The workability and mechanical properties of mortar containing shredded automobile and truck tyres were evaluated. Two different shapes of rubber particles were used as constituents of mortar : (1) granules about 2 mm in diameter, and (2) shreds having two sizes which were, nominally, 5.5 mm x 1.2 mm and 10.8 mm x 1.8 mm (length x diameter). As expected, the geometry of the rubber particles influenced the fracture behaviour of rubber-containing mortar. The addition of rubber led to a decrease in flexural strength and plastic shrinkage cracking of mortar. The crack width and crack length due to plastic shrinkage were reduced for mortar containing the 10.8 x 1.8 mm rubber shreds compared with a mortar without shreds. The rheological properties of the mortar containing rubber shreds were comparable to those of a mortar without rubber and yielded lower plastic viscosity than a mortar containing 25.4 mm x 15 mu m (length x diameter) polypropylene fibres. The alkaline stability of rubber in mortar was also evaluated by immersing rubber shreds in NaOH and Ca(OH)(2) solutions for 4 mon and the results showed that there is less than 20% change in stress and strain value. The findings of the research suggest that automobile and truck tyres can be recycled by shredding and incorporating them into mortar and probably concrete for certain infrastructural applications.