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Concrete Accelerator and Retarder; and Striking off

posted Jul 20, 2019, 7:42 AM by jeffery jim

Few days ago one of the stake holders' representative visited the site and requested if some of the bridge girder processes can be expedite in order to have higher productivity at site. I declined that sort of proposal without putting resistance with technical arguments since it is futile. I just told him, it should be reflective from a trial mix. Further to that, he is a respected figure and considered knowledgeable in bridge construction. A positive summation from literature and backed by trial mixes may point toward a desirable outcome. But that is less likely; it is like proving alternative hypothesis against a theorem.

Well, nothing wrong with high productivity provided that the package contractor is able to cope with the production rate and resource crashing. Resource crashing is not ideal since the process only happened for one quarter of the overall process which resulting in resource wastage three quarters of the time. It is important to leverage resources to be accessible at most time but also fit with allowable lag which does not affect the critical path. Further to that, specialized or highly skilled workers are tradesmen who cannot be deployed for multitask purposes. That is from planning point of view.

Expediting process in fabrication is achievable but at times, it will result in low productivity at the end of the day. Errors and the law of diminishing return will come in and spoil everything and even lead to economic or financial losses. So economics approach is dire important in fabrication or manufacturing for work in progress which have no significant impact on the overall project time.

When everything is assumed to be important and the production of concrete bridge girder need to be done in one day, there are trade-offs. Just like economics, we have it ceteris paribus where all the variables are set as constants except for the production speed. We all know that vertical surface of a beam can be removed in matter of days or hours but it all depends on the type of material used.

Accelerator can be used to increase the concrete setting speed but at the same time, the delivery of the concrete have to be timely before it start to set and become stiff. Unless these concrete girders are fabricated at the yard and the batching plant is a fathom away, this would be ideal. In less than perfect condition, the specification also allow delay in casting by 45 minutes by introducing superplasticizer and retarder. In some cases, time of delivery is extended up to 3 hours with the assistance of data acquired during the trial mixes. There is no certainty in productivity for in-situ concrete casting and therefore the concept of just in time (JIT) cannot be adopted without proximate consideration of encumbrance, impediment or delay during or throughout the process.

Of course it is a little bit obstinate to mix accelerator into concrete design mix which come with retarder and superplasticizer since accelerator's function is the opposite of a retarder. It may seem to be neutralize the effect but segregation of these compounds will affect the concrete curing and early development within the structural member and perhaps behavior at certain planar until 120 days. Mixing both will only benefit for learning purposes by monitoring the heat from hydration and monitoring the bleeding and stiffening of the concrete.

The striking off period also depends on the shape of the structural member. I-beams have better chance to cure independently without the need of formwork and will definitely stay in shape. It is a compressive member. T-beam however may succumb deformation due to additional stress at the joint between flange and web from tension or tensile stress which needed additional support(s) for the upper flange to avoid deformation due to self-weight. Although the flange is not critical during post-tensioning works, irregularities may affect the process of constructing the decking for the bridge and affect the desired level. If girder's hogging exceeded certain degree, these joints may remain in plastic state until it returns into equilibrium after launching and loading.

Sadly, interpretations are very much depending on how professional individuals deem the structure and cases to be. Beam with bottom support and rest on ground (bearing) may not need 4 days to strike but a T-beam definitely need props for additional days for the thin flange at the top.

The detail on concrete deficiency on early (curing) days and the case of strains will be the next topic.

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