Adhesion of stainless steel rebar after elevated temperatures due to fire

By Vicente Albero¹*, David Hernández-Figueirido¹, Marta Roig-Flores¹, Toni Melchor-Eixea¹, Ana Piquer¹; ¹Universitat Jaume I, Castellón, Spain. Email: valbero@uji.es

Concrete coatings protect reinforced concrete structures from corrosion, acting as a physical barrier to protecting steel rebar from external erosion. Passivation also protects steel embedded in concrete from corrosion. This passivation is achieved due to the high alkalinity of the concrete mass (pH value between 12.6 and 14). Current reinforced concrete regulations require minimum coatings based on the corrosive nature of the external environment. In many cases, this measure, combined with the correct concrete mix, design, placement, compaction and curing, is sufficient to protect the steel from corrosion and ensure its durability throughout the structure’s life. However, the durability of reinforced concrete elements may be undermined if aggressive substances penetrate the concrete’s pore network and the passivation layer becomes unstable.
In recent decades, strategies developed to achieve better corrosion protection of steel bars can be broadly divided into two categories: those acting on concrete and those acting on steel bars. Currently, the most widely used strategy is to act on the steel bars. Stainless steel reinforcements with a chromium content of >12% have self-passivating properties, providing excellent performance for long-term corrosion protection. The increased use of stainless steel rebar is reflected in the publication of specific European standards, such as prEN 10370:2023, for stainless steel bars.

The measures used to protect reinforced concrete structures from corrosion can significantly impact the bonding behaviour between steel and concrete. This bonding behaviour is a key factor in stress transfer between the two materials. The International Federation of Beton (FIB) Model Specifications for Concrete Structural Structures 2010 and FIB Bulletin No. 10 describe liability issues associated with certain methods of corrosion protection, such as reinforcements with epoxy coatings or the use of fibre-reinforced polymer (FRP) reinforcements. Most international standards (ACI, BS) specify a 20% to 50% extension of the anchorage length for epoxy-coated corrosion-resistant steel bars. On the other hand, the current discussion in the scientific literature on the bond strength of galvanized reinforcements is controversial^[1-3]. Certain tests show reduced adhesion strengths, and research suggests lower initial adhesion in the early stages but comparable adhesion in the later stages of concrete^[4]. In the case of galvanized steels, the decline in adhesion is commonly linked to hydrogen evolution at the contact interface, arising from the chemical reaction between the zinc coating and the fresh cement matrix of the concrete.
Conversely, the adhesion of stainless-steel reinforcements, though less explored so far, is approached from a safety standpoint in current design codes^[5,6].

Alleviating impact of industrial fires

Extensive research has been conducted on the adhesion of protected reinforcements in reinforced concrete structures under typical project conditions. Yet, considerable uncertainty arises in accidental scenarios like fires, where elevated temperatures can substantially compromise the mechanical properties of structural materials. The Model Code 2010 stands as the singular standard explicitly stating that, in the case of a fire impacting unprotected reinforced concrete structures, a reduction in adhesion can be expected proportionally to the decrease in the tensile strength of concrete at high temperatures. Regarding the accidental occurrence of fires, it is essential to recognize it as a crucial design scenario for various structural systems. Indeed, all national and international standards related to structural calculations incorporate diverse specifications in this context. Additionally, emphasizing the substantial economic impact associated with this hazard is crucial.

As per recent reports from the Spanish Insurance Association (UNESPA)^[7], from July 2020 to June 2021, 5.16% of the 73,000 registered fires occurred in the industrial sector. These incidents accounted for the most significant economic impact, averaging approximately €50,000 per event. Notably, these statistics point to an average of about ten industrial fires daily in Spain, resulting in a daily economic loss of €500,000. This considerable loss is attributed to the typical intensity of industrial fires, associated with significant economic consequences and related to the high combustible load usually present in such establishments. Any efforts to enhance structural fire resistance can yield positive outcomes in alleviating these substantial economic impacts.
Returning to the subject of adhesion, the late 1970s and early 1980s marked the initiation of experiments on adhesion at elevated temperatures. These experiments employed pull-out tests adapted for high temperatures or conducted pull-out tests post-exposure to temperatures reaching up to 800°C. These tests^[8–10] covered smooth, corrugated, and drawn steel reinforcements, revealing a more pronounced loss of adhesion for smooth bars and a loss of adhesion of a similar magnitude to the reduction in compressive or tensile strength of concrete for corrugated bars. However, to date, no known study has explored the adhesion of stainless steel reinforcements affected by high temperatures. Therefore, the CECOM research group at Universitat Jaume I in Castellón, through the GVA research project (CIGE/2021/116) led by Professor V. Albero^[11], presents the first prospective study on this aspect.

Valuable study results

This study could prove valuable for expert assessments and decisions regarding repairs or demolitions following a fire incident in reinforced concrete structures with stainless steel reinforcements. The experimental campaign entailed executing pull-out tests following the requirements outlined in standard EN 10080 Annex D, using unprotected B500S carbon steel bars as a reference and duplex stainless steel bars ACX915 (EN 1.4362). Evaluations were conducted for both 12 mm and 16 mm diameters in both cases. The repeatability achieved in the tests was 2 specimens per temperature and diameter, resulting in the creation of 16 specimens for B500S steel and an additional 16 for ACX915 stainless steel. The concrete mixes were designed with a water-to-cement ratio of 0.44 and targeted a strength of C25/30. This quality was assessed for each test and sample, yielding an average compressive strength value of 35.4 MPa for specimens with B500S steel reinforcement and 35.6 MPa for ACX915 stainless steel specimens.
Adhering to the specifications of standard EN 10080 Annex D, all fabricated specimens are of cubic form, measuring 200 mm on each side. In this configuration, a 600 mm steel bar is inserted and securely bonded to the concrete at a distance of 5d (60 mm for 12 mm diameter bars and 80 mm for 16 mm diameter bars). A metallic separation sleeve was incorporated to prevent adhesion in the remaining contact zone. After 28 days of concrete curing, the specimens underwent heating in an oven at a rate of 100°C/min to the designated temperature (200°C, 400°C, 600°C), and maintained for 3 hours. Subsequently, natural air cooling was initiated until ambient temperature was reached.
Several days post-cooling, the specimens were subjected to pull-out testing, employing a controlled force increase of 80 N/s for 12 mm diameter reinforcements and 143 N/s for 16 mm diameter ones. Throughout the test, both the applied load and the penetration of the bar at its free end or slippage were meticulously recorded (slip = ∆0 – ∆1). Examining the tension-slip curves recorded for various test series (Figure 5) shows a clear decrease in the maximum adhesion stress can be identified with the increase in the exposure temperature. This decrease is more pronounced from 400°C onwards. It should be noted that the maximum adhesion stress values are closely aligned with the models stipulated in Model Code 2010 and EN 1992, which estimate this maximum stress according to the following equation:
τb, max = 2,5 f cm

For an average compressive strength of concrete (fcm) of 35.5 MPa, this maximum adhesion stress would result in 14.9 MPa, which, as can be observed, aligns conservatively with the values obtained for 20°C and 200°C.
Figure 6 displays the average values for all tested series, presenting maximum adhesion stress values relative to the compressive strength of concrete (τb/fc). Error bars denote the standard deviation across the obtained results. Notably, at room temperature (20°C) for ACX915 stainless steel reinforcements, slightly lower average values of maximum adhesion stress are evident compared to those for B500S carbon steel reinforcement. This discrepancy is likely attributed to the distinct corrugation configuration of ACX915 steel. However, following exposure to elevated temperatures, ACX915 reinforcements consistently demonstrate superior adhesive behaviour compared to B500S carbon steel reinforcements.

Conclusions

To date, there is no known study on the adhesion of stainless steel reinforcements affected by high temperatures. These temperatures may occur during a potential fire situation, causing irreversible damage to the structure and compromising the subsequent serviceability of the reinforced concrete structure. Following the initial preliminary studies, duplex stainless steel reinforcements ACX915 have shown better adhesive behaviour after exposure to high temperatures than conventional carbon steel reinforcements.

About this Tech Article

This tech article appeared in Stainless Steel World, April 2024 magazine. To read many more articles like these on an (almost) monthly basis, subscribe to our magazine (available in print and digital format) – SUBSCRIPTIONS TO OUR DIGITAL VERSION ARE NOW FREE.

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