Client: Parvati Girls Hindu College – Block B | Post Tension Slab Foundation
Location: Clark Road, Penal
Contractor: NH International Caribbean Limited
In December 2013, numerous cracks were noticed in the ground floor slab of the classroom block of the new school. This lead to an eventual work stoppage by the project manager representing the Education Facilities Company Limited (EFCL). The EFCL hired an independent consultant to investigate the chain of events that led to the problem and propose a method of repair. Aleron Limited was not aware of exactly how much experience the independent consultant had in the design and construction of foundations using the post tensioning technology; but as the engineer of record for the design-build project, it was ultimately our responsibility to conduct our own independent investigation.
The project entailed the design and construction of post tensioned slab on grade foundations for blocks A (Administration), B (Classrooms), D (Multi-Purpose Hall) and E (Science Building) as well as the value engineered superstructures for blocks A, D and E. Aleron Limited had successfully designed and supervised the construction of several post tensioned slabs on several projects across the nation in both the housing and education sectors. The cracking of the unloaded PT slab on the Parvati Project was not only cause for much concern and but a phenomenon we did not have a solution for because we did not know what it was happening.
This unsettling situation prompted the immediate consultation with a post tension design guru based in the United States. It was revealed that the aspect ratio of the slab in question which was 7:1, coupled with the incorrect sequencing of stressing the tendons contributed to the cracking of the slab. With all the PT slabs done prior to this project, never before was a slab with an aspect ratio that made the slab somewhat very slim in nature was done before. Furthermore, PT construction as a value engineering tool was essentially a contractor led initiative and the specialist contractor was permitted to operate on site with very little oversight and quality control. The lessons learned from this incident resulted in Aleron Limited taking the following actions on all future post tension projects;
- Using construction joints to acquire an aspect ratio of less than 4:1;
- Becoming a certified level 1 field fundamentals PT installer/inspector;
- Preparing a detailed technical specification and installation manual;
- Preparing fabrication drawings to accompany all PT construction drawings;
- Upgrading the construction drawings to include internationally recognized details and notes;
- Preparing tendon stressing and elongation records for use as a quality assurance tool;