Use of grout bags in Strengthening reinforced concrete bridges

Use of grout bags
in Strengthening
reinforced concrete bridges
They provide continuous contact
between soffits and beams
BY PHILIP ADRIAN HARRIS
PROJECT ENGINEER
W.S. ATKINS AND PARTNERS
WOODCOTE GROVE
EPSOM, SURREY, ENGLAND
he only road link between west Wales and other
parts of the country is the Carmarthen Bridge, a
reinforced concrete structure opened to traffic
in 1938. It carries two major highways over a river, a railway and a road. Today, like many other such
s t ru c t u re s, the bridge and its approach viaducts must
carry a far heavier load than it was designed for—in this
case increased traffic resulting from the growth of
tourism in west Wales, as well as traffic generated by the
oil industry.
Recent inspection revealed evidence of distress. Traffic was restricted to avoid overloading the bridge, and
work was begun to strengthen one traffic lane pending
construction of a bypass around Carmarthen.
Two different methods of strengthening the reinforced concrete beam/slab and slab decks were used.
Spans in public view were to be strengthened with
stressed external strand, while other spans were to be
strengthened with weathering steel support frames,
beams or towers. The use of grout bags above these steel
frames and beams, to take up the variable space and
provide uniform support, proved the key to the success
of this second operation.
The design analysis assumed that the concrete deck
and the steel frames or beams act together, although not
compositely, along their full lengths so that applied
loading is shared in proportion to their stiffnesses. Continuous contact between the steel frames and beams
and the concrete deck was needed for the strengthening system to work.
T
Getting continuous contact
Se ve ral methods of achieving continuous contact
were considered, but the real problem was the difficulty
* Numbers in parentheses refer to metric equivalents listed with this article.
of access after the steel beams were installed. Fu rt h e rmore, the soffits of the decks tended to undulate, resulting in varying gaps. Packing with a sand-cement mortar
or a sand-epoxy resin mixture seemed impractical; the
beams were 12 inches(1)* wide and the deck soffit over the
top flange was inaccessible. A rubber strip bearing between the top flange of the beam and the deck soffit
would take up the desired shape, but its flexibility would
p re vent simultaneous deflection from occurring between the two structural systems.
What was needed was a system that
• would be easy to install where access was difficult
• could take up any shape to fill the gap
• could provide enough rigid contact to transmit load
from the concrete deck to the frames and beams
The use of bags pumped full of cement-sand grout to
fill varying gaps between the steel frames and beams
and the deck soffits was then investigated. Heavy-duty
polyethylene was considered first, even though it
seemed possible that this material would be too flexible
and likely to stretch excessively under pressure. A manufacturer agreed to make two 28-inch-circumference(2)
nylon-reinforced polyvinyl grout bags for test purposes.
Testing
Tests were carried out by the British government’s Department of Transport. They involved pumping a 1:3 cement-sand grout into a bag sandwiched between two
beams approximately 11⁄2 inches(3) apart. Building sand
(Zone 2++) and ordinary portland cement were used.
In the tests 1⁄2-inch-diameter(4) brass plumbing fittings
were used for the inlet and bleed connections. Water
was added to the sand-cement mixture until a
pumpable consistency was obtained. The first bag failed
when its longitudinal welded joint split open. In the second test this joint was placed against the beam flange.
This proved successful.
For the custom-made bags for the project it was specified that all longitudinal joints were to be stitched as
well as welded for extra strength. Because the brass fittings used in the test were cumbersome and expensive
a suitable alternative was found in the plastic skin fittings sold by yacht supply houses.
+ British Standard 882.
Figure 1. Elevation of the
Carmarthen Bridge.
Figure 2. Typical bridge
sections showing grout
bags in position.
A grout bag, seen as a broad white line (top,
center), is being filled. A second bag, with
inlet pipe and nozzle, can be seen on the
right.
Viaduct
The viaduct structure was reinforced concrete slaband-beam and slab decks supported on reinforced concrete piers 26 feet(5) on centers. Weathering steel frames
were designed. They were supported by the existing
piled foundations with a gap of approximately 11⁄2 inches(3) to be left between the top frame member and the
soffit of the existing concrete beams. Grout bags were
placed in this gap and a sand-cement mixture pumped
in. The bleed tubes were sealed when the grout bags
were filled.
Railway span
In the case of the 44-foot (6) railway span, 24- by 12inch(7) weathering steel beams weighing 160 pounds per
lineal foot(8) were installed 261⁄4 inches(9) on centers between the existing reinforced concrete beams. The highway over the railway is on a vertical curve; the gap be-
tween the strengthening beams and the soffit of the deck
slab varied from approximately 11⁄4 to 3 inches.(10) The
grout bags were installed and fitted from a scaffolding
platform erected near the railway tracks.
On site, the 1:3 cement-sand grout was hard to pump
and frequent blockages occurred in the pump. It was
found that the contractor had used very fine grained
sand. After changing to Zone 2 sand and varying the proportion to 1:1.5 the sand-cement grout was satisfactory.
However, admixtures were used to improve the pumping
properties of the mixture.
Although pressures of up to 100 psi(11) at the pump
were used, visual inspection of the sides of the grout
bags was needed to ensure that the gaps were completely filled. Too much pressure can have the effect of stressing the existing span upward against its dead load, and
the engineer had to satisfy himself beforehand that this
pressure would not harm the structure.
So far, forty-two 28-inch-circumference(2) grout bags
and eight 19-inch-circumference(12) bags have been used
between the soffits of beams and slabs and the top
flanges of steel beam or strengthening frames. Lengths
vary from 6 feet 7 inches(13) to 19 feet 8 inches.(14) Generally, three inlet and three bleed connections have been
used for each bag. The bags are providing the necessary
continuous contact between soffits and beams on the
Ca rm a rthen bridge structure; they are making the project succeed.
Metric equivalents
(1) 300 millimeters
(2) 700-millimeter-circumference
(3) 40 millimeters
(4) 13-millimeter-diameter
(5) 8 meters
(6) 13.5-meter
(7) 600- by 300-millimeter
(8) 240 kilograms per lineal meter
(9) 665 millimeters
(10) 30 to 75 millimeters
(11) 0.70 megapascals
(12) 460-millimeter circumference
(13) 2.0 meters
(14) 6.0 meters
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