Some people refer to these as ‘spiderweb’ cracks but the technical name is ‘mapping cracks’.
That’s because they can look like a street map.
Generally they are much more obvious after rain as water seeps doown into the cracks.
These cracks are typically only 2-3mm deep and are as a result of the surface concrete shrinking more, and sometimes faster, than the underlying concrete.
The reasons for this greater/faster shrinkage include:
■ Adding water to the mix, when the delivery truck arrives on site, to improve concrete workability.
■ Starting finishing while ‘bleed water’ (standing water after placed concrete levelled) is on the surface
■ Sprinkling cement powder to soak up ‘bleed water’.
■ Letting the surface dry without Proper Curing .
Although the cracking does spoil the look, it is a cosmetic issue and it doesn’t significantly affect the strength of the concrete.
See Concrete for more posts.