protects the steel from rust.

Causes of honeycombing

• Poor compaction with the vibrator: if each layer is not compacted well, air voids stay inside the concrete and on its surface.
• Mix water leaking from the formwork: loose formwork joints let water and cement escape, leaving only the gravel.
• Dense steel and tight spacing: the concrete cannot easily get between the bars, especially where beams meet columns.
• Dropping concrete from a height: in columns the concrete falls from the top and separates, so the gravel collects at the bottom of the column.
• A dry mix or a delayed pour: a mix that has started to set before it is placed does not fill the formwork properly.

How is honeycombing found in time?

The best time to find it is right after the formwork is removed and before any plaster. Check the bottom of columns, the sides of beams and the underside of the slab, and look for exposed gravel, voids and visible steel or spacers. If it is found, the treatment is decided by the structural engineer depending on its depth and where it is in the element, and it is not covered before then.

Preventing honeycombing starts before the pour: tight formwork joints, enough spacers around the steel, and enough vibrators and workers for a continuous pour. That is why the Fahison team agrees these points with the concrete contractor in pre-planning before the pour day.

What is spalling in concrete?

Spalling is the common name for the rusting of the reinforcing steel inside concrete. When steel rusts it expands and pushes on the concrete from inside, so cracks appear along the line of the steel, then the concrete flakes, pieces fall off and the rusty steel shows. As the rust continues, the steel section itself gets smaller, and at that point it becomes a structural issue, not just a cosmetic one.

Causes of spalling

• Not enough concrete cover: steel close to the surface, because of missing spacers or bars moving during the pour, is reached by moisture quickly.
• Honeycombing and cracks: every void or crack is an open path for water and air to reach the steel.
• Ongoing water leaks: under bathrooms, around water tanks, at AC condensate drains, and on poorly waterproofed roofs.
• Humidity and salts: coastal humidity and salts in the soil and groundwater speed up rusting, especially in footings, tie beams and columns near the ground if their waterproofing is weak.
• Poor curing after the pour: concrete that is not sprayed or covered in the heat cracks on its surface and becomes weaker against moisture.

Signs of spalling to watch for

• Brown or orange rust stains on the plaster or paint.
• Long cracks along the line of the steel in slabs, beams or columns.
• Bulging and flaking plaster or paint, especially on ceilings under bathrooms.
• A hollow sound when you tap the concrete or plaster lightly.
• Pieces of concrete falling off and rusty steel showing.

Where does spalling usually show in villas?

• Ceilings under bathrooms and kitchens: because of leaks from the pipes or weak waterproofing of wet floors.
• The top-floor ceiling: if the roof waterproofing is weak or water collects on the roof.
• Balconies and concrete canopies: exposed to sun, rain and humidity from more than one side.
• External columns and beams: especially those near the ground or near water sources.
• Around water tanks and AC bases: because of constant moisture and condensate water.

When is honeycombing or spalling serious?

Not every case of honeycombing or every rust stain is a danger to the building, but do not judge it yourself. Ask a structural engineer for an assessment in these cases:

• The honeycombing is deep, shows steel, or is in a load-bearing element such as a column or beam.
• The steel is exposed and rusty, or pieces of concrete have fallen off.
• Cracks run along a beam or column, or grow over time.
• The slab is sagging or the concrete is clearly bulging.

The repair method is decided by the structural engineer for each case, because it ranges from a surface repair to treating or strengthening the element.

Mistakes to avoid before treatment

• Do not cover honeycombing or exposed steel with plaster or paint before the assessment. The defect disappears from view and keeps developing underneath.
• Do not start the repair before stopping the water source, or the spalling will come back in the same place.
• Do not break or cut any part of a structural element without a structural engineer's opinion.
• Do not put off dealing with exposed steel; rust does not stop on its own.

How can the Fahison team help?

If your villa is under construction

In Construction Stage Inspections, every pour gets two inspections: supervising the pour (delivery tickets, no water added to the mix, slump test, cube samples and vibrator compaction), then the inspection after the formwork is removed (honeycombing, exposed steel, dimensions, cracks and curing). Inspections start from 700 AED per item:

• Footings concrete inspection
• Tie beam concrete inspection
• Columns concrete inspection
• Slab concrete inspection

Most importantly, the steel and formwork should be inspected before the pour, because the spacers and the concrete cover are the first line of defence against spalling: footings inspection, columns inspection and slab inspection before the pour.

If the house is already built or you are about to buy it

In a pre-purchase inspection, our engineers examine the visible structural elements such as columns, walls and ceilings, record signs of spalling, cracks and damp, and show them in the report with photos. The inspection is visual and does not break anything open, so it cannot vouch for the hidden steel and concrete, but it reveals the signs before you buy. If the spalling is caused by a leak, you can start with water leak detection to stop its source.

For more details, read: Villa construction inspection: 8 stages you should not leave uninspected, How to tell if plaster is done correctly, and Defects that appear after moving into new villas.

Frequently asked questions about honeycombing and spalling

Q: What is honeycombing in concrete?
A: Voids and exposed gravel on the concrete surface that show after the formwork is removed, usually caused by poor compaction with the vibrator or mix water leaking from the formwork joints. It weakens the concrete and exposes the steel to moisture.

Q: What is spalling in concrete?
A: The rusting of the reinforcing steel inside concrete. When steel rusts it expands and pushes on the concrete from inside until it cracks, flakes and pieces fall off, exposing the steel.

Q: Does honeycombing cause spalling?
A: It can be one of the causes. Honeycombing leaves the steel without enough concrete cover, so moisture and salts reach it easily, and over time rust starts.

Q: Is it enough to cover honeycombing with plaster?
A: No. Plaster hides the defect without treating it, and the steel stays exposed to moisture underneath. The treatment is decided by the structural engineer depending on the depth and location of the honeycombing.

Q: Is spalling dangerous?
A: It depends on where it is and how far it has gone. A small rust stain is not the same as exposed steel in a column or beam. If steel shows or pieces of concrete have fallen, a structural engineer should assess it before any repair.

Q: When should I book a concrete inspection?
A: During construction: on the pour day and right after the formwork is removed, before the plaster. In an existing house: before buying, or when cracks, rust stains or flaking appear on ceilings and columns.

Book a concrete inspection at the right time

Have a pour coming up, or the formwork has been removed and you want to check before the plaster? Book a concrete inspection on WhatsApp. If you have noticed rust stains or flaking in your home, send us photos on WhatsApp and we will arrange the inspection with you.