Case Histories

Parking Garages
Bridges
High-Rise Buildings
Concrete Mix Design Optimization
Pavements
Pre-Cast Concrete

Parking Garages

Barnes Hospital, St. Louis (2000-2001):

Con-Cure conducted maturity testing for 33 pours at a post-tensioned parking structure in St. Louis during November, December, January, February, March and April.

Mix: Metro Concrete 6000psi mix with Microsilica and FiberMesh.
Required strength to begin stessing operations: 3450psi.
Average in-place age to reach 3450: 27.1 hours. Range: 16-48 hours. Ambient temp plays a role in this range. Also, amount of heating has tremendous impact.

Result: Faster construction because in-place strength higher than field-cured cylinders.
Better structure due to less heat being used. Saved at least one day for each pour, plus tremendous amounts of propane and labor to switch out tanks and for fire watch. Saved more than $5000 in one weekend alone.

The Hot Cure Box:

We found out firsthand just how beneficial maturity testing can be. The contractor had placed sensors throughout the deck of a post-tensioned parking garage. The temperatures during the pour were typical of February in St. Louis - mid-thirties or so. Blankets, jacketing and plenty of propane "salamanders" below the deck protected the concrete from freezing.

At about 8pm, a fierce windstorm arrived, dropping temperatures by 20 degrees, blowing down most of the tarps and extinguishing the heaters. The project manager knew the maturity meters would track temperatures, and that the blankets were still intact over the deck.

The next morning, the testing lab broke test cylinders and called to report strengths over 4000psi, which meant stressing operations could begin. However, the maturity tests showed in-place strengths of just over 2000psi, much too low to stress PT. Why the huge difference?

The test cylinders had been stored in a heated cure box - not even close to the same conditions as the deck that cold night. Had stressing begun it almost certainly would have blown out the cables, with great risk of injury to workers.

Bridges

Creve Coeur Lake Memorial Park Bridge, St. Louis:

A large double box girder cast-in-place segmental bridge constructed using a balanced cantilever method, this project provided many opportunities to explore the benefits of maturity testing on a wide range of applications.

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Primarily, the Con-Cure System was used to determine early-age in-place strength. Once sufficient strength had been reached, the contractor was allowed to begin stressing operations, moving formwork and was able to cease external heating operations. This project is being highlighted as a foremost use of maturity testing in a segmental cast-in-place bridge and is featured in a paper by Bruce Kates, lead engineer on the project. In Mr. Kates' words:

"[The Maturity] method lends itself to those conditions which require high early strength to allow the partial removal of forms for cycling of the equipment to the next placement location, or the application of post-tensioning forces into the structure. Besides providing reliable strength assessment, the procedure includes a record of the concrete temperature during the entire curing period, which helps to assess cold weather procedures when minimum temperatures are specified....The...advantage is in cost savings due to reduced construction time and/or avoidance of costly repairs resulting from low-strength concrete which may be loaded too early as the result of a mishandled cylinder test."

Kiefer Creek Overpass, St. Louis:

Maturity Testing and cylinders used simultaneously.

Only two pours--each monitored for 10 days.

Problem: One truck driver added 2.5 gallons of retarder instead of superplastisizer.
Bridge deck remained plastic for 6 days (!) in area where retarder was placed.
Temperature history showed virtually flat line in that area, indicating very little strength gain, while the other areas showed normal strength gain.
No cylinders were taken from problem truck--reliance on cylinders exclusively would have been erroneous and possibly dangerous if forms had been stripped

What all of this means is that while Con-Cure couldn't tell the contractor what was wrong, only that something was different. The contractor was amazed that the system could pick up such differences. The key is in interpreting the data.

High-Rise Buildings

Residential 30-story tower, Clayton, MO (2001):

The Con-Cure System was used to allow early stressing of PT and stripping of forms on a very tight schedule. The client reported saving at least one day per pour, and could have saved more but, in his words, "We couldn't work that fast."

This client reported saving an average of $57,000 per pour based on early completion bonuses.

Concrete Mix Design Optimization

One client related an amazing story to us recently. His company was building a large mass concrete project where more than 50,000 cubic yards of concrete would be used. The engineers provided the contractor with a recommended mix design, one which would naturally attempt to control the substantial heat of hydration expected of such massive placements.

Using the Con-Cure System, the client was able to develop maturity curves of 9 different mixes--each slightly different--to try to find the optimal mix design based on the critical factors of heat, shrinkage and workability. This testing was not cheap, but the end result was worth it.

The client reported finding an alternate mix design to the one recommended by the engineer which met all of the performance requirements and saved the contractor more than $10 per cubic yard. THAT'S HALF A MILLION DOLLARS saved. He told us this story because he said that while it would have been possible to do this testing without Con-Cure, our system made it MUCH easier. His words, not ours. We also helped out during the testing as consultants, just to make sure everything went smoothly.

Pavements

Interstate 70, St. Charles County Missouri

April-May 2001

Early opening times are the buzzword around the country and nothing aids this like maturity testing. Here's a story to warm the heart of any paver:

Contractor is tasked with shutting down lanes of I-70 (overnight on weekends only) to place more than 260 full depth patches. A mix is chosen that, according to the state, reaches sufficient strength (2500psi compressive) in 5 hours. So, the contractor has to get to the site, close lanes starting at 7 pm, tear out the bad concrete, prepare the "hole," place new concrete and let it sit there for at least 5 hours. All of this has to be done by 6 am the next morning. This just does not leave a lot of room for actually paving, since they have to be done pouring the concrete by 1 am.

Using the Con-Cure system, we demonstrated that due to the much larger mass of the concrete in the patch, the in-place concrete was reaching 2500 psi in only 3 hours. This is a huge deal to the paving company, because now they have 2 extra hours to work overnight. They were able to patch more holes and earned a $1.2M bonus by completing the entire project early.

To those paving companies who don't believe this works, we suggest having us demonstrate it to you.

Pre-Cast Concrete

Maumee River Crossing, Toledo OH:

More info later, but here's the bottom line: The contractor reports saving a minimum of 4 hours per segment using maturity testing. 4 hours may not seem like much to you and me, but to them it's huge. This project involves casting 3100 huge segments in a yard. 8 segments a day every day for two years or more. Turnaround of the forms is paramount and only maturity testing is giving the contractor the edge on this project. It's in Toledo where cold is very common.