This paper discusses the effects of fire on concrete and provides a methodology to assess, evaluate and repair concrete structures. Though concrete is among the best fire-resistant building materials, it does get damaged when exposed to fire. Depending on the intensity and temperature of the fire, concrete can suffer various types of damages ranging from minor cosmetic blemishes to severe cracking, spalling and chemical changes. Such damage may affect the structural integrity of a concrete structure. Thus, fire-damaged structures require damage assessment, evaluation, and repair. This process includes a comprehensive evaluation consisting of visual inspection, nondestructive testing (NDT), and laboratory testing. The objective of the evaluation is to identify the type and extent of fire damage, as well as any changes in the physical or material properties in the reinforcing steel and concrete. The process of assessment evaluation and repair of a fire-damaged structure is demonstrated through a case study of a wastewater treatment plant that suffered extensive fire damage.
Concrete is highly resistant to fire damage, primarily due to its high specific heat capacity and its poor thermal conductivity (slow rate of heat transfer). This protects the concrete from a rapid increase in temperature and minimizes damage. Unlike other building materials, fire-damaged concrete members are usually reused after evaluation and repair. Thus, substantially reducing the cost and time lost in demolition and rebuilding. A proper and thorough assessment of fire damage is required before any repair design can be developed. This article presents an overview of how to conduct an evaluation of a fire-damaged structure. A case study of evaluating and repairing a fire-damaged waste water treatment plant is presented.
In the aftermath of a fire, several critical questions must be investigated. Is the structure safe for use or entry? Immediate measures must be taken for securing public safety. What is the extent of the damage? What is the effect of fire on the various materials? How are the service life and long-term durability of the structure affected? What repairs are required? The answers to these questions are derived from a comprehensive assessment and evaluation of the fire-damaged structure. The results of the structural evaluation will dictate the type and extent of the required repairs.
Guidance for the assessment and repair is available in ACI 562 “Code Requirements for Assessment, Repair and Rehabilitation of Existing Concrete Structures”1. ACI 562 provides assessment, design, construction and durability requirements for repair and rehabilitation of existing concrete structures. The requirements of this document are mandatory where it is adopted by local jurisdictions. A proper investigation requires a thorough understanding of the effects of fire on various building materials. This paper will concentrate on the effects of fire on concrete and reinforcing steel.
Effects of Fire on Concrete
Depending on the intensity and temperature of the fire, concrete can suffer various types of damage ranging from minor cosmetic blemishes to severe cracking, spalling and chemical changes. The high heat resistivity of concrete prevents the rapid travel of heat within the concrete interior mass; thus, the exterior face temperatures decrease rapidly with concrete depth. Thus, the damage is usually confined to the near surface zones. The effects of fire on concrete are summarized in Table 1.
Reinforcing steel bars exposed to temperatures up to 1100 °F (≈600 °C) can lose approximately 50% of their yield strength. However, they will recover almost all their strength upon cooling. Exposure to higher temperatures beyond 1100 °F (≈600 °C) results in a reduced yield strength after cooling. The behavior of prestressed strands with temperature is more complex. In addition to the reduction of yield strength, permanent relaxation losses in the steel can occur. These relaxation losses are not recovered upon cooling. At temperatures 400 to 800 °F (≈200 to 425 °C), prestressed steel shows considerable loss of strength. Lower than 400 °F (≈200 °C) temperatures show minimal loss of strength (National Codes and Standards Council, “Fire Protection Planning Report”, August 1994)2.
In general, the absence of buckled or distorted exposed bars may indicate that the steel most likely does not reach 1100 °F (≈600 °C) and the concrete spalling most likely occurred after the fire. Reinforcing bars that lack signs of severe distortion are unlikely to have suffered significant permanent reduction in yield strength. Similarly, if the spalling does not extend to the steel (cover remains in place), the structural strength of the concrete member is relatively unaffected.