Temas Variados

Tailor Made Concrete Structures – Walraven & Stoelhorst (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47535-8

Slipforming of advanced concrete structures

ABSTRACT: Slipforming is a method of constructing tall concrete structures based on known parameters and proven technology. The method encompasses several activities and the successful execution of a slipform operationdepends on proper understanding of the mechanisms involved, careful planning and work preparation and the skills of the operator. Slipform operations are often executed by specialist subcontractors. Recent advances in concrete technology, pump line equipment and formwork design enable the method to be employed in the rapid erection of tall and demanding concrete structures, but schedule and qualityadvantages can also be achieved for structures of more ordinary size and complexity. The paper describes the important parameters for successful slipforming and contains some examples of slipforming of advanced concrete structures.

1 INTRODUCTION Slipforming is a highly efficient method of constructing tall concrete structures. Rates of construction of several meters per day of varying geometricalshapes and cross sections and containing multiple inserts and openings can be achieved within strict geometrical tolerances. Slipforming is labour intensive over short periods, but when properly planned and executed it offers significant advantages with respect to overall construction time, quality and safety. The method has made great progress during the past decades, particularly in relation tothe large concrete platforms and storage tanks delivered to the oil and gas industry and for tall concrete buildings, chimneys and pylons. Progress in the field of high performance concrete, formwork design and pump line technology are some of the key elements. Slipforming is a construction process of many interdependent items. The essential elements are proper planning, a well designed concretemix with uniform and predictable properties, a distribution system ensuring uniform and timely delivery of all components, and a strong and durable slipform assembly lifted at a predetermined speed commensurate with the setting time of the concrete mix. The system must have adequate robustness to cope with the changing conditions and occasional conflicts of interest for the many activities on aconstruction site.

For tall structures involving considerable overhead crane activities special attention must be paid to safety by proper fences and railings, clearly marked accesses and proper housekeeping on the work decks. In this respect a slipform assembly with a permanent lay out from start to finish offers some potential for improved safety compared to conventional forms installed insections and served by constantly moving crews and equipment.

2 THE BASIC PRINCIPLE OF SLIPFORMING The essential elements of a slipforming assembly are two parallel wall panels (about 1, 2 m tall) supported by steel frames and horizontal yokes connected to hydraulic jacks – ref. Figure 1. The jacks climb on vertical steel rods at a rate of some 15 mm per stroke. The spacing of the two panelsdetermines the wall thickness and can be varied by screw type controls. The geometry of the concrete structure can vary from straight walls to circular cells and towers of varying diameter and geometrical shape. The steel reinforcement, inserts and box outs are placed inside the wall panels as they are continuously being lifted by the jacks and as the concrete is placed in layers of uniform thickness.The main key to a successful operation is the ability to synchronise and control the concrete setting time

831

Figure 1. Typical slipform assembly.

Figure 2. Slipforming of the Telecom Centre Munich BRD (2003) showing large box outs and openings.

and the lifting rate of the assembly, so that the layers of concrete set and harden shortly before they emerge below the wall panels and not...