Strength of Concrete in Basement wall

[gp238]

I have a reinforced concrete basement wall for a new home poured by a contractor at Amherst, NY on December 14th. The high and low temperatures on that day were 32 and 20 degree. The high & low tempreatures on the next day were 40 and 24. The top half of the wall was covered by insulated blankets, and the form work along with the blankets were removed after 24 hours. The water-cement ratio of concrete was 0.54 and type III cement was used with super-plastisizer and 5-8 % entrapped air. After 7 days I tested the concrete strength of the wall by a Swiss rebound hammer. I tested at least at 20 different points and the correlated compressive strength was 1000-1200 psi. Also, when I tapped the surface of the wall by a small hammer it left a dent on the wall. When I contacted the contractor about the problem, he informed me that the concrete supplier has tested one cylinder from the batch and the 7-day strength is 2000 psi. In my opinion , the tested cylinder is not cured at the site and thus does not reflect the in-situ strength of the concrete wall. I am very worried about the basement wall and hesitant to let the builder start framing. Could you or someone please advice me.
Thanks!
gp238

 

[Rjeffery]

do you see any signs that the concrete froze? ie are there pretty fern like impressions on the walls after the formwork was removed? If there were then your surfaces will not likely ever gain strength!