Understand The Tension

Post-tensioned reinforced concrete – Yup, that’s a mouthful. Most insurance professionals are familiar with conventional reinforced concrete (rebar) used to construct slabs, walls, columns and piers. But how does concrete reinforced with post-tensioning steel differ from the conventional system? And what are the unique post-disaster considerations for this type of construction?

Post-tensioned concrete was first developed in the late 1950s in Europe. Over the years, it has been used in parking garages, bridges, storage facilities, stadium/grandstands, offices and apartment buildings to create structures using less concrete and with fewer cracks. Post-tensioned concrete is generally more versatile than conventional reinforced concrete, but it has benefits and disadvantages.

For example, it requires fewer support beams and columns, thus offering greater design flexibility. It also uses thinner slabs (less concrete) with overall lower building weight. However, post-tensioned reinforced concrete is a more complex structural system that is susceptible to deterioration from moisture. It is also vulnerable to tendon breakage and sudden ruptures, which necessitates more frequent inspections.

There are two basic types of post-tensioning systems – unbonded and bonded. Unbonded post-tensioned systems typically comprise a series of high strength steel strands formed by 7 braided wires packed in the grease-filled plastic sheathing, which forms a tendon. Bonded post-tensioned systems are composed of steel strands that become encased in the poured concrete (no sheathing or grease) and are typically stressed before the concrete is placed. In North America, unbonded systems are more commonly used in building structures; as such, this article focuses on this method.

Unbonded post-tensioned concrete is an “active” technique of reinforcing concrete whereas conventional steel reinforcement (rebars) is considered a “passive” method. The post-tensioning tendon with the plastic sheathing is placed into the formwork and then the concrete is poured around it. After the concrete has sufficiently cured/hardened, the steel tendons are drawn tight (stressed) by a hydraulic jack placed at one or both ends of the tendon. Then, the steel tendons are secured by steel-casted anchoring systems at each end and grouted.

The tendon ends when locked/secured engage the concrete by a steel cast anchoring system and transfers the tensile force in the tendon into a compressive force in the concrete. This intentionally compresses the concrete structural member (i.e. slab, beam). Because of this “active” reinforcement method, it is inherently more dangerous in a post-accident or post-disaster situation should the structure become damaged. The tendons could de-stress and shoot out of the end of the slab or out of the surface or underside of the slab, which would not only affect the structures integrity but also cause potential injuries or fatalities.

An insurer’s concern should start with proper knowledge if a structure is post-tensioned and then the proper assessment and remediation of damage post-tensioned tendons should a catastrophic event occur or in case of accidental overloading. However, during renovation work, a contractor may inadvertently drill through a concrete slab cutting through a stressed steel tendon, potentially causing injury as well as creating a structural deficiency (see image below).

The biggest concern with post-tensioning, aside from physical damage, is exposure to moisture. If moisture gains access into the sheathing, the protective grease will break down and the high strength steel strands could suffer stress cracking. In older post-tensioned structures, if water gains access into the system through leaking walls, roofing systems, balcony slabs or failed garage membranes, the strands will experience corrosion (which may or may not be excluded from an insurance claim).

In a flood, assessment and remediation efforts on a post-tensioned structure should go beyond the visible surface elements of the structure. The tendon sheathings could become contaminated and lead to tendon corrosion or allow contaminants into the tendon sheathings. Cleaning up and drying the exterior of the concrete structure is not enough to remediate the flood damage in post-tensioned structures.

After fire exposure, the steel tendons could be heat damaged and, as such, no longer capable of carrying the required tensile force. Assessment of fire damaged post-tensioned concrete structures will involve observing many of the tell-tale signs indicating concrete damage from fire or heat to that of conventional reinforced concrete. These include pink/red discoloration (determined to be from iron content in aggregate oxidizing in documented studies) of the concrete if it was exposed to a fire above 300deg. C, surface crumbling and layer separation called spalling and delamination.

Signs of fire damage unique to post-tensioned concrete structures would be tendons undergoing relaxation and a loss of strength, melted plastic sheathings on tendons or contaminated water containing chemicals from fire suppression activities becoming trapped inside the tendons. This could affect the condition of the tendons. Under normal circumstances, cutting a hole in a post-tensioned concrete slab has to be done with great care and usually with the aid of non-destructive techniques, such as x-raying or Ferroscanning.

During fire suppression activities, fire fighters, unaware that a roof slab or apartment building is post-tensioned, may cut holes through roof decks for venting. A tendon under high tensile force could be severed causing injury to those nearby or compromising the structural integrity of the roof slab. As such, it is crucial for fire-fighting departments to be aware if a building structure is post-tensioned due to the inherent risks with these systems.

Accidental overloading could involve a loaded concrete truck or large garbage truck accessing a suspended parking garage slab or podium deck surrounding a building with no vehicle height barrier or signage. Overloading could also occur from building debris collapsing onto or piling up on the post tensioned slab such as during a fire.

It is important for insurance adjusters to know whether their insured’s building/structure has post-tensioned concrete for risk management. In a catastrophic event (fire, flood, vehicle impact into a structure) adjusters are the representatives of their insured client and often the first to speak with emergency personnel. Merely mentioning that the structure is post-tensioned could be seen as vital risk management step to prevent a property loss from turning into a much greater disaster.

In post-fire forensic investigations, a forensic structural engineer experienced with post-tensioned concrete would provide invaluable assistance to a fire investigator. Working in conjunction, they may even complement each other efforts to achieve their own objectives.

Forensic investigation of damaged post tensioned structures requires special techniques and expertise. Repair/rehabilitation also poses challenges unique to the construction industry that requires a level of flexibility, creativity and ingenuity beyond normal design and construction. Canada Mortgage and Housing Corporation (CMHC) commissioned a report entitled Investigation Protocol for Evaluation of Post-Tensioned Buildings, which provides a basis for forensic investigation of damaged post tensioned concrete structures.

Since the early 1980s, Read Jones Christoffersen Ltd. (RJC) has developed assessment and rehabilitation techniques for post tensioned concrete structures at risk. Through assessment and monitoring of several hundred structures across North America, RJC has gained unique insights in predicting the performance of these types of structures and responding with repair and protection programs.

Derek Gibson, B.Eng, P.Eng., is a Project Engineer with Read Jones Christoffersen Ltd. (RJC) specializing in forensic engineering. Philip S
arvinis, B.A.Sc., P.Eng., is the Managing Principal of the Building Science and Restoration practice areas in RJC’s Toronto office and President of the Building and Concrete Restoration Association of Ontario.