Abstract

This article explores significant technological advancements in bridge construction. The study highlights how technological advancements in bridge construction, such as precast concrete, prestressed systems, advanced grouting, rebar couplers, and pot bearings are transforming bridge construction practices thereby improving safety, durability, efficiency, sustainability and longevity. By showcasing real-world applications in Kenya, it demonstrates how these innovations enhance bridge design and construction, leading to more resilient and cost-effective infrastructure. Precast concrete technology, including elements like pier caps, shafts, and superstructures, has revolutionized bridge building by enabling rapid assembly, improving quality, and reducing construction time. Prestressed concrete innovations, such as high-strength steel tendons, facilitate the design of slender, longer-span bridges with superior load-bearing capacity and increased durability. Advanced grouting systems, incorporating expansive and non-shrink materials, ensure thorough duct filling and enhanced tendon bonding, significantly improving structural longevity. The adoption of rebar couplers has notably reduced labor and material costs compared to traditional lap splicing, especially for large diameter bars. Pot bearings, essential for large and heavily loaded bridges, provide high load-bearing capacity and accommodate larger rotations than elastomeric bearings, with modern designs enhancing sealing and durability against harsh conditions. Case studies of notable projects in Kenya, including the Nairobi Expressway, Mbogolo Bridge, and Mtwapa Market Bridges, illustrate the practical benefits and effectiveness of these advancements. Additionally, the ongoing construction of the New Mtwapa Bridge, a 345-meter (100+145+100) cast-in-place balanced cantilever box girder bridge, will further demonstrate the advantages of these innovations.

Introduction

A bridge is a structure that spans over rivers, valleys, creeks, roads, and railways, facilitating the crossing of people, animals, vehicles, and trains. There are various types of bridges, classified based on factors such as the materials used in construction, the bridge’s purpose, the construction method, and its structural form.

In Kenya, bridge construction has evolved significantly over time. In the pre-colonial and early post-colonial periods, small timber bridges were commonly used to cross rivers, streams, and other obstacles. The limited availability of construction materials, primarily timber and stones, resulted in bridges that were not very sturdy, had short lifespans, and could not accommodate large spans.

With the introduction of steel into the construction industry, timber bridges were gradually replaced by steel structures such as Bailey bridges, many of which are still found across the country. The advent of reinforced concrete further revolutionized the bridge construction industry, enabling the construction of longer, more durable spans.

In the 20^th and 21^st Century, technological advancements brought about the construction of long-span bridges designed to last up to 120 years, capable of supporting heavier loads and offering greater durability. This article highlights these advancements in bridge construction in Kenya, showcasing real projects that exemplify the modern methods made possible by these technological innovations.

Objectives

The primary objective of this study is to showcase the technological advancements in the bridge construction industry in Kenya, which have facilitated the construction of long-span bridges that are more durable and safer.

Additionally, the study aims to compare these modern technological advancements with the traditional methods used in the past, highlighting the benefits and improvements they have brought to the bridge construction industry, particularly in terms of efficiency, longevity, and structural safety.

Methodology

The main methods and approaches utilized in this article are as follows;

• Desk Study: This involved a review of previous articles, journals, and online resources to gain a comprehensive understanding of the existing knowledge related to advancements in bridge construction.
• Interviews: Engaged with Bridge Engineers involved in the case study projects, providing first-hand insights and expert perspectives on the recent developments and technological advancements in bridge construction.
• Analysis of Design Reports and Construction Drawings: Examined design reports and construction drawings used during the bridge construction to understand the technical specifications, methodologies applied, and the innovations integrated into the construction process.

Results and Findings

Pot Bearings

Bearings are essential components in bridge construction, used to transfer loads from the superstructure to the substructure. These loads may arise from vehicular traffic or environmental factors and can be categorized into vertical loads, horizontal loads, bridge movements in the horizontal direction, and rotations about the bridge’s main axis.

Early timber bridges in Kenya were constructed without bearings. Over time, steel bearings such as roller, pin, and sliding bearings were introduced to accommodate the necessary rotations and movements. Today, more advanced bearings, capable of sustaining large vertical and horizontal forces, as well as significant rotations and displacements, are common in the bridge construction industry. One such bearing is the pot bearing.

Pot bearings consist of an elastomeric disc enclosed in a steel pot. When subjected to loads, the disc is compressed by a piston, causing it to act like a viscous material. The bearing includes seals to prevent the elastomeric rubber from escaping the pot and is equipped with a low-friction material known as Polytetrafluoroethylene (PTFE), which facilitates sliding. Pot bearings are classified into three types: fixed pot bearings, which allow only rotations without movements; guided pot bearings, which permit rotation with translational movement in one direction; and free pot bearings, which allow rotations and translational movements in all directions.

Pot bearings were extensively used in the construction of the Nairobi Expressway Project. Their ability to allow large rotations (up to 0.03 radians), their suitability for skewed and curved bridges expected to bear substantial loads, and their capacity to handle large displacements in confined spaces made them ideal for this project. Pot bearings are also planned for use in the New Mtwapa Bridge, currently under construction. This bridge, which will have a dual carriageway with a 23.6-meter deck width, is expected to transfer significant forces from the superstructure to the substructure, primarily due to the weight of the superstructure and vehicular traffic loads. The image below shows pot bearings that have been installed on the bridge bearing pedestal, awaiting the hoisting of the superstructure. The photo was taken during the construction of an interchange along the Mombasa – Mtwapa – Kilifi (A7) Road.

One of the features that makes pot bearings particularly suitable for modern bridge construction is the protection of their main operating components, which are enclosed to prevent degradation. They also perform exceptionally well under high load magnitudes and in varying temperature conditions, ensuring their reliability and longevity in diverse environments.

Rebar Couplers

Rebar couplers are mechanical devices used to join reinforcement bars in construction, allowing the continuation of reinforcement without the need for overlapping. When compared to traditional methods like lapping, rebar couplers reduce reinforcement congestion, particularly where large diameter bars are used. This is a significant advantage in bridge construction, as it reduces reinforcement congestion thus improving concrete flow.

Additionally, couplers reduce the tonnage of steel required, minimizing wastage by allowing offcuts to be reused. According to Harinkhede et al. (2016), large diameter steel reinforcing bars in concrete structures require around 15% more steel at a lap due to overlapping length, making couplers an efficient alternative. Debate exists regarding the cost-effectiveness of couplers versus lapping. A cost comparison study by Damsara P. and Kulathunga T. (2018) found that couplers are more cost-effective for larger diameter bars, such as 32mm and 40mm.

In Kenya, bridge construction often involves large diameter reinforcement bars ranging from 20mm to 40mm. Couplers are particularly beneficial in pile cage reinforcements, as they reduce the total weight of the skeleton and alleviate congestion, allowing for easier concrete flow. The Nairobi Expressway project notably utilized couplers in pile and pier cap reinforcements. The image below shows a 1.5m diameter pile reinforcement cage segment equipped with couplers that will aid in joining the rebars of other segments together.

Grouting sleeves, a type of coupler used for connecting precast members, were employed in joining pier shaft and pier cap precast elements on the nairobi Expressway Project. The sleeves, embedded within the precast elements, are filled with high-strength non-shrink grout, facilitating a robust connection. These sleeves feature two holes—one for grout inlet and another for overflow, ensuring complete grouting. The image below shows details of a grouting sleeve that was used in joining of precast pier shafts to the pile cap (Starter Bars) during the construction of the Nairobi Expressway Project.

Precast Concrete

Precast concrete is manufactured off-site in a controlled environment before being transported to the construction site. This process involves casting concrete using reusable molds or formworks, which are then cured in a controlled setting. While most existing bridges in Kenya were built in situ, the trend is shifting toward the use of precast concrete members due to their numerous advantages over traditional methods.

One significant benefit of precast concrete is the guarantee of high-quality members. The close monitoring and repetitive manufacturing techniques employed during production ensure consistent quality control. Moreover, since precast elements are produced off-site, they require less working space on-site, which is particularly advantageous in urban areas where space is often limited. A prime example of this is the construction of the Nairobi Expressway and Mtwapa Market Roundabout Junction Bridge, situated in the heart of Nairobi City and Mtwapa Town respectively.

Precast members can also be stored and delivered precisely when needed in the construction schedule. Each precast element can have a barcode sticker attached, providing essential information to ensure the correct member is transported and hoisted into its designated position. This technique was used in the Nairobi Express Way Project which involved a large number of precast box girders. The barcode was used to show; the type of the box girder, the casting date, post-tensioning date, and the exact location where the girder was to be transported and hoisted.

Additionally, the use of precast concrete reduces the overall construction time of bridges, as the production of precast elements can occur simultaneously with other on-site activities. During the Nairobi Expressway Project, for instance, while piles and pile caps were being constructed, precast pier shafts, pier caps, and box girders were simultaneously produced in a dedicated precast yard.

The effects of creep and shrinkage are significantly minimized when using precast segments, as they are allowed to mature to full strength in a controlled environment.

Precast elements can be launched using girder launching gantry cranes, which eliminates the need for extensive falsework and allows for erection from the top of completed bridge portions. This method is particularly beneficial for high-level crossings or in urban areas where minimizing interference with the surroundings is crucial.

Finally, tight quality controls during factory production ensure that the cover levels for reinforcement are consistently met, resulting in an aesthetically pleasing surface free from rust. Precast concrete’s inherent strength also allows for the creation of long clear spans without the need for additional structural support.

Prestressed Concrete

Prestressed concrete is a principle used in the concrete industry whereby concrete members are subjected into a state of compression by tensioning of tendons embedded in them. The concrete members are subjected to compressive stress before any loading is applied. Tendons are usually tensioned in prestressed concrete members so as to induce stresses into the concrete member which will later be counteracted by external loadings applied onto the structure. Tendons are high strength steel strands or cables made from winding several wires together.

Prestressed concrete has really revolutionized bridge construction industry in Kenya and is now widely used across the country. Prestressed concrete is slowly replacing reinforced concrete especially in the construction of the super structure of bridges. This is due to its advantages over reinforced concrete.

Most prestressed bridge members in Kenya are precast. Some of the precast prestressed concrete bridge members that have been widely used in Kenyan bridges include; AASHTO I Girders (Type I – VI) and precast box girders. Example of notable bridges that have been constructed using Prestressed Precast I girders include Mbogolo and Mtwapa Markets Bridges along the Mtwapa – Kilifi Road where by 25m type (IV) and 40m type (VI) PSC I Girders formed part of the superstructure. Cast in situ prestressed concrete box girders have also been used to construct major bridges in Kenya such as the Mtwapa, Nyali and Kilifi bridges. The super structure of the New Mtwapa bridge will also be composed of a single box double cell prestressed concrete cast in place box girder superstructure.

There are two methods prestressing concrete:

1. Pre- Tensioning

Pre tensioning involves stretching of steel tendons before casting concrete and then cutting them after the concrete cures(sets), resulting into a more durable and efficient structure. The process involves; tensioning of the tendon strands using hydraulic jacks to the desired force, concreting of the member and finally after the concrete attains a certain specified designed strength, the tendons are cut. 

2. Post – Tensioning

Post tensioning on the other hand is a method of prestressing in which prestressing steel is tensioned after concrete has hardened. The process involves; Concreting the concrete member, curing until the concrete attains the desired strength and finally tensioning the tendons to the desired force. The images below showcase posttensioning process being done on a 30m precast box girder used in the construction of the Nairobi Expressway and 40m I girder used in the construction of Mtwapa Market Roundabout Bridge.

Some of the advantages of prestressed concrete include;

1. Cost-effective for various conditions and spans: Post-tensioning allows the construction of high-quality bridges over a wide range of conditions and span lengths at a lower cost.
2. Minimizes falsework and reduces public interference: Precast elements can be launched using girder launching gantry cranes, which reduces or eliminates the need for falsework. This technique, used in projects like the Nairobi Expressway, also minimizes disruption to the public and the surrounding environment, making it especially effective for high-level crossings and urban projects.
3. Accelerates construction time: High-strength concrete allows post-tensioning to be done 4 to 10 days after casting. This enables grouting and hoisting of girders immediately, speeding up the entire construction process.
4. Minimizes site congestion: Precasting offsite and hoisting into place streamlines the process, reducing site congestion and time spent on site.
5. High durability and strength: Prestressed concrete provides enhanced durability and load-bearing capacity compared to traditional reinforced concrete.
6. Ideal for large quantities and repeatable designs: Precast prestressed concrete is cost-effective when dealing with large volumes of girders or repeated design elements, such as in large bridge projects.
7. Adaptable to complex construction staging: It is suitable for projects with constraints like limited construction time, traffic disruptions, environmental impact requirements, and utility relocation.
8. Ensures long spans without additional support: Prestressed concrete allows the creation of long spans, reducing the need for intermediate supports and allowing greater design flexibility.
9. Post-tensioning makes possible the cost-effective construction of high-quality bridges over a wide range of conditions and span lengths.

Grouting.

Grouting is the process of injecting a composite material with fluid-like properties into cavities or spaces to fill voids, strengthen structures or provide sealing.  The main components of grout include cement, admixtures and water. It hardens over time to provide support, bonding or prevention of ingress of moisture and/or chemicals.

Grouting has revolutionized the bridge industry in Kenya, particularly in bridge repair works and post-tensioning. This technology has been effectively applied in major projects, including the repair of the Existing Mtwapa Bridge. As the trend in Kenya moves towards post-tensioned concrete bridge designs, grouting plays a critical role in ensuring the complete sealing of voids inside tendon ducts or sheaths. By preventing water ingress into the ducts, grouting helps protect tendons from corrosion, thereby enhancing the durability and safety of the structures.

Advancements in technology have enabled more extensive testing of admixtures used in grouting, such as plasticizers, stabilizers, and retarding agents. These admixtures offer several benefits, including improved flowability at a given water-cement ratio, the elimination of bleed water, prevention of segregation during high-pressure grouting, and the controlled retardation of grout setting time. These improvements enhance the overall effectiveness and reliability of the grouting process, ensuring better performance in construction applications.

Pre-mixed grouts aslo, manufactured in controlled environments, ensure consistent quality and reduce errors in on-site mixing, improving the overall reliability of the grouting process.

The integration of advanced digital machines equipped with pressure gauges and high-capacity pumps has significantly enhanced the bridge construction industry. These machines allow for precise control and monitoring during the grouting process, ensuring that the operation is carried out according to the required standards. With the ability to regulate pressure and flow accurately, these machines improve the overall quality of grouting, ensuring optimal filling of voids, better bonding, and enhanced structural integrity. The images below show grouting processes being done on pier shaft and pile cap connection, during the construction of the Nairobi Express Way and grouting of tendon ducts after post tensioning process for I girders for Mtwapa Market Bridge.

Discussion

Technological advancements in bridge construction have markedly enhanced the quality, durability, and efficiency of modern infrastructure projects in Kenya. Innovations such as precast concrete, prestressed systems, advanced grouting techniques, rebar couplers, and pot bearings have become essential components of contemporary bridge construction. These technologies facilitate faster project timelines, improve safety, reduce material wastage, and enhance overall structural performance.

Precast concrete revolutionizes the construction process by enabling off-site production of bridge components, which leads to superior quality control and decreased on-site labor needs. Prestressed concrete, particularly through post-tensioning and pre-tensioning methods, provides higher load-bearing capacities and allows for longer spans without intermediate supports, essential for large-scale projects like the Nairobi Expressway, Mbogolo Bridge, Mtwapa Market Roundabout Bridge and the upcoming New Mtwapa Bridge.

Rebar couplers effectively minimize steel congestion in high-reinforcement areas, improving concrete flow and reducing material usage, making them especially valuable in large projects with heavy reinforcement.

Advanced grouting techniques enhance post-tensioning applications by ensuring that tendon ducts are fully sealed, thereby extending the longevity of structures and preventing corrosion.

Pot bearings are crucial for managing significant vertical loads and accommodating large rotations in heavily loaded and skewed bridges, offering improved durability and lower maintenance requirements compared to traditional bearing systems.

These advancements, exemplified in key projects such as the Nairobi Expressway, Mbogolo Bridge, and the Mtwapa Market Bridges, underscore a shift toward more sustainable, cost-effective, and resilient bridge construction practices in Kenya. This progress represents a significant evolution from traditional methods, enabling the development of infrastructure that meets modern demands.

The images below illustrate sections of the Nairobi Expressway, as well as the Old and New Mbogolo bridges. These serve as clear examples of how technological advancements have significantly transformed the bridge construction industry in Kenya.

Conclusions

The technological advancements in bridge construction in Kenya have fundamentally transformed the industry, enhancing efficiency, durability, and cost-effectiveness. Innovations like precast and prestressed concrete, rebar couplers, advanced grouting techniques, and pot bearings have facilitated the construction of longer, more resilient bridges that address contemporary infrastructure needs.

These technologies not only expedite construction but also improve the longevity and safety of bridges nationwide. Projects such as the Nairobi Expressway, Mbogolo Bridge, Mtwapa Market Roundabout Bridge, and New Mtwapa Bridge exemplify the tangible benefits of these advancements. As Kenya continues to expand its infrastructure, the integration of these technologies will be crucial for ensuring sustainable, durable, and efficient bridge construction for future generations.

Acknowledgment

The author would like to acknowledge the Engineers involved in the construction of the Nairobi Expressway Project, whose invaluable insights contributed to the writing of this article. The author also acknowledges Tekfen Engineering, in joint venture with East African Engineering Consultants for permitting the use of their project as a case study in this article.

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This article was originally presented as a paper at the 31^st IEK Conference held between October, 29 to November, 1 2024, in Mombasa, Kenya. Click here to download the original article in PDF format.