Need help with low 7 and 28 field breaks

[ETIcorp]

I am starting to get low 7 and 28 field break. The labs for the same sets are at times 50% more than the fields. All the slumps and air are within specs. The initial curing is being done correct. Transportaion is correct (box is padded,cylinders are capped during trip, trip is less than 4 hours) The contractor blames the fact that we just recently switched to 4x8s from 6x12s. I have plenty of data from other jobs over the past 1 1/2 years since the switch where the fields are making full strenght. I am at a lose here, any help would be appreciated.

 

[henri2]

I'd like to ask a few questions in order to get a better understanding of the situation.

1. For 6 x 12-inch cylinders, what was the compressive strength of the field cured cyliners compared to the lab cured cylinders under different weather conditions (cold weather, temperate weather, hot weather)?

2. Do you have any set of results where companion 4 x 8-inch cylnders were cast alongside 6 x 12-inch cylinders made from the same concrete mix? This may indicate what role specimen size plays.

3. You wrote "I have plenty of data from other jobs over the past 1 1/2 years since the switch where the fields are making full strenght." Were the specimen sizes 4 x 8-inch or 6 x 12-inch

A study investigating the effect of curing on compressive strength of 4 x 8-inch cylinders was conducted by NRMCA.

http://www.nrmca.org/research/NRCQ0404_57-59.pdf

check the 28 day results on page 2.