High Pozzolan Mix Designs for Mass Concrete Placements

At the American Concrete Institute convention this spring 2011, American Engineering Testing (AET) will be discussing its new ability to trial batch and model mass concrete. The abstract is as follows:

Many state departments of transportation (DOT) require a maximum curing temperature of ~160⁰F, and maximum temperature differential of ~35⁰F for the curing of mass concrete. The reasons for these requirements are the prevention of delayed ettringite formation (DEF) and thermal cracking, respectively. To fulfill these requirements, there has been a recent use of high pozzolan mix designs, due to their lowering of the heat of hydration. AET has conducted research on a mix design with 55% slag and 20% fly ash replacements to determine if the MN-DOT requirement of 45⁰F, 50⁰F, and 60⁰F temperature differentials in the first 48 hours, next 2 to 7 days, and next 8 to 14 days, respectively, is fulfilled for one of Lafayette Bridge’s pier footings in Saint Paul, Minnesota. This footing was assumed to be cured with two thermal blankets and under moderate ambient conditions. First, testing was compiled on thermal conductivity, thermal diffusivity, isothermal calorimetry, semi-adiabatic calorimetry, and unit weight. Then the results were imputed into a finite element analysis program, and a simulation of the mass element’s heat generation and dissipation was created. Additionally, data on a non-pozzolan type I/II mix were taken from ACI 207.1R-05 and ACI 207.2R-07, and this data was inputed into the software under the same conditions. Thus, a comparison was made between the two simulations. Finally, the compressive strengths of the pozzolan mix were correlated to tensile strength, and the influence of low initial strengths on thermal cracking was discussed.     

The purpose of this blog is to update on the presentation's progress, and discuss the topic.

Slides

So my name is Benn Stutrud, and I am Sister Mary Concrete. I am the one giving this presentation at ACI. I have finished my slides and thought it would be interesting to post and talk about them…

In St. Paul, Minnesota the Lafayette Bridge is being constructed with mass concrete elements. To give you an idea for what this entails consider the following definitions:

Precautions have to be taken while curing these mass elements due to potential thermal cracking and delayed Ettringite Formation (DEF).

The remedies for this are optimizing the mix design for lower heat generation, and using thermal insulation during the curing process.

Sister Mary Concrete’s strategy was to first create a mix design with high Pozzolans content, and large aggregate yield. This effectively lowers the heat of hydration. The following shows the information for our footing mix:

 Being that I had a math background, I had to go in depth into the heat equation. So, I showed the variables, Numerical analysis, and figure as follows:

Before we could create any modeling, there was a number of thermal tests that we had to conduct. The slides on this are fairly self-explanatory…   

I then go through a number of results obtained for a footing and pier:

For the conclusion, I really want to talk about the importance of Pozzolans in the mix design for mass concrete. Also, I made some rough calculations for thermal cracking as a conversation piece for future considerations with high pozzolan mixes.

Well that’s my slideshow. Feel free to post comments/questions.