This article illustrates and describes cracks that can occur in prestressed hollow-core slabs during casting, stripping, or shipment. The types of cracks covered, and their causes are:
Longitudinal crack at void--causes: transverse shrinkage, improper handling, lack of differential compaction, placement and eccentricity of prestressing steel, flange too thin due to movement or misalignment of void forms, or overinflation of rubber void forms.
Longitudinal crack at web--causes: subsidence over void, or shrinkage due to improper curing and mix proportions.
Corner cracks--causes: saw blade pinches when member cambers, sawcut not deep enough, saw blade "wobble" due to excessive use, uneven dunnage, support blocks not providing even distribution of load during storage, uneven handling due to pickup devices not being level, or excessive tension stress during stripping.
Web crack at or near the strands--causes: excessive bursting stresses, lack of concrete consolidation around strands, strand masking improperly placed, layers not bonded, or sawcut not deep enough or not complete across sides.
Transverse crack--causes: longitudinal shrinkage, contraction due to delayed detensioning after uncovering heat cured product, excessive top fiber tension, or insufficient cover on transverse reinforcing bar (when required for special designs)
Web crack above the strands--causes: excessive prestress force relative to cross section of member, insufficient release strength, expansion of rubber void forms due to increased temperature, bottom surfaces of member sticking to form during stripping, sawcut not deep enough, lifting inserts or end stirrups misplaced, mix too wet or too dry, or insufficient vibration.