Comprehensive condition assessment program on the fire damaged structure – a project case in Singapore

A fire damaged structure at the eastern part of Singapore was assessed. Some concrete spalling exposing corroded steel reinforcements were noted on the post tensioned concrete beam and reinforced concrete slab, raising a concern about the structural integrity of the overall floor. A comprehensive condition assessment was performed on the affected structural elements to determine the extent of the damage, which included some on-site destructive and non-destructive tests as well as some laboratory testing on the collected concrete and steel samples. Testing data revealed that the concrete was still in consistently good condition with the average residual compressive strength of 36.51MPa. Petrographic examinations suggested that the top 5mm of the concrete surface might be exposed to a temperature not more than 450o C. The steel reinforcement and post tension strands were found to be still in good condition as well. The findings of this assessment will then be used for further structural assessment to determine the most effective structural rehabilitation program.


LITERATURE REVIEW
• The extent of concrete damage, such as carbonation depth, the existence of void and micro cracks, and estimation of concrete temperature during a fire, can be examined using Petrographic Examination on the concrete core sample [ASTM C856-04] • Although the concrete surface may look to be in a good condition, with no crack and spalling, some internal separation (delamination) may occur, which is quite dangerous if not properly assessed. Structural repairs are required for this area to prevent concrete spalling in the future. Acoustic impact testing was used to detect the concrete area with shallow delamination [ASTM D4580] • The residual concrete compressive strength might be the most important thing to be assessed to ensure the affected structures still have the required capacity to take the load. The compressive test was conducted on the extracted core samples [BS 1881: Part 120]

LITERATURE REVIEW
• After the fire reached a certain temperature, the steel mechanical properties, including its tensile strength, ductility, and hardness will change. The tensile strength test is required to determine the residual strength of the steel rebar to ensure that it has the required strength as per design requirement. Steel bend test is one method to qualitatively evaluate ductility. Vickers hardness test is used to evaluate the steel rebar hardness

Petrographic Examination
• Petrographic Examination was performed in accordance to ASTM C856-04 on a ground section using a stereo microscope and on a thin section with a polarizing and fluorescent microscope (PFM), under transmitted and reflected light.
• Through an examination of the ground section, the assessment was made on the homogeneity of the concrete, compaction and types and distribution of large particles.
• Under transmitted light on the examination of a thin section, various components (type of cement and aggregates), air voids content, compaction pores and damage phenomenon in the sample were identified.
• Under reflected light, the fluorescent microscopy made it possible to study the homogeneity of the mix and the cement paste, capillary porosity, micro cracks and other defects in the sample.

LITERATURE REVIEW
Acoustic Impact Testing • Using the principle of emission of elastic sound waves, the impacted surfaces exhibit either a sharp metallic ring or a dull hollow sound representing "sound" and "unsound" concrete conditions, respectively

Steel Bend Test
• Steel bend test is one method to qualitatively evaluate ductility. It is done by bending the steel sample to a 45 o angle and then heating it up to 100 o C for at least 30 minutes. After it cools down the specimen is re-straightened to at least a 23 o angle and it should not show any damage.

Steel Hardness Test [Vickers Hardness Test]
• Vickers hardness test is used to evaluate the steel rebar hardness. A constant force of 10kg is often used to obtain the Vickers Hardness Value (VH10), which can be indicatively correlated to give an estimation to its yield strength.

CONDITION ASSESSMENT
3.1.1. Visual Inspection • Accessible areas of the concrete structure were visually examined.
• Some concrete spalling exposing corroded steel reinforcements were noted on the roof storey beam and slab soffit. • No sign of concrete defect was found on the 3 rd storey beam and slab where the fire occurred. • No damage was noted on the PT Tendon ducts, even at the most severely spalled concrete.
3.1.2. Acoustic Impact Testing • Generally, unsound (i.e., delaminated) areas were in the immediate proximity of cracks in the beams. No concrete delamination was noted outside the spalled concrete area.

Ferroscan Pachometer Survey
• The ferroscan pachometer surveys were performed to estimate the core sample locations. • No scans were performed on the concrete surface with exposed rebars as the core sample location can be visibly determined.

Concrete Core and Steel Rebar Sample Extraction
• Fifteen (15) concrete core specimens were collected using wet rotary diamond core drilling techniques at selected locations. Concrete core samples were visually examined and photographed prior to concrete laboratory testing. Concrete core holes were patched with shrinkage-compensating repair mortar subsequent to sample collection. • A total of nine (9) steel rebar samples were collected on site. Seven (7) samples were collected from the roof level which is grade 460 rebar, and two (2) samples (3B1 and 3B2) were collected from the 3 rd storey level which is A6 BRC. The collected steel samples were sent to the accredited laboratory for further laboratory tests

Petrographic Examination
• Petrographic examinations were performed on seven (7) submitted core samples to determine the extent of the concrete damage. All samples were analyzed starting from the sample surface exposed to the fire. • Carbonation was noted within the 5mm depth.
• Some micro cracks were noted on the cement paste. Some of these were not fire-induced cracks which occurred before the fire.