Visit with us at the ACI Sprint 2010 convention in Chicago March 21st - 25th! Jim Mikulanec and Tom Luby will be attending and you can catch them during the week in the Exhibit area. They would be happy to discuss concrete maturity or temperature profiling technology and share their experience from the field.
In addition, Tom Luby, P.E. will be presenting ways to use concrete maturity to reduce material costs (Monday at 3pm). Bidding and starting the job with multiple approved and calibrated mix designs can really make a difference on your bottom line! He'll show you how it's being done in the northestern US. For continuing education credits, use the session attendance tracking form located at the back of the program book provided at the convention. This form can be submitted to state boards that allow self-reporting of Continuing Education activities as evidence of participation. In most cases, 1 contact hour is equal to 1 Professional Development Hour (PDH). Check with your state board for acceptance criteria.
For more information on the convention, click here.
If you would like to schedule a time to chat with Jim or Tom, you can get their contact information here.
We'll see you there!
Clayco, Inc. and concrete sub Concrete Strategies, Inc. recently completed a 2 story parking garage for online brokerage firm Scottrade in St. Louis, Missouri. Information from intelliRock concrete maturity sensors was used to time critical workflow activities such as post tensioning.
Left: Scottrade Building HQ and garage site in St. Louis, MO.
Right: Post tension cables which were stressed in two stages, 1600 PSI and 3000 PSI
Concrete Strategies VP, Barclay Gebel explains “For crack controlling reasons we stressed the PT cables in two stages. The first stage was when the concrete achieved 1600 psi and the second stage was 3000 psi. We use intelliRock to determine when to do the stressing.”
Project engineer Curt Costello continues, ”We were able to determine when different crews would come in to start stripping forms and preparing the PT cables for stressing. Once the strengths were met, we were able to stress the cables without losing much time. This was because we were able to anticipate concrete strengths based on information from intelliRock.”
The graph above is an example of the information available to Concrete Strategies in real-time at the jobsite. intelliRock sensors evaluated the concrete’s maturity every 60 seconds. The maturity reading could be correlated to a concrete strength using a calibration curve for the mix design. This gave Concrete Strategies up-to-the-minute concrete strength values.
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Clayco, Inc. is one of the nation's largest, privately owned real estate, architecture and engineering, design/build and construction firms. Read more about Clayco at www.claycorp.com.
Concrete Strategies is a leading edge, full service, design/build and general contracting firm specializing in all forms of concrete and structural steel construction. The firm operates nationally and has an unparalleled track record in architectural site cast concrete, structural concrete, flatwork and all types of self-perform construction related to commercial projects. For more information see their website at www.concretestrategies.com
"With the performance-based mass concrete in hand, and intelliRock loggers in the concrete, Walsh uses the new wireless data transmission system to overcome the data collection challenges."
"About the Author: Eric Hayes is an assistant project manager for Walsh Construction, currently on-site managing the construction of the twin Allegheny River Bridges. For more information about Walsh Construction, visit www.walshgroup.com."
Read the case study from Modern Contractor magazine here!
If you’re ever wondering what a maturity calibration curve (graph shows strength vs maturity in °C-Hrs) could tell you about the time needed to reach a target strength, there is a simple calculation can give you enough information to at least see what’s feasible.
Maturity, using the Nurse-Saul method, is given in units of °C-Hours. The calculation is simply “Time x Temperature” with the units of time being hours, and units of temperature in °C. Assuming the common datum temperature of 0 °C, the math gets that simple: Maturity = Time x Temperature. Just be sure you get the units right.
Let’s assume you need 4,000 PSI and your calibration curve says the maturity needed for 4,000 PSI is 3,500 °C-Hrs. How long is that? A convenient first approximation is to see how long 3,500 °C-Hrs is at a comfortable 23 °C (73 °F) temperature. Maturity/Temperature = time, 3500 °C-hrs/23 °C = 152 hours, which is a little over 6 days. If your goal is 4,000 PSI in 7 days and it’s warm outside, then you’re probably fine with that mix design. If you need the 4,000 PSI in 3 days, then what do you need to do to get there? You need higher temperatures! How high? Temp = Maturity/Time, Temp = 3,500 °C-Hrs/(3 days* 24hrs/day) = 49 °C (104 °F). Is it reasonable for the concrete to have an average curing temperature of at least 49 °C? Mass concrete in Florida during July – you’re just fine. A thin elevated deck during January in Chicago – you’ll either have to supply supplementary heat or use a “hotter” mix.
This simple calculation is especially insightful when considering leaner, lower cost mix designs. Take the example where you need to achieve 3,000 PSI in 2 days and your maturity data says you are achieving 5,000 PSI in 2 days using your expensive high-early mix. Would a standard lower-cost mix design still get you there? Grab the calibration curve for the leaner mix, see what maturity is necessary, divide maturity by 48 hours and see what average curing temperature you need (remember, that’s the temperature of the concrete, not the ambient temperature). Compare the calculated temperature to the temperatures profile you’re currently getting with the high-early mix. Considering that the leaner mix will run somewhat cooler, is it likely that the average 48 hour concrete temperature will be at or above what the maturity calculation said? This simple calculation can at least tell you yes, no or maybe. For a 60 second effort, that’s a lot of insight.
The current AASHTO maturity specification is designated T 325-04(2008) “Standard Method of Test for Estimating the Strength of Concrete in Transportation Construction by Maturity Tests."
This specification, as most, is built around ASTM C 1074 and is intended to be used for estimating the strength of concrete in pavements as well as structures. Specific uses are the timing of:
• Opening to traffic
• Form Removal
• Post Tensioning
• Termination of curing procedures
• Destructive methods of evaluating concrete strength
Absent in most other specifications, T 325 does recommend the minimum number of temperature/maturity sensors to be used on a concrete placement.
• Slabs, beams, and abutment walls: 5 per 100 cubic meters
• Small columns: 1
• Large columns: 2
• Pavements and overlays: 2 per 1000 sq meters
• Pavement repairs: 2 per 750 cu meters or one per repair
The AASHTO specification also addresses situations where not every lot of concrete is tested.
One interesting recommendation by SHRP researchers is the usage of the Arrhenius function as opposed to Nurse-Saul. I’ll skip the Arrhenius versus Nurse-Saul soapbox speech for now, but will say that if Arrhenius models are used one should perform a rigorous calibration procedure at multiple temperatures, and be sure that the mix and materials are extremely consistent. As with any maturity technique, validate the mix often and follow the recommendations of the engineer of record on each jobsite.
Copies of the specification are available for purchase at several sites online including:
http://global.ihs.com and http://www.techstreet.com
ASTM C 1074 "Standard Practice for Estimating Concrete Strength by the Maturity Method" is the basis for virtually all concrete maturity specifications in the U.S. The document provides procedures for estimating concrete strength using a maturity index as either a “time-temperature factor” or “equivalent age.” The resulting strength information can be used to allow the start of construction activities such as:
1. Removal of formwork
2. Post-tensioning
3. Cold weather protection termination
4. Opening roadways to traffic
When using maturity on workflow-related activities maturity is replacing or enhancing information typically given by field-cured cylinders. It is important to realize that maturity does not replace all usage of concrete test specimens (cylinders or beams). The maturity method is based on information from cylinders or beams and destructive testing of specimens must continue for quality control purposes, to ensure consistency of the concrete mix.
The overall procedure is comprised of the following steps:
CALIBRATION
1. Select a concrete mix design
2. Prepare test specimens (beams, cylinders, cubes, etc). At least qty 15.
3. Embed a maturity sensor in the center of two test specimens.
4. Cure the specimens.
5. Perform breaks, typically at 1, 3, 7, 14, and 28 days and read the specimen’s maturity from the sensors in the instrumented specimens.
6. Compile a strength vs maturity calibration curve from t he data.
ESTIMATING IN-PLACE STRENGTH
1. Embed a maturity sensor either before, or immediately after concrete placement.
2. Begin logging temperature and maturity information
3. As the concrete cures, monitor the maturity reading until the maturity index indicates that the target strength is attained. The target strength is typically 75% or 100% of the specified strength.
4. Convert the maturity reading to compressive or flexural strength as needed.
5. Validate the delivered mix to be sure the delivered concrete is consistent with the expected mix design. There are multiple ways to accomplish this step, but you do NOT need to wait on test specimens to reach target strength.
6. If the mix is validated, the strength reading based on the maturity index can be used for timing construction operations.
For more information, review the current version of ASTM C 1074. You can purchase a copy online at: http://www.astm.org/Standards/C1074.htm
Over the years intelliRock has been used on jobsites with extremely harsh conditions for electrical instrumentation. For these applications we developed “armored” cables to protect the download wires from being damaged as the concrete was being placed. These tough cables, suitably dubbed “yellow wire loggers”, were so reliable we started using the tougher wire on all loggers with cable lengths over 4 ft. Today 4ft red-black cables are still available on loggers as a cost-effective solution on small concrete placements. However, on large placements we highly recommend that 8ft or longer cables be used to maximize durability and reliability even if the placement isn’t considered a “mass concrete” pour.
intelliRock loggers are currently available with the following cable lengths: 4ft, 8ft*, 15ft*, 30ft*, 50ft*, and 100ft* (* denotes loggers with tough “yellow wire” cables) . Cables longer than 100ft are available by special order.
Special loggers are available for users needing to monitor mass concrete temperatures over "years." These loggers were originally developed to monitor long-term temperatures on the Tom Sauk Reservoir rebuild where engineers wanted a way to conveniently monitor mass concrete temperatures as long as possible. The "MEGA" loggers are physically larger than standard intelliRock loggers because they contain larger batteries. Although only guaranteed for three years, the larger battery theoretically gives the loggers enough power to log for 5 to 10 years.
If you have a mass concrete monitoring project where you could employ MEGA loggers, give us a call and we can discuss needs and capabilities!