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How Tall, How Fast, How Safe?

How Tall, How Fast, How Safe?

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    One of the 40-foot-tall walls for SCC placement.

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    Formwork pressure measuring system

For the purpose of calculating pressure head in the model, the filling rate for each test was measured as listed in the Table. Also listed in the Table was the maximum pressure recorded by three sensors from the tests. Using the formwork pressure model, we can obtain the estimated pressure exerted by SCC onto the forms. During these tests, several observations have been noticed.

1. Pressures jump when localized disturbance occurred. For instance, hammering on form walls and other sources of vibration reliquefy the gelling concrete and therefore increase pressure.

2. The drop chute from the concrete pump creates vibration. If the drop chute was slightly submerged below the concrete surface, agitation is reduced and a more laminar, smooth flow of concrete occurs.

3. Interruptions to concrete pumping operations were not beneficial. Steadily flowing SCC reduces the initial pile up of concrete by the drop chute and improves the laminar flow of concrete.

Recommendation for form filling rate

The IlliForm formwork pressure model can help engineers to determine SCC placement rate. The pressure predictions can be calculated for various pouring rates. Faster filling rates produce higher maximum pressures. As a rule of thumb, temperature is a significant factor, and filling rate in summer could be faster than that in winter when using the same SCC mixture.

Summary

SCC is emerging as an increasingly popular type of concrete for fast, tall, and safe applications. A new SCC formwork pressure model provides engineers a rational method to avoid overly conservative assumptions when designing formwork and planning for construction. As such, IlliForm assists decision-making, economic optimization, and improvement of construction safety.

David A. Lange is a professor and Yi Shi Liu is a graduate research assistant in the Department of Civil and Environmental Engineering at the University of Illinois at Urbana-Champaign, Ill. This article was originally published in CONCRETE CONSTRUCTION in November 2008.

To learn more about this topic, visit CONCRETE CONSTRUCTION'S Articles section.