Tell us more about the mix....Strait cement? Ash? MRWR? You stated you have had this problem for 2 years there is an issue with the mix not weather. Is the problem just in the summer? If it is do you use fly ash? If not why not in the summer it helps control this issue during hot weather. email me with the mix if you would like I can take a look at it and possibly make a few suggestions.

You should probubly take your own advice....20 gallons to 11 yards that would change the W/C by 0.02 that is NOT why it is cracking, You must be an admix salesman for GRT or something....