Common problem

Cracks or rust in the concrete?

Some cracks are only on the surface; some matter to the structure. Rusting rebar swells and splits the concrete from the inside. Find the cause first, then choose the repair that fits.

Consult on LINE Send photos

What the cracks and rust tell you

What you see	Why it happens	How to tell
Fine surface cracks	The concrete dried too fast when it was placed, or shrank as it dried.	Fine, often in a net pattern, usually only at the surface and not widening — but a fine crack can still be deep, so it is checked
Cracks that open and close	Heat and moisture make the concrete move, and there was not enough room for it.	The width changes with the time of day or the season
Diagonal cracks, cracks in beams and columns	Diagonal cracks at window and door corners are often from shrinkage and heat stresses that gather at the corner. Settlement, a weak lintel or overload can also cause them.	Diagonal cracks from door and window corners, cracks in beams or columns, cracks that keep widening — have a structural engineer look
Cracks along the rebar, rust stains	Carbonation or salt breaks down the concrete's protection of the steel, sooner where the cover is thin. Rust takes up more room than steel and pushes the concrete apart.	A crack running straight along a bar, with brown rust stains
Hollow or spalled concrete	The rust has gone further and the cover concrete comes away.	A hollow sound when tapped, exposed rebar
Attack by salt or chemicals	Salt from the sea mainly rusts the rebar. Acids, sulfates and corrosive gases at plants soften the concrete itself.	A weak, crumbly surface, a lot of rust

First, find the cause

Look and record

Cracks, rust stains, spalls and old repairs are mapped.

Tap for hollow spots

A hammer finds the concrete that has come loose by the change in sound.

Measure whether it moves; test where needed

Crack widths are measured over time. Where needed the cover, carbonation and salt are tested, and a structural engineer is brought in when the cause or the risk is unclear.

These are examples of how leaks are traced. We choose the checks to suit the site.

Choosing the repair

These are the possible repairs. Which one fits is decided after we see the roof.

Repair	When it fits	Keep in mind
Surface sealing	Fine cracks that are not moving.	It only closes the surface, so a moving crack opens again.
Epoxy injectionCrack injection Details →	Dry cracks that have stopped moving, to bond the concrete back into one piece.	It comes back if the cause is not removed, and it does not suit cracks that move.
PU injectionLeak-sealing injection Details →	Cracks that leak or are wet.	Flexible PU stops water and follows small movement, but does not restore strength.
Rout and seal (flexible sealant)	Cracks that keep moving.	It does not restore strength; whether to treat a crack as a joint is decided with a structural engineer.
Patch repair (remove damaged concrete, treat the rebar, repair mortar)Concrete patching Details →	Rust has made the concrete hollow or spall.	Damaged concrete and rust are removed, and the space behind the bars is filled too.
Stop the rust coming back (corrosion inhibitors, protective coatings)	Salt or carbonation remains in the concrete.	After a patch, the steel around it can start to rust. The protection is chosen after testing; once salt has reached the steel, the effect is limited.
Add steel or strengthen (CFRP)CFRP strengthening Details →	The bars have lost section, or the load capacity is in doubt.	It needs a structural engineer's design.

One possible way to repair spalled concrete

Remove loose and spalled concrete back to sound concrete
Clean the rust off the rebar and apply corrosion protection; add steel where the bars have thinned
Where needed, apply a corrosion inhibitor
Rebuild with repair mortar
Inject or seal the cracks
Where needed, finish with a protective coating

This is one example. We decide the actual work after seeing the roof.

Questions

What decides the price?

The length, width and number of cracks, how large and deep the spalls are, how badly the rebar has rusted, access and scaffolding, and whether tests are needed. Send photos on LINE.

Is this crack dangerous?

Width alone cannot tell you. Cracks in beams or columns, diagonal cracks, exposed rebar and cracks that keep widening should be seen by a structural engineer. We also look on site to find the cause.

Can it just be covered over?

Not if the cause is still there — the crack comes back and the rust keeps working inside. A fine crack that is not moving and shows no rust can be sealed at the surface.

Send us a photo of the crack.

Send photos of the cracks or rust, where they are and whether they are growing. We check the site and propose the repair that fits.