The development of indirect-tensile, flexural and compressive strengths has been followed fromn an age of 18 hoiurs to 1 year on various concrete mixtures containing up to 180 kg/m3 of ash. The low early strengths normally associated with such mixtures were as much as one-third less than control mix strengths at about 7 days but some ash modified mixes became more than 30 percent stronger than the appropriate control mix at one year. Values of strain capacity have been determined by two methods and have shown that at early ages the risk of cracking which is mainly the result of thermally induced strains is increased in ash-modified mixes