Evaluation of Innovative and Sustainable Fire Protection Systems for Reinforced Concrete Structures ^†

Abstract

This study presents a comprehensive overview of recent advancements in fire protection technologies for reinforced concrete (RC) structures, with a focus on sustainable and high-performance solutions. As climate change and urban densification continue to shape modern construction, the need for fire-resilient and environmentally responsible building systems has never been more urgent. This study examines traditional fire protection practices and contrasts them with emerging innovations. Emphasis is placed on their thermal performance, structural integrity post-exposure, and long-term durability. Case studies and laboratory findings highlight the effectiveness of these systems under standard and severe fire scenarios. This paper will present the results of a research study on the assessment of different fire protection systems for RC columns retrofitted with fiber-reinforced polymer (FRP) jacketing. To quantify how insulation can preserve confinement, three commercial fire protection schemes were tested on small-scale CFRP- and GFRP-confined concrete cylinders: (i) a thin high-temperature cloth + blanket (DYMAT™-RS/Dymatherm), (ii) an intumescent epoxy-based coating (DCF-D + FireFree 88), and (iii) cementitious mortar (Sikacrete™ 213F, 15 mm and 30 mm). Specimens were exposed to either 60 min of soaking at 200 °C and 400 °C or to a 30 min and 240 min ASTM E119 standard fire; thermocouples recorded interface temperatures and post-cooling uniaxial compression quantified residual capacity. All systems reduced FRP–interface temperatures by up to 150 °C and preserved 65–90% of the original confinement capacity under moderate fire conditions ($\le$400 °C and $\le$30 min ASTM E119) compared to 40–55% for unprotected controls under the same conditions. The results provide practical guidance on selecting insulation types and thicknesses for fire-resilient FRP retrofits.

1. Introduction

Fiber-reinforced polymer (FRP) composites offer substantial benefits for strengthening reinforced concrete (RC) columns, such as high strength-to-weight ratios, corrosion resistance [1], and enhanced seismic performance due to effective confinement [2]. However, a significant challenge is FRP’s inherent vulnerability to fire [3]. The polymer matrix, typically room temperature-cured epoxy, is degraded at elevated temperatures, especially above its glass transition temperature ($T_g$, commonly 50–120 °C [4]), leading to a drastic loss in mechanical properties [3]. Ignition of the polymer matrix in carbon FRPs (CFRPs) can occur between 250 and 300 °C [5]. Consequently, ACI Committee 440 guidelines [6] specifies that, under fire exposure, the confinement provided by FRP shall be taken as zero unless its retention can be demonstrated, necessitating effective passive fire protection (PFP) systems [7]. The fire performance of such protected elements is often evaluated using standardized fire tests, such as the ASTM E119 standard fire [8].

This paper quantitatively evaluates and comparatively analyzes the protection mechanisms and failure modes of three distinct PFP systems, drawing exclusively from experimental data presented in the PhD dissertation of the first author [9]. The focus is on understanding how each system preserves FRP confinement and their physical integrity post-exposure and, critically, discussing the influence of varied experimental thermal protocols on the observed mechanisms and providing a comparative interpretation. For clarity and conciseness, the systems primarily evaluated are referred to as System A (DYMAT™-RS cloth and Dymatherm™ blanket), System B (FIRECOAT: DYMAT™ DCF-D epoxy and FireFree 88 intumescent), and the Sikacrete™ 213F system (cementitious mortar). This nomenclature streamlines discussion from the varied system labels used within the source dissertation [9]. A brief contextual note will also be made regarding the performance of what is referred to in the source dissertation [9] as “System C” (REARLOCK primer with FireSet60 intumescent topcoat).

2. Materials and Methods

2.1. Specimen Configuration and Base Materials

Concrete cylinders of varying dimensions were used: Ø200 × 450 mm for evaluating System A and System B, as detailed in distinct experimental series within the source dissertation, and Ø100 × 200 mm and Ø150 × 300 mm for the Sikacrete™ 213F system, investigated in a separate experimental series [9].

Crucially, different concrete batches were employed across these test series, impacting baseline unconfined compressive strengths (f′_c) and necessitating series-specific controls for normalization:

• Batch #1 (used for evaluating System A under ramp-and-hold conditions): $f_c^{\prime }$ ≈ 21–22 MPa at 28 days. FRP-confined room-temperature compressive strength ($f_{cc,RT}^{\prime }$) ≈ 55.6 MPa (CFRP).
• Batch #2 (used for evaluating System B under initial ASTM E119 conditions and Systems A, B, and C under extended ASTM E119 conditions): $f_c^{\prime }$≈ 30 MPa (at 28 days). FRP-confined $f_{cc,RT}^{\prime }$ ≈ 86.1 MPa (CFRP).
• Batch #3 (used for evaluating the Sikacrete™ 213F system under ramp-and-hold conditions [9]): $f_c^{\prime }$≈ 42 MPa (at 28 days). FRP-confined $f_{cc,RT}^{\prime }$ varied with FRP type (e.g., ≈126.6 MPa for 2-ply CFRP).

The loading rates for post-exposure compression testing also varied: 0.6 MPa/s for the experiments involving Systems A and B under ramp-and-hold and ASTM E119 exposures and 0.25 MPa/s for the experiments involving the Sikacrete™ 213F system under ramp-and-hold exposures. FRP systems primarily involved two plies of CFRP or Glass FRP (GFRP) applied via wet layup.

2.2. Fire Protection Systems, Thermal Exposures, and Performance Metrics

The fire protection systems investigated included the following:

• System A Key configuration: T3 (inner RS cloth + 2 Dymatherm™ layers + outer RS cloth) and T4 (inner RS + 4 Dymatherm™ layers + outer RS).
• System B key configurations: T1 (1 epoxy + 3 intumescent layers) and T2 (2 epoxy + 6 intumescent layers).
• Sikacrete™ 213F System: Applied at 15 mm and 30 mm thicknesses.
• System C (REARLOCK+FireSet60): REARLOCK primer with FireSet60 intumescent topcoat.

Distinct thermal protocols were used:

• Ramp-and-hold (used for the initial evaluation of System A): Heating at 2.5 °C/min to 200 °C or 400 °C, then 60 min hold.
• ASTM E119 standard fire (used for the initial evaluation of System B and comparative evaluation of Systems A, B, and C): Durations of 30 min (furnace peak ~821 °C), 60 min (furnace peak ~925 °C), and 4 h (240 min, furnace peak ~1090 °C).
• Ramp-and-hold (used for the evaluation of the Sikacrete™ 213F system): Specimens heated in furnace at 400 °C or 700 °C for 90 or 180 min.

Systems A, B and C were sourced from Dymat, Austin, TX, USA. Sikacrete™ was sourced from Sika, Rabigh, Saudi Arabia.

Instrumentation included thermocouples for the furnace and FRP/insulation interface temperatures ($T_{interface}$) and pyrometers for post-exposure surface temperatures. Post-exposure uniaxial compression tests were performed after cooling, with some tests on “hot” specimens, particularly in the comparative evaluation series involving extended ASTM E119 exposures [9].

Table 1. Summary of experimental program for key fire protection systems.

System	Materials	Nominal Specimen Size (s)	Primary Experimental Context
System A	DYMAT™-RS cloth and Dymatherm™ blanket	Ø200 × 450	Ramp-and-hold at 200/400 °C; ASTM E119 standard fire for 30–240 min
System B	FIRECOAT: DYMAT™ DCF-D epoxy and FireFree 88 intumescent	Ø200 × 450	ASTM E119 standard fire for 30–240 min
Sikacrete™ 213F	Cement-based insulative mortar (15 mm, 30 mm thicknesses)	Ø100 × 200, Ø150 × 300	Ramp-and-hold at 400/700 °C for 90–180 min

summarizes the key experimental parameters.

The performance metrics are detailed in

Table 2. Key performance summary of selected PFP systems under specific exposures.

System	FRP Type	Insulation Config.	Thermal Exposure	SRR	Unprotected SRR
System A	CFRP	T4 (4-layer blanket)	400 °C (60 min)	0.79	0.50
	CFRP	T4 (4-layer blanket)	4 h ASTM E119	0.36	0.08
System B	CFRP	T2 (2 epoxy + 6 FireFree88)	30 min ASTM E119	0.88	0.40
	CFRP	T1 (1 epoxy + 3 FireFree88)	4 h ASTM E119	0.08	0.08
Sikacrete™ 213F	CFRP	15 mm	400 °C (180 min)	0.74	0.52 1
	C2G2	30 mm	400 °C (180 min)	0.98	0.57 1

¹ Measured after 300 °C exposure.

. Performance was evaluated using the following:

• Strength Retention Ratio (SRR): SRR = $f_{cc,T}^{\prime }$/$f_{cc,RT}^{\prime }$, where $f_{cc,T}^{\prime }$ is the post-exposure confined strength and $f_{cc,RT}^{\prime }$ is the room-temperature strength of an identical companion specimen from the same concrete batch, FRP configuration, and test series.
• FRP interface temperatures $\left(T_{interface}\right)$: Maximum recorded temperature ($T_{interface,max}$) and temperature profiles.
• Observed protection mechanisms: Inferred from visual state and thermal data (e.g., insulation, intumescence, sacrificial action; see Figure 1 for representative post-exposure conditions).
• Failure modes and physical integrity: Spalling, cracking, and debonding of PFP system; degradation and charring of FRP system; concrete conditions.

3. Results and Discussion

3.1. Baseline: Unprotected FRP-Confined Specimens

Unprotected FRP-confined cylinders, as investigated in the baseline tests of the source dissertation [9], consistently showed significant fire vulnerability. Exposure led to FRP charring, matrix loss, and potential ignition (CFRP matrix at ~250–300 °C), resulting in very low SRRs. For Batch #1 concrete (controls for initial System A tests), unprotected CFRP cylinders exposed to the 400 °C ramp-and-hold protocol yielded SRR ≈ 0.50. For Batch #2 concrete (controls for ASTM E119 comparative tests), an unprotected CFRP exposed to the 4 h ASTM E119 fire had SRR ≈ 0.08.

3.2. System A (Flexible Insulation Barrier—DYMAT™-RS/Dymatherm™)

This system functions as a thermal insulation barrier, with the Dymatherm™ blanket providing primary insulation, contained by DYMAT™-RS cloth layers. In the 400 °C ramp-and-hold test (Batch #1 concrete), its T4 configuration kept $T_{interface,max}$ for CFRP at ~90–95 °C, achieving an SRR of ≈0.79. Under the more severe 4 h ASTM E119 standard fire exposure (Batch #2 concrete), the T4 configuration resulted in $T_{interface,max}$ for CFRP reaching ~320–350 °C, yielding an SRR of ≈0.36. Visually, as observed in the experiments documented in [9], the outer cloth was charred, but the blanket system remained largely intact. Well-protected FRP systems showed minimal charring, and failure was often induced by cohesive concrete crushing, indicating preserved confinement.

3.3. System B (Intumescent/Epoxy Coating—FIRECOAT)

System B relies on intumescence, where the FireFree 88 intumescent forms a porous char over a DYMAT™ DCF-D epoxy barrier. This system exhibited significant PFP integrity issues, especially under severe or prolonged exposure, including cracking, spalling, and explosive detachment (notably in the 4 h ASTM E119 test, as documented in [9]). Such PFP failure exposed the underlying FRP system to charring and degradation; hence, limited direct T_interface data are available from [9] for the most severe tests and performance is primarily inferred from SRR and integrity observations.

For Batch #2 concrete, specimens with T2 insulation achieved SRR ≈ 0.88 after a 30 min ASTM E119 standard fire exposure. After a 60 min exposure, T1 insulation yielded SRR ≈ 0.22 (T2 specimens were too damaged for testing). For the 4 h ASTM E119 fire, T1 provided SRR ≈ 0.08 (T2 specimens experienced explosive spalling).

3.4. Sikacrete™ 213F System (Cementitious Mortar)

The Sikacrete™ 213F cementitious mortar provides robust passive fire protection primarily through its significant thermal mass, low thermal conductivity, and its function as a sacrificial insulating layer that delays heat transfer to the FRP system.

This system’s effectiveness was clearly demonstrated at elevated temperatures. A 15 mm layer of Sikacrete™ 213F applied to CFRP-confined cylinders (Batch #3) resulted in a high Strength Retention Ratio (SRR) of 0.74 after a demanding 180 min ramp-and-hold exposure at 400 °C, as shown in Table 2. The typical post-exposure condition of these protected specimens, characterized by the mortar remaining largely intact with only superficial surface cracking, is illustrated in Figure 2b. The successful protection of the underlying FRP system contrasts sharply with the extensive degradation observed in unprotected specimens, as shown in Figure 2a.

Furthermore, increasing the protection to a 30 mm thickness on C2G2 hybrid FRP-confined cylinders yielded an excellent SRR of 0.98 under identical 400 °C and 180 min exposure. These high SRRs strongly suggest that the Sikacrete™ effectively maintained FRP interface temperatures below critical degradation points for the duration of the test. The system also exhibited considerable resilience even under more extreme conditions; for example, 30mm of Sikacrete™ on hybrid C2G2 FRP cylinders retained an SRR of approximately 0.41 after a 180 min isothermal test at 700 °C, although with more pronounced, yet generally not detrimental, mortar damage [9].

3.5. System C (REARLOCK+FireSet60)

This intumescent system, comprising a REARLOCK primer and FireSet60 topcoat, experienced significant explosive spalling during the 4 h ASTM E119 fire test (as part of the comparative ASTM E119 test series in the source dissertation [9]). This PFP failure compromised its protective function and precluded meaningful post-fire mechanical testing for this severe exposure.

3.6. Comparative Discussion: Mechanisms, Failure Modes, and Protocol Influence

Direct numerical comparison of the SRRs across these systems is inappropriate due to the different concrete batches, specimen sizes, loading rates, and, critically, the disparate thermal exposure protocols employed in the experimental programs documented in [9]. Each protocol challenges PFP systems in unique ways, influencing their apparent effectiveness and failure modes.

System A and Sikacrete™ are passive insulators, relying on low thermal conductivity and/or high thermal mass. Systems B and System C are reactive intumescents, depending on stable char formation for insulation. System A generally showed stable PFP integrity. Sikacrete™ was robust, with only minor cracking or localized spalling at extreme temperatures. In contrast, Systems B and System C were prone to PFP spalling or detachment, especially in severe fires, compromising FRP protection.

The varied thermal exposure protocols significantly influenced the observed PFP performance. The ramp-and-hold protocol (used for System A’s initial evaluation and Sikacrete [9]) tested sustained insulation capacity under moderate, stable elevated temperatures, where System A (T4 configuration) and Sikacrete both performed well. In contrast, the ASTM E119 standard fire simulated more aggressive building fire scenarios. Short durations (30/60 min, used for System B’s initial evaluation [9]) evaluated initial reaction and char formation; System B (T2) excelled at 30 min but faltered at 60 min. The long-duration, 4 h ASTM E119 test challenged endurance and char stability; here, System A (T4) maintained integrity and provided protection, while both intumescent systems (B and C) largely failed by spalling.

These protocol differences mean that PFP performance is protocol-dependent. An intumescent might perform well in a short fire but poorly in a long one if its char degrades or detaches. Therefore, SRR values must be interpreted within the context of their specific exposure. Normalized metrics allow for comparison with their unprotected counterparts within the same series and support qualitative, mechanism-focused comparisons across systems.

4. Conclusions

This evaluation of three distinct PFP systems for FRP-confined concrete cylinders, based on varied experimental protocols from a comprehensive dissertation [9], yields the following key conclusions:

• System A (DYMAT™-RS cloth and Dymatherm™ blanket) provides effective thermal insulation and maintains good physical integrity. Its T4 configuration achieved SRR ≈ 0.79 (CFRP, 400 °C ramp-and-hold, $T_{interface,max}$ ≈ 90–95 °C) and SRR ≈ 0.36 (CFRP, 4 h ASTM E119, $T_{interface,max}$ ≈ 320–350 °C).
• System B (FIRECOAT) relies on char formation. It demonstrated high initial performance (SRR ≈ 0.88 for CFRP T2, 30 min ASTM E119) but suffered PFP cracking and spalling in longer/more severe fires, leading to very low SRRs (e.g., ≈0.08 for T1, 4 h ASTM E119).
• Sikacrete™ 213F (cementitious mortar) offers robust thermal mass and insulation, achieving excellent SRRs such as 0.74 (CFRP, 15mm, compared to 0.52 for unprotected) and 0.98 (C2G2, 30 mm, compared to 0.57 for unprotected) after 400 °C (180 min) exposure and maintaining good resilience even under more extreme conditions (e.g., SRR ≈ 0.41 for hybrid C2G2 FRP with 30 mm protection after 180 min at 700 °C).
• All PFP systems improved performance over unprotected FRP. However, efficacy and failure modes are highly dependent on the PFP nature and, critically, thermal exposure characteristics. The varied protocols highlight that simple system ranking is inappropriate; evaluations must be relevant to specific fire scenarios. Understanding how a PFP system behaves and fails under different fire conditions is essential for reliable design.