High performance concrete: field observations of cracking tendency at early age

In addition to the early age cracking problem, several other and possibly interrelated field problems, have been observed in practical use of concrete with low w/b-ratios. These include the following:

1. To obtain proper workability. 2. To retain workability for a reasonable period of time. 3. Trowelling of free concrete surfaces. 4. Frequently an abnormal drop in strength when the air content exceeded

a certain limit, usually 5,0-5,5% for m=0,40. 5. Testing often showed that water-impermeability was not achieved.