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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

First introduced in the 1980s and now widespread in the United States, self-consolidating concrete (SCC) is a high-performance material designed to flow under its own weight, completely filling formwork, and achieving full consolidation, even in the presence of congested reinforcement. SCC has an advantage over conventional portland cement concrete in that it is easily placed without vibration or mechanical consolidation, thus reducing the labor cost associated with concrete placement.

Some concerns related to the formwork design have been raised because it's a common expectation that SCC exerts greater pressure on the forms. The formwork constitutes one of the major costs of reinforced concrete construction, and may even exceed the cost of concrete and steel materials combined. Failure of formwork always is dangerous and costly. Form problems also can create flawed surfaces that require expensive repair. Due to these concerns, SCC users for tall wall applications often adapt highly conservative assumptions that lead to limitations on lift height and overly strong forms. There is need for an effective, practical method to evaluate formwork pressure of SCC for tall forms to increase project efficiency and ensure safety.

Concrete pressure versus hydrostatic pressure

The common assumption for SCC formwork design is that forms must withstand the full hydrostatic pressure in the belief the concrete behaves as a liquid. For a liquid, pressure is computed as the unit weight times the height. This assumption limits the lift height—the taller the wall, the higher the calculated pressure. The instant after SCC or normal concrete is placed, it behaves as a fluid. As time passes, however, concrete progressively evolves into a solid, building up an internal structure capable of self support. Even before set of concrete, this gelation process reduces the pressure of concrete exerted against formwork.

Many factors affect formwork pressure. These factors can be classified into three principal categories: material properties, such as the characteristics of the concrete mixture; placement conditions, such as the procedure used in the field for pouring and consolidating the concrete, as well as other parameters such as weather; and formwork characteristics, such as dimensions of the element and formwork surface material.

Studies conducted at the University of Illinois found that a pressure decay curve can be established for any SCC mixture. This characteristic pressure decay curve accounts for all material parameters that influence how the SCC gels and becomes self supporting in the first few hours after placement. The decay curve also reflects temperature effects on concrete pressure. In the study, concrete pressure was measured using commercially available pressure sensors mounted at various heights in the form. Many different SCC mixtures were tested, encompassing a range of admixtures, temperatures, and aggregate gradation.

Formwork pressure model

Researchers at the University of Illinois have developed IlliForm, a computer tool for predicting formwork pressure when using SCC. IlliForm guides users when designing formwork and specifying maximum pour rates for field construction. Users provide several input parameters, including the SCC characteristic decay signature, formwork geometry, formwork strength, and form-filling rate. Before using the worksheets, the concrete pressure data from a representative sample of the SCC should be acquired. A 3-foot PVC test column is used in both laboratory and field testing to obtain the concrete pressure decay curve.

Tall wall application

The formwork pressure model has been employed in several projects involving tall wall SCC placement, one of which was in cooperation with Mortensen Construction on the OSF SFMC Milestone Project in Peoria, Ill. In this project, 40-foot concrete walls were poured using formwork rated at 1650 psf. If the full hydrostatic pressure assumption was imposed, the lift height would be limited to 11 feet, requiring more than three separate concrete placements. In actual practice, each wall was completed in a single continuous pour in one day.

To monitor the pressure exerted on the forms, sensors were installed at different heights. For instance, in the Table, Test 1-1 placed sensors located at 1, 7, and 24 feet above the wall foundation. The sensors were connected to a datalogger (see pictures on page 34). The 3-foot tall test column mentioned also was used for every wall pour to obtain the characteristic decay curve.