Cincinnati Ready-Mix Presents: By: Rick Roth
First: What is “High Early” Concrete????
“High Early” Concrete is concrete that reaches its strength earlier then 28days! Any PSI concrete can be made to be High Early.
How is “High Early” Made? • H/E is achieved by using a Type III cement. An extra bag of cement can also be used in place of the Type III since most R/M producers do not have the silo space for Type III.
Here are a few mixes that are already designed for “High Early” applications used by the Ohio Dept of Transportation! • Class F.S. • Class M.S.
Class F.S. • This mixture is a fast-setting Portland cement concrete for accelerated setting and strength development. The minimum cement content shall be 534 kg/m3 (900 pounds per cubic yard) and the maximum water cement ratio shall be 0.40. The rigid replacement may be opened to traffic after four hours provided test beams have attained a modulus of rupture of 2.8 MPa (400 psi)
Class M.S. • Class MS. This mixture is a moderate-setting Portland cement concrete for accelerated strength development. The rigid replacement may be opened to traffic after 24 hours provided test beams have attained a modulus of rupture of 2.8 MPa (400 psi). The minimum cement content shall be 475 kg/m3 (800 pounds per cubic yard) and the maximum water-cement ratio shall be 0.43.
More common “High Early (H/E)” mixes used in the City of Cincinnati today are: • Class C H/E • Class C H/E 2%
Class C H/E • Class C is deigned to reach 4000psi in 28 days! • With the H/E added to it you are shooting to reach 100% of that strength in 7 days • Most cases you will reach 50%-70% of that strength in 3 days!
Class C H/E 2% • Same as Class C H/E • 2% will help it along on colder raw days! • More then likely needed on days under 50 • degrees
Ok what about our question??? When can i blacktop on concrete??
What about Class C 2%? • Class C is widely mistaken for a H/E concrete. Just adding accelerators doesn’t give the Type III Cement that is needed for the early strength! Accelerator will help it set up quicker and start gaining strength quicker, but will level out much quicker then having the Type III Cement.
If you allow Concrete to be blacktopped on when it reaches 50% of its strength. • Class F.S. - 5 hours after set • Class M.S.- Next Day! • Class C H/E 2 Days • Class C H/E 2% 2 Days (use on cold raw days.) • Class C 2% 3 or 4 days
Misc Notes • H/E will not necessarily make the concrete set faster as accelerator will! It will gain its strength faster after its initial set! • Don’t use the term “fast set” it covers any mix out there you want it to. • Don’t call Class F.S. anything but Class F.S.