You don't say what your design compressive strength is, but usually ready mix concrete will achieve its design compressive strength in less than 28 days. You also indicate that the concrete is in the foundation. It is likely that the foundation will not receive its design loading in 28 days, unless you are planning some blazingly fast construction! So with that in mind, you probably don't need to achieve the design strength any sooner than would normally happen with well designed, properly produced, and properly placed concrete.
Many times, design specification will call for testing at necessary intervals, 7days or 28 days etc. However, like Ron stated in his repsonse: ready-mix conrete will usually achieve the desired strength prior to the 28-day mark. Knowing this many contractors wishing to wreck forms early or procede with some other construction involving that concrete will ask the testing company to make extra cylinders and have them broke at 3 or 5 days. Generally, the contractor bears the extra cost as he will be able to keep his crew working and job progress going. If the concrete breaks with 28-day strength on the 3 or 5 day, great, let the contractor move on with his operation. Otherwise, you still have the remaining cylinders.
Some operations state that forms may be wrecked at an early stage but no loading for a minimum period of time. Check all the specifications before letting the contractor move on when it involves loading. [sig][/sig]
Thank you for your responses.
The design strength I need is f'c = 2500 psi. Yes, I realize that full compressive strength is expected at 28 days and I believe almost 50% capacity can be expected in 14 days. I want to maximize this.
This project involves moving a timber frame church from a closing military base. The church was constructed by the Army during WWII. (Great engineering, simple, elegant timber trusses spanning 36 feet with bays at 12' o.c.)
Now, a church group has purchased the building for $1 and will pay about $100k to have it moved by truck. The structure will be sliced into approx. 14' sections and rebuilt at the new site. New stud walls with HardyFrame light gage steel shearwalls will be addes as well as stronger PSL floor beams. The raised foundation I designed includes spot footings, combined footings and gravity load bearing retaining walls.
Naturaly, the rush is on to avoid the rains because the interior hard wood floor is at risk. So, the contractor wants to begin to lift the sections onto the foundation only about two weeks after the pour.
Again, my question: which add-mixture would be best to recommend and how much compression will the foundation take? [sig][/sig]
In the highway industry it is not uncommon for various sections of pavement to be removed and the concrete replaced with traffic back on the roadway within a few hours. In many applications, a Sika product is used such as Sikaset. In addition, the High Early Strength (HES)concrete is used which uses typically between 6 and 8 sacks of concrete, a low water/cement ratio, and type III cement. Data has shown the concrete without admixture to provide over 2500 psi within 12 hours. Using the admixtures, you will probably cut that down by 1/3.
You can achieve essentially the same effect by specifying a higher strength concrete. For typical portland cement concrete (using only portland cement as the cementitious material)about 50 percent of the strength will be gained in about 3 days, about 70 percent at 7 days. Using this relationship and buying 4000 psi concrete, you would have your required f'c in 5 or 6 days. I would caution you against using 2500 psi ready mix concrete as that is the lowest strength typically produced, and it is usually ordered for residential applications, not commercial applications. It is much easier to compromise the properties and quality of lower strength concrete than mid-range strengths. I would opt for the higher strength material as its durability will be greater, you have more latitude for error, and the strength will be there as needed. [sig][/sig]
I agree with Ron. The concrete I was talking about for highway applications is usually 4000 psi. For prestressing applications in which HES is used the concrete specified is 5000 or 6000 psi.
The cost should not be an issue if the contractor is able to move on with the operation saving both time and perserving the church's interior. [sig][/sig]
THANK YOU, GENTLEMEN. YOU HAVE BEEN MOST HELPFUL. BY SOLVING THE PROBLEM AT THE OTHER SIDE OF THE EQUATION, MEANING BY INCREASING THE SPECIFIED STRENGTH, I WILL REST EASY WATCHING THEM BUILD ON THE NEW FOUNDATION. NOW IF THE STEEPLE DOESN'T BLOW OFF IN THE NEXT WIND STORM, I'LL PROBABLY STILL BE IN BUSINESS.
