Beyond the Pavement Patch. Applications of Maturity Testing for Bridges, Structures and Pre-Cast Concrete. July 19, 2006. Presented by John P. Gnaedinger, Pres.Con-Cure Corporation, St. Louis, MO. Willow Island Cooling Tower Collapse.
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Beyond the Pavement Patch
Applications of Maturity Testing for Bridges, Structures and Pre-Cast Concrete
July 19, 2006
John P. Gnaedinger, Pres.Con-Cure Corporation, St. Louis, MO
51 Workers were killed when a cooling tower collapsed. It was shown that the primary cause was “green” concrete. In-place strength was lower than test cylinder strength, leading to interest in better in-place strength determinations.
Beyond the Pavement Patch,
there are many more applications that
benefit from Maturity Testing!
ESPECIALLY IN COLD WEATHER
Effect of Curing Temperature on the rate of strength gain
Relative Strength (to 3d@70F)
* require correlation
method to determine
Typically, Underestimate In-place Strength
Structure =Larger mass =warmer temps & better hydration
Test Cylinders= smaller mass = cooler temps
“The Bathtub Test”
The deck is hot
The cylinders are not
Plante, et. al., “Influence of Curing Conditions on Concrete Specimens at Construction Site,” ACI Materials Journal V. 97, No. 2, March-April 2000, pp. 120-126.
Richardson, D. N., “Review of Variables that Influence Measured Concrete Compressive Strength,” N.A.A. Circular No. 132, 1991, p. 112.
But New Techniques Exist:
The construction industry relies on field-cured cylinder testing to determine strength of curing concrete.
Using an established ASTM Standard, in-place strength can be determined accurately and instantly.
The temperature at which concrete is cured determines the strength of the concrete at any given point in time.
The relationship between the temperature history of a concrete and its strength can be empirically determined, and is called its maturity index.
Concrete of a given mix at the same maturity has approx. the same strength, regardless of the temperature and time history that made up that maturity.
Concrete of a given mix at the same maturity has the same strength, regardless of the temperature and time history that made up that maturity.
Maturity (EA or TTF)
The sensors are placed in the structure by drilling a small hole in the formwork and threading the sensor plug through the hole. The meter is attached below.
Here, the sensor tip is protected from damage during placement operations by securing the wire and sensor to the underside of the rebar
View showing sensor and meter together, with wire running through formwork.
There is no limit to the number of meters that can be deployed for a given pour.
Why Do Maturity Testing?
Pull cables and strip forms ASAP =Project Acceleration.
Curing times are usually cut dramatically, esp. in the winter. Test cylinders typically lag far behind the structure.
Extend the construction season.
Save money by assessing cold weather protection to ensure sufficient temperatures for curing without wasted heating.
Can also allow early termination of external heating or thermal protection.
Open pavements to traffic sooner=get paid sooner
Determine optimal saw-cut times
Save money by reducing or eliminating reliance on field-cured test samples.
Improve site safety by not stripping forms or stressing cables too soon
Improve concrete quality by learning the temperature history of the concrete. Compensate for changes in field conditions on-the-fly.
When Should You Do Maturity Testing?
Expressed another way:
Any time-sensitive placement where knowing the in-place strength would be beneficial for quality, engineering or economic reasons
3.4 days EA
Using maturity testing allowed the contractor to continue placing concrete even in very cold weather
Prevented a complete shutdown of the project over the winter months, saving the contractor more than $2,000,000 in costs and liquidated damages
State gained valuable experience and data to support the use of maturity testing on a wide variety of projects
Almost 400 CY of concrete, poured in 6 hours
On day 3, the cylinder with the Con-Cure meter attached was broken (4310psi) and the meter was read (4300psi)
Maturity meter, with temperature probe wired to rebar in wing of segment
A Stunning Real-World Example and Lessons Learned
Contact: John Gnaedinger, PresidentCon-Cure CorporationSt. Louis, Missouri636-386-4800
The Mini-CD contains PDF versions of all of our product literature as well as some more technical information, which you can print out at your convenience.
Also, our web site contains a good deal of information about the science of maturity testing.