Bremerton Marina, Bremerton, Washington
A 1,440 foot-long floating concrete breakwater was constructed
in the state of Washington using TXI Expanded Shale & Clay fine
graded lightweight aggregate produced at TXI’s Boulder, Colorado
facility. The $8 million breakwater project was designed for the
Port of Bremerton Marina to attenuate wind driven waves and ferry
wakes. Additionally, it was designed to upgrade the marina’s boat
capacity, and enlarge the waterfront public space extending from
an upland public park area.
Twelve individual floating bodies, with a maximum weight of 950
tons, are rigidly connected by post-tensioning to act as a single
floating structure moored with 50 different mooring lines. Unlike
fixed breakwaters, the floating structure is fish-friendly and allows
marine life to pass underneath the structure.
Concrete benches, light fixtures and future artistic sculptures were figured into the design to add
counterweights and aesthetic elements consistent with the existing downtown waterfront park.
Construction
In fabricating the breakwater structure, the
Bremerton Floating Breakwater is the first largesize
floating concrete breakwater in Washington to
use cast-in-place ready-mixed concrete with four
different concrete mixtures. Each concrete mixture
design includes TXI fine graded lightweight aggregate
to optimize the final density of the structure.
To enhance performance of the structure the engineer
specified four different concrete mixtures to
address design considerations depending on the
desired density of the individual floating bodies.
The 12 concrete bodies ranged in weight from 131
pounds per cubic foot to 144 pounds per cubic foot.
The final concept design required to meet the target
strength, free board and stability was achieved by
blending lightweight fine aggregate with normal fine
aggregate. TXI assisted the engineering firms of
Art Anderson Associates and Reid Middleton in the
design of the concrete proportions to economically
meet the rigorous design criteria.
“Lightweight aggregate provided us the flexibility
we needed to design individual concrete bodies to
maintain stability even in the event of flood damage
in one open cell of the breakwater. The lightweight
aggregate also allowed us to accommodate add-on
elements, such as benches and light poles, and still
achieve the desired strength and stability,” said
Senior Engineer Willy Ahn of Reid Middleton.
Several experiments, using scaled model tests, were conducted
to analyze different aggregate blends and concrete
densities in the individual concrete sections. All four concrete
mixes were batched at Glacier Northwest’s ready-mix
plant in Everett, Washington, which is located near the casting
site. The concrete was then cast-in-place by the pumping
method.
“There seems to be a common misconception that lightweight
concrete is extremely difficult to pump. However, we
were very pleased with the consistency, ease of pumpplacement,
and finishing characteristics of all four mixes,”
said Dustin McClure, Project Manager at McClure & Sons.
The Bremerton Floating Breakwater project was completed
with less than one percent change order costs, contributing
to the overall timely installation of the breakwater.
Internal Curing
The Bremerton Floating Breakwater was designed to last
more than 50 years. Lightweight aggregate (LWA) concretes
have generally exhibited excellent performance under
severe weathering conditions. One of the reasons cited for
this is the high integrity of the interfacial transition zone
formed between the LWA and the neighboring hydrating cement paste.
All four concrete mixtures will benefit from the highly absorptive microstructure of the lightweight
fine aggregate. For internal curing to occur, the lightweight aggregate was pre-wetted prior to
batching. The water is absorbed into the lightweight aggregate and therefore does not increase the
net water/cement ratio, and eliminates the initial absorption of batchwater. The absorbed water
does not initially contribute to hydration. Only after initial set does this water migrate out of the
lightweight aggregate particles and into the cement matrix, contributing towards continuous hydration
and increased long-term durability. This process is known as internal curing.
American Society of Civil Engineers 2007 Region 8 Project of the Year Awards:
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