EVALUATION and Repair of Roof slab - Post-Tensioning … Convention/Naja... · The subject...

EVALUATION & REPAIR OF CORROSION DAMAGED ROOF LIFT SLAB WITH WIRE POST-TENSIONING SYSTEM

(BUTTON HEADS)

2006 PTI TECHNICAL CONFERENCE –SESSION 2 - REPAIR WALID NAJA

(C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHT RESERVED 1 of 23

The following presentation discusses the review, evaluation and repair of a corrosion damaged post-tensioned roof lift slab. The process used may be applicable to many similar structures that are in service today throughout the country. The subject structure is a three story post-tensioned lift slab residential structure along the Central Coast of California. The 8” thick slabs were lifted in two sections (approximate section area of 5000 sq. ft). Built in or around 1960, the roof slab due to its location and a combination of poor detailing, lack of proper maintenance and old technology suffered serious corrosion deterioration to significant portion of its prestressing steel.



1. Visual Inspection and Site Assessment

2. Review of As Built Structural Plans

3. Destructive and Non-Destructive Testing to allow Inspection of

Post-Tensioning Wires, Mild Reinforcement, Anchorages, Embeds, Etc…

4. Structural Analysis and determination of possible Repair Options:

a. Evaluation of Original Capacity b. Evaluation of Existing Capacity c. Determination as to the Need of Repair d. Review of possible repair options e. Analyze roof slab with the different retrofit options

5. Execution of Chosen Repair Method

6. Durability Considerations



1. Visual Inspection and Site Assessment:

a. Scan the site for spalled or excessively cracked concrete b. Document locations of visibly broken tendons c. Inspect tendons live and dead ends locations d. Document locations of apparent rust / corrosion e. Review slab to column connections

It is worth stating that in this case, the extent of spalled concrete at the slab to column connections, prompted the engineer to require that shoring be installed at all locations with apparent damage. Further investigation was needed to assess the damage and determine the appropriate action.

Patched Area at Slab Collar



Spalled Concrete at a Broken Tendon near a Column



Broken wire Tendon exposed Upon Removal of Roofing Material

Broken Tendon due to severe corrosion



2. Review of As Built Structural Drawings

Often times, as-built plans for privately owned buildings are not well kept and finding structural information can prove to be a difficult task. In this case, a partial set of structural drawings for a twin building was located. Though certain connections were done differently based on the actual field conditions (slab collars to column connections in this case), useful information was obtained from these documents:

a. Material properties for concrete, mild reinforcement and prestressing b. Member sizes and reinforcement cover c. Reinforcement size and quantities d. Post-tensioning system used e. Post-tensioning forces and profiles

This information was vital in assessing available capacities and the extent of structure retrofit work. In their absence, a tremendous amount of destructive and non-destructive testing would be required to develop a sense of the slab capacity.



3. Destructive and non-destructive Testing to allow inspection of Post-Tensioning wires, mild reinforcement, anchorages, and slab collar to column connections:

Even though the as-built plans are helpful in providing the conditions of the building at the time of construction completion, destructive testing is essential in determining the condition of the embedded items within the concrete at the time of the investigation. Rust stains, spalling of concrete are the symptoms and exposing these various elements can expose the root of the problem and its extent. As the investigative engineer, it is our responsibility to develop a comprehensive testing plan that provide the necessary information to provide an opinion upon which an owner and his contractor can determine the feasibility of structure repair or replacement. Depending on the evidence of corrosion, sufficient number of tendons need to be exposed and examined at high points, and possibly low points as well near their anchorages. In the case of the roof slab herein, every tendon was exposed at one of their high points. Additionally, the anchorages at the tendons dead and live ends were exposed at several spots. Furthermore, all accessible slab collar to column connections were exposed at the slab soffit with some at the slab surface as well to ensure that the connection was not undermined by corrosive action of the salt water. Furthermore non destructive testing of the welds was performed (Magnetic particle imaging). This is important as the failure of these connections would be catastrophic. The result of the testing showed that the slab column connections were not impaired. However, due to the use of a continuous 8” wide pour strip at the tendons stressing ends, and pour water proofing conditions, extensive damage was observed near the tendons live ends and along the tendons near the slab surface (high points). The investigation showed that for the 202 tendons in the roof slab, there were 15 broken tendons, 64 corroded tendons and 123 rusted tendons (no significant pitting).



Exposed Slab / column connection (slab soffit)

Exposed Slab / column connection (roof slab surface)



Pitted Wire Tendon

Another example of Pitted Wire Tendon



Exposed Anchor at Dead End



Severely Corroded Anchor Plate

Completely Corroded ¼” wires at Live End of Tendon



Exposed Tendons at High Point of Profile



Preparation for lift off of a Wire Tendon



4. Structural Analysis and selection of retrofit scheme:

The structural analysis of the existing roof slab involved four stages:

a. Determine required reinforcement based on self weight and code prescribed live load slab. Also, compute service stresses and deflection assuming all PT tendons are fully effective (to establish the available level of redundancy).

b. Determine the present load carrying capacity based on the loss of the broken and the severely corroded tendons (based on all testing and investigation results). Determine need of repair and its extent

c. Review of possible repair options d. Analyze roof slab with the different retrofit options

a. Determine required reinforcement based on self weight and code prescribed live load slab.

This step is performed early prior to completion of the bulk of investigation of visual inspection but subsequent to the execution of visual inspection. It is important as it serves as a clear indication of the redundancy of the slab (specifically in lean designs where little redundancy is available). As lift slabs are designed for construction loads and the stresses they are subjected to during the lifting operation, the analysis, in this case showed that for the load (floor dead load and code live load), the concrete experienced no tension stresses for all practical considerations. This in turn allowed the engineer more options in the retrofit schemes.

b. Determine the present load carrying capacity based on the loss of the broken and the severely corroded tendons (based on all testing and investigation results).

The investigation showed that a significant portion of the tendons were corroded or broken (more than one third of the total). This portion of the analysis showed that the residual slab reinforcement, with the damaged tendons considered ineffective, is not adequate to support the roof dead and live load and that the slab needs to be repaired.



c. Review of possible repair options. Three repair options were considered. The first two options considered a long term solution to the corrosion issue. These options addressed the fact that all the tendons had incurred some damage (since the investigation did not look at the entire length of every tendon). Option one involves the introduction of a network of upturned beams to carry the roof slab and it called for a portion (to be determined by analysis) of the existing tendons to be replaced with 0.6” diameter strands. The second option called for all existing wire tendons to be replaced with 0.6” diameter strand tendons. The third option considered the immediate need for repair and called for all the broken or severely damaged tendons to be replaced using 0.6” diameter strand. d. Analyze roof slab with the different retrofit options The analysis of the roof slab using newly placed network of upturned beams showed that 25% of the existing tendons need to be replaced using 0.6” diameter strand tendons. With this option, the loss of all remaining wire tendons would have no impact on the capacity of the retrofitted roof system to carry its service loads. The analysis of the roof slab considering the replacement of all wire tendons with strand tendons did not yield positive results and led to the withdrawal of this option. The analysis of the roof slab considering the replacement of approximately 80 wire tendons (effective force of 46.4 kips) with 0.6” strand tendon (effective force of 40 kips) showed that the repaired slab met the code strength requirement for the roof service loads.



5. Execution of chosen repair method:

With option one and three to choose from, the owner opted to deal with their immediate needs and decided to repair the broken or severely corroded tendons. Since those tendons requiring replacement were spread out throughout the structure, the work was performed with little inconvenience to the residents. Tendons replacement was sequenced as to eliminate shoring of the slab. Tendons anchorage areas were chipped out and removed allowing access to remove the existing wires. The void was then pumped full with grease. Bare 0.6” diameter strands were further greased and pulled through the existing void. New encapsulated anchorages were applied in the field. High strength grout was poured behind the anchorages and the tendons were stressed and anchored.

New 0.6” diameter Strand Tendon at Live End



New 0.6” diam. Strand Tendon with Encapsulated Anchor at Live End



New 0.6” diameter Strand Tendon at Dead End



6. Durability considerations: It is interesting to note that the removed wire strands exhibited corrosion damage for portion that were closest to the top of the concrete (high points). Also, due to the type of roof water proofing moisture appeared to cause serious corrosion damage at the cold joint between the main roof slab and the stressing pockets of the wire strands (no grease was observed to be left at these locations). Additional concrete top cover and proper detailing of water proofing at the cold joint as well as periodic maintenance of the roof membrane could very likely prevented much of the corrosion damage observed.



EVALUATION and Repair of Roof slab - Post-Tensioning … Convention/Naja... · The subject...

Documents

Transcript of EVALUATION and Repair of Roof slab - Post-Tensioning … Convention/Naja... · The subject...