Concrete - Geomaterials: Under the Microscope - page 93

Table 22 Causes of cracking in concrete (after Concrete Society, 1992)

Time of crack Crack type Causes

After hardening Physical Shrinkable aggregates

Dying shrinkage
Crazing

Chemical Reinforcement corrosion

Alkali-aggregate reactions (AAR)
Delayed ettringite formation (DEF)
Cement carbonation

Thermal Freeze-thaw cycles

External seasonal temperature variations
Early thermal contraction

Structural Accidental overload

Creep
Design loads

Before hardening Early frost damage

Plastic Plastic shrinkage

Plastic settlement

Constructional movement Formwork movement

Subgrade movement

196

Table 23 Classification of cracks

Crack Nominal Typical

type width length

Fine <1 μm A few millimetres

microcracks

Microcracks 1-10 μm 1-30 mm

Fine cracks 10-100 μm Up to 300 mm

Cracks 100 μm- Up to several

1 mm metres

196 Plastic cracking of slab concrete. The crack
appears as a series of tension gashes (yellow); PPT,
×35.

Large cracks >1 mm wide Up to several

metres

Cracks that occur in fresh concrete include plastic
cracks. Plastic shrinkage cracks are caused by excess
bleeding and plastic settlement cracks are usually
attributed to rapid early drying. In thin section, plastic
cracks may appear as a linear series of tension gashes

that run through the cement matrix and around
aggregate particles ( 196 ). Another type of early cracking
is known as 'crazing', which can occur on slab and wall
surfaces as an irregular network of fine cracks (often with
close spacing).