THINGS TO CONSIDER WHEN OPTING IN OR OUT FOR BUILDING PAD REMOVAL
The need to later remove the pad after FEMA is done and prior to reconstruction will add considerable cost to the project which must then be born by the property owner.
1. Concrete absorbs toxins.
2. Concrete weakens at high heat.
3. Steel in steel reinforced concrete gets brittle under high heat and further weakens the pad. It may also be bent and unable to tie into the structure. Above is a picture of the twisted steel frame of a former garage giving an idea of the temperatures the structure endured.
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Concrete Absorbs Toxins.
Concrete can absorb toxins during a high-heat fire in a building, but its ability to do so depends on several factors, including the type of fire, the materials involved, and the structure of the concrete itself.
Here’s how it happens: Mechanisms of Toxin Absorption in Concrete:
1. Porosity of Concrete: • Concrete is naturally porous, meaning it has tiny voids and capillaries that can trap gases, liquids, and particulates. During a fire, these pores may absorb toxic gases, smoke residues, and combustion byproducts released from burning materials like plastics, chemicals, or furnishings.
2. Chemical Reactions: • At high temperatures, the chemical reactions within the concrete can increase its ability to bind or interact with certain toxins. For example: • Carbon dioxide (CO₂): Released in fires, it can react with calcium compounds in concrete to form calcium carbonate. • Heavy metals and pollutants: Some can adhere to or chemically bind with the concrete’s matrix.
3. Soot and Hydrocarbons: • Burning organic materials release soot and hydrocarbons, which can deposit on the surface and penetrate the outer layers of concrete. This contamination often leaves the concrete permanently stained or damaged.
4. Absorption of Volatile Organic Compounds (VOCs): • Fires involving synthetic materials (like plastics or insulation) release VOCs, which can be absorbed into the surface of concrete through its pores.
5. Retention of Waterborne Toxins: • Firefighting efforts often involve spraying water, which can carry toxins and residue from burned materials. This contaminated water may be absorbed into the concrete, especially if it cools the structure.
Post-Fire Effects:
1. Structural Degradation: • Toxins absorbed during a fire can weaken the concrete chemically, causing long-term structural degradation. • For example, acids from combustion can react with calcium hydroxide in the concrete, leading to further damage.
2. Health Hazards: • Concrete contaminated with toxins can pose health risks to workers and occupants during post-fire demolition, repair, or occupancy due to the potential release of hazardous residues.
3. Environmental Concerns: • If the contaminated concrete is demolished and discarded, toxins trapped in the material can leach into the environment, requiring proper disposal methods. Mitigation and Cleanup: •
After a fire, testing of the concrete for toxins is essential, especially in industrial settings or where hazardous materials were burned. • Contaminated concrete often needs remediation (e.g., sealing, resurfacing) or removal and disposal following environmental regulations.
In Summary:
Concrete does absorb toxins during high-heat fires due to its porosity and interaction with the byproducts of combustion. While this can sometimes preserve the structural integrity in the short term, the absorbed toxins may pose long-term challenges for structural safety, health, and environmental impact.
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Concrete loses strength at high heat.
Typical Temperatures in a House Fire:
1. Early Stage (Incipient Stage): • Temperature: 200–400°F (93–204°C) • At this stage, the fire is small and localized, with relatively low heat as it begins to grow.
2. Growth Stage: • Temperature: 400–1300°F (204–704°C) • As the fire spreads, it heats the surrounding materials, causing them to ignite. Heat transfer occurs through convection, conduction, and radiation.
3. Fully Developed Stage (Flashover): • Temperature: 1100–2000°F (593–1093°C) • This is the hottest phase of the fire. Almost all combustible materials in the space are burning, and temperatures can reach their peak, potentially melting metals and weakening structural materials like steel and concrete.
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Consequence of heat damage.
High heat can severely weaken concrete and steel, the two primary materials used in structural construction, by altering their physical and chemical properties. Here’s how:
Concrete:
1. Loss of Moisture: • Concrete contains water in its structure. At around 100–200°C (212–392°F), this water evaporates, causing microcracks and a reduction in strength.
2. Decomposition of Hydration Products: • The calcium silicate hydrate (C-S-H) gel, which gives concrete its strength, begins to break down at around 300–400°C (572–752°F), further reducing the structural integrity.
3. Thermal Expansion and Cracking: • Concrete expands unevenly under high heat, leading to internal stresses and cracking. At temperatures above 500°C (932°F), severe spalling (surface chipping and flaking) can occur, exposing inner layers to the heat.
4. Carbonate Decomposition: • If limestone is present in the concrete (as part of the aggregate or cement), it begins to decompose into calcium oxide and carbon dioxide at temperatures around 700–900°C (1292–1652°F), which weakens the structure.
Steel:
1. Loss of Strength: • Steel begins to lose its strength significantly at temperatures above 400°C (752°F). At 600°C (1112°F), it retains only about 50% of its strength, and by 800°C (1472°F), it retains just 10% of its strength.
2. Thermal Expansion: • As steel heats up, it expands. This can cause buckling or deformation, especially in structural elements like beams and columns.
3. Softening and Melting: • Steel softens and becomes ductile at high temperatures, making it unable to support loads. Its melting point is around 1370–1510°C (2500–2750°F), but failure occurs well before that due to the softening.
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from ChatGTP:
Why remove a building pad after a house fire.
Removing a building pad after a house fire that fully engulfed and destroyed the structure is often necessary for several reasons, ranging from safety and environmental concerns to ensuring the structural integrity of a future rebuild. Here are the key reasons:
1. Contamination and Toxic Residues • Absorbed Toxins: Building pads, often made of concrete, can absorb toxic residues from burned materials such as plastics, chemicals, and synthetic construction materials. These contaminants can include: • Heavy metals (e.g., lead, mercury). • Volatile organic compounds (VOCs). • Hydrocarbons from soot and smoke. • Asbestos, if present in the structure. • Health Hazards: Contaminated pads can release toxins over time, posing health risks to workers, future occupants, and the surrounding environment.
2. Structural Compromise • Heat Damage: Intense heat from the fire can weaken or degrade the structural integrity of the pad, even if it appears intact. This may include: • Spalling: The surface layer of concrete can flake off, exposing the inner layers. • Chemical Changes: High heat can break down the internal composition of concrete, reducing its strength and load-bearing capacity. • Reinforcement Damage: Steel reinforcement (rebar) inside the pad can warp or lose its tensile strength due to extreme heat. • Foundation Cracking: Stress from the fire or firefighting efforts (e.g., water cooling the pad unevenly) can cause cracks that weaken the pad.
3. Compliance with Building Codes • Rebuilding Requirements: Local building codes often require that a damaged or compromised pad be removed before reconstruction to ensure the new structure meets current safety and engineering standards. • Environmental Regulations: Fire-damaged pads may need to be removed to comply with environmental laws that prohibit the reuse of contaminated or weakened materials.
4. Fire Debris Cleanup • Embedded Debris: Fire debris, such as charred wood, melted materials, and ash, can embed into the pad. These materials can be difficult to remove entirely without demolition. • Proper Disposal: Removing the pad allows all fire-related debris to be disposed of properly in accordance with hazardous waste regulations.
5. Geotechnical Concerns • Soil Contamination: Toxins from the fire can leach through the pad into the underlying soil, requiring remediation. Leaving the pad in place may prevent proper cleanup of the soil beneath. • Future Foundation Stability: A fire-damaged pad may not provide a stable or level surface for new construction, potentially leading to structural issues in the future.
6. Insurance and Liability • Insurance Requirements: Insurance companies may mandate the removal of a damaged pad as part of the claim settlement to ensure proper rebuilding and prevent future liabilities. • Liability Risks: Leaving a compromised pad in place could expose property owners to liability if it fails or causes harm during or after reconstruction.
7. Aesthetics and Property Value • Visual Damage: Fire-damaged pads are unsightly and can lower the overall value of the property. • Clean Slate for Rebuilding: Removing the pad provides a clean, safe, and stable foundation for future construction, ensuring the property is restored to its full potential. In Summary: Removing a building pad after a house fire is often necessary to address contamination, ensure safety, comply with regulations, and provide a clean and stable base for reconstruction. This process is typically part of fire debris cleanup and rebuilding efforts, ensuring the long-term integrity and habitability of the property.