When reinforced concrete is exposed to fire, both the concrete and reinforcement may be altered, which can result in structural and material damage. The repair strategy for fire-damaged structures depends on the severity of damage. While both can be characterized by laboratory testing of extracted samples (concrete cores and steel reinforcing), the scale of material sampling can often be very large, expensive and time consuming. The longer and hotter the fire, the more severe the damage to the concrete.
BHATE uses nondestructive evaluation (NDE) methods to understand the extent of damage followed with appropriate laboratory testing. Once the extent of damage is characterized by NDE, targeted laboratory testing can then be used to determine the depth and extent of damage to develop repairs.
The repair strategy for fire damaged structures depends on the severity of damage. At relatively high temperatures, there may be loss of compressive strength due to microcracking and expansion of concrete including thermal effects on yield strength of reinforcing steel.
Initial assessment of fire-exposed concrete structures starts with a review of the original construction drawings and specifications, if available, followed by visual inspection to identify signs of cracking, spalling, color changes, and surface scorch marks.
Non-destructive testing methods such as acoustic sounding, impact-echo and ultrasonic pulse velocity measurements can be used to verify the residual strength of the affected concrete and the condition of the reinforcing steel; this allows for a proper understanding of the structural integrity and necessary repair strategies.
Key aspects of a fire-damaged concrete assessment conducted by BHATE include:
- Qualified Construction Materials Professionals: BHATE has on staff construction materials professionals with significant NDE and laboratory evaluation experience in damage assessment.
- Visual Inspection: Observe patterns of cracking, spalling (concrete chunks breaking off), and surface discoloration. Look for signs of deformation of structural members.
- Non-Destructive Testing (NDT)
- Rebound Hammer or Windsor Probe Tests: Measures the surface hardness of concrete, providing preliminary indication of the damage depth.
- Impact Echo or Ultrasonic Pulse Velocity: Measures the travel time of ultrasonic waves through concrete, helping to assess the concrete’s integrity and identify areas of significant damage.
- Ground Penetrating Radar (GPR): GPR is used to locate reinforcement bars and assess the depth of damage in specific situations.
- Destructive Testing (Laboratory Analysis)
- Concrete Core Sampling: Extract concrete cores from affected areas to perform compressive strength tests and analyze the microstructure of the damaged concrete.
- Petrographic Examination: Microscope analysis to examine the aggregate composition, cement paste, and potential damage to the concrete matrix.
- Chemical Analysis: Investigate potential chemical changes in concrete due to high temperatures.
- Reinforcement Details: The size, spacing, and cover depth of the reinforcing steel influence the structural behavior under fire exposure.
- Repair Recommendations: Based on the assessment results, working in conjunction with the project Structural Design Engineer and Architect, a detailed repair plan can be developed, including necessary concrete removal, reinforcement replacement, and patching.