Illustration of concrete undergoing thermal stress and internal moisture pressure during a fire.

Concrete Under Fire: How to Protect Buildings From Explosive Spalling

Reese Marlowe in Tech & Innovation September 2025 • 4 min read.

"Discover the secrets to safeguarding ultra-high performance concrete structures from fire damage using innovative material combinations and microscopic analysis."

Ultra-High Performance Concrete (UHPC) stands as a testament to modern engineering, boasting compressive strengths exceeding 150 MPa. Its superior performance is achieved through meticulously designed microstructures, maximizing packing density with fine minerals, quartz powder, and silica fume. However, this very density renders UHPC susceptible to a dangerous phenomenon under fire conditions: explosive spalling.

Explosive spalling is a critical concern for UHPC structures, potentially leading to catastrophic failures. Unlike traditional concrete, UHPC's low capillary action makes it highly sensitive to fire. This sensitivity arises from two primary mechanisms: thermal gradients and moisture clog.

Researchers are exploring innovative solutions to mitigate these risks, focusing on material modifications and a deeper understanding of the underlying mechanisms. One promising approach involves incorporating synthetic fibers, such as polypropylene (PP) or acrylic (PAN) fibers, to enhance fire resistance. This article delves into the behavior of a specific UHPC, known as BCV, under fire conditions, examining the effects of various synthetic additions and their impact on spalling.

Understanding Fire Spalling: The Thermal Gradient and Moisture Clog Effects

Illustration of concrete undergoing thermal stress and internal moisture pressure during a fire.

When fire strikes, the thermal gradient within the concrete creates stress that can exceed the material's failure limit. This is exacerbated by the 'moisture clog' effect, where water within the concrete vaporizes and creates pressure, weakening the structure. Both thermal gradient and moisture clog must be addressed to prevent spalling.

The addition of polypropylene (PP) fibers has emerged as a popular strategy for improving concrete's fire resistance. However, the exact mechanisms behind their effectiveness remain a topic of ongoing research. It is also not fully understood why PP fibers are better than other polymers. The goal is to understand what parameters enable fibers to be efficient or not.

Thermal Gradient: Fire causes uneven heating, leading to internal stresses that can crack the concrete.
Moisture Clog: Vaporized water builds pressure inside the concrete, exceeding its tensile strength.
Material Composition: Ultra-dense UHPC is more prone to spalling due to its low permeability.

To explore these factors, researchers conducted a series of experiments using a blowtorch test on prismatic BCV specimens with varying synthetic additions. They also performed scanning electron microscopy (SEM) observations after heating and mercury intrusion porosimetry (MIP) to analyze changes in the concrete's microstructure. The focus was on reducing the moisture clog effect.

Critical Factor for Spalling Resistance

This research introduces a groundbreaking method for assessing the ability of BCV compositions to withstand fire. By developing a simple, efficient high-temperature blowtorch test and analyzing the microstructure changes with SEM and MIP, the study identifies a critical factor (Fzc) that predicts spalling resistance. This new indicator, based on cumulative pore area, offers a more straightforward approach to determine fire resistance compared to traditional fire tests, paving the way for safer and more resilient UHPC structures.

About this Article -

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This article is based on research published under:

DOI-LINK: 10.1016/j.cemconres.2018.10.005, Alternate LINK

Title: Fire Spalling Of Ultra-High Performance Concrete: From A Global Analysis To Microstructure Investigations

Subject: General Materials Science

Journal: Cement and Concrete Research

Publisher: Elsevier BV

Authors: L. Missemer, E. Ouedraogo, Y. Malecot, C. Clergue, D. Rogat

Published: 2019-01-01

Everything You Need To Know

What is explosive spalling, and why is it a concern for Ultra-High Performance Concrete (UHPC)?

Explosive spalling is the sudden and violent ejection of concrete from a structure when exposed to fire. For Ultra-High Performance Concrete (UHPC), it poses a significant threat because its high density and low capillary action make it particularly vulnerable. This means that UHPC can experience both a significant thermal gradient, leading to internal stresses, and a moisture clog effect, where trapped water vaporizes and builds pressure. Both of these factors contribute to the potential for catastrophic failure in UHPC structures under fire conditions.

How does the 'moisture clog' effect contribute to explosive spalling in concrete, and why is it relevant to UHPC?

The 'moisture clog' effect occurs when fire heats the water trapped within concrete, causing it to vaporize. In Ultra-High Performance Concrete (UHPC), which has very low permeability, this vapor gets trapped, building up pressure inside the material. This pressure increase can exceed the concrete's tensile strength, leading to cracking and spalling. The moisture clog is particularly relevant to UHPC due to its dense nature, which limits the escape pathways for the vapor, thus exacerbating the pressure buildup and increasing the likelihood of explosive spalling.

What are the key differences between Ultra-High Performance Concrete (UHPC) and traditional concrete in terms of fire resistance?

Ultra-High Performance Concrete (UHPC) differs from traditional concrete in its response to fire due to its unique properties. UHPC's higher density and finely designed microstructure, which are key to its strength, also make it more susceptible to explosive spalling. Traditional concrete, with its higher permeability, allows for some moisture and heat to escape, reducing internal pressure. UHPC, with its lower permeability, traps moisture, intensifying the 'moisture clog' effect and increasing the risk of spalling. Moreover, the thermal gradient is more pronounced in UHPC due to its density.

What role do synthetic fibers like polypropylene (PP) and acrylic (PAN) fibers play in enhancing the fire resistance of UHPC?

Synthetic fibers, such as polypropylene (PP) and acrylic (PAN) fibers, are incorporated into Ultra-High Performance Concrete (UHPC) to enhance its fire resistance. They work by melting at high temperatures and creating channels for the vaporized water to escape, thereby mitigating the 'moisture clog' effect. This process reduces the internal pressure within the concrete, lessening the likelihood of explosive spalling. The effectiveness of these fibers depends on various parameters, and research is ongoing to fully understand the mechanisms behind their performance.

How are researchers using techniques like blowtorch tests, SEM, and MIP to improve the fire resistance of concrete, and what is the significance of the critical factor (Fzc)?

Researchers are employing a combination of experimental and analytical techniques to enhance the fire resistance of concrete, especially Ultra-High Performance Concrete (UHPC). Blowtorch tests simulate fire exposure on concrete specimens. Scanning electron microscopy (SEM) is used to analyze the microstructure of the concrete after heating, identifying changes caused by the fire. Mercury intrusion porosimetry (MIP) measures the pore structure of the concrete, which is crucial for understanding the 'moisture clog' effect. The critical factor (Fzc), identified through these tests, offers a way to predict spalling resistance based on the cumulative pore area, which provides a more straightforward and efficient approach to assess the fire resistance of UHPC compared to traditional fire tests, leading to safer UHPC structures.