Concrete Redux (1/27/16)
Welcome back; and what a surprise we received to inaugurate the new semester! I hope you all had a pleasant winter holiday and that you all enjoyed the snow and the extra time to gear up for the next five months.
Much work was accomplished on the I 395 project while you were gone; but the Blizzard of January 2016, though magnificent and mighty, was an unwelcome surprise for the project. It brought the work to an abrupt halt. Most of this week will be spent clearing snow so that work can begin anew. Work should begin again in earnest later this week.
Looking forward into the new year, we can expect to see the steel girders that will ride the columns begin to arrive in early February. They will be set in place from north to south by huge cranes beginning around the middle of the month. Precast walls between the columns will begin to be set around the second or third week in February, and the concrete planks that ride upon the steel will be set shortly after the steel beams are in place. I will write more about steel and these precast concrete planks in a future Construction Note.
Since you have been gone, utility work has continued on Massachusetts Avenue and 3rd Street. A 30 inch water line has been installed and pressure tested along Massachusetts Avenue and an 8 inch water line is being installed along 3rd Street. Thus, traffic continues to be redirected from those areas. Fire hydrants have been installed on the highway surface, work continues on the new Massachusetts Avenue entrance to the highway, and a temporary bridge has been placed over the excavation on Mass. Avenue.
If you take the time to walk around the perimeter of the project, you will notice that the blue slurry tanks are gone from 2nd Street and the excavating drills, metal support sleeves, pumps, and generators have been demobilized, all signaling that the installation of the deep foundation caissons has been completed. Columns down the center of the highway have progressed further south, as have the footings that will support the walls between the columns. Columns along the east and west side of the highway surface have begun to rise, and the cap beam is being installed along the top of the slurry wall on the west side of the highway. Piers to extend the sidewalk area of the Massachusetts Avenue Bridge have been built. Piles and lagging along the 2nd Street exit portal are installed, side walls have gone up alongside the ramp, and concrete has been poured for much of the ramp surface.
Given the storm and the cold weather, several people have asked how Balfour Beatty can continue to pour concrete. Why doesn’t it freeze if it is mostly liquid? When construction engineers refer to “cold weather,” they mean that the average daily temperature falls below 40 degrees Fahrenheit for more than three consecutive days. As I mentioned in a previous note, the chemical reaction within the concrete generates heat. For concrete to set properly and harden, a chemical reaction called hydration or curing must take place. Hydration occurs when the chemicals in the concrete react with water to bind the mixture together. In order for curing to occur properly, the temperature of the concrete has to be 40º F. Anything below this temperature will slow the curing process and may even cause it to stop. In colder temperatures, proper care must be taken to ensure ice crystals do not form during the curing process.”
On very large projects, the internal temperature of concrete foundations will rise to 145 degrees Fahrenheit while curing. Although the heat in the I-395 concrete will be much lower, engineers must still prevent its internal heat from escaping too rapidly when the cold or freezing temperatures surround the outer edges of the slabs and columns. If the core and outer temperatures of the concrete cannot be maintained within 35 degrees of each other, the concrete will crack or weaken.
To ensure the structural integrity of the columns, piers, walls, and ramp slabs, the concrete arrives at the site bearing a temperature of about 65 degrees. Prior to the pour, the ground must be thawed and any surfaces to which the concrete will adhere must be cleared of snow, ice, or frost. If concrete is placed on frozen ground, uneven settling may occur when the ground thaws, causing cracks and instability. After the pour, insulated curing blankets and wind screens are placed over the slab or around the column for at least seventy-two hours, the critical period for curing the concrete. During the cure, thermometers are kept between the blankets to monitor the temperature and keep it constant. If the temperature of the concrete drops below or rises above about 60 degrees, the blankets are adjusted. Finally, a sample of each pour is saved in a separate testing core. Those cores are tested to ensure that the concrete is maintained at a proper temperature and that the structural integrity of the columns is preserved. As I mentioned above, the curing blankets must remain on the column for a minimum of three days to ensure proper curing; but because the I-395 columns are load bearing structures, the covers will likely remain on for a longer period of time.
Workers from the Facchina Concrete Company, the same company that provided concrete for our Hotung International Law Building and the Ginsberg Sport and Fitness Building, erected a polyethylene “lean-to” or tent around each of the steel members connecting the columns. They placed heaters under these tents to warm the steel. As soon as the steel reaches the proper temperature, steel tubes within the beams will be filled with self-consolidating concrete to stiffen the beams. The cylinders on top of the column connected to the steel beam will also be filled with grout. These measures will add stability to the columns and provide support for the steel beams and precast concrete planks that will later rest upon them.
If a building were being erected instead of a tunnel, you might see heaters being used to warm the rebar and the floor slabs. Depending on the temperature outside, propane heaters are set beneath the forms upon which the concrete will be poured. These heaters produce about 250,000 BTUs of energy and raise the temperature under the slab to about 75 or 80 degrees. The heaters also warm the reinforcing steel rods so that the concrete does not freeze immediately when it comes in contact with the rebar.
As I said last time, concrete is an amazing substance. If you want to learn even more about it, you can listen to the Gimlet Broadcast #3 Concrete — Did you just argue that cement is the reason for modern civilization?” in theirSurprisingly Awesome series. https://gimletmedia.com/episode/3-concrete/Thanks to Jessica Gallagher for bringing it to my attention. The podcast is informative and very funny and was produced around the same time I was writing the earlier Construction Note about concrete.
I did make an error in my last Construction Note about concrete. I said that “Writers… seldom compose odes to concrete; there are no poems to explore its splendor, no songs to attest to its endurance and magnificence.” Iwas wrong. Mary Nodine, a geotechnical engineer who writes poetry about soil and who is published regularly in the ASCE Geo-Institute magazine Geo-Strata, and who somehow learned about my Construction Notes, sent me a poem she wrote about Soils, another surprising topic for poetry. About soils he writes,
Soil is for poets
With its vast array of colors,
Microscopic grains to
Capable of creating
Vast plains, and
And about concrete, she muses,
Concrete is burly, Messy.
From simple ingredients
Come mighty strengths,
A mixture malleable
Into any form one could imagine.
Mad science meets brute force.
Mad science meets brute force indeed, combined now with literary and lyrical beauty to spur our imagination and renew our faith in the creativity of humanity. Think about that as you pursue your study of law.
Cement Making Process, http://www2.cement.org/basics/images/flashtour.html
Gimlet Broadcast #3 Concrete — Did you just argue that cement is the reason for modern civilization?” in their Surprisingly Awesome series, November 27, 2015. https://gimletmedia.com/episode/3-concrete/
W. Mlyniec, Construction Notes, Transforming a Campus in Washington D.C. (2006)
Mary Nodine, P.E., Soil is for Poets, Geo-Strata — Geo Institute of ASCE, Vol. 18, №3, May/June 2014, pg. 6
Pioneer Building Materials, What Precautions Should be Taken When Pouring Concrete ion Winter, http://www.stixnstones.com/blog/bid/75732/What-Precautions-Should-You-Take-When-Pouring-Concrete-in-the-Winter
Report of the American Concrete Institute Committee 306, Standard Specification for Cold Weather Concreting (306.1–90), (reapproved 2002)