Precast Concrete Elements Quality in Precast Concrete Design
Principles of Precasting
Experience has identified a series of detail considerations critical to the success of precast operations. These must be studied if the advantages claimed for precast concrete construction are to be achieved. They include those given under the headings Precast Concrete Elements Quality below.
In design
1. The producer must be consulted early in the design stage
2. Elements must be designed for precasting, not modified from insitu practice.
3. Design should be based upon tried and established precast practice, utilizing preferred dimensions where possible.
4. Methods of connection and joint detail must be suited to the conditions in which the work is to be carried out at site,
5. Connections must be arranged to ensure instant Erectability” freeing expensive cranage for further duties.
6. Tolerance on fit must be clearly established, whether a high degree of accuracy and fit is intended or whether wide tolerances are acceptable.
7. Early validation of detail and establishment of quality must be achieved by the erection of a mock up or model of the structure.
8. Detail should incorporate a safe method of erection.
9. Handling arrangements must be suited to both production and erection requirements.
10. Interaction between elements must be such that component parts of the structure complement each other,frame edge detail and cladding, for example.
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In production
1. Product standards must be established at an early stage and made clear to production personnel by samples at the work place.
2. Moulds must provide optimum number of reuses, work in mould alteration being balanced against costs of in-built provision for variety.
3. Casting programmes and mould alterations must be clearly established and adhered to by production and service departments.
4. Daily casting cycles must be maintained in order that service over-heads and those of heating, lighting and similar costs may be absorbed.
5. Concrete must be available on demand. Back-up arrangements must be established to ensure supply in case of breakdown.
6. The clear shed principle: production centres must be self-sufficient in handing equipment. Production teams must be able to strip and prepare moulds unhampered by previous production.
7. Only finished products must be transferred into the stackyard. All stock must be clearly identified and its inspection status clearly indicated.
8. Works problems must not be passed on to the site such as by delivery of substandard products.
9. Constant feedback must be maintained on quality and outputs, site to works and quality assurance department to works.
10. Production personnel must receive clear instruction on the safety implications of security of embedded fixings and fastenings.
In Transit
1. Vehicles must be suited to the loads they are to transport and to the access conditions at site.
2. Routes to site must be clearly defined.
3. Cradles and stillages must be provided to support elements at designed position, avoiding wracking and damage.
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In erection at site
1. Prior to commencement of erection a survey must be carried out of the accuracy of foundations, fixings and similar work carried out by the contractor, and corrective work to be carried out before erection commences.
2. A method statement should be prepared, indicating the methods to be employed in erection and the responsibilities of those concerned.
3. Site personnel must be instructed in erection techniques tested during construction of mock-ups and models.4. Shear walls and bracing members, whatever their construction, must be installed as work proceeds.
5. There must be adequate access for the selected means of delivery to site.
6. An engineer or draughtsman fully conversant with the design and detail should attend at site during early stages of erection.
7. All temporary works and supports must be properly designed and detailed.
8. A named person must be made accountable for the quality of the workmanship in jointing and weather proofing.
9. Erection should, as far as possible, be carried out from the delivery vehicle, avoiding the build-up of a site stock of units vulnerable to damage.
10. Handling must be carried out using spreader beams and carefully designed lifting connections; All fixings for braces and props must have been proved in the works.
11. Adjustment to detail, cutting of elements or modification of connections must not be allowed at site other than under the instruction of an engineer.
12. Clear and concise progress reports must be made to production; site stocks of elements and any substandard elements must be clearly identified and works informed accordingly.
