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Concrete Bridges

Concrete is the most-used construction material for bridges in the United States, and

indeed in the world. The application of prestressing to bridges has grown rapidly and

steadily, beginning in 1949 with high-strength steel wires in the Walnut Lane Bridge in

Philadelphia, Pennsylvania. According to the Federal Highway Administration’s 1994

National Bridge Inventory data, from 1950 to the early 1990s, prestressed concrete

bridges have gone from being virtually nonexistent to representing over 50 percent of all

bridges built in the United States.

Prestressing has also played an important role in extending the span capability of

concrete bridges. By the late 1990s, spliced-girder spans reached a record 100 m (330 ft).

Construction of segmental concrete bridges began in the United States in ly,

close to 200 segmental concrete bridges have been built or are under construction, with

spans up to 240 m (800 ft).

Late in the 1970s, cable-stayed construction raised the bar for concrete bridges. By

1982, the Sunshine Skyway Bridge in Tampa, Florida, had set a new record for concrete

bridges, with a main span of 365 m (1,200 ft). The next year, the Dames Point Bridge in

Jacksonville, Florida, extended the record to 400 m (1,300 ft).

HIGH-PERFORMANCE CONCRETE

Compressive Strength

For many years the design of precast prestressed concrete girders was based on

concrete compressive strengths of 34 to 41 MPa (5,000 to 6,000 psi). This strength level

served the industry well and provided the basis for establishing the prestressed concrete

bridge industry in the United States. In the 1990s the industry began to utilize higher

concrete compressive strengths in design, and at the start of the new millennium the

industry is poised to accept the use of concrete compressive strengths up to 70 MPa

(10,000 psi).

For the future, the industry needs to seek ways to effectively utilize even higher

concrete compressive strengths. The ready-mixed concrete industry has been producing

concretes with compressive strengths in excess of 70 MPa for over 20 years. Several

demonstration projects have illustrated that strengths above 70 MPa can be achieved for

prestressed concrete girders. Barriers need to be removed to allow the greater use of these

materials. At the same time, owners, designers, contractors, and fabricators need to be

more receptive to the use of higher-compressive-strength concretes.

Durability

High-performance concrete (HPC) can be specified as high compressive strength

(e.g., in prestressed girders) or as conventional compressive strength with improved

durability (e.g., in cast-in-place bridge decks and substructures). There is a need to

develop a better understanding of all the parameters that affect durability, such as

resistance to chemical, electrochemical, and environmental mechanisms that attack the

integrity of the material. Significant differences might occur in the long-term durability

of adjacent twin structures constructed at the same time using identical materials. This

reveals our lack of understanding and control of the parameters that affect durability.

NEW MATERIALS

Concrete design specifications have in the past focused primarily on the

compressive strength. Concrete is slowly moving toward an engineered material whose

direct performance can be altered by the designer. Material properties such as

permeability, ductility, freeze-thaw resistance, durability, abrasion resistance, reactivity,

and strength will be specified. The HPC initiative has gone a long way in promoting

these specifications, but much more can be done. Additives, such a fibers or chemicals,

can significantly alter the basic properties of concrete. Other new materials, such as

fiber-reinforced polymer composites, nonmetallic reinforcement (glass fiber-reinforced

and carbon fiber-reinforced plastic, etc.), new metallic reinforcements, or high-strength

steel reinforcement can also be used to enhance the performance of what is considered to

be a traditional material. Higher-strength reinforcement could be particularly useful when

coupled with high-strength concrete. As our natural resources diminish, alternative

aggregate sources (e.g., recycled aggregate) and further replacement of cementitious

materials with recycled products are being examined. Highly reactive cements and

reactive aggregates will be concerns of the past as new materials with long-term

durability become commonplace.

New materials will also find increasing demand in repair and retrofitting. As the

bridge inventory continues to get older, increasing the usable life of structures will

become critical. Some innovative materials, although not economical for complete

bridges, will find their niche in retrofit and repair.

OPTIMIZED SECTIONS

In early applications of prestressed concrete to bridges, designers developed their

own ideas of the best girder sections. The result is that each contractor used slightly

different girder shapes. It was too expensive to design custom girders for each project.

As a result, representatives for the Bureau of Public Roads (now FHWA), the

American Association of State Highway Officials (AASHO) (now AASHTO), and the

Prestressed Concrete Institute (PCI) began work to standardize bridge girder sections.

The AASHTO-PCI standard girder sections Types I through IV were developed in the

late 1950s and Types V and VI in the early 1960s. There is no doubt that standardization

of girders has simplified design, has led to wider utilization of prestressed concrete for

bridges, and, more importantly, has led to reduction in cost.

With advancements in the technology of prestressed concrete design and

construction, numerous states started to refine their designs and to develop their own

standard sections. As a result, in the late 1970s, FHWA sponsored a study to evaluate

existing standard girder sections and determine the most efficient girders. This study

concluded that bulb-tees were the most efficient sections. These sections could lead to

reduction in girder weights of up to 35 percent compared with the AASHTO Type VI and

cost savings up to 17 percent compared with the AASHTO-PCI girders, for equal span

capability. On the basis of the FHWA study, PCI developed the PCI bulb-tee standard,

which was endorsed by bridge engineers at the 1987 AASHTO annual meeting.

Subsequently, the PCI bulb-tee cross section was adopted in several states. In addition,

similar cross sections were developed and adopted in Florida, Nebraska, and the New

England states. These cross sections are also cost-effective with high-strength concretes

for span lengths up to about 60 m (200 ft).

SPLICED GIRDERS

Spliced concrete I-girder bridges are cost-effective for a span range of 35 to 90 m

(120 to 300 ft). Other shapes besides I-girders include U, T, and rectangular girders,

although the dominant shape in applications to date has been the I-girder, primarily

because of its relatively low cost. A feature of spliced bridges is the flexibility they

provide in selection of span length, number and locations of piers, segment lengths, and

splice locations. Spliced girders have the ability to adapt to curved superstructure

alignments by utilizing short segment lengths and accommodating the change in direction

in the cast-in-place joints. Continuity in spliced girder bridges can be achieved through

full-length posttensioning, conventional reinforcement in the deck, high-strength

threaded bar splicing, or pretensioned strand splicing, although the great majority of

applications utilize full-length posttensioning. The availability of concrete compressive

strengths higher than the traditional 34 MPa (5,000 psi) significantly improves the

economy of spliced girder designs, in which high flexural and shear stresses are

concentrated near the piers. Development of standardized haunched girder pier segments

is needed for efficiency in negative-moment zones. Currently, the segment shapes vary

from a gradually thickening bottom flange to a curved haunch with constant-sized bottom

flange and variable web depth.

SEGMENTAL BRIDGES

Segmental concrete bridges have become an established type of construction for

highway and transit projects on constrained sites. Typical applications include transit

systems over existing urban streets and highways, reconstruction of existing interchanges

and bridges under traffic, or projects that cross environmentally sensitive sites. In

addition, segmental construction has been proved to be appropriate for large-scale,

repetitive bridges such as long waterway crossings or urban freeway viaducts or where

the aesthetics of the project are particularly important.

Current developments suggest that segmental construction will be used on a larger

number of projects in the future. Standard cross sections have been developed to allow

for wider application of this construction method to smaller-scale projects. Surveys of

existing segmental bridges have demonstrated the durability of this structure type and

suggest that additional increases in design life are possible with the use of HPC.

Segmental bridges with concrete strengths of 55 MPa (8,000 psi) or more have been

constructed over the past 5 years. Erection with overhead equipment has extended

applications to more congested urban areas. Use of prestressed composite steel and

concrete in bridges reduces the dead weight of the superstructure and offers increased

span lengths.

LOAD RATING OF EXISTING BRIDGES

Existing bridges are currently evaluated by maintaining agencies using working

stress, load factor, or load testing methods. Each method gives different results, for

several reasons. In order to get national consistency, FHWA requests that all states report

bridge ratings using the load factor method. However, the new AASHTO Load and

Resistance Factor Design (LRFD) bridge design specifications are different from load

factor method. A discrepancy exists, therefore, between bridge design and bridge rating.

A draft of a manual on condition evaluation of bridges, currently under development

for AASHTO, has specifications for load and resistance factor rating of bridges. These

specifications represent a significant change from existing ones. States will be asked to

compare current load ratings with the LRFD load ratings using a sampling of bridges

over the next year, and adjustments will be proposed. The revised specifications and

corresponding evaluation guidelines should complete the LRFD cycle of design,

construction, and evaluation for the nation's bridges.

LIFE-CYCLE COST ANALYSIS

The goal of design and management of highway bridges is to determine and

implement the best possible strategy that ensures an adequate level of reliability at the

lowest possible life-cycle cost. Several recent regulatory requirements call for

consideration of life-cycle cost analysis for bridge infrastructure investments. Thus far,

however, the integration of life-cycle cost analysis with structural reliability analysis has

been limited. There is no accepted methodology for developing criteria for life-cycle cost

design and analysis of new and existing bridges. Issues such as target reliability level,

whole-life performance assessment rules, and optimum inspection-repair-replacement

strategies for bridges must be analyzed and resolved from a life-cycle cost perspective.

To achieve this design and management goal, state departments of transportation must

begin to collect the data needed to determine bridge life-cycle costs in a systematic

manner. The data must include inspection, maintenance, repair, and rehabilitation

expenditures and the timing of these expenditures. At present, selected state departments

of transportation are considering life-cycle cost methodologies and software with the goal

of developing a standard method for assessing the cost-effectiveness of concrete bridges.

DECKS

Cast-in-place (CIP) deck slabs are the predominant method of deck construction in

the United States. Their main advantage is the ability to provide a smooth riding surface

by field-adjustment of the roadway profile during concrete placement. In recent years

automation of concrete placement and finishing has made this system cost-effective.

However, CIP slabs have disadvantages that include excessive differential shrinkage with

the supporting beams and slow construction. Recent innovations in bridge decks have

focused on improvement to current practice with CIP decks and development of

alternative systems that are cost-competitive, fast to construct, and durable. Focus has

been on developing mixes and curing methods that produce performance characteristics

such as freeze-thaw resistance, high abrasion resistance, low stiffness, and low shrinkage,

rather than high strength. Full-depth precast panels have the advantages of significant

reduction of shrinkage effects and increased construction speed and have been used in

states with high traffic volumes for deck replacement projects. NCHRP Report 407 on

rapid replacement of bridge decks has provided a proposed full-depth panel system with

panels pretensioned in the transverse direction and posttensioned in the longitudinal

direction.

Several states use stay-in-place (SIP) precast prestressed panels combined with CIP

topping for new structures as well as for deck replacement. This system is

cost-competitive with CIP decks. The SIP panels act as forms for the topping concrete

and also as part of the structural depth of the deck. This system can significantly reduce

construction time because field forming is only needed for the exterior girder overhangs.

The SIP panel system suffers from reflective cracking, which commonly appears over the

panel-to-panel joints. A modified SIP precast panel system has recently been developed

in NCHRP Project 12-41.

SUBSTRUCTURES

Continuity has increasingly been used for precast concrete bridges. For bridges with

total lengths less than 300 m (1,000 ft), integral bridge abutments and integral

diaphragms at piers allow for simplicity in construction and eliminate the need for

maintenance-prone expansion joints. Although the majority of bridge substructure

components continue to be constructed from reinforced concrete, prestressing has been

increasingly used. Prestressed bents allow for longer spans, improving durability and

aesthetics and reducing conflicts with streets and utilities in urban areas. Prestressed

concrete bents are also being used for structural steel bridges to reduce the overall

structure depth and increase vertical clearance under bridges. Precast construction has

been increasingly used for concrete bridge substructure components. Segmental hollow

box piers and precast pier caps allow for rapid construction and reduced dead loads on

the foundations. Precasting also enables the use of more complex forms and textures in

substructure components, improving the aesthetics of bridges in urban and rural areas.

RETAINING WALLS

The design of earth retaining structures has changed dramatically during the last

century. Retaining wall design has evolved from short stone gravity sections to concrete

structures integrating new materials such as geosynthetic soil reinforcements and

high-strength tie-back soil anchors.

The design of retaining structures has evolved into three distinct areas. The first is the

traditional gravity design using the mass of the soil and the wall to resist sliding and

overturning forces. The second is referred to as mechanically stabilized earth design. This

method uses the backfill soil exclusively as the mass to resist the soil forces by engaging

the soil using steel or polymeric soil reinforcements. A third design method is the

tie-back soil or rock anchor design, which uses discrete high-strength rods or cables that

are drilled deep into the soil behind the wall to provide a dead anchorage to resist the soil

forces.

A major advancement in the evolution of earth retaining structures has been the

proliferation of innovative proprietary retaining walls. Many companies have developed

modular wall designs that are highly adaptable to many design scenarios. The innovative

designs combined with the modular standard sections and panels have led to a significant

decrease in the cost for retaining walls. Much research has been done to verify the

structural integrity of these systems, and many states have embraced these technologies.

RESEARCH

The primary objectives for concrete bridge research in the 21st century are to

develop and test new materials that will enable lighter, longer, more economical, and

more durable concrete bridge structures and to transfer this technology into the hands of

the bridge designers for application. The HPCs developed toward the end of the 20th

century would be enhanced by development of more durable reinforcement. In addition,

higher-strength prestressing reinforcement could more effectively utilize the achievable

higher concrete strengths. Lower-relaxation steel could benefit anchor zones. Also,

posttensioning tendons and cable-stays could be better designed for eventual repair and

replacement. As our natural resources diminish, the investigation of the use of recycled

materials is as important as the research on new materials.

The development of more efficient structural sections to better utilize the

performance characteristics of new materials is important. In addition, more research is

required in the areas of deck replacement panels, continuity regions of spliced girder

sections, and safe,durable, cost-effective retaining wall structures.

Research in the areas of design and evaluation will continue into the next

use of HPC will be facilitated by the removal of the implied strength

limitation of 70 MPa (10.0 ksi) and other barriers in the LRFD bridge design

specifications. As our nation’s infrastructure continues to age and as the vehicle loads

continue to increase, it is important to better evaluate the capacity of existing structures

and to develop effective retrofitting techniques. Improved quantification of bridge system

reliability is expected through the calibration of system factors to assess the member

capacities as a function of the level of redundancy. Data regarding inspection,

maintenance, repair, and rehabilitation expenditures and their timing must be

systematically collected and evaluated to develop better methods of assessing

cost-effectiveness of concrete bridges. Performance-based seismic design methods will

require a higher level of computing and better analysis tools.

In both new and existing structures, it is important to be able to monitor the “health”

of these structures through the development of instrumentation (e.g., fiber optics) to

determine the state of stresses and corrosion in the members.

CONCLUSION

Introduced into the United States in 1949, prestressed concrete bridges today

represent over 50 percent of all bridges built. This increase has resulted from

advancements in design and analysis procedures and the development of new bridge

systems and improved materials.

The year 2000 sets the stage for even greater advancements. An exciting future lies

ahead for concrete bridges!

混凝土桥梁

在美国甚至在世界桥梁上，混凝土是最常用的建设材料。从1949年开始在宾夕法尼亚州的费城随着高强度的钢丝应用于核桃里桥上，桥梁预应力应用得到了迅速稳步的增长。从1950年到90年代初，根据美国联邦高速公路管理局的1994年全国桥梁的库存数据中得知，在美国超过50%已建桥梁上预应力混凝土桥梁已经有了很大的发展。

在延长混凝土桥梁跨度的能力上预应力也发挥了重要作用。到90年代末，拼接梁跨度达到创纪录的100米（330英尺）。在1974年节段混凝土桥梁的建造工程于美国开始兴起。到现在接近200节段混凝土桥梁已建成或正在建设，其跨度达240米（800英尺）。

到了70年代后期，对混凝土桥梁，斜拉桥建设开始运用了。到1982年，在佛罗里达州的坦帕市阳光高架桥其主跨有365米（1,200英尺），打破混凝土桥梁的新纪录。第二年，在佛罗里达州的杰克逊维尔市达梅斯点大桥更新了纪录达到400米（1300英尺）。

高性能混凝土

抗压强度

多年来，预制预应力混凝土梁的设计是基于混凝土34至41兆帕（5000至6000

PSI）的抗压强度上。这种强度有很好的耐久性能，并提供了建设美国预应力混凝土桥梁工业的基础。在20世纪90年代该行业开始利用更高的混凝土抗压强度设计值，并在新千年开始之际，业界准备接受使用抗压强度高达70兆帕（10,000 PSI）的混凝土。

对于未来，该行业需要设法有效利用更高抗压强度的混凝土。超过20年预拌混凝土行业已经能生产70兆帕斯卡以上混凝土抗压强度。几个示范项目说明，70兆帕斯卡以上的优势，可以促成预应力混凝土梁实现。允许更多地使用这些材料来排除障碍。同时，业主，设计师，承包商和制造者必须接受更高抗压强度的混凝土。

耐久性

高性能混凝土（HPC）可以定为抗压强度高（例如，在预应力梁）或传统的耐

久性抗压强度（如铸造，就地桥面和子结构）。这就有必要对所有参数有更好了解，如抗化学，电化学耐久性，环境机制，攻击完整的材料。显著性差异可能发生在同一时间使用相同的材料双相邻结构的长期耐用性上。这揭示了我们缺乏理解和控制影响耐久性的参数。

新材料

在过去，混凝土设计规范关心的重点主要是抗压强度。混凝土是慢慢走向的一个精心设计的材料，其性能可直接由设计者改变。材料特性，如透气性，延展性，抗冻融，耐久性，耐磨损性，反应性和强度都有所规定。高性能计算的倡议在促进这些规范方面已有很长时间，但还有很多可以做。添加剂，这种纤维或化学物质，可以显着改变混凝土的基本属性。其他新材料，如纤维增强聚合物复合材料，非金属加固（玻璃纤维增强和碳纤维增强塑料等），新金属增援，或高强度钢筋也可以用来提高这些被认为是传统的材料的性能。当与高强度混凝土配合时，高强度钢筋可能是特别有效。正如我们的天然资源在减少一样，替代总来源（如再生骨料）及再生产品与胶凝材料替代目前正在进一步研究。高活性水泥和聚合反应将给与关注，就像长期的耐久性新材料一样变得司空见惯。

在维修和改造方面，新材料的需求也会增加。正如已有的桥会继续变老，那么增加结构的寿命将变得至关重要。在改装及维修上，一些创新的材料，对整个桥梁来说虽然不是经济的，但是不久将发现他的好处。

截面优化

在预应力混凝土桥梁早期应用中，设计者开发出他们自己的最好梁段的想法。其结果是，每个承包商使用时都略有不同的梁的形状。它太昂贵，设计者不能为每个项目都定制大梁。

因此，对公共道路局的代表（现联邦公路管理局），国家公路官员（AASHO）美国协会（现AASHTO标准）和预应力混凝土协会（PCI）开始进行标准梁部分的设计。这AASHTO-PCI标准的第一到第四类型的梁段是在50年代末产生的，和第五和第六类型是在六十年代初开发的。毫无疑问，标准化的梁简化了设计者的疑问，以致预应力混凝土桥梁得到更广泛的的使用，更重要的是导致成本的降低。

随着在预应力混凝土设计和施工技术进步，许多国家开始改善其设计和开发自

己的标准部分。因此，在70年代末，联邦公路管理局赞助的一项研究，以评估现有的标准梁段，并确定最有效的大梁。这项研究的结论是，气泡似的梁是最有效的部分。在相同跨径下，这些部分可能导致与AASHTO第六标准型相比减少高达百分之三十五的重量，和比AASHTO-PCI型梁相比成本节省高达百分之十七。在联邦公路管理局研究的基础上，PCI发展成PCI bulb-tee的标准型，这些是由桥梁工程师通过AASHTO标准在1987年年度会议做出的。随后PCI –气球型截面在几个州被采用。此外，类似的截面产生了并在佛罗里达州，内布拉斯加州和新英格兰州被采用。此外，类似的截面是发达国家和佛罗里达州，内布拉斯加州和新英格兰州通过。这些高强度混凝土截面的梁性价比高，其跨度达60米（200英尺）。

主梁拼接

拼接的并且跨度在35到90米（120到300英尺）范围内的混凝土I -梁桥效能价格合算的。其他形状除了I型梁包括U，T和矩形梁，虽然在迄今为止应用中占主导的是I -梁，主要是因为其成本相对较低。拼接桥梁的特点是灵活，他们在跨长，数量和桥墩位置的选择上提供段长和接头。拼接梁通过使用短部分梁段来适应弯曲的地方和容许在方向和接头的变化。在拼接梁桥的连续性可以通过全长后张来实现，传统的钢筋在甲板，高强度螺纹杆拼接，或预应力钢绞线剪接，虽然绝大多数申请利用全长后张法来施工。抗压强度高混凝土的可用性比传统的高34兆帕（5000 PSI）显着提高拼接梁的设计性能，其中在靠近桥墩高弯曲部分剪切应力会有集中。在被动区域标准化腋梁墩部分需要提高性能。目前，这部分形状从逐渐增厚底部凸缘变化到一个有固定大小的底部凸缘和可变深度的弯曲腰身部分。

桥梁预制节

预制节段混凝土桥梁已成为在限制用地建设项目和高速公路上制定的类型。典型的应用包括在交通运输系统在现有的城市街道和公路，现有的交汇处和桥梁，交叉或项目环境敏感地点。此外，节段施工已被证明是可行的，如长水道渡口或城市快速路高架桥或该项目的美学尤其重要桥梁。

目前的事态发展表明，在未来节段施工将在更多的项目上使用。标准断面已经制定，以便更广泛地在小规模的工程上运用这些建筑方法。现有段桥梁的调查表明这种结构类型的耐用性，建议在设计使用年限与额外增加的高性能计算成为可能。

在过去的5年中，混凝土的强度达到55兆帕斯卡（8000平方英寸）或更高的多节段桥梁已建成。设备安装工程的开销已扩展应用更拥挤的城市地区。预应力钢和混凝土桥梁的使用减少了上层建筑的自重，并且增加了跨度。

现有桥梁的额定负荷

目前，维护机构利用工作压力，负荷率，或负载等测试方法对现有的桥梁进行评估。每个方法给出不同的结果，有几个原因。为了获得国家的一致性，联邦公路管理局要求所有国家的桥梁评级报告使用负载因子的方法。但是，新的AASHTO标准荷载和抗力系数设计（荷载抗力系数）的桥梁设计规范是不同于负载系数的方法。因此在桥梁设计和桥梁评估上存在差异。

一本关于桥梁评估的手册草案，目前正在为AASHTO标准的发展，已有桥梁的负载和抵抗因素评价规范。这些规范代表了现在的重大变化。在明年美国将要求目前的荷载等级与带有荷载抗力系数的荷载等级进行比较，并且将做出调整。修订后的规范和相应的评价的准则应覆盖全国桥梁的设计，施工荷载抗力系数周期，和评价。

生命周期成本分析

公路桥梁的设计和管理的目标是确定和实施最佳的战略，确保在尽可能最低的生命周期成本下达到足够可靠的水平。对于桥梁基础设施的投资，最近的几项要求考虑周期成本分析。然而，迄今为止，把生命周期分析与结构可靠性分析结合在一起是受到了限制的。这里没有公认的方法来制定现有的和已存在的桥梁的生命周期的设计和分析的标准。从生命周期成本的角度战略上，如可靠性水平的目标，全寿命绩效评估规则，以及合适的检查维修和桥梁更换等问题必须加以分析和解决。为了达到这个设计和管理的目标，国家运输部门必须以一种有条理的方式开始收集需要确定桥梁寿命周期的成本数据。这些数据必须包括检查，保养，修复和重建的支出，以及这些支出的时间安排。目前，国家有关部门选定的交通正在考虑生命周期成本的方法，以发展评估的成本混凝土桥梁的成效的标准方法的目标和软件。

甲板

在美国甲板建设的主要方法是现浇位（CIP）甲板砖。他们的主要优势是在混凝土浇筑通过道路表面的可调区域能够提供平稳的乘坐表面场。近年来，混凝土浇筑

和完成的自动化使这种该系统合理化。然而，CIP板有缺点，包括支持梁过度收缩和缓慢的建设。在桥面最近创新集中在提高具有成本竞争力，快速建造，和耐久性的替代系统的发展和CIP甲板的现行做法。重点是发展混合和固化的方法，那就是生产诸如冻融性的抵抗，耐磨性高，低硬度，低收缩，而不是高强度等特点。全面而深入的预制板有显着的降低收缩作用和提高建设速度的优势，并已在国家用于甲板更换项目的高交通流量的地方使用。公路合作研究计划报告407条关于桥面甲板的迅速更换方面提供了与在横向预应力板提出全面而深入的带有改变方向的先张和纵向的后张面板系统。

有几个国家使用原位张拉的预应力面板并结合CIP板，这即为建设也为甲板更换。该系统具有CIP甲板的成本竞争力。这种结构模板作为抹平混凝土，也是作为对甲板结构深入的一部分。该系统可以大大减少建筑领域形成的时间，因为只有外部梁悬需要。这种结构模板会有反射裂缝，它通常出现在面板与面板的接缝之中。一种改进的SIP预制板系统最近已经在公路合作研究计划项目12-41中开发出来了。

下部结构

连续性日益被应用于预制混凝土桥梁。对于总长度小于300米（1000英尺）桥梁，桥墩上的整体式桥台和整体式隔膜允许在施工上简单和消除易膨胀节点的需要。虽然大多数桥梁下部结构继续是钢筋混凝土建造的，但是预应力已被越来越多地采用。预应力排架允许更长的跨度，即提高耐久性和美学并且减少在城市地区与街道和公共事业的冲突。预应力混凝土排架也被用于钢结构桥梁，以减少整体结构的沉降和增加桥梁的净高。预制结构已越来越多地用于混凝土桥梁下部结构组件的建设中。分段中空箱桥墩及预制墩帽能够迅速建设和减少基础上的恒载。在下部结构部件中，预制还能允许更复杂的形式和材质的使用，改善城市和农村地区的桥梁美学。

挡土墙

在上个世纪，地下支挡结构的设计发生了巨大变化。挡土墙的设计已经从简单的石头重力到结合新材料的混凝土结构演变过来，如土工合成材料的土壤加固措施和高强配合回泥土锚钉。

围护结构的设计已演变成三个不同的领域。第一个是传统的重力设计中使用的土壤和墙总重量来提供抗滑力和倾覆力。第二个被称为机械的稳定土壤设计。此方

法通过使用钢或聚合土壤的增强来使回填的土抵抗土壤的压力。第三种设计方法是配合回土壤或岩锚设计，使用分立的高强棒或电缆入钻到深层土壤，并在墙背后提供死锚来抵抗土壤的推力。

在支挡结构演变中的一个最大提高就是创新的专有挡土墙扩散。许多公司已经开发模块化设计，墙体高度适应多种设计方案。标准的模块化和面板相结合的创新挡土墙设计导致成本大幅的下降。大量的研究已经进行来核实这些系统的结构完整性，并且许多国家已经接受了这些技术。

研究

在21世纪混凝土桥梁研究的主要目标是开发和测试新材料，它们能够重量更轻，更长，更经济和更耐用的混凝土桥梁结构，并这种技术转移到桥梁设计师手中。到20世纪末开发的高性能计算机由于耐用性的增强而得到了提高。此外，高强度预应力钢筋更有效地利用可以达到的更高强的混凝土强度。低松弛钢筋对锚固区有利。此外，后张肌腱和电缆更好的设计以便最终的维修和更换。正如我们的天然资源减少一样，再生材料的调查和新材料的研究一样重要。

更有效的结构部分以便更好地利用新材料的性能特点的发展是重要的。此外，在一些领域需要更多的研究，如在甲板上更换面板，拼接梁地区的连续性，和安全耐用且成本效益合理的挡土墙结构。

在设计和评价方面的研究将持续到下一个千年。这种高性能计算机也会带来方便，即消除70兆帕斯卡（10.0 ksi）的限制和在荷载抗力系数的桥梁设计规范中的其他一些障碍。随着我们国家的基础设施不断老化和车辆负载不断增加，重要的是要更好地评估现有机构的能力，并制定有效的改造技术。桥梁系统的可靠性改进量化，预计通过的体制因素，校准，以评估作为程度的冗余功能的成员的能力。有关检查，保养，维修和复原的支出及其时间的数据必须系统地收集和评价，以发展评估混凝土桥梁成本效益的更好的方法。基于性能的抗震设计方法，将需要更高的计算水平和分析工具，。

在新的和现有结构，重要的是能够通过监测仪器的发展（如光纤）以确定各成员的应力和腐蚀的状态。

结论

在美国从1949年到现在，今天的预应力混凝土桥梁取代了超过50%已架起的桥梁。这种增加是由于在设计和分析程序的进步和新桥梁系统和改善材料的发展而导致的。

2000年为更大的进步奠定了基础。那么混凝土桥梁会有一个令人兴奋的未来前景！