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RM50,000 Jetty - Why not concrete?

posted May 8, 2022, 5:53 PM by jeffery jim
I get this kind of question early in the morning. Well, I can explain it easily to those who are eager to learn and not make judgment like every learned people. Why don't JKR Sarawak use concrete instead as this is a cheaper option?

2. For those who are not familiar with construction, they may not know properties of this particular wood as we are now living in an era with soft wood being use for our furniture. It is understandable why many feels that this is an overspending exercise and to certain extend thought there is a misconduct or misappropriation by certain parties. Let me guide you to understand things that are factual.

3. Wood properties. As I have mentioned yesterday, Belian or Ironwood is categorized as S.G. 2 in wood group and considered as very hard wood. This particular wood is durable and comes with good engineering properties which people often forgotten. It is harder than most wood and have no problem to be used as scaffolding access, walkway and working platform.

4. Durability and susceptibility to attacks. Belian in one of the best woods when it comes to exposure to marine, especially borer bugs that infested riverine and estuaries. Hence, this provides better durability than concrete. If we look at the site and its remoteness in logistics, it is one of the best construction option available. Then this particular grain properties which is great against shear and other load-related exposure. Unlike reinforced concrete which highly dependent on reinforcement bars when it comes to tensile load.

5. Concrete is not as good as Belian. Commercial premixed concrete is not a good option for this kind of construction since most of these type of concrete are not specially designed with trial mix conducted. If one is familiar with concrete specification, typically a concrete will start to stiffen after 40 minutes and that is why concrete cold joints are permitted for work under 45 minutes with exception of trial mix extended to 3 hours like how we do it in Pan Borneo with proper documentation of retarding properties and procedures to ensure concrete is not stiffen and cause low workability. Imagine those hours required for a concrete mixer to transport heavy load through undulating and plantation road with high gradient. Dosing using water to increase workability is not permissible as this will increase water cement ratio and further reduce concrete durability.

Apart from that, lower grade concrete such as G30N have very high cement water ratio. This issue is a durability issue which was going to and fro for many years in concrete technology. For this particular matter, revisions of concrete durability from CP110 to BS8110 (several revision for Part 1) and Eurocodes have tried to fix this issues and is still a matter for debate. Now, this answers why substructures previously are required and additional 5kN/m2 to 10kN/m2 for foundation than the actual characteristic strength for the structure.

After hydration, CSH gel will form and microcrystalline will leave voids which make concrete very fragile against the ingress of corrosive elements from the saline water as well as in contact with pyrites from soil with poor conductivity. The lost of passivity of concrete allows corrosion and hence, the concrete jetty's integrity is in doubt. This is why Belian have been used in the older days for marine and boat construction.

6. Density is key. The other thing that one have to consider is the density and the bulk of transportation cost. Belian have a very low density between 9KN/m3 to 13kN/m3 while concrete is almost twice the density in the range of 19kN/m3 to typical 24kN/m3. This mean by volume of transportation, it is cheaper to transport Belian wood rather than all materials involved for a concrete jetty construction. With this kind of weight for the topside, imagine the bearing capacity required on soft ground when the dead weight is twice and that does not include the imposed load. Hence, there is saving for that when it comes to foundation criteria and design.

7. Constructibility. A Belian wood jetty is easily constructed where in this case, the jetty was completed in one month while the contract period allowed is 3 months. Every contract stipulates that time is of the essence and constructibility is one of the important elements which is required and dictate the project delivery and brief. Second, it is pricy to construct concrete jetty as concrete need to be constructed in stages. Each stage, there is a need for concrete curing window that may need 5 working days or maybe 12 working days depending on the concrete mix and cement used.

For QAQC or quality control, it is a hustle to bring concrete cube specimen for compressive test to Miri. Imagine 6 cubes weighing 8kg each to Miri just to test for material conformity in six different trips (7th day and 28th day). Counter productive? Now, how about reinforcement tensile test? How about bolts? Here is the thing or issue when it comes to QAQC.

Now, raw materials for reinforced concrete required to be logistically moved and as mentioned earlier, concrete delivery is already an issue. Now, bar bending process is also an issue as it may require mobilization of machines just to bend and shape reinforcement bars based on the schedule. Imagine the surplus of resources required just to stage the form of the concrete jetty. Why not use Belian wood which comes in various sizes conform to standard size which is in accordance to all dimensions required in construction drawing. Besides this, imagine the superintending and supervision required to ensure compliance of the construction.

8. Maintenance. It is much cheaper to replace decks or bracing parts which are degraded or dilapidated compare to concrete repair and rehabilitation which is labor intensive and much more expensive to maintain.

9. There are more to write but I believe this is sufficient to give laymen the logic behind the choice of Belian Wood as main material for this particular jetty construction.

May be an image of body of water and tree