Concrete Cracks: Types, Causes and Ways to Prevent Them

Fine lines appearing on a slab a few hours after the pour, or a fissure opening in a beam months later? Both are cracks, but their meanings are entirely different. Concrete is a material that is strong in compression and weak in tension; it cracks when its volume changes and that change is restrained. When a crack appeared, in which direction it runs and whether it grows over time tell you its cause almost on their own. A correct diagnosis protects you both from needless injection costs and from an overlooked structural problem.

Why Does Concrete Crack?

Concrete has high compressive strength; its tensile strength, however, is only about a tenth of its compressive strength. Concrete shrinks as it dries, expands as it warms, contracts as it cools and deforms under load. None of these movements causes a crack on its own. A crack is born when movement is restrained: a slab rubbing against the ground, a beam squeezed between two rigid columns, a foundation hot inside and cold outside. A restrained volume change generates tensile stress in the concrete; the moment that stress exceeds the tensile strength at that time, the concrete cracks.

This is why the first question in crack diagnosis should not be "was the concrete bad" but "which movement was restrained, when and by what". The tensile strength of fresh concrete is almost zero, so in the first hours even very small stresses open cracks. In hardened concrete, a far greater force is needed to open the same crack. Timing is the crack's identity.

Dividing cracks into two broad groups makes your job easier. The first group is volumetric cracks arising from the concrete's own internal movement: plastic shrinkage, plastic settlement, drying shrinkage and thermal cracks. The second group is structural cracks arising from external effects: overload, ground settlement, reinforcement corrosion. Those in the first group are mostly a cosmetic and durability issue; the second group is a load-bearing issue and calls for an engineer.

Plastic Shrinkage Cracks: A Problem of the First Hours

When: Between the first 30 minutes and 6 hours after the pour, while the concrete is still plastic. It usually starts after screeding, at the moment the surface loses its sheen.

Appearance: Short, shallow, fine lines, parallel to one another or randomly scattered. They are mostly 20-100 cm long, and their width can be hairline or reach 2-3 mm. Their depth is usually small, although it sometimes extends to half the slab thickness. They mostly run at right angles to the wind direction. They do not follow the reinforcement; they concentrate at corners and on large exposed surfaces.

Cause: When the amount of water evaporating from the surface exceeds the bleed water rising to the surface from within the concrete, the surface dries and shrinks. The concrete beneath is still wet and immobile; the surface layer pulls against this restraint and tears. Hot weather, low humidity, strong wind and a high concrete temperature multiply evaporation. At the same temperature, a windy day produces several times the evaporation of a still day. In Bodrum, the wind off the sea brings this risk into play particularly on spring and autumn pour days.

Prevention: Anything that slows evaporation helps. Move the pour to the cool hours; wet the formwork and the ground in advance; spray an evaporation-retarding curing compound without delay after screeding, or keep the surface moist with a fine water mist; put up a windbreak; on large slabs, cover as soon as finishing is complete. If cracks are noticed before trowelling is complete, the surface can be re-screeded and closed up; once the concrete has set, that chance is lost. For detailed practical steps on hot-weather curing, see our article on curing and pouring in hot weather.

Plastic Settlement Cracks: Lines that Trace the Reinforcement

When: Within the first few hours after the pour, before setting.

Appearance: The crack forms above an obstruction inside the concrete and follows its line. It appears directly above the top reinforcement bars in slabs, at the stirrup spacing in beams, and in the upper parts of columns and walls. When you see regularly spaced, straight cracks that match the reinforcement layout exactly, it is most likely plastic settlement. Sometimes a slight depression is also seen on either side of the crack.

Cause: After placing, fresh concrete keeps settling under its own weight; the solid particles move down and water moves up. If there is an obstruction inside the concrete, such as reinforcement, a pipe or a projection of the formwork, the concrete above the obstruction cannot settle while the concrete beside it does. This difference creates a line of stress at the surface, and the concrete tears along the obstruction. High-consistency, over-wet, inadequately compacted mixes and insufficient cover provoke this mechanism. In deep sections (deep beams, column heads) settlement is greater, and so is the risk of cracking.

Prevention: Do not choose a higher consistency than necessary; never add water on site. Keep the cover at the design value; do not seat the reinforcement close to the formwork. Apply the vibrator for long enough and at close enough spacing. One of the most effective methods is revibration: if a second vibration is applied after placing, while the vibrator poker can still sink under its own weight, settlement cracks close up. In columns and walls, pouring the lower part first, waiting for it to settle and then moving on to the upper part also works.

Drying Shrinkage Cracks: Months of Contraction

When: In hardened concrete, weeks or even months after the pour. Most of the shrinkage takes place within the first three months, but it continues for up to a year.

Appearance: Usually straight, regularly spaced cracks running through the full section. In long slabs they repeat at set intervals, in walls and shear walls they appear vertically, and they run diagonally from corners and the edges of openings (windows, doors, manholes). Their width opens and closes with the seasons; when measured they are narrow in summer and wide in winter.

Cause: As the free water in hardened concrete gradually evaporates, the paste shrinks. If the concrete were free to move, this shrinkage would cause no problem; but the slab is tied to the ground, the wall to the foundation, the beam to the column. Restrained shrinkage turns into tensile stress, and when that stress exceeds the strength, the concrete cracks. The more water in the mix, the greater the shrinkage; a high water/cement ratio, excess cement paste, fine-grained aggregate and early drying in low humidity all increase shrinkage. We explained this relationship between water and strength in detail in the water/cement ratio article.

Prevention: Drying shrinkage cannot be eliminated; the aim is to bring the cracking under control. There are three tools for this. The first is the mix: the lowest possible water content and a high proportion of coarse aggregate. The second is curing: keeping the concrete moist in the first days both raises strength and delays shrinkage, so that the concrete shrinks when it is stronger. The third is joints: in slabs, you decide where the crack will be. Contraction joints cut to one third of the slab thickness confine the crack to a neat line. Joint cutting must be done within 6-18 hours of the pour; if it is left too late, the concrete opens its own joints at random. Shrinkage reinforcement does not prevent cracking, but it distributes the width and keeps the cracks small.

Thermal Cracks: Hot Inside, Cold Outside

When: In the first days after the pour; usually between days 1 and 7. Seen in thick sections (raft foundations, retaining walls, large column heads, bridge piers).

Appearance: Vertical or slightly inclined cracks in walls, and in rafts usually regularly spaced cracks perpendicular to the long side. They start at the surface and go deep. They resemble drying shrinkage cracks; the difference is that they appear much earlier and only in thick sections.

Cause: Cement gives off heat as it reacts with water. In a thin slab this heat dissipates immediately; in a thick mass, however, the heat building up inside cannot escape and the core temperature can climb to 60-70 °C. The outer surface, in contact with the air, cools. When the temperature difference between inside and outside exceeds 20 °C, the outer skin, which wants to cool and contract, is restrained by the still hot and expanded core, and cracks. If the formwork is struck early the surface cools suddenly and the risk increases. A high cement content, rapid-setting cement and hot concrete provoke this mechanism.

Prevention: It works on two fronts. To reduce heat generation, a cement with a low heat of hydration (slag or fly-ash blends) and the lowest cement content the design strength allows are chosen; on hot days the concrete temperature is lowered with cold water and aggregate kept in the shade. To reduce the temperature difference, the surface is insulated: the formwork is struck late, exposed surfaces are covered with insulating blankets, and mass concrete pours are staged if necessary. The aim is not to stop the concrete cooling, but to make the inside and outside cool at roughly the same rate. On critical mass pours, monitoring core and surface temperatures with thermometers shows when the insulation can be removed.

Structural Cracks: Load, Settlement and Corrosion

When: After the structure has come into use, at any time over the years. Sometimes it also appears during formwork striking.

Appearance: There are three distinct signatures depending on the cause. Flexural cracks form in the tension zone of a beam or slab (at the bottom at mid-span, at the top over supports), run perpendicular to the axis of the element, are wide on the tension face and taper off towards the neutral axis. Shear cracks run at roughly 45 degrees near the supports; these are the dangerous ones, because they point to a sudden failure mechanism. Corrosion cracks run parallel to the reinforcement, accompanied by rust-coloured staining from the crack and, in time, spalling of the concrete cover. Cracks caused by foundation settlement appear diagonally in walls in a stepped pattern and widen towards the settling zone.

Cause: Loads not allowed for in the design (heavy equipment, added storeys, change of use), insufficient or wrongly placed reinforcement, early formwork striking, differential settlement of the ground, and reinforcement corrosion caused by chlorides or carbonation. In corrosion, rust occupies several times the volume of the steel; this expansion bursts the concrete from the inside. On the coast, in salty, humid environments such as Bodrum, inadequate cover and high permeability make corrosion inevitable over the years.

Prevention: Structural cracks are prevented at the design stage, not on site: correct load analysis, appropriate reinforcement, adequate cover. What falls to you on site is ordering the concrete in the strength and exposure class specified in the design, using cover spacers, not striking the formwork before the design period and not adding water to the concrete on site. Impermeable concrete with a low water/cement ratio is the cheapest armour protecting the reinforcement from chlorides.

Which Cracks Are Dangerous, Which Are Cosmetic?

Ask the same questions of every crack you see on site; the answers will lead you to the right category.

• When did it appear? Within the first 24 hours, it is most likely plastic (shrinkage or settlement); within the first week, thermal; after weeks or months, drying shrinkage; if it appeared years later or with a change in loading, structural.
• Which direction? If it follows the reinforcement, settlement or corrosion; if it is perpendicular to the axis of the element, flexure or shrinkage; if it is inclined at 45 degrees, shear; if it is stepped, settlement.
• Is it growing? Place a mark on either side of the crack and measure its width over several weeks. A crack that stays constant is a "dead" crack and is usually cosmetic. A crack that keeps opening points to an active cause.
• How wide? Roughly speaking, cracks below 0.3 mm are generally considered acceptable in interior elements; outdoors and in water-retaining structures the limit is tighter (0.1-0.2 mm). These are not definitive legal values but general rules of thumb that vary with the exposure class in the project.
• Is water leaking through, are there rust stains? Leakage shows that the crack passes right through the section; rust staining shows that the reinforcement is affected. Either one takes the crack out of the cosmetic category.
• Where? Cracks in load-bearing elements such as columns, beam support zones, walls and foundations are not put in the same category as hairline cracks on a slab surface.

In brief: plastic shrinkage cracks and fine drying shrinkage cracks are mostly cosmetic, but they should be sealed for durability because they let water and chlorides through. Plastic settlement cracks open directly above the reinforcement and are therefore a gateway for corrosion; they should be sealed. Thermal cracks can pass through the full thickness and are taken seriously in water-retaining structures. Flexural, shear, settlement and corrosion cracks are structural; regardless of their width they must be assessed by a civil engineer. When you see a shear crack, do not wait.

Repair Approach: Cause First, Then Filling

The first rule of repair: a crack whose cause has not been removed opens again after filling. If the ground is still settling, if the overload has not been removed, if corrosion is advancing, injection only buys time. Once the cause has been identified and stopped, the method is chosen according to the nature of the crack.

• Epoxy injection: For dry, non-moving structural cracks. Low-viscosity epoxy resin is injected into the crack under pressure; once hardened it makes the concrete monolithic again and restores the continuity of the section. It is not used in active, moving cracks; the hardened epoxy simply opens a new crack right beside it.
• Polyurethane injection: For cracks that leak water, are damp and move slightly. The flexible resin foams and expands on contact with water, stopping the leak and tolerating small movements. Preferred for cracks in foundations, basement walls and water tanks.
• Surface sealing and joint forming: For cosmetic, shallow, fine cracks. The crack is opened out into a V, cleaned, and filled with polymer-modified repair mortar or an elastic sealant. Elastic sealant is preferred for moving cracks; rigid mortar cracks again.
• Stitching and external strengthening: In widened structural cracks, the section is strengthened with steel staples set across the crack, carbon-fibre strips or steel plates. This is an engineering project, not a site decision.
• Corrosion repair: Before the crack is sealed, the concrete around the rusted reinforcement is broken out, the bars are cleaned and supplemented if necessary, an anti-corrosion primer is applied and the section is rebuilt with repair mortar. Sealing the crack alone hides corrosion; it does not stop it.

However much effort you put into repair, there is no solution as cheap and effective as concrete poured with the right mix, at the right consistency, without water added on site, and properly cured. To decide on the strength and exposure class suited to your project and to have the mix adjusted to the temperature and wind conditions on pour day, talk to the Bodrum Beton technical team before the pour; you can find which concrete class suits which element on our products page.

Frequently asked questions