The 35 °F (20°C) maximum temperature gradient specification has been a “cut-and-paste” specification on mass concrete projects for decades. This specification often causes tremendous challenges for the contractor, and adds unnecessary costs and delays to projects. In most cases the flat 35 °F (20°C) requirement is an overly-restrictive requirement after a short period of time.
ACI 207.2R discusses methods of calculating temperature gradients and tensile stresses that cause thermal cracking in mass concrete. These techniques can be used to calculate safe temperature gradients based on concrete mix design, placement conditions, and concrete properties. One result from this analysis is performance-based temperature difference limit based on actual jobsite and concrete mix-design parameters. This calculated limit is much more pertinent and attainable than a flat 35 °F (20°C) specification.
Using intelliRock, the placement's temperature and strength can be monitored in real-time. This allows the in-place strength to be considered when determining what temperature gradient is currently safe.
Baker has used intelliRock's technology for years, but two recent projects really stand out.
First, Baker used intelliRock to complete the new CEMEX headquarters building on schedule, under budget, and with LEED points. Even while using lean mix designs with fly ash, Baker was able to stress PT beams in 18 hours and strip decks in 20 hours. And the lean mix design saved CEMEX approximately $130,000 in material costs. "The CEMEX headquarters project should serve as the poster-child for intelliRock because everything went exactly according to plan and we kept right on schedule." says Baker Senior Project Executive Aaron Smith. Case Study
Most recently, Baker completed the University of Texas, MD Anderson Cancer Center Mid-Campus Building 1 (MDACC) project. Located in Houston's Medical Center, the project consisted of three structures - 26 stories, 21 stories, and 5 stories. Not only was intelliRock used to monitor temperatures on what was the second largest mat pour placed in the united states, intelliRock technology was used to monitor deck strength in real-time greatly reducing the time that would be otherwise be necessary if using only traditional concrete cylinders to determine concrete strength. "Using the intelliRock system allowed the crew to gain half a day on almost every pour, which is significant when there are three pours per floor and 26 floors. Baker brought the project in three weeks ahead of our original schedule even after an additional floor was added to our scope," said Garret Benson, Baker Project Manager. Read more in Baker's newsletter (reprinted here with permission from Baker).
Baker has been ranked among ENR's top 3 specialty contractors for the past 20 years. Learn more about Baker on their website
As concrete maturity technology gains popularity, users are continuing to leverage its benefits. Today's application trend offers substantial cost saving opportunities on mass concrete pours. And if you could shave 10%, or even 20%, off of your concrete material cost before bidding, wouldn't that offer you a substantial competitive advantage?
Temperature management is a key risk-management task on mass concrete pours. Usually, maximum temperatures need to be kept below ~160F to avoid DEF issues and thermal gradients must be kept below some threshold value, such as 35F, to minimize the thermal cracking. These requirements are often viewed as challenges, BUT the root cause of these thermal control "problems" can also be viewed as a tremendous opportunity. Since cement hydration generates heat, and that heat gets trapped in mass pours (which is what causes the temperature rise), it takes a lot less cement to get the job done. And less cement means lower temperatures and substantial reductions in material cost! Even if cement content is reduced in the mix-design, the trapped heat will continue to drive the hydration reactions and strength gain. So this really creates a win-win opportunity to both reduce concrete temperatures and material costs at the same time.
To "tune" your mix design, use intelliRock concrete maturity sensors in your pre-pour test blocks. By instrumenting blocks made with multiple mix designs, and simultaneously calibrating the maturity values using cylinder breaks, intelliRock maturity sensors can determine the anticipated temperature rises AND strength gain profiles. Once you have real data defining the temperatures and strengths for each mix design, you can make some cost-saving decisions by using lower-cost mix designs!
For more information see the blog post below. And remember that these opportunities are compounded by the retained heat in mass concrete placements!
http://www.engius.com/blog/post/Using-intelliRock-to-Lower-Material-Costs.aspx