The Weibull modulus of mortar specimens containing 35% (w/w of cement content) of NaOH-treated rubber particles was calculated showing a value of 9.1 for the control and 9.4 for the specimens with rubber, indicating that the incorporation of a high amount of rubber does not change the casting reproducibility. Since the flexural strength of these rubber-containing specimens was reduced by 43%, new composites were prepared using 10% of rubber as addition or aggregate. Water sorption by immersion, resistance to acid attack, flexural strength and freeze-thaw experiments were performed. Transport properties were improved for the addition-rubber-containing composites; the best results were obtained with the aggregate-rubber-containing composites. A reduction of 15% in flexural strength was observed for addition-rubber-containing composites and 25% for the aggregate-rubber-containing composites, roughly as expected if the flexural strength varies linearly with the rubber content. Furthermore, after 60 freezing and thawing cycles, a reduction of 75% in flexural strength was observed for the control specimens and only 20% for the addition-rubber-containing specimens. (C) 2004 Kluwer Academic Publishers.