Characteristics of Reinforced and Post-tensioned Concrete Slabs during Construction

摘要

The temporary formwork system is one of the most important features of construction interms of safety and economy, especially in multi-storey buildings for which post-tensioned slab structures are very common. However, general practices for formworksystems, including removal times, in both post-tensioned and reinforced concretebuildings, have to comply with the standards established for reinforced concretestructures. Although a recommended formwork removal times for reinforced concretestructures is specified in these standards such as in Table 5.4.2 (AS 3610), there is nosuch table which can be easily put into practice by the industry in post-tensionedstructures. Nevertheless, some standards, such as AS 3600 and ACI 347R, specifycertain requirements for removal of the formwork system from under a post-tensionedslab.Post-tensioning causes the negative curvature of a slab segment to increase, whichincreases its cracking moment capacity in the service load range. This may allowformwork supports to be removed earlier from post-tensioned than from reinforcedconcrete structures. In order to improve current formwork operating practices in post-tensioned building construction, this study has examined the characteristics ofreinforced and post-tensioned slabs during construction by taking extensive on-sitemeasurements. Data was collected from two post-tensioned building constructions inCanberra, Australia – a residential 20-storey building and a commercial low-risebuilding. The average slab load distribution on a typical suspended post-tensioned slabwas determined by monitoring the prop forces, strains in the slab sections, deflections,and ambient and concrete slab temperatures over the construction period of foursuspended floor levels. The slab load distribution is generally represented by the ratio ofthe applied load to the self-weight of the slab and is referred to as the slab load ratio(SLR). There are several methods for estimating the SLR for the reinforced concretemulti-storey building construction. However, very little work has been conducted in thearea of SLR estimation for post-tensioned building construction. These SLR estimationmethods are usually applied to all formwork systems adopted by the industry in buildingconstruction in order to validate decisions regarding the removal of formwork supportsduring construction.SLR estimation methods originally proposed for reinforcediiiconcrete structures are not necessarily appropriate to post-tensioned structures as theybehave differently at the early age.This work proposes a new SLR estimation method designed specifically for post-tensioned structures. It is based on two methods: the Simplified Method used forreinforced concrete structures; and Kajewski's modified Method which has beenproposed for post-tensioning structures (Kajewski 1998). An analysis of the collecteddata is reported and is used in proposing a new SLR estimation method, which is moresuitable for post-tensioned structures. It is then applied to determine the loaddistributions of the post-tensioned slabs in the five most prominent types of formworksupport systems.The fundamental concept of this new method is that, if the maximum applied loadobtained from the calculated SLR does not exceed the cracking slab load capacity at thedesired removal time, the temporary formwork supports may be removed. However, if itdoes, the slab needs to be supported longer to achieve sufficient strength.Applying the proposed SLR method in the five most commonly employed formworksystems has enabled suitable formwork removal times for post-tensioned multi-storeyconstruction to be proposed. Under this proposal, the props or backprops at the lowestlevel of support for a suspended post-tensioned slab may be removed one to three daysearlier than they are currently, depending on the type of formwork system used. Theseproposed removal times are validated by investigating the serviceable limits of theslabs: allowable deflection and cracking moment.The particular significance of this work for the construction industry is its potential forimproving the economy and speed of construction. It also encourages the practice ofapplying more suitable shoring and reshoring removal time to post-tensionedconstruction. A table of prop removal time is provided for the use of post-tensionedstructures to go along with AS 3610 – Table 5.4.2 which is more suitable for traditionalreinforced concrete construction. The proposed new SLR method offers a greateraccuracy in determining the slab load distributions of post-tensioned structures in multi-storey building construction.