Mass Concrete Case Study
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Mass Concrete: A Massive Beam in the San
Francisco Bay
By John Gajda, PE, Principal Engineer, CTLGroup
The new San Francisco-Oakland Bay Bridge is one step closer to
completion. American Bridge/Fluor, JV recently constructed the E2
Beam, which is one of three primary supporting members of the main
suspension span of this landmark bridge.
Massive Beam
The E2 Beam will support the east portion of the self-anchored suspension
span. The beam is 60½ m (200 ft) long, 6 m (20 ft) high,
and up to 5½ m (18 ft) wide, and sits on two 33 m (110 ft)
tall columns on a footing in the San Francisco Bay. The beam was
constructed in a continuous 14 hour long concrete placement with
1,410 m3 (1,843 yd3) of 60 MPa (8700 psi)
self-consolidating concrete (SCC). SCC was used because large quantity
of closely spaced reinforcing steel and areas of high rebar congestion.
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| Formwork installation for the E2 Beam in the
foreground |
Mass Concrete
Due to its dimensions, the beam was designated as mass concrete,
where temperatures and temperature differences were to be controlled.
Control was required from the time of placement until the hottest
portion of the beam cooled back to near-ambient conditions. During
this period, concrete temperatures were to be kept under 65°C
(150°F) and thermal cracking was to be prevented.
While many specifications provide specific temperature difference
limit (such as the often used generic limit of 20°C [35°F]),
Caltrans did not specify a specific temperature difference limit
for the project mass concrete. Instead, they specified that thermal
cracking was to be prevented. Thermal cracking short circuits the
benefits of the low permeability high performance concrete, which
was specified to achieve the desired 150 year service life. Not
specifying a definitive temperature difference limit allowed the
necessary latitude to select an appropriate limit based on the properties
of the concrete, and the requirement of that thermal cracking was
to be prevented. In this case, CTLGroup developed an engineered
temperature difference limit and tailored it to the properties of
the project concrete.
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| Placement of the SCC concrete |
Careful Planning
As part of the mass concrete designation, the project specifications
required that a thermal control plan be developed to demonstrate
how temperatures and temperature differences would be kept within
the specified limits during placement and curing.
CTLGroup worked closely with American Bridge/Fluor staff to develop
the thermal control plan based on detailed thermal modeling. Because
of the high temperature rise of the concrete, the thermal control
plan was developed based on having cooling pipes uniformly installed
at about 700 mm (27 in.) on-center in the beam to remove internal
heat during the curing and thermal control periods. The cooling
pipes allowed the concrete to be placed with an initial temperature
as high as about 25°C (75°F) without fear of exceeding the
specified maximum temperature limit, and for thermal control to
be completed in about 1½ weeks. Without cooling pipes, modeling
showed that the initial concrete temperature would need to have
been limited to about 10°C (50°F) and the time of thermal
control would have continued for well over 1 month.
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| Manifolds and cooling pipe connections. Due
to the large number of pipes, supply lines were color-coded
blue and return lines were color-coded red. |
Conclusions
During the time of construction, temperatures were monitored on
an hourly basis at numerous locations in the beam. The monitoring
showed that temperatures and temperature differences were kept within
the specified limits, and the temperatures and temperature differences
were in reasonable agreement with the thermal modeling and thermal
control plan. More importantly, after the formwork was removed,
no thermal cracking was noted. Through careful and detailed planning,
placement was a complete success.
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| Temperatures at some of the measured locations
in the E2 Beam |
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| The completed E2 Beam, with most of the formwork
removed |
John Gajda, PE,
Principal Engineer
CTLGroup, Skokie, Ill.
JGajda@ctlgroup.com
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