Explosive spalling of pieces of concrete from the heated surface is considered to be the most dangerous effect of damage of concrete subjected to intense fire attack, especially when it occurs in restricted areas such as underground tunnels. Recent investigations have revealed that the amount of explosive spalling and the extent of cracking can considerably be reduced by use of suitable amount of polypropylene fibres. However, little attention has been given to exactly how the fibres behave in the matrix of the concrete when exposed to fire. A good insight into the behaviour of the fibres when applied in concrete, especially when subjected to fire, can help optimise their use to reduce explosive spalling. This paper deals with a case study, in which an integrated microscopic method consisting of stereomicroscopy and polarising and fluorescent microscopy were used to assess the effectiveness of the pp-fibres in reducing explosive spalling in concrete elements subjected to fire attack. Rapid Chloride Migration test was also performed on standard specimens to establish whether the presence of the fibres might adversely affect the permeability and durability of the elements.