jumpform
A kind of formwork; a temporary mold, made from planks, into which concrete is poured, in order to create vertical concrete structures that rise with the building process.
jumpform: climbing mold that rises with each concrete pour
A jumpform is a self-climbing formwork system used to cast tall vertical concrete structures, typically in bridge pylons, tower cores, and high-rise buildings. The form consists of a steel or timber framework with facing materials (plywood or steel panels) that holds fresh concrete. Once the concrete sets to sufficient strength, hydraulic jacks or strand jacks attached to the form lift the entire assembly vertically, and the cycle repeats. The form literally jumps to the next casting height as the structure grows.
The system eliminates the need to dismantle and rebuild formwork at each level, which saves time and labor on large repetitive structures. A typical cycle takes 3 to 7 days depending on concrete strength requirements and lift height. Jump heights are usually 2 to 3 meters per cycle, though some systems operate on 4-meter lifts. The form self-supports its own weight plus the weight of wet concrete and temporary loads during the pour and setup phases.
How it works and what can go wrong
The form is held against the structure by a system of horizontal or radial supports (called yokes or brackets) that transfer loads into previously cast concrete. These supports must be released and repositioned as the form climbs. Proper sequencing is critical: concrete must develop enough strength to carry the form's dead load, plus any live loads from workers and equipment, before jacking begins. Jumping too early can crack the concrete or cause the form to tilt. Synchronized lifting across multiple jacks is essential to prevent binding or tilting.
Jumpforms require precise planning and coordination with concrete strength gain. Temperature, cement type, and admixtures affect cycle time. Cold weather extends cure periods and delays the next jump. The system also demands skilled operators familiar with hydraulic jacking procedures and load monitoring.
The term comes directly from the cyclic action: the form "jumps" upward once each casting phase is complete. This vocabulary contrasts with slip forms, which move continuously as concrete hardens, and conventional formwork, which is removed and moved by external equipment. Jumpforms are most economical on structures with sufficient repetitive height to justify the setup cost and the specialized labor required.