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What triggered such cracks?

posted May 8, 2022, 6:05 PM by jeffery jim

I saw this article and it is quite intriguing where some comments which were made by readers are controversial. Whose fault is it when there are such cracks grazing around the end of the beam? Is it the contractor, consultant or client?

2. Most of the comment made directed such structural failure to consultant. It is definitely convenient as it renders a perspective where cracks triggered when there is lack of reinforcement bar (stirrup or nominal links) at the said area. This led to the reaction of beam(s) under duress and sheared under loads (sic). This is most probable scenario for most structural engineers when they look at this particular photo in the first place. Is it really the case?

3. Typically, such crack patterns are related to shearing but it is hard to establish it is the outcome solely due to the reinforcement issue. There are other issues that one must consider before ultimately putting such failure as design failure.

4. Causal and outcome should be listed as possible mechanisms which trigger such crack. In this case there are several.

4A. Post Tensioning. The construction of such bridge utilizes a lot construction methodologies in post tensioning fabrication in comparison to typical in-situ short span bridge. In this case, usually tendons take precedence when talking about beam performance before the reinforcements. In this type of construction, tendon profile is essential and depends on the end of both tendons. If both ends are live end, then that is not an issue. Nevertheless, if one dead end, this should be visible when stressing is conducted. Failure during stressing where displacement exceed 6% should indicate the failure of tendon or the anchorage bulb (spiral).

4B. Another factor one should consider is the concrete characteristic strength and reaction during the curing period. Low concrete strength will have very low performance for most engineering properties. Second, the hydration and formation of ettringite which may trigger cracking when Delayed Ettringite Formation (DEF) occurs due to exposure of concrete under high heat and inappropriate curing procedure.

4C. The third possibility is the formation of alkali-silica reaction (ASR) or alkali-carbonate reaction (ACR). The low maturity aggregates maybe susceptible to chemical reaction and lead to crack formation. This incident is the initial mechanism which leads to cracks propagation and not the main reason for such failure to occur.

5. There are several other reasons which may propagate or trigger such crack but degradation in the long run. If we put all of these into a desktop study, it pointed all three parties; consultant, contractor and client as defaulting parties. Contractor might have failed to diligently check their work before commencing to another stage. The consultant have failed to supervised the job with low workmanship or failed to interpret test records and reports. Client? Yes. In this case, the client can be blamed if they failed to stipulate or mentioned about the suitability of materials in their specification and allows recommended material (in this case, aggregate) which is highly reactive to be used as construction material.

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