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 form-work 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.
Formwork pressure measuring system
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.
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.
- 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.
- 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.
- 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.
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.