Demolition Vs. Cfrp Strengthening: A Brief Life-Cycle Cost Evaluation
IntroductionA large portion of old structures all over the world, specifically bridges, require immediate repair or demolition due to aging. The deterioration of bridges occurs due to environmental conditions, climate, location and usage. Damage to bridges can include the following:
- Deck deterioration
- Corrosion of steel or of steel reinforcement within the concrete when a bridge’s piers are in direct contact with water
- Aging and deterioration of materials, as well as problems due to dynamic response (wind or earthquake)
Therefore, many bridges need to be repaired and strengthened or demolished and replaced with new ones. Demolishing existing bridges and building new ones would be so expensive and time consuming that it is not a viable option. However, it is possible to use long-lasting methods of repairing and strengthening which result in low maintenance bridges at a reasonable lifecycle cost (LCC).
The usage of CFRP in bridge construction and repair has increased over the past few decades, and it is a great choice for strengthening existing bridges.
Figure 1: Demolition vs. CFRP strengthening of a bridge
ADVANTAGES OF CFRP THAT DIRECTLY OR INDIRECTLY REDUCE THE CUMULATIVE COSTS OF BRIDGE CONSTRUCTION AND REPAIRAside from the outstanding mechanical properties of CFRP, here are some of its advantages that can reduce bridge repair costs:
- Reduces the dead load of bridge decks by about 80% so they have a higher seismic resistance.
- CFRP is prefabricated so its installation is completed at a higher rate therefore reducing costs.
- No need for heavy lifting equipment and requires smaller construction vehicles.
- Lightweight, easy transportation and lower labor costs and construction time.
- Lower traffic disturbance as the repair can be completed quickly.
- Resistant to aggressive environmental impacts and high temperatures.
- Corrosion and frost resistance (freeze-thaw cycles).
- Non-magnetic so it has high dielectric strength and acts as an insulator.
- Low thermal conductivity. Due to the low coefficient of thermal expansion in CFRP, its thermal conductivity is about 200 times slower than in conventional materials.
- Long-term durability as well as a high resistance to chemicals results in minimal maintenance.
SUSTAINABILITY AND ENVIRONMENTAL IMPACTS OF CFRPDue to the importance of bridge sustainability, the environmental impacts of bridge design are considered during the decision-making, design and repair phases.
Negative impacts of bridges on the environment are as follows:
- Global warming potential (GWP)
- Ozone depletion (ODP)
- Terrestrial acidification (AP)
- Freshwater eutrophication (EP)
- Fossil depletion (FD)
- Human toxicity cancer (HTC)
- Human toxicity non-cancer (HTNC)
- Ecotoxicity (ET)
One of the major negative effects of demolition and rebuild is the emission of carbon during these phases due to the increased use of energy. There is also a high usage of raw materials for reconstruction, such as water, concrete, steel, timber, glass and many other materials. 35% of all waste in the UK belongs to the construction industry, which is another major environmental disadvantage of demolition. ²
By using recyclable CFRP to strengthen existing or to build new structures, we can help the environment by reducing waste material.
DISADVANTAGES OF DEMOLITION
- Bridge demolition is a high-tech task. While planning to demolish a bridge, engineers must be extremely careful to have minimal debris escape onto roads or other surfaces such as railway tracks, waterways, etc.
- Demolition is truly a complicated task, for example bridges are made of different materials, each of which requires its own method of demolition.
- When compared to repairing, demolition is very costly. One of the most expensive parts of demolition is cleaning up the debris of a demolished structure.
- Overall, demolition generates dust (that sometimes contains asbestos), gas, noise and vibration. Consequently, the risk factor on the health of inhabitants in the area increases considerably after demolition.
- High cost and labor to remove the demolition waste and/or the high cost and labor to bring in the large quantities of water and materials required to rebuild a structure.
LIFE-CYCLE COSTS OF BRIDGESLCC (Life-Cycle Costs) is a technique of assessing the costs that occur over a building’s lifespan, from construction, through operation and maintenance/repair, to end-of-life and disposal.
In an LCC assessment of a bridge, its design lifespan is generally considered to be 75-100 years, but considering the durability of the pavement, it is less than 40 years in reality.
In the figure below, the life-cycle cost composition of bridges is shown. ³
Figure 2: The composition of life cycle costs of bridges
By strengthening a damaged bridge with CFRP, LCC can be reduced as follows:
- The inspection costs will be reduced due to a reduction in man-hour costs (for instance, 2 workers can inspect a bridge in one day).
- Since there will be no limits imposed on the bridge’s road during the inspection, there are no user costs (See figure 2).
- In CFRP-reinforced deck bridges, in the case that there is a need to partially resurface areas, small surface corrections can be done at night, and therefore “user costs” are removed.
- The whole CFRP-reinforced deck needs to be resurfaced every 25 years, which means that its sustainability is significantly higher than steel-reinforced decks.
Figure 3: Strengthening a damaged concrete bridge girder with CFRP strips
CONCLUSIONCFRP strengthening is the more cost and environmentally effective alternative when compared to demolition/rebuild. CFRP-strengthening is also more economical than other retrofitting methods such as welded steel jackets, internal strand splices, external post-tensioning and replacement of damaged girders. These other techniques are heavy in weight, labor-intensive and vulnerable to future corrosion and traffic disruption which increase a bridge’s LCC in comparison to CFRP.
The initial construction cost of a CFRP-reinforced bridge is higher than a traditional steel-reinforced bridge; however, when the first significant deck repair is needed on the steel bridges in year 20, the cumulative cost exceeds the cost of the CFRP bridge. ³ By using CFRP reinforcement, the agency LCC is reduced by 12% compared to epoxy-coated reinforcement and reduced by 23% compared to black steel. ⁴
Using CFRP to repair and strengthen existing structures and/or to build new structures is not only the most cost-effective technique, but also the fastest. It is easier to achieve the desired strengthening of a structure using CFRP.
Parastoo Azad and Dr. Mehrtash Soltani (July 30, 2021)
Parastoo Azad and Dr. Mehrtash Soltani (July 30, 2021)
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