Title: Concrete Q & A: Does Curing Time or Strength Control in Cold Weather?
Author(s): Steve Morrical and Dave Suchorski
Publication: Concrete International
Appears on pages(s): 92
We’re working under a very tight schedule to construct a multistory building. This requires us to place a floor a week under cold weather conditions. The specified 28-day compressive strength is 4000 psi (28 MPa). Field-cured cylinders prepared in accordance with ASTM C31 are consistently reaching 1-day compressive strengths of 3000 psi (21 MPa). Our contract documents reference ACI 308.1-98, “Standard Specification for Curing Concrete,”1 and ACI 306.1-90, “Standard Specification for Cold Weather Concreting.” 2 We want to start removing shores and recycling our forms after 1 day, but the construction manager says we have to keep the concrete protected against damage from freezing for 3 days, citing the following section from ACI 306.1-90: “3.4.4 Protection against freezing—Cure and protect concrete against damage from freezing for a minimum period of 3 days, unless otherwise specified…”Enclosing and heating the floors below the ones we’re forming is costly and makes it difficult for us to remove shores and forms efficiently. Do we need to provide protection against freezing even when the concrete has gained more than 70% of the specified 28-day strength?