Lao PDR shares common borders with five other South East Asian countries. In order of descending border length1, they are Vietnam, Thailand, China, Cambodia & Myanmar. Though it is nowhere near an ocean, Lao PDR is fortunate to have hundreds of thousands of kilometers of fresh water flowing through three large rivers & their many tributaries. There are currently four large operating bridges connecting Lao PDR to Thailand over the mighty Mekong River.
Affectionately named ‘Friendship’ bridges, there is one in the north (linking Houayxay, Bokeo Province with Chiang Khong, Chiang Rai Province), one in the center (linking Vientiane, Vientiane Province with Nong Khai, Nong Khai Province) & two in the south (linking Savannakhet, Savannakhet Province with Mukdahan, Mukdahan Province & Thakek, Kammouane Province with Samat, Nakhon Pathom Province). The bridges have each been a tremendous success, as steadily increasing traffic transiting them will attest.
There are already plans in place to build three more bridges across the Mekong to Thailand: linking Bolikhamsai to Bueng Kan; Salavan to Ubon Ratchathani; & Vientiane Province to Loei Province. Construction on the first is expected to begin in 2019. Bridges across rivers are just one way to facilitate the free movement of people & products within our own country as well as moving forward to welcome the new economic reality of a vital, ‘borderless’ Asian Economic Community (AEC); resulting in increased travel, trade & commerce across borders with our nine ASEAN neighbors.
Lao bridge building employs common construction materials & methodologies. The type of material used to build a bridge depends entirely on the type of bridge being built, that is, the type of purpose the proposed bridge will serve. Here is a broad outline of the most common materials that have been used for the bridge construction.
Stone has been used for thousands of years, as it is exceptionally strong. Stone bridges have been built since the 13th century BCE. The Romans were probably the greatest users of stone in their structures in antiquity. They built bridges & aqueducts that were immensely strong & became stronger as they were loaded, due to the ability of stone (& brick) to endure large compressive forces.
Timber has also been used for a long time, but due to its weather-dependent nature, it can easily snap under pressure. Lao bridges have traditionally been used for very light traffic or pedestrian applications & have evolved from simple suspension bridges with floors made from tree limbs & branches, strengthened with twisted vines from nearby jungles.
As a Lao bridge building material, timber is abundant & highly sustainable. It was probably the very first building material used by humans for bridge building. Timber is strong, light-weight & has energy-absorbing qualities that have lent themselves favorably to smaller application structures. Currently, timber is being used on the ground to keep the railroad rails in place (known in railroad parlance as ‘sleepers’) in the construction of the 4,500 km Beijing to Singapore rail link. Tunnels, stations & 154 bridges2 will need to be completed in Lao PDR before the first high-speed train can ride the rails.
Concrete is perhaps the most widely used Lao bridge building or construction material. It is extremely strong in compression but must be fortified with rebar (reinforced steel bars or rods) to add strength as concrete eventually cracks. Proper curing of concrete is integral to its strength. We suggest a look at PCA’s excellent website for a thorough familiarization of the importance of the concrete curing process:
Iron. The first cast iron bridge was built in England in the 18th century & predated steel. Until the 19th-century iron construction material was either cast or wrought, or a combination of both. In the early 20th century the process of welding was discovered & applied to a bridge in Poland. However, iron didn’t have the tensile strength to support large loads, & with the invention of steel, rapidly gave way to it. Steel made its first entry into the construction of a bridge in the USA in 1883.
Steel in Lao bridge building has tremendous advantages. It is lighter, more ductile & more flexible. It’s also environmentally friendly as it is sustainable; recyclable (almost 100%); & virtually maintenance-free for 30 years. Steel, being an alloy of iron & carbon, is the 4th most common element on the earth, so its continued production & use is assured due to stable prices. Steel has also been made to be stronger in recent years. Metals are stronger under tension than under compression.
Corrosion3 is a serious threat to the long-term function and integrity of a steel bridge. Steel is much more resilient to corrosion, however, structural steel will corrode if left unprotected or inadequately protected from the natural environment. This corrosion can take the form of general uniform thickness loss or concentrated pitting depending on exposure to the environment and the steel design detail in question. Bridge designers should view corrosion as a long-term threat to the integrity of the bridge structure, and is a critical consideration that must be addressed in an intelligent manner during the design process.
While there are several proven strategies for corrosion protection of steel bridges, there is no universal solution. The proper system must be chosen to accommodate cost, fabrication and productivity, and long-term performance and maintenance. Additionally, each corrosion protection system must be selected based on the anticipated exposure of the structure to corrosive elements over its lifetime. Harsh environmental conditions such as annual monsoonal rainfall is a prime factor in Lao bridge building.
Galvanized Steel. Galvanization is one method of corrosion protection. It is a multi-step process of preparing & then dipping completed or fabricated steel structures in a bath of liquid zinc. The zinc coating provides a barrier to weathering & corrosion. Zinc is the 24th most readily available element, is natural & completely recyclable.
Composite, as applied to bridge building, used to mean making the best use of steel & concrete together, synergizing each of the materials’ best qualities to produce the best outcome; to build a better bridge. However, today the term ‘composite’ is increasingly used to describe Fiber Reinforced Polymer (FRP). There are already hundreds of pedestrian & vehicular bridges worldwide that are constructed of FRP decks entirely. As with many newer hi-tech materials, FRP offers corrosion & fatigue resistance, high strength, light weight, faster installation, lower maintenance & longer service life.
1 Border lengths between Lao PDR & Vietnam: 2,069 km; Thailand: 1,835 km; China: 505 km; Cambodia: 435 km; Myanmar: 236 km.
Source: Asia Pacific Parliamentary Forum, 17th Annual Meeting, Vientiane, Lao PDR, January 11-15, 2009
2 www.shanghaiist.com website
3 Reprinted in whole from the US Department of Transportation Federal Highway Administration Steel Bridge Design Handbook, ‘Corrosion Protection of Steel Bridges’ Publication # FHWA-IF-12-052-Vol. 19, November 2012