CONCRETE CONSTRUCTION MAGAZINE Online
Wednesday, February 26, 2014
Concrete Insights Good News for Fly Ash
How many of you believe that we are running out of high quality fly ash? According to Rafic Minkara, president of Headwaters Resources, none of us should raise our hand because he insists there is plenty. But he also notes that the use of fly ash in concrete has been declining slightly, at least in part due to uncertainty over whether the Environmental Protection Agency will declare fly ash a hazardous waste. Perhaps that will change though following a recent report from the U.S. Environmental Protection Agency that states very directly that fly ash used in concrete is safe—better than safe since it consumes a byproduct that would otherwise end up in a landfill and reduces cement manufacturing which generates greenhouse gases.
“Environmental releases of constituents of potential concern from coal combustion residuals fly ash concrete and FGD gypsum wallboard during use by the consumer are comparable to or lower than those from analogous non-CCR products, or are at or below relevant regulatory and health-based benchmarks for human and ecological receptors,” the EPA said in its report. “EPA supports the beneficial use of coal fly ash in concrete and FGD gypsum in wallboard.” Click here to read the full report.
This is a significant endorsement from the EPA. Thanks to the leadership of the American Coal Ash Association and its executive director Tom Adams, this should dissipate the cloud that has been hanging over fly ash for the last several years. From the concrete industry’s viewpoint, this is much more than simply an environmental coup; more important to us is the improvement fly ash imparts to concrete properties. Even a small percentage of cement replacement results in concrete with lower permeability, less susceptibility to alkali-silica reactivity, lower heat of hydration, and a smaller carbon footprint. EPA has committed to finalizing the fly ash rules by the end of 2014 and it looks likely now that we will see more fly ash use in the coming years.
Very Large Scale Seismic Testing of a 4 Span Bridge
In 2012, testing was conducted on one of the largest “shake tables” in the world. The quarter scale, 100 foot long simulation incorporated shape memory alloys (SMA), engineered cementitious composites (ECC), post-tensioned columns, and built-in elastomeric pads within it. The results of the testing were contrasted against comparable standard reinforced concrete structures. The details of the test are as follows and are excerpted from the website of Carlos A. Cruz Noguez, PhD (https://sites.google.com/site/ccruznoguez/home):
“At the North bent of the bridge, SMA material was selected because of its ability to undergo large strains but recover its shape upon stress removal, while ECC was chosen due to its high ductility and tensile strain capacity. At the Middle bent, the columns were post-tensioned to enhance the recentering capability for the columns. At the South bent, a built-in rubber pad was used to replace concrete in the bottom plastic hinges of columns to avoid concrete damage; post-tensioning was used to enhance the recentering ability of the columns. The remainder of the columns in the SMA and ISO piers was constructed with conventional RC to provide a response comparison between innovative materials and RC construction within the same column.
Figure 3 Bridge specimen and side-view column details (Cruz-Noguez and Saiidi, 2012a)
The bridge was modeled the bridge with both commercial (SAP2000) and research-oriented finite-element packages (OpenSEES) to replicate the bridge response (the objective was to show that commonly used software could be used to model the high-performance materials). Computer simulation was crucial for the success of the experiment at the pre-test and post-test stages. The response of the innovative materials was successfully captured by existing element and material formulations. Close agreement was found between the calculated and measured bridge response parameters. The comparison between the model and the experimental response of the bridge can be seen in the following videos:
Video: Bridge Response During Final Test (Run 7, PGA=1.00g)
Results obtained in this study (Fig. 5) showed that plastic hinges detailed with the ECC/SMA combination and elastomeric pads exhibited no damage compared to conventional construction. Furthermore, all innovative bents had no significant residual displacements (Cruz and Saiidi, 2011; 2012). These findings can be used in the design of new bridge structures that must remain fully operational after strong seismic excitation, with only minor repairs needed (as in the case of critical lifelines or essential structures). Therefore, the potential impact of this work is deemed to be very significant, since it shows that 1) innovative details can be incorporated in real structures, 2) some of them perform better than conventional RC in terms of minimizing damage and residual deformations, 3) innovative materials and details can be accurately modeled using computer software available in most engineering firms, and 4) while it is true that these details are relatively more expensive than conventional RC, a damaged bridge with innovative components does not need significant repairs after seismic events, which offsets the initial cost. These are significant benefits for the public and the society. Results from this study have been presented to the departments of transport of the states of California (CALTRANS) and Nevada (NDOT), and work for the design of the first bridge with innovative materials is currently underway.
Figure 5 Damaged state at top and bottom plastic hinges after final test (Cruz-Noguez and Saiidi, 2012b)
(End of excerpted portions from Carlos A. Cruz Noguez, PhD (https://sites.google.com/site/ccruznoguez/home):
A Representative Article from the ACI STRUCTURAL JOURNAL Entitled “Cyclic Response of Concrete Bridge Columns Using Superelastic Nitinol and BendableConcrete” article #106-S08 BY M. SAIID SAIIDI etal., 2009