Dear colleagues
I hope you had a pleasant Thanksgiving holiday and are looking forward to the end of the
semester. Although the workers at BBC were also able to take some time away from the project
during the Thanksgiving holiday, work is again continuing at an accelerated pace. Utility work
continues along Massachusetts Avenue and 3rd Street. A 30 inch water main will be installed
under Massachusetts Avenue later this month and an electrical vault and line is scheduled to be
installed by the end of February.
BBC will soon begin working in the existing tunnel north of Massachusetts Avenue.
Because the Capitol Crossing platform will connect with the north tunnel, it will, in effect, create
a single very long tunnel. Thus the ventilation and fire safety protection for the old tunnel has to
be upgraded and integrated into the new systems. This work will continue into the summer.
Another new phase of the project has begun. When the project is completed, the current
southbound entrance to I-395 at 3rd Street will be closed. In the future, cars will enter the
highway from the center lane of Massachusetts Avenue just east of 4th Street, NW. The ramp will
dip into a tunnel between 3rd Street and 2nd Street NW., curve to the right and under the eastbound lanes of Massachusetts Avenue, and continue on until it meets the highway surface.
Construction on the tunnel portion of the ramp has already begun. As you walk up Massachusetts
Avenue toward 3rd Street, you can see the curve of the ramp and see that the old road surface is
being demolished. Some piles have also been driven to support construction.
Preliminary demolition for the new southbound entrance
The last of the deep foundation caissons were completed at the end of last week. This has
brought some relief to our Gewirz residents because the completion of the caissons signals the
end of night work along the south end of 2nd Street until sometime in February. Moreover, we
believe the decibel level of the sounds during the future night work will be considerably lower
than the levels we have been experiencing since school began. Construction on the highway
surface continues, and we are beginning to visualize the contours of the platform that will
support the buildings. We can also see the design of the exit ramp which rises from the highway
onto 2nd Street and leads to Massachusetts Avenue. The ramp has been slightly relocated from
the position of the old ramp. The ramp in the tunnel will now end just north of F Street. It will
continue to rise until it meets the surface of 2nd Street just south of G Street. The concrete for the
ramp is poured in two foot thick mats that measure about 30 feet by 30 feet. Seven of the eight
mats on the rising portion of the ramp have been poured. Three other mats of varying lengths that
total about 100 feet long by 30 feet wide will be poured sometime later at the same level as the
highway. Concrete will also be poured for the walls that line the tunnel and portal of the exit
ramp. The forms and reinforcing steel for the concrete walls are already in place in some
locations.
Concrete Mat preparation
Poured Concrete Mat
For the near future, BBC will be pouring the concrete for the columns that will support
large steel beams that will traverse the length of the highway. The beams in turn will support
concrete planks that form the platform itself. The steel beams and precast concrete planks will be
the subjects of later Construction Notes. Columns are currently being built along the center of the
highway and others will be constructed along the east and west sides of the highway. To create a
column, a cylindrical form is placed on top of a caisson or on the concrete cap of the slurry wall.
The form is made of steel and is coated with a lubricant so that the concrete will not adhere to the
form. Reinforcing steel is placed into the cylindrical form to add strength to the column and then
concrete is poured into it. Once the concrete sets, the form is removed and the column continues
to cure. Reinforcing steel brackets are fitted over the columns connecting them to one another to
further strengthen and stabilize the columns.
Yellow Column Forms
Concrete Columns Resting Atop the Caissons
Concrete footings are being poured at the base of the columns to support additional walls that
will rise between them. A total of 149 columns and three piers (rectangular-shaped supports
resembling a thick concrete wall) will support the platform. There is a pier going up now in the
highway median at the Massachusetts Avenue Bridge. The pier is necessary because the bridge is
being widened to make a more spacious sidewalk on Massachusetts Avenue. A new abutment is
also being built at the west end of the bridge to help support the new sidewalk. Sometime in
February, the big cranes will begin to load the steel beams that will rest upon the piers and
columns, even as additional columns are being built. The plan is to erect the platform on the
north block as the columns for the south and center blocks are poured.
Pier Support
Piles for the Bridge Abutment
For the next few months, we will see dozens of concrete trucks arrive and depart from the
site. Modern construction projects owe much to the developers of concrete. Concrete is simply a
mixture of gravel, pebbles, broken stone, sand, water, and cement that hardens into a water- and
fire-resistant solid of great compressive strength. Cement, often mistakenly used as a synonym
for concrete, is a powdery substance made with clay and calcined lime, that is, thermally
decomposed lime. It is mixed with water to form mortar or mixed with sand, gravel, and water to
make concrete. Despite its simplicity, concrete’s development significantly altered the way large
buildings and major public structures are built. Clay was one of the earliest bonding substances
used for construction. The Assyrians and Babylonians, civilizations existing in Mesopotamian
Valley beginning almost 4,000 years ago, used it to erect their buildings. But clay is not very
durable and their early buildings are lost in antiquity. The ancient Egyptians developed a
bonding material more closely resembling modern concrete. They used lime and gypsum as
binders and used this concrete extensively. It was the Romans, however, who dramatically
expanded architectural methods by using a stronger concrete made from volcanic ash cement.
With this form of concrete and a greater understanding of compression, Roman architects were
able to develop domes, arches, and vaults to cover large spaces, and sturdy foundations and walls
to support large public buildings, bridges, and sewers. Some of the earliest surviving examples of
Roman buildings made with concrete are the Temple of Sybil, built at Tivoli in the first century
B.C.E., and the walls of the Camp of the Praetorian Guard, built in Rome in 21 A.D. The
Emperor Nero also used concrete to build an octagonal domed vault in the Golden House, built
in Rome in 68 A.D.
Temple of Sybil
Walls of the Camp of the Praetorian Guard
The Domed Vault of Nero’s Golden House
The use of concrete declined in the Middle Ages but was revived in 1824 with the
development of Portland cement by Joseph Aspdin, a bricklayer in Leeds, England. Aspdin
called it Portland cement because it resembled the oolitic limestone on the Isle of Portland off the
coast of Britain. It is made by heating a combination of limestone and clay containing oxides of
calcium, iron, aluminum, and silicon, and then pulverizing the resultant fused matter. Concrete
made with Portland cement produced stronger bonds than former concretes, thus permitting the
construction of larger buildings. In 1867, Joseph Monier, a French commercial gardener,
patented a method of strengthening thin concrete flowerpots by embedding iron wire into the
concrete. This process ultimately led to the development of reinforced concrete. Francois
Hennebique, a French engineer, began using reinforced floor slabs and railroad ties in 1879 and
by the end of the century, the notion of reinforcing concrete with iron bars to create a great
tensile strength to complement its compressive strength had become common. As a result, the
stone edifice gave way to less expensive reinforced concrete and brick buildings. The first
concrete bridge using reinforced concrete was built in 1894 in Wiggen, Switzerland, and since
then reinforced concrete has become a standard product in all types of construction. You can see
an overview of the cement manufacturing process in this 7-minute animation.
http://www2.cement.org/basics/images/flashtour.html
The architectural value of reinforced concrete cannot be underestimated. It is inexpensive
and easy to obtain when compared to the quarried stone that was used for foundations and
structures through the early nineteenth century. It has great load-carrying capacity and thus can
support large structures. For example, the concrete for the caissons and the columns in the
Capitol Crossing Project were poured to a compressive strength of 4,500 pounds per square inch.
That means the caissons and columns will withstand over 7.1 million pounds of pressure without
crumbling.
To obtain all of these advantages, however, concrete must be poured carefully and in
good weather. Excess water in the mixture and cold and hot weather during the curing period
affect the strength and drying time of concrete. If the internal temperature of the poured concrete
is too hot or too cold when compared to the external temperature, the concrete may fail and crack
or collapse.
Although you may see a worker walking on the surface of the concrete pad soon after it is
poured, it will continue to cure at high but decreasing internal temperatures for several days. For
many months after that, the concrete remains chemically active inside, growing stronger with the
passage of time.
The concrete work on the Capitol Crossing project has produced some interesting
facts. As I mentioned in an earlier email, the slurry wall and the caissons contain 22,500 cubic
yards of concrete (2,500 truckloads). The columns resting on the caissons and slurry wall will
add an additional 2,350 cubic yards of concrete (260 truckloads) and the exit ramp will add
another 3915 cubic yards (435 truckloads). The caissons and slurry wall were reinforced by
1,200,000 linear feet of reinforcing steel weighing 3,250,000 pounds. I am awaiting the
calculations for the number of feet and the weight of the reinforcing steel used for the 2nd Street
exit ramp and the columns.
Concrete is an amazing substance. It is simple, yet complex; rugged, yet elegant. Its place
in architectural history and in the rise of urban culture is subtle but dominant. Writers, however,
seldom compose odes to concrete; there are no poems to explore its splendor, no songs to attest
to its endurance and magnificence. Yet it is the tool of an engineer for highways, bridges,
flyovers, and foundations; it is an art form for an architect, in itself or as an element of design.
The ancients found it mesmerizing, almost magical, but we moderns take it for granted. In an age
of technological and digital marvels, concrete stands solid, holding our world together. But
concrete is usually unnoticed and always unappreciated. As you take your study breaks, walk the
streets of this city or even just this campus, and see what marvels are produced by humans using
the simplest elements of nature. Look long and deeply -- take your time to contemplate centuries
of magic, science, and human ingenuity; but most of all, take nothing for granted and never lose
your sense of wonder at life’s intricate web of simplicity and complexity.
Good luck with your finals and have a pleasant holiday break.
Wally Mlyniec
SOURCES
Cement Making Process, http://www2.cement.org/basics/images/flashtour.html
ENCYCLOPAEDIA BRITTANICA, Concrete, http://www.britannica.com/technology/concretebuilding-material
History of Innovation, “1892: Hennebique Method of Reinforced Concrete,”
https://aehistory.wordpress.com/1892/10/05/1892-hennebique-method-of-reinforced-concrete/
Today in Science History, “Joseph Aspdin,”
http://todayinsci.com/A/Aspdin_Joseph/AspdinJoseph-Cement.htm
W. Mlyniec, CONSTRUCTION NOTES, TRANSFORMING A CAMPUS IN WASHINGTON D.C. (2006)