- Cold Weather Concreting
WHAT is Cold Weather?
HOW to Place Concrete in Cold Weather?
Figure 1 Effect of Temperature on Set Time (1a)
Cold weather is defined as a period when the average daily
temperature falls below 40°F [4°C] for more than three successive
days. These conditions warrant special precautions
when placing, finishing, curing and protecting concrete against
the effects of cold weather. Since weather conditions can
change rapidly in the winter months, good concrete practices
and proper planning are critical.
WHY Consider Cold Weather?
Successful cold-weather concreting requires an understanding
of the various factors that affect concrete properties.
In its plastic state, concrete will freeze if its temperature falls
below about 25°F [-4°C]. If plastic concrete freezes, its potential
strength can be reduced by more than 50% and its durability
will be adversely affected. Concrete should be protected
from freezing until it attains a minimum compressive
strength of 500 psi [3.5 MPa], which is about two days after
placement for most concrete maintained at 50°F [10°C].
Low concrete temperature has a major effect on the rate of
cement hydration, which results in slower setting and rate
of strength gain. A good rule of thumb is that a drop in concrete
temperature by 20°F [10°C] will approximately double
the setting time. The slower rate of setting and strength gain
should be accounted for when scheduling construction operations,
such as form removal.
Concrete in contact with water and exposed to cycles of
freezing and thawing, even if only during construction,
should be air-entrained. Newly placed concrete is saturated
with water and should be protected from cycles of freezing
and thawing until it has attained a compressive strength of
at least 3500 psi [24.0 MPa].
Cement hydration is a chemical reaction that generates heat.
Newly placed concrete should be adequately insulated to retain
this heat and thereby maintain favorable curing temperatures.
Large temperature differences between the surface and
the interior of the concrete mass should be prevented as cracking
may result when this difference exceeds about 35°F [20°C].
Insulation or protective measures should be gradually removed
to avoid thermal shock.
Cold Weather Concreting Guidelines
1. Use air-entrained concrete when exposure to moisture and freezing and thawing conditions are expected.
2. Keep surfaces in contact with concrete free of ice and snow and at a temperature above freezing prior
to placement.
3. Place and maintain concrete at the recommended temperature.
4. Place concrete at the lowest practical slump.
5. Protect plastic concrete from freezing or drying.
6. Protect concrete from early-age freezing and thawing cycles until it has attained adequate strength.
7. Limit rapid temperature changes when protective measures are removed.
Sunday, 18 January 2009
Saturday, 17 January 2009
SUSPENSION BRIDGE INFORMATION
CATWALK AND HAULING SYSTEM
Walkways (or catwalks) are provided just below each main cable to access during the spinning and subsequent erection operation. Typically these are about 3 m wide, consisting of hardwood tread boards mounted on galvanised mesh all carried on wire strands, with two additional strands acting as handrailing. Stiffening frames are spaced regularly (approximately 50 m) along the span and cross bridges provided to link the two walks. Also two tramway support strands are stretched above the catwalk between the main towers and from towers to abutments to carry the stabilising cross beams (at approximately 90 m spacings) and spinning wheel haul rope sheaves. The whole system is stabilised by means of strong ropes anchored to the towers and abutments. The walkway strands are the first to be placed in position using barges. The free end of each is raised and connected to the top of the tower while the rest is unwound off the reel along the bed of the river, and subsequently fixed to the abutment or other tower. Adjustment is generally necessary to produce the desired sag for the spinning operation.
The rest of the catwalk equipment such as mesh, stiffening frames, main bridging beams, etc., are usually slid down the strands working from the tower tops. 'A small working carriage running on the handrail strands is generally set up to transport the workmen.
The tramway support strands are next erected in a similar manner to those of the footbridge, with the cross beams also being slid into position under control from the tower tops and tied to the catwalk frames. With these operations complete the tramway haul rope can be winched along the catwalk.
Walkways (or catwalks) are provided just below each main cable to access during the spinning and subsequent erection operation. Typically these are about 3 m wide, consisting of hardwood tread boards mounted on galvanised mesh all carried on wire strands, with two additional strands acting as handrailing. Stiffening frames are spaced regularly (approximately 50 m) along the span and cross bridges provided to link the two walks. Also two tramway support strands are stretched above the catwalk between the main towers and from towers to abutments to carry the stabilising cross beams (at approximately 90 m spacings) and spinning wheel haul rope sheaves. The whole system is stabilised by means of strong ropes anchored to the towers and abutments. The walkway strands are the first to be placed in position using barges. The free end of each is raised and connected to the top of the tower while the rest is unwound off the reel along the bed of the river, and subsequently fixed to the abutment or other tower. Adjustment is generally necessary to produce the desired sag for the spinning operation.
The rest of the catwalk equipment such as mesh, stiffening frames, main bridging beams, etc., are usually slid down the strands working from the tower tops. 'A small working carriage running on the handrail strands is generally set up to transport the workmen.
The tramway support strands are next erected in a similar manner to those of the footbridge, with the cross beams also being slid into position under control from the tower tops and tied to the catwalk frames. With these operations complete the tramway haul rope can be winched along the catwalk.
SADDLES
The cable is usually spun in place, strand-by-strand. When the required number of wires in a strand is achieved they are grouped together with straps or bands placed at about 2 m intervals. The saddle at the top of each tower and the splay saddles are cast in steel and grooved, stepped and divided with steel spacers to aid wire location during spinning. A cover plate is is provided for protection against corrosion. The whole unit is bolted down to resist movement.
The cable is usually spun in place, strand-by-strand. When the required number of wires in a strand is achieved they are grouped together with straps or bands placed at about 2 m intervals. The saddle at the top of each tower and the splay saddles are cast in steel and grooved, stepped and divided with steel spacers to aid wire location during spinning. A cover plate is is provided for protection against corrosion. The whole unit is bolted down to resist movement.
ERECTION METHODS
The two systems most commonly selected for the deck placing sequences are:
1. Start erection from centre of of main span.
2. Start erection from the towers.
The deck section joints are left unconnected until the last unit is in position as significant displacement of the main cable and towers takes place while the dead loading is progressively increased. Calculations should therefore be made of the cable tension and shape, and distortion of the towers during the erection sequence to ensure tolerable limits of deformation are maintained. A typical sequence of operations is as follows:
(i) Construct piers and anchorages.
(ii) Erect towers and saddles.
(iii) Hoist catwalk cables into place and build catwalk.
(iv) Place cross bridges and fix storm ties to stabilise catwalk.
(v) Haul out tramway ropes for spinning wheels.
(vi) Erect suspension cable by air spinning method adjusting wires and strands for sag within tolerance specification.
(vii) Compact cable and temporarily clamp.
(viii) Fix permanent bands and hangers to approximate positions operating from work carriages running on the tramway cable.
(ix) Erect deck units.
(x) Connect deck joints when all units are in place and complete tightening of hanger bands.
(xi) Wrap main cable.
(xii) Complete road surfacing, etc.
SUSPENSION BRIDGE INFORMATION
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ANCHORAGES
The anchorage is fundamental to the stability of a suspension bridge, as all the load in the cable must be transferred to a fixed anchorage (a. few small bridges with selfanchorages similar to cable-stayed bridges have been used). There are commonly three types:
1. Rock anchors.
2. Tunnel anchorage.
3. Gravity anchorage.
Rock anchorages simply involve drilling into the rock and grouting in large bolttype anchors to which the strands are subsequently attached. Where suitable rock is available, a U-tunnel can be constructed and the two cables joined to form a loop. However, the gravity anchorage has proved to be most popular with designers. The basic arrangement consists of looped over strand shoes attached to anchor bolts located in the concrete. The cable forces are thus resisted by a combination of overburden, dead weight and bearing friction.
The anchorage is fundamental to the stability of a suspension bridge, as all the load in the cable must be transferred to a fixed anchorage (a. few small bridges with selfanchorages similar to cable-stayed bridges have been used). There are commonly three types:
1. Rock anchors.
2. Tunnel anchorage.
3. Gravity anchorage.
Rock anchorages simply involve drilling into the rock and grouting in large bolttype anchors to which the strands are subsequently attached. Where suitable rock is available, a U-tunnel can be constructed and the two cables joined to form a loop. However, the gravity anchorage has proved to be most popular with designers. The basic arrangement consists of looped over strand shoes attached to anchor bolts located in the concrete. The cable forces are thus resisted by a combination of overburden, dead weight and bearing friction.
CABLE WRAPPING
The hangers are given a final tightening whereafter the cable is wrapped in galvanised mild steel wire (3-4 mm diameter) to provide corrosion protection. The wrapping machine consists of two drums encircling the cable and the wire is unwound as the unit in winched along. Wrapping can be achieved from either end of the machine in order that wire may be brought tight against a hanger band. The drum should contain just enough wire to cover between two hanger bands and allow the drums to be slit apart to pass the bands. The whole unit is hung from the tramway support strands and travels at about 0.5 m/min. The final operation requires the catwalk and other strands to be dismantled, this is a very difficult and dangerous operation requiring the workmen to raise the mesh panels by hand and winch them to the deck below.
The hangers are given a final tightening whereafter the cable is wrapped in galvanised mild steel wire (3-4 mm diameter) to provide corrosion protection. The wrapping machine consists of two drums encircling the cable and the wire is unwound as the unit in winched along. Wrapping can be achieved from either end of the machine in order that wire may be brought tight against a hanger band. The drum should contain just enough wire to cover between two hanger bands and allow the drums to be slit apart to pass the bands. The whole unit is hung from the tramway support strands and travels at about 0.5 m/min. The final operation requires the catwalk and other strands to be dismantled, this is a very difficult and dangerous operation requiring the workmen to raise the mesh panels by hand and winch them to the deck below.
HANGERS
Hanger cables are clamped directly to the suspension cable with tightly bolted bands. The main cable wrapping only extends up to the bands as these are fixed prior to the wrapping operation.
Hanger cables are clamped directly to the suspension cable with tightly bolted bands. The main cable wrapping only extends up to the bands as these are fixed prior to the wrapping operation.
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