The sudden deflection of part of the fourth-floor post-tensioned deck slab in a parking garage owned by Montgomery College in Takoma Park, Maryland, prompted emergency closure and shoring of the structure.

  • Name of the owner
    Montgomery college
  • Name of the client
    Plano-Coudon LLC
  • Delivery date of the project
    August 2014

Structural assessment and damage report

A full structural survey of the five-storey building, which dates from 1979, was subsequently carried out to assess its structural condition and identify what repairs were needed.
The condition of the post-tensioned concrete structure was revealed to be severely deteriorated, with many of the tendons being corroded in the vicinity of the construction joints. The most significant problem was found on the fourth floor where one of the slabs had deflected by some 25 mm across the full length of the joint.

Time constraints

An action plan to bring the garage back into use was established, but work on site had to be planned to minimize the disruption to students and was restricted to the summer vacation period. The footprint of the garage measures 71 m by 66 m with post-tensioned slab span lengths of approximately 16 m and 2.7 m height between floor slabs.
Specialist contractor Freyssinet was invited to bid for the work as a result of having previously been in contact with the building owner, and to propose appropriate techniques and procedures for the work.

Three main types of repairs were required; post-tensioning tendons and intermediate anchors in the sunken slab had to be repaired or in some cases replaced, patches of concrete delamination or spalling had to be repaired, and epoxy injection was required for some concrete repair.
A silane sealer was subsequently applied to the deck slabs to prevent them being further damaged by water contaminated with chlorides – and new drains and pipework was installed to assist with this.

  • 155
    PT anchor repairs
  • 17,700
    m² of area shot blasted and sealed

Pocket treatment

A three-month programme of work was carried out with the garage closed to the public, enabling pockets to be opened up in the concrete in order to lock-off and de-stress the existing PT tendons before new ones were installed, with new intermediate anchors or ‘dog-bones’. The same tendons were then stressed back to full capacity. Repairs were needed on all levels of the building, to a greater or lesser degree, and on columns as well as deck slabs.