Geotechnical engineering, tanks & infrastructure

Rusted reinforcement in concrete? Why does concrete crack and how can corrosion be stopped?

Last updated: 16 October 2026

Rust expands and causes the coating to crack. Anti-corrosion treatment for the reinforcement, R3/R4 repair compound and surface protection to prevent further cr

The system at a glance

Solutions & Applications · Concrete Repair

Rust takes up many times the volume of the steel it corrodes. Inside the concrete, it has nowhere to go, so it pushes against the covering until it cracks and detaches it. That is why exposed, rusted reinforcement is not an aesthetic problem but a symptom — and that is why a simple cement mortar applied over the rust will flake off again within a few years. A lasting repair first stops the corrosion and then restores the cross-section.

Why does concrete crack?

Fresh concrete is highly alkaline, and in this environment the steel is covered by a thin protective film. As long as this film remains, the reinforcement does not rust. Two mechanisms cause it to deteriorate:

Mechanism	What’s happening?	Where can we find him?
Carbonation	CO₂ in the air penetrates through the surface and gradually reduces the alkalinity. When the front reaches the reinforcement, the protection is lost	Slight overlap, porous concrete, old buildings, piles, balconies
Chlorides	Chloride ions penetrate the film even in alkaline concrete and cause localised, deep corrosion	Coastal buildings, salt, swimming pools, reservoirs

With water and oxygen, corrosion progresses, and the expanding rust paves the way for even more water. It is a cycle that accelerates.

The symbols, in the order in which they appear

Mark	What does this mean?
Brown rust spots on the surface	Corrosion has already begun and the products are corroding from the inside out
Cracks running parallel to the reinforcement	Rust has started to push through the coating
A ‘hollow’ sound when tapped, flaking	The covering has come away — it is going to fall off
Exposed reinforcement, thinner than the original	The cross-section is missing; verification by a structural engineer is essential

Why a simple patch job fails

When a patch is applied with cement mortar over rust, three things remain as they were: the rust continues beneath the patch, the ordinary mortar shrinks and detaches from the old concrete, and the surrounding concrete remains carbonated. Often, the corrosion simply shifts to the edges of the patch and spalling reappears alongside it.

A repair that lasts: step by step

Delimitation. Check for hollow areas using a hammer and mark the repair area slightly beyond the visible problem.
Demolition. Removal of all loose concrete, including that behind the reinforcement bars, so that the reinforcement is exposed along its entire perimeter within the corroded zone.
Cleaning of reinforcement. Removal of rust down to bare metal. If the cross-section has been significantly reduced, the engineer shall decide on additional reinforcement before work continues.
Corrosion protection. Apply SINTOCRETE AntiCorr ST to the reinforcement (~0.14 kg per metre of Ø14 bar). The same material is also applied to the concrete as a bonding layer (≥ 0.5 kg/m²), inhibiting corrosion and restoring alkalinity in the zone.
Wetting. The substrate must be thoroughly wetted prior to application of the repair compound.
Cross-section repair. Fibre-reinforced thixotropic mortar in accordance with EN 1504-3: SINTOCRETE (R4, ~2 kg/m²/mm) or SINTOCRETE Basic (R3, ~17 kg/m²/cm). These may also be applied to vertical surfaces and beneath beams without formwork.
Wet curing. 6–24 hours for R3, 24–48 hours for R4 — longer in hot and dry conditions.
Surface protection. A flexible cementitious compound that delays carbonation throughout the entire element, not just at the repair site: Ceresit CR 166 or VANDEX CEMELAST 100 (sD for CO₂ greater than 50 m).

R3 or R4?

	SINTOCRETE Basic · R3	SINTOCRETE · R4
Class EN 1504-3	R3	R4 — the highest
Typical use	Repairs due to corrosion in beams, columns, cantilevers and masonry	Structural repairs to load-bearing elements, coastal zone, industrial environment
Special features	Dimensionally stable, non-shrinking	Compensated for shrinkage, impermeable even under pressure, resistance to sulphates
Consumption	~17 kg/m² per cm	~2 kg/m² per mm
Wet maintenance	6–24 hours	24–48 hours

When repair is not enough: reinforcement

If the reinforcement has lost a significant portion of its cross-section, restoring the geometry does not restore the load-bearing capacity. In such cases, following a structural analysis, the member is reinforced with FRP composite materials:

S&P C-Sheet 240 — carbon fibre fabric for column wrapping and beam reinforcement, with epoxy impregnation resin SINMAST S2 WV.
S&P C-Laminate — carbon fibre sheets for flexural reinforcement, bonded with epoxy paste SINMAST P 103.

The reinforcement is applied to concrete that has already been properly repaired: first, the corrosion is halted, then the element is reinforced.

Prevention in fresh concrete

The cheapest repair is the one that isn’t needed. In fresh concrete, corrosion is prevented by adequate cover, good compaction and admixtures: TECNOS 95 for a lower water-to-cement ratio, SINTOMIX SM N for a watertight mass and, in a coastal environment, MuCis ad 19/L — an inhibitor that diffuses through the porous structure to the reinforcement and remains effective even in the presence of chlorides.

ARGYNET Note

Exposed reinforcement in a column or beam is not a job for a ‘handyman’. Before carrying out any removal work on a load-bearing element, the engineer must assess the damage: how much cross-section has been lost, whether shoring is required during the work, and whether reinforcement is necessary.

Repairs do not end with the mortar. Without corrosion protection for the reinforcement and without surface protection, the same process begins again — usually at the edges of the repair.

At ARGYNET, we specify the repair system on a component-by-component and environment-by-environment basis, and calculate quantities per m² and per thickness.

Mistakes that cost

Incorrect	Consistency
Cement screed over slag	Corrosion continues from below; the patch is falling off again
Demolition only in front of the bar	The rear side of the reinforcement remains rusted and unprotected
Reinforcement without anti-corrosion treatment	The new mortar contains an active source of corrosion within it
Repair compound for dry substrates	Old concrete ‘absorbs’ the water from the mortar: poor adhesion, cracks
No wet maintenance required	Shrinkage cracks in repairs
Repair without surface protection	Carbonation progresses through the remainder of the element and corrosion appears alongside it