Chapter 5: Preliminary Considerations
Anchor: #i1014740Section 1: Materials
Anchor: #i1014745Overview
Availability of materials is generally not a factor in determining the most suitable type of structure for a given location. Concrete and steel are the basic ingredients of most structures, and they are available to every county in the state. While bridges are primarily concrete and steel, aluminum is used very sparingly in railing and pipe; plastics are used for small diameter pipe; asphalt is used for overlays; neoprene is used for bearings; and butyl rubber is used for railroad underpass waterproofing.
Except for reinforcing steel, only brief descriptions are given here.
Anchor: #i1014762Concrete
Concrete is described by class, which identifies its strength, cement content, water/cement ratio, and coarse aggregate type according to the item “Portland Cement Concrete” of the Texas Department of Transportation (TxDOT) Standard Specifications for Construction of Highways, Streets, and Bridges. Concrete may be made from many different sources of cement, fine and coarse aggregate, and water, but all materials must meet the requirements of the specification.
Various additives are allowed or required for certain conditions of use. The use of fly ash to augment or replace some of the cement is gaining acceptance. Silica fume has also been used. It has been demonstrated that high strength concrete, around 13,000 psi compressive strength, can be produced from Texas aggregates and successfully placed in the forms for certain bridge members. These ingredients are also being used to develop a “high performance concrete” (HPC) with emphasis on density to provide better resistance to chloride attack.
Reinforced concrete is the term applied to concrete containing reinforcing bars designed to resist any tension that may occur in the member. Virtually all bridges contain some reinforced concrete. The concrete is usually mixed nearby and trucked to the job site.
Prestressed concrete is the term applied to high-strength concrete containing very high-strength steel that has been stretched and anchored to the concrete with sufficient force to significantly reduce tension from occurring in the member. When the concrete is placed before the steel is stretched, the member is said to be “post-tensioned.” When the steel is stretched before the concrete is placed, the member is said to be “pretensioned.” Post-tensioned structures are used sparingly, but pretensioned precast concrete beams are the mainstay of Texas bridge construction. Type IV beams up to 135 ft. long are possible to manufacture and transport. Greater lengths are possible with the use of high performance concrete. Type VI (MOD) beams can be used for spans up to 175 ft. in special circumstances. Use of long beams, however, depends on the accessibility of the bridge site by transporting trucks.
Anchor: #i1014794Structural Steel
Structural steel is available in many shapes and sizes. Much of the structural steel is manufactured elsewhere, but fabrication is usually performed in or near the state. Further discussion of structural steel can be found in Chapter 7 of this manual.
Anchor: #i1014808Prestressing Steel
Prestressed steel is a very high-strength material, which is discussed further in Chapter 7 of this manual.
Anchor: #i1014822Reinforcing Steel
Background. There have been many changes in the strength and configuration of reinforcing bars in the history of TxDOT. Smooth bars cold twisted to improve bond were used early, but soon outlawed by the specifications. All bars were square for awhile and, even into the late 1940s, #9, #10, and #11 bars were square. Oil well sucker rods were used occasionally during World War II because of a scarcity of regular reinforcing bars. For non-specification work it was possible to find anything from barbed wire to old car parts reinforcing the concrete.
Variations through the years in the specification requirements for reinforcing bars are shown in tables for years 1918-1953 and 1953-1988.
Ductile structural grade steel was used until the early 1950s. Rail steel was added, only to be removed in the late 1970s and added back in the 1980s. Deformations were a big concern of the 1940s but the questions were put to rest by ASTM 305-47T. The early 1960s saw the availability of #14 and #18 bars established. The 1973 American Association of State Highway and Transportation Officials (AASHTO) specification ushered in high-strength reinforcing steel and put a limit on stress range to avoid fatigue problems. Weldable reinforcing steel was covered by ASTM A706. Grade 75 bars were considered for concrete but abandoned because of the absence of a yield plateau in the stress/strain diagram. Grade 75, size #18S bars were used for anchor bolts by one light pole manufacturer. Epoxy coating of reinforcing bars was introduced in the late 1970s.
Design of reinforcing steel requires analysis of the complex interaction with concrete slabs, beams, columns, and footings. For service load design, an allowable stress is specified that accounts for a reasonable factor of safety. Load factor design, which has become the standard method, allows the reinforcing steel to reach yield under the action of loads factored up to provide safety. With load factor design, service load stresses must usually be calculated to insure that crack width and fatigue stress limits are not exceeded.
|
AASHTO Specification |
Material Specification |
Special Requirements |
fs allowable (ksi) |
|---|---|---|---|
|
1918 (T.H.D.) |
Yield ≥ 33 ksi Bend 180° over one diameter pin |
O.H., Mild, or Medium Plain, Twist, and Deform |
16 |
|
1926 (T.H.D.) |
A15-14 |
O.H., Deform, or Plain Twist with engineer’s approval |
16 |
|
1931 |
A15-30 |
O.H., Str. Only No Twist |
LL 16 DL 24 |
|
1935 |
A15-33 (Mod.) |
O.H., Str. Only No Twist |
16 |
|
1941 |
A15 |
O.H., Str. or Int., Deform approved by engineer No Twist |
18 |
|
1944 |
M31-42 |
O.H., Str. or Int., Deform approved by engineer No Twist |
Str. 18 Int. 20 |
|
1949 |
M31-38 (Deform) |
O.H., Str. or Int., All Deform No Twist |
Str. 18 Int. 20 |