Abstract
Steel and polypropylene hybrid fiber-reinforced concretes have been widely considered for structural applications due to its superior mechanical performance compared to plain and mono fiber-reinforced concretes. Fire is one of the most serious potential risks to concrete structures. The fire resistance of the steel and polypropylene hybrid fiber-reinforced concretes cannot be ignored when assessing the safety of concrete structures. This paper reviews the available studies on the mechanical performance of thermally damaged steel and polypropylene hybrid fiber-reinforced concretes. The deterioration mechanism and the influence of the test factors were discussed. The temperature-dependent mechanical properties of the hybrid fiber-reinforced concretes were analyzed, including compressive elastic modulus, compressive strength, flexural strength, and fracture toughness. In addition, the effect of the post-fire re-curing on the mechanical performance of the thermally damaged steel and polypropylene hybrid fiber-reinforced concretes was also reviewed.
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1 Introduction
Concrete, the most widely used construction material, is quasi-brittle [1]. The quasi-brittle nature not only affects the long-term durability but also limits the application of concretes [2]. Numerous studies have shown that the addition of short fibers is an effective way to reduce the brittleness of concretes and improve the tensile-related mechanical properties [3], such as flexural strength [4,5,6,7], shear strength [1], energy-absorbing ability [8,9,10], and seismic resistance [11]. The improved performance of the fiber-reinforced concrete is attributed to fiber’s bridging effect. The randomly distributed fibers, across the cracks, restrain the propagation and coalescence of the micro cracks, delaying the formation of the macro cracks. The effectiveness of the bridging can be controlled by the shape [12], material [13], volume fraction [14], and combination of fibers [13, 15]. The improved mechanical performance of fiber-reinforced concretes has been exploited for structural applications, particularly for tunnel shotcrete, precast tunnel segments and beams, and industrial pavements and slabs [10, 16].
Apart from the brittleness of concretes, fire is one of the most serious potential risks to most concrete constructions [17]. Concrete structures are likely to experience high temperatures by accidental causes or for special applications [18]. The components of concretes, such as aggregate and cement, are non-combustible. Therefore, concretes are commonly considered to have good fire resistance, but their thermal response is dictated by its constituents, moisture and porosity. Compared to other construction materials, such as timber, steel, and polyester, concretes have a better performance at high temperatures due to its low thermal conductivity and non-combustibility properties [19]. However, the non-combustibility property does not mean that concrete structures are safe in the event of fire. When exposed to high temperatures, the dense micro-structure inhibits the evaporation of physical and chemical water, resulting in high vapor pressure in concretes [17, 20]. Meanwhile, the thermal gradient produces considerable thermal stress in a concrete component. The vapor pressure and thermal stress motivate the onset and propagation of micro- and macro-cracks. In addition, the chemical changes also deteriorate the performance of the concrete. When the temperature exceeds a certain level [21, 22], the cement matrix and aggregates experience decomposition, leading to a serious reduction in the load-carrying capacity of the concrete. In the past, large concrete structures have collapsed during the fire exposure with catastrophic consequences, e.g., the tunnel connecting France and England in 1996 [23].
To mitigate the deterioration of concretes exposed to high temperature, a feasible approach is to reduce the internal pressure caused by high temperature and to resist the propagation of cracks during the fire and cooling period [24,25,26]. Studies [25, 27,28,29,30,31] have highlighted the role of polypropylene fibers on concrete performance at elevated temperatures, especially in reducing or eliminating the spalling. Steel fibers improve the strength [32, 33] and energy absorption capacity [34, 35] by reducing the propagation of cracks in concretes. However, the effect of steel fibers on the fire resistance of concretes is controversial. The steel fibers limit crack propagation [17, 36, 37] and increase the heat transfer capacity [38] to some extent. However, the addition of steel fibers can create voids and micro cracks, increasing the chance of micro crack propagation at high temperatures [39]. In general, it has been observed that the addition of dispersed fibers in a cement matrix causes an increase in porosity (see e.g., [2]) and, consequently, and may reduce mechanical properties.
There is a general agreement on the favorable influence of fiber hybridization on the mechanical performance of fiber-reinforced concretes [17, 40, 41]. The advantage of the hybrid fiber on the fire resistance of concretes has been shown by some researches [42, 43]. However, a more in-depth research is needed for the fire resistance of the steel and polypropylene hybrid fiber-reinforced concretes. Existing fire design codes [19, 44, 45] are based on plain concretes and steel bar-reinforced concretes. Extrapolation of the guidelines to hybrid fiber-reinforced concretes may not be appropriate. The presence of the hybrid fiber changes the internal structures of the concretes, leading to a variation of the mechanical behavior, namely the stress–strain relationship and the failure model at high temperatures.
Restoring the fire-damaged concrete buildings to the safety level is an effective way to reduce concrete consumption compared to demolition and rebuilding [46]. To achieve this goal, a suitable rehabilitation method is required. Apart from increasing the stiffness of the thermally damaged concrete structure by newly introduced structural enhancement, the restoration of the thermally damaged concrete is a potential method. A few methods, such as water re-curing [47], normal air re-curing [48], concrete repair [49], and fiber-reinforced polymers [50], have been proposed for the rehabilitation of thermally damaged concretes. Among them, water re-curing is a cost-effective and technically easy method. The advantages of the water re-curing on the thermally damaged concrete, such as better strength recovery and lower cost, have been identified [48, 51, 52].
However, it should be noted that although the mechanical behavior of thermally damaged fiber-reinforced concretes has been extensively investigated, the studies on thermally damaged hybrid fiber-reinforced concretes, especially for steel and polypropylene hybrid fiber reinforcement, and the factors influencing the mechanical performance are limited. To enhance fire resistance, polypropylene fibers have been proposed because, at elevated temperatures, melt creates additional channels that allow a release of the water vapor pressure generated within the concrete at elevated temperature. In addition, the effect of the post-fire re-curing methods on the thermally damaged hybrid fiber-reinforced concrete has been analyzed to some extent, but their real applications are very limited.
Therefore, this review presents the state of the art of studies dedicated to the mechanical behavior of both thermally damaged and post-cured hybrid fiber-reinforced concretes. In particular, the paper provides a comprehensive overview of the effects of elevated temperature and water re-curing on the performance of thermally damaged steel and polypropylene hybrid fiber-reinforced concretes. The review covers: the fibers adopted for hybrid fiber-reinforced concretes (Sect. 2), the description of deterioration mechanisms related to physical and chemical changes (Sect. 3); the influence of the testing factors on the thermal damage (Sect. 4); the mechanical properties of the thermally damaged hybrid fiber-reinforced concretes (Sect. 5); the post-fire re-curing effects on the mechanical performance recovery (Sect. 6).
2 Types of the hybrid fiber reinforcements
The performance of hybrid fiber-reinforced concretes is significantly influenced by its reinforcement, namely short fibers, which are only considered here. Hybrid fiber-reinforced concretes comprise no less than two sets of fibers with different: sizes (from macro to micro), shapes (straight, twist, and hooked-end), and materials (steel, organic, and inorganic). Hybrid fiber-reinforced concretes can have a wide range of applications, thanks to the optimal fiber hybridization which can enhance the mechanical behavior.
Based on the research on the hybrid fiber-reinforced concretes in recent years, the typical fiber hybridizations and their percentage of publications indexed in Google Scholar in the last 10 years are shown in Fig. 1. It can be seen that hybrid fibers including steel fiber (SF) account for 68.8%. The hybridization of steel fiber and polypropylene fiber (PP) accounts for 27.3%. The hybrid fibers including steel fiber and polyvinyl alcohol (PVA) fiber or basalt fiber (BF) have also attracted attention for their good reinforcing effect. Other fibers (such as carbon fiber and nylon fiber) were barely considered. For the purposes of this review, only steel fiber and polypropylene fiber are considered the most commonly used hybrid fiber reinforcement for structural concretes.
Percentage of publications in the last 10 years dealing with the different hybrid fiber-reinforced concretes (data from Google Scholar)
3 Deterioration mechanisms of steel and polypropylene hybrid fiber-reinforced concretes under high temperature
Steel and polypropylene hybrid fiber-reinforced concretes are a composite containing water, cement matrix, aggregate, and short fibers. These components and micro structures experience physical and chemical changes under high temperatures, leading to the deterioration of the mechanical performance of concretes (see Fig. 2). The physical-related thermal damage is caused by the vapor pressure, thermal stress, mismatch of the thermal expansion, and melting of the polypropylene fibers. The chemical-related thermal damage is produced by the decomposition of cement matrix and aggregate and oxidation of steel fibers. In general, there are five degradation mechanisms, namely vapor pressure, thermal stress, decomposition, oxidation, and melting of fibers [23, 26, 40, 52].
Fire damage and water re-curing healing process of hybrid fiber-reinforced concretes: a before heating, b after heating, c after water re-curing. ITZ indicates the interfacial transition zone
3.1 Vapor pressure
In concretes, there are three kinds of water, namely, free water, gel water, and chemical water [53, 54]. With the increase of temperature, water in concretes becomes steam and evaporates. The dense concrete structure confines the water evaporation, leading to the high vapor pressure in the concrete. The initiation, propagation, and coalescence of the micro cracks are activated by the increasing vapor pressure [55]. The vapor pressure can be measured by pressure–temperature gauges in the concrete as suggested in [26].
Free water evaporates when the temperature approaches to about 150–400 °C [56, 57]. In [42, 58, 59], a slight improvement in compressive strength and flexural strength was observed. It was attributed to the initial increase of the vapor pressure. The steam works as an “internal autoclave,” causing additional hydration of the unhydrated cement in the concrete, reducing the original defects [60].
When the temperature exceeds 200 °C, gel water, existing between the layers of the calcium–silicate–hydrates (C–S–H), breaks the bonding and evaporates [40]. The evaporation of the gel water shrinks the C–S–H structure and increases the vapor pressure, causing further damage to the concrete. When the temperature exceeds 500 °C, the hydrated composites experience decomposition, leading to the evaporation of chemical water (Fig. 2b). During this stage, the micro structure collapses with many cracks in the concrete, leading to a sharp decrease in the load-bearing capacity.
When the heating rate is high (above 10 °C/min), the spalling of concretes may appear for high-strength concretes with a dense micro structure. In general, two main explanations are used for the spalling mechanism of concretes at high temperatures.
The first one is the pore pressure spalling mechanism [23, 61, 62]. The foundation of this hypothesis is the dense structure and low permeability of concretes. Under heating, the capillary water evaporates to the heated surface and flows into the inner part of the concrete with low vapor pressure. The steam condenses in the inner region with a temperature around 100 °C, forming a saturated layer. The saturated layer, like the moisture clog wall, blocks the evaporation of the water, increasing the pore pressure near the saturated layer. When the vapor pressure and thermal stress in this region exceed the tensile strength of the concrete, spalling happens as shown in Fig. 3. According to the laboratory test and theoretical prediction, the pore pressure is under 1 MPa for normal-strength concretes and 2–3 MPa for high-strength concretes [62,63,64,65,66].
Illustration of the mechanism of spalling of concretes, adapted from Zeiml et al. [62]
The second spalling mechanism relates to the thermal stress produced in concretes which is analyzed in the following section.
3.2 Thermal stress
There are two mechanisms for thermal stress deterioration: the first is the thermal gradient, and the second is the thermal dilatation mechanism.
The thermal gradient mechanism relates to the low thermal conductivity of concretes [23, 67]. At high temperatures, the low thermal conductivity creates a thermal gradient from the heated surface to the center of the concrete, resulting in compressive stress parallel to the heated surface [68] and a reduction in tensile stress in the concrete near the heated surface. Meanwhile, the tensile stress is formed in the direction perpendicular to the heating surface. The spalling appears when the tensile stress exceeds the tensile strength of the concrete, as shown in Fig. 4. It is the second spalling mechanism in Sect. 3.1. The intensity of damage caused by the thermal gradient depends on the heating rate, thermal conductivity, and tensile strength of the concrete. Therefore, adding short steel fiber is a proper way to reduce the damage caused by the thermal gradient, by bridging effect, and to improve the thermal conductivity.
Spalling caused by the thermal gradient, adapted from Ozawa et al. [68] (T is the temperature and σ is the stress in the heated concrete)
The thermal dilatation mechanism relates to different thermal dilatation coefficients between aggregate and cement matrix [23, 69, 70]. Under high temperatures, aggregates expand, while cement matrix shrinks. The thermal deformation produces tensile stress in the cement matrix near the aggregate, leading to the onset and propagation of micro cracks in the interfacial transition zone (ITZ), see Fig. 2b. Compared to vapor pressure and thermal gradient, the damage caused by thermal dilatation is only produced in the ITZ. The deterioration of the ITZ causes the reduction of the modulus of elasticity and compressive strength [23, 71]. The intensity of the damage caused by the thermal deformation mismatch depends on the aggregate. For example, concretes containing siliceous aggregates have more serious deterioration in the ITZ than that of concretes including calcareous aggregates [23, 40]. For hybrid fibers including the steel fibers, cracks also appear in the ITZ between the steel fiber and mortar (Fig. 2).
3.3 Decomposition of concrete components
For mortar, the main composites include calcium hydroxide (Ca(OH)2), C–S–H, and calcium carbonate (CaCO3) [23, 40]. They undergo decomposition under high temperatures, causing the generation and propagation of both micro and macro cracks in the concrete. Coupling with the physical-related damage, the whole micro structure collapses, leading to a sharp decrease of the load-bearing capacity [72].
The Ca(OH)2, existing as portlandite, dehydrates to calcium oxide (CaO) when the high temperature exceeds 500 °C [73, 74]. The dehydration of portlandite generates cracks in the cement matrix due to the volume difference between Ca(OH)2 and CaO. During the post-fire stage, CaO reacts with water and produces calcium hydroxide [75]. The volume of Ca(OH)2 is much larger than that of CaO, leading to the further increase of cracks in thermally damaged concretes [52].
C–S–H, the main phase of the cement matrix, determines the load-bearing capacity of the cement matrix [40]. The decomposition of the C–S–H causes serious deterioration in the mechanical properties of the concrete. It was reported that the energy for the dehydration activation of C–S–H varied from 83.69 to 371.93 kJ/mol, implying that the decomposition of C–S–H happens in a wide range of temperatures [76]. It depends on the complex structure of C–S–H and its amorphous nature [77]. Studies [21, 22, 73, 78] indicated that the C–S–H starts dehydration when the temperature reaches 200 °C and finishes at 750 °C. When the temperature increases from 200 to 300 °C, some C–S–H layers depolymerize into shorter chains and some dehydrate to anhydrous CnS. C–S–H is completely depolymerized when the temperature reaches 450 °C [79]. When the temperature reaches 750 °C, C–S–H completely dehydrates to CnS, leading to the collapse of the micro structure. Above 750 °C, CaCO3 decomposes to CaO. The decomposition of the calcium carbonate leads to the further decrease of the loading capacity of the thermally damaged concrete [80, 81].
The mineralogy and porosity of aggregate are key parameters that influence the behavior of concretes at high temperatures [82, 83]. Under high temperatures, aggregates with high porosity may crack due to evaporation of the water. The mineralogy of aggregates determines their decomposition. For siliceous aggregates, such as granite and sandstone, decomposition appears when exposed to a temperature of about 573 °C, changing the form of quartz [72]. For carbonate aggregates, when the temperature exceeds 700 °C, aggregates experience the decomposition process [40], as shown in SEM (scanning electron microscopy) images of Fig. 5. Therefore, concretes containing aggregates with different mineralogy and porosity have different thermal responses.
SEM analysis of concretes at 20 °C (left) and 750 °C (right), reproduced from Biolzi et al. [84]
3.4 Oxidation and melting of fibers
Fibers, especially steel fibers, can effectively inhibit the propagation of cracks in concretes. But, under high temperatures, the physical and chemical modifications of fibers deteriorate the inhibiting effect on cracking.
Polypropylene fibers melt and evaporate at relatively low temperature (low melting point), losing the bridging effect on the crack propagation [84] and leading to the fast propagation and coalescence of micro cracks (see Fig. 2).
For concretes reinforced with steel fibers, their oxidation deteriorates the mechanical performance of hybrid fiber-reinforced concretes [85]. Below 700°C, the high temperature produces a negligible effect on the mechanical properties of steel fiber, except for the lower bonding strength caused by the decomposition of the cement matrix. When the temperature exceeds 700 °C, steel fiber starts to experience oxidation (Fig. 6), causing the weakening of the mechanical properties [84].
Steel fiber at 750 °C, reproduced from Biolzi el al. [84]
4 The influence of the testing factors
The testing conditions have a considerable influence on the behavior of concretes exposed to high temperature, in particular: heating method, heating process, and testing condition.
4.1 Heating method
The heating method refers to the equipment used for heating the specimens. The heating method influences the accuracy of the heating rate and target temperature during the heating process. According to the equipment used in several studies [1, 17, 21, 22, 25, 26, 37, 53, 86,87,88], there are three heating methods: electric furnace, fire, and radiant heater (Fig. 7).
The electric furnace is the most widely used heating device [1, 17, 21, 22, 37, 53, 86]. The heating rate, target temperature, holding time, and cooling rate can be precisely controlled. Heating by fire is the best heating method to simulate real condition [25, 91]. However, the heating rate and target temperature cannot be precisely controlled. The radiant heating [26, 87, 88] can heat the specimens at a distance. Therefore, the various changes in concrete specimens during the heating process can be monitored. The holding time and target temperature can be controlled. However, it is not easy to control the heating and cool rate precisely [26, 88].
4.2 Heating process
The heating process, including the heating rate, holding time at the target temperature, and the cooling rate, has an important influence on the mechanical behavior of concretes exposed to high temperature.
The heating rate has a significant influence on thermal gradient and vapor process in concretes [92, 93]. In general, a higher heating rate produces a larger thermal gradient and a higher vapor pressure in concretes, accelerating the propagation of cracks at high temperatures. To analyze the residual performance of the thermally damaged concrete, a relatively low heating rate, ranging from 0.5 to 10 °C/min, is widely [17, 21, 28, 37] adopted for keeping the integrity of the specimen for the mechanical tests.
It has been reported that longer holding time produced more serious deterioration in compressive strength [94]. When the lower heating rate, ranging from 1 °C/min to 10 °C/min, is adopted, it is recommended to keep the target temperature for 1 or 2 h to achieve a uniform thermal field in the specimen [30, 92]. A higher thermal gradient is generated from the center to the surface with a fast cooling rate. It produces a thermal shock and causes further deterioration. The cooling methods influence the release of heat and the post-fire chemical reactions of the thermally damaged concrete. In several studies [17, 53, 68, 86], the heated concretes were naturally cooled in the furnace, with cooling rate ranging from 0.2 °C/min to 2.5 °C/min. In [95, 96], the specimens were put in water directly after heating to achieve a fast cooling rate. Fast cooling rates can cause higher thermal stress, which can further deteriorate thermally damaged concretes. The water cooling provides enough water for post-fire chemical reactions.
4.3 Testing condition
The testing conditions of the thermally damaged specimens include stressed, unstressed, and unstressed residual [40, 84, 97, 98].
For the stressed test, the specimens are heated with an imposed stress level and tested when the targeted temperature is reached [84]. During the unstressed test, the specimens are heated without pre-stress and tested at the target temperature. From the fire environment perspective, the stressed test can better describe the performance of concretes compared to the other two testing conditions [30]. Under the stress test, specimens have higher compressive strength, as shown in Fig. 8 [84]. The nominal compressive strength is the ratio of the compressive strength of the thermally damaged concrete to that of the unheated one. The external load produces a restriction effect on the extension of micro cracks caused by the thermal stress and the vapor pressure. For concretes including steel fibers, the fracture energy under the stressed testing condition is lower than that under the residual testing condition. The hybridization of the PP and steel fiber produces a synergistic effect on compressive strength and fracture energy of the thermally damaged hybrid fiber [99, 100]. For the unstressed residual testing condition, micro cracks are introduced in the ITZ by the thermal shrinkage during the cooling period, leading to the further deterioration of the thermally damaged concrete [40].
The effect of the testing condition on compressive strength of concretes with different aggregates: a unstressed, b stressed, c unstressed residual strength (data from Mehta et al. [40])
5 Mechanical performance of thermally damaged hybrid fiber-reinforced concretes
The mechanical properties of hybrid fiber (PP and steel fiber)-reinforced concretes under the unstressed residual test condition are detailed here, considering the heating rates ranging from 0.5 to 13.3 °C/min. Table 1 summarizes several reported experimental tests in the literature on thermally damaged steel and polypropylene hybrid fiber-reinforced concretes, where the mechanical properties are compared in the sub-sections.
5.1 Compression properties
The compression stress–strain relationship is an important property of fiber-reinforced concretes [37]. High temperature deteriorates the micro structure and affects the damage evolution, which is directly reflected by the compression stress–strain curve. Randomly distributed fibers, especially steel fibers, in the thermally damaged concrete bridge cracks and change the relationship between the compression stress and axial strain, lateral strain, and volumetric strain.
Figure 9 shows the nominal stress–strain relationships of thermally damaged steel and polypropylene hybrid fiber-reinforced concretes under different pre-heating levels. Nominal stress is the ratio between the stress and the compressive strength of the concrete mixture. The temperature up to 200 °C had a negligible effect on the developing trend of the compression stress–strain curve, except for a decrease in the compressive strength. When the temperature was increased to 300 and 400 °C, the stress–axial strain curve reflected the internal material deterioration and the considerable reduction of stiffness and strength. And, the stress–lateral strain curve presented an obvious nonlinearity before peak stress, indicating the appearance of the micro structure deterioration. The stress–strain curves presented a wider strain range when the temperature was increased to 600 and 800 °C, with a sharp reduction on the compressive strength as a consequence of the spread macro damage.
The nominal residual compressive strength of steel and polypropylene hybrid fiber-reinforced concretes is shown in Fig. 10 (according to Table 1). The nominal residual compressive strength is adopted. It is calculated by dividing the residual compressive strength of the thermally damaged concrete by the unheated one. The temperature in most of the reference studies ranges from 200 to 900 °C. The temperature above 900 °C is not considered in this study. As shown in Table 1, the thermally damaged hybrid fiber-reinforced concrete includes both normal-strength concrete (NS) and high-strength concrete (HS). The compressive strength of the unheated concrete varies from 40 to 130 MPa. The nominal residual compressive strength decreases with the elevated temperature above 300 °C, with a 90% drop when the temperature reaches 900 °C. For temperatures below 300 °C, the nominal residual compressive strength has a small fluctuation around the unheated concrete level. In addition to the high temperature, the compressive strength of the thermally damaged concrete is also affected by many other factors such as the aggregates and cement, as discussed in Sect. 2. As expected and detailed in reference [103], the nominal residual compressive strength of the normal-strength concrete is lower than that of the high-strength concrete.
Nominal residual compressive strength of thermally damaged steel and polypropylene hybrid fiber-reinforced concretes
Under high temperatures, the propagation of the micro cracks and the decomposition of the bulk compositions significantly change the micro structure, leading to the lower stiffness of the cement matrix [40]. For the hybrid fiber including the synthetic fiber with a low melting point, the melting and evaporation of PP fibers increase the porosity of concretes, leading to the decrease of the elastic modulus.
Figure 11 presents the relationships between the temperature and the nominal residual compression elastic modulus of the thermally damaged hybrid fiber-reinforced concrete according to the studies reported in Table 1. The nominal residual compression elastic modulus is the ratio of the modulus of elasticity at high temperature to that at room temperature. The nominal residual elastic modulus decreases with the high temperature. Compared to the nominal residual compressive strength, the elastic modulus decreases more significantly with the high temperature. The nominal residual elastic modulus experiences a drop of at least 30% when the temperature increases from 20 to 300 °C, which has a negligible effect on the residual strength. It may relate to the propagation of the micro cracks in ITZ.
Nominal residual elastic modulus of thermally damaged steel and polypropylene hybrid fiber-reinforced concretes
It is worth noting that some differences in nominal residual elastic modulus appear for different studies at the same high temperature when the temperature exceeds 200 °C. On the one hand, the heating process has a significant influence on the nominal residual elastic modulus. For example, the materials and compressive strength in [107] and [109]-1 (see Table 1) were similar, but the nominal elastic moduli were quite different. It might be attributed to the different holding times at the targeted temperature. On the other hand, other factors relating to the mixture and fibers also influence the residual elastic modulus. The replacement of the cement with silica fume and slag reduces the residual elastic modulus. In [31], partially replacing the ordinary Portland cement (OPC) with metakaolin and silica fume decreased the nominal residual elastic modulus at 600 °C and 800 °C, as shown in Fig. 11. It is related to the change of porosity [114]. In general, silica fume and slag reduce the porosity, leading to a higher vapor pressure at the same high temperature. The volume fraction of steel fiber has a negligible effect on the nominal residual elastic modulus according to [109].
5.2 Flexural strength
The tensile-related mechanical properties of concretes are strongly dependent on the onset, propagation, and coalescence of both micro and macro cracks [1, 40]. The presence of fibers improves the tensile performance of concretes at room temperature. Steel fibers with high tensile strength produce a strong restriction effect on the extension of cracks under tension or flexural loadings [8, 9]. Hooked-end steel fiber provides a higher enhancement to the tensile strength than that of straight steel fiber. PP fiber with relatively low tensile strength produces negligible enhancement to the tensile strength [115]. The PP fibers are more likely to debond with the cement matrix or break, losing the bridging effect. At high temperatures, steel fiber-reinforced concretes have a better tensile performance than that of plain concretes and PP fiber-reinforced concretes due to the thermal stability of the steel fibers [116, 117]. Steel fibers can still bridge the cracks at high temperatures, delaying crack propagation. For the PP fiber-reinforced concrete, the reduction of the tensile strength is lower compared to the plain concrete when the temperature is under 400 °C. For higher temperatures, PP fibers melt and evaporate, creating extra paths for water evaporation. The damage caused by the vapor pressure can be reduced by exploiting the synergy effect of steel and PP fibers.
Figure 12 shows the typical flexural performance of the thermally damaged steel and polypropylene hybrid fiber-reinforced concrete. At room temperature, the flexural stress–CMOD (crack mouth opening displacement) curves show an almost linear increase and then a slight nonlinear hardening up to the peak point. For a temperature of 200 °C, the peak stress slightly grows, probably due to further curing of the matrix. When the temperature reaches 300 °C and 400 °C, the curves are more nonlinear before the peak load, which have a considerable reduction with a larger CMOD. The temperatures of 600 °C and 800 °C produce a more serious deterioration on the material with a reduced bonding of the steel fiber due to the decomposition of the matrix, and then a drastic worsening of the flexural response.
Figure 13 presents the nominal residual flexural strength of steel and polypropylene hybrid fiber-reinforced concretes. The nominal flexural strength is the ratio of the residual flexural strength at high temperature to that at room temperature. The nominal residual flexural strength decreases with the increase of the elevated temperature. In general, the nominal residual flexural strength decreases slowly when the temperature increases from 20 to 200–300 °C. After that, the nominal residual flexural strength decreases quickly, reaching about 0.2 at 800 °C. A slight increase of the nominal residual flexural strength was observed in [100] and [103]-2 when the temperature was increased to 200 °C and 450 °C. It might be attributed to the hydration of the unhydrated cement particles, which reduced the original defects in the concrete.
Nominal residual flexural strength of thermally damaged steel and polypropylene hybrid fiber-reinforced concretes
The volume fraction of the hybrid fiber also influences the nominal residual flexural strength of the hybrid fiber-reinforced concrete. In [105], when the volume fraction of the PP fiber increased from 0.11 to 0.22%, a slightly higher nominal residual flexural strength was observed when the temperature increased from 20 to 600 °C. The strength of concretes is also a key factor for the residual flexural strength. In [103], the hybrid fiber-reinforced normal-strength concrete had lower nominal residual flexural strength than that of the hybrid fiber-reinforced high-strength concrete. It might relate to the differences in porosity and hydration of the unhydrated cement particles between normal-strength and high-strength concrete.
5.3 Fracture toughness
Fracture toughness is an important parameter for evaluating the energy absorption capacity of cementitious materials. In this section, only the flexural fracture toughness is analyzed. The fracture toughness of cement-based materials is generally defined as the area under the load–deflection curve [118, 119]. The presence of the steel fiber produces a positive effect on the fracture toughness [120, 121]. However, the PP fiber has no significant effect on the fracture toughness [122,123,124]. Under high temperatures, steel fiber still produces an enhancement in the residual fracture toughness due to the high thermal stability [122]. For concretes reinforced by PP fibers, no significant enhancements in the fracture toughness are observed at high temperatures. The PP fibers melt and evaporate at high temperatures, losing the bridging effect.
Figure 14 shows the nominal residual fracture toughness of the thermally damaged hybrid fiber-reinforced concrete. The nominal residual fracture toughness is the ratio of the fracture toughness at high temperature to that at room temperature. In general, the nominal residual fracture toughness decreases with the temperature. When the high temperature reaches 600 °C and 800 °C, the nominal residual fracture toughness decreases to 0.3 and 0.2. The compressive strength of the concrete has a significant influence on the nominal residual fracture toughness of the hybrid fiber-reinforced concrete. The thermally damaged hybrid fiber-reinforced normal-strength concrete [103]-1 had higher nominal residual fracture toughness than that of the thermally damaged hybrid fiber-reinforced high-strength concrete [103]-2. It might relate to the difference in the porosity between normal- and high-strength concrete. In general, high-strength concrete contains silica fume or slag. They reduce the porosity of the cement matrix [40], leading to the serious thermal damage caused by the higher vapor pressure.
Effect of high temperature on nominal residual fracture toughness of steel and polypropylene hybrid fiber-reinforced concretes
6 Post-fire re-curing effect
For thermally damaged hybrid fiber-reinforced concretes, the mechanical performance deterioration relates to the physical and chemical changes [125]. The chemical changes are caused by the decomposition of the cement matrix and aggregates [23, 126]. The decomposition process produces some compositions that can react with water and carbon dioxide during the post-fire re-curing stage, producing compositions similar to those of the hydrated cement [40, 127] matrix. The rehydrated compositions fill or narrow the cracks caused by the high temperature, leading to the partial recovery of the mechanical properties, as shown in Fig. 2.
6.1 Post-fire re-curing methods
The recovery of the thermally damaged hybrid fiber-reinforced concrete is related to the chemical reaction of the decomposition products. The rehydration of dehydrated products is a water-ingress-controlled process. The water content in the post-fire re-curing environment influences the rehydration level and the rehydrated products. The post-fire re-curing can be obtained by three different methods. The first method is the water re-curing [52, 128]. The naturally cooled concrete is submerged in water for few or dozens of days. The second method is the water–air re-curing [129]. After some days of water re-curing, the specimens are re-cured in air for some days. The third method is the air re-curing [78, 130]. The naturally cooled thermally damaged specimens are cured in the air with a certain level of humidity for some days. The post-fire re-curing method has significant effect on the rehydration process as shown in Fig. 15.
X-ray powder diffraction (XRD) analyses of different thermally damaged concretes with different re-curing method, data from Akca et al. [78]
6.2 Chemical mechanism of post-fire re-curing
As stated in Sect. 3.3, the cement matrix experiences a decomposition process, producing CaO, CnS, and other compositions, as shown in Fig. 16. During the post-fire re-curing stage, these decomposition products react with water and carbon dioxide, reproducing some hydrated compositions. CaO reacts with water and carbon dioxide, re-generating Ca(OH)2 and CaCO3 [30, 78]. The CnS, such as C2S and C3S, reacts with water, forming C–S–H in the damaged cement matrix [81]. These rehydrated compositions fill or narrow the thermal-induced cracks, increasing the integrity and the strength of the thermally damaged cement matrix (see Fig. 2). However, the rehydrated compositions may also produce a negative effect. During the post-fire re-curing stage, CaO reacts with water and produces Ca(OH)2. The volume of the Ca(OH)2 is larger than that of CaO, leading to the expansion of cracks in the thermally damaged concrete [131]. Therefore, the formation of the Ca(OH)2 leads to further deterioration of the thermally damaged concrete.
Chemical changes of the hardened cement paste at hydrated process, high temperatures, and after post-fire-curing
According to [78], the proper post-fire re-curing produces a positive effect on the mechanical properties of the thermally damaged concrete. However, it could only partially recover the strength compared to the pre-heated concrete. It mainly relates to the rehydration process and rehydrated products. Only some cracks can be partially healed by the rehydrated compositions [128]. Large cracks may not be completely filled by the rehydrated composition (see Figs. 2 and 17). For the cracks in ITZ, the rehydration process can only start from the side of the cement matrix. The aggregates are unlikely to be filled by the rehydrated products. The aggregate cracks caused by the physical and chemical changes cannot be repaired by post-fire re-curing. The micro structure of the rehydrated cement matrix is not as tight as the pre-heated cement matrix, as shown in Fig. 18. It is related to different hydration speeds [132]. The hydration speed of the dehydrated products is faster than that of the unhydrated cement grains [133]. For unhydrated cement grains, generally, it takes about 2 months or longer time to finish the hydration process. The dehydrated compositions complete the rehydration process by 2 or 3 weeks.
Cracks partially filled with new forms after water re-curing, reproduced from Akca et al. [134]
Micro structures of cement matrix before heating (left) and after water re-curing (right), reproduced from Akca et al. [134]
6.3 Recovery effect of the post-fire re-curing
Studies dedicated to the effect of the post-fire re-curing on the mechanical properties of thermally damaged steel and polypropylene hybrid fiber-reinforced concretes are limited.
Studies indicated that water re-curing produced a positive effect on the mechanical recovery [47, 51, 78, 113], while air re-curing had a negligible or a negative effect on the recovery of the thermally damaged hybrid fiber-reinforced concrete. In [47] after exposure to 1000 °C, the compressive strength and elastic modulus decreased to 57% and 24% of the pre-heated ones, respectively. After 28 days of water re-curing, the compressive strength and elastic modulus recovered to 66% and 44% of the pre-heated ones, respectively. However, for air re-curing, a negative effect was observed. The compressive strength and elastic modulus decreased to 44% and 14% of the pre-heated values, respectively. The hybrid fiber containing more steel fibers had a better recovery effect. In [78], the dry air and water re-curing were compared. The compressive strength, tensile strength, and Young’s modulus did not recover clearly after 3 months of dry air curing. However, water re-curing provided a significant recovery effect on the thermal damage. The nominal strength raised from 57 to 82% and the elastic modulus from 23 to 60%. The water re-curing partially repaired the thermal damage, recovering the initial stress, critical stress, compression stress, and toughness [113], as shown in Fig. 19. The effect was related to the change in the micro structure. During the water re-curing, the rehydrated compositions filled the cracks and voids, enhancing the strength of the cement matrix [135]. The recovery of ultrasonic pulse velocity and permeability of the thermally damaged hybrid fiber-reinforced concrete after 3 months of water re-curing was shown.
Initiation stress, critical stress, and compression strength (a) and toughness (b) of thermally damaged and water re-cured steel and polypropylene hybrid fiber-reinforced concretes, data from He et al. [113] (P and S are the thermally damaged hybrid fiber-reinforced concrete including polypropylene fiber and wave-shaped steel fiber, respectively, PW and SW are the water re-cured P and S, respectively)
Water re-curing changed the fracture process of the thermally damaged hybrid fiber-reinforced concrete as shown in Fig. 20. Water re-curing reduced the number of branching crack and eliminated the multiple main cracks for thermally damaged hybrid fiber-reinforced concretes. After water re-curing, the fracture paths were similar to that before heating. During the water re-curing stage, the rehydrated phases, like C–S–H, bridged the micro cracks and narrowed the macro cracks, increasing the bonding strength between fibers and matrix, which enhanced the restriction effect on the propagation of crack. The enhanced bonding strength improved the stress transfer ability across the cracks by the fibers, reducing the stress concentration at the crack tip.
The fracture morphology of hybrid fiber-reinforced concretes exposed to high temperature and water re-curing, data from He et al. [51]
The limited studies indicated the better recovery effect of the water re-curing on the thermally damaged hybrid fiber-reinforced concrete. However, the corrosion of the steel fiber, if adopted, caused by the water re-curing and the related deterioration have not been reported. Therefore, further research should be conducted to investigate the corrosion of the steel fiber during the water re-curing stage and to evaluate the durability of the water re-cured thermally damaged hybrid fiber-reinforced concretes.
7 Conclusions
In this paper, the effects of elevated temperatures and post-fire re-curing on the performance of steel and polypropylene hybrid fiber-reinforced concretes were reviewed. The thermal damage mechanism and the associated factors influencing the residual mechanical performance were analyzed. The partial recovery effects of different post-fire re-curing methods were compared. Based on the review and analysis of previous studies, some conclusions were drawn.
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1.
The deterioration of the hybrid fiber-reinforced concrete is the joint effect of physical and chemical damages. The physical damages are mainly caused by the vapor pressure, thermal stress, thermal expansion, and evaporation of synthetic fibers. The chemical ones refer to the decomposition of cement matrix and aggregation and oxidation of steel fiber. The high temperature produced a significant effect on the physical and chemical damage.
-
2.
The performance of the thermally damaged hybrid fiber-reinforced concrete can be affected by testing factors, such as heating method, heating process, and testing condition. The variation of these factors results in discrepancies in the mechanical performance. The residual compressive strength with the stressed testing method is higher than that with unstressed one. A higher heating rate leads to a more severe deterioration on the thermally damaged hybrid fiber-reinforced concrete.
-
3.
In general, thermal damage of the hybrid fiber-reinforced concrete increases with the temperature. The nominal residual compressive strength keeps almost the unheated value when temperature is below 200 °C and deteriorates when the temperature exceeds 200 °C. The nominal residual flexural strength, elastic modulus, and fracture toughness also decrease when the temperature increases from room temperature to elevated temperatures. The temperatures above 300 °C lead to the sharp deterioration on the mechanical strengths of the hybrid fiber-reinforced concrete.
-
4.
Water supply is a key factor influencing the post-fire re-curing effect. Water re-curing produces a better partial recovery effect on mechanical properties of the thermally damaged concrete than air re-curing. Water re-curing provides the water for the rehydration process in the thermal-induced cracks.
8 Recommendations for future research
Based on the present review of the performance of thermally damaged steel and polypropylene hybrid fiber-reinforced concretes, further studies are recommended.
-
1.
The mechanical performance by the stressed testing method should be further investigated. The stressed test can better describe the performance of concretes compared to the other test conditions. Most studies have been carried out using the unstressed residual method.
-
2.
Further analyses on the post-fire water re-curing of thermally damaged hybrid fiber-reinforced concretes are needed. The limited studies indicate the better recovery effect of the post-fire water re-curing. However, the corrosion of the steel fiber caused by the water re-curing and the durability of the thermally damaged hybrid fiber-reinforced concrete have not been thoroughly investigated. The influence of the water re-curing on the corrosion of steel fibers as well as steel rebars needs to be investigated in detail to assess the long-term mechanical performance of the thermally damaged reinforced concrete. These further understandings are indispensable for real-time applications of the water re-curing method to fire-damaged concrete structural components.
-
3.
The performance of full-size hybrid fiber-reinforced concrete structural elements at high temperatures is worth investigating. The main concern is the retention of the load-bearing capacity of full-scale hybrid fiber-reinforced concrete beams and columns after exposure to high temperatures. The small-scale standard laboratory test specimens cannot fully and accurately represent the actual performance of the full-scale structures.
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Acknowledgements
F. He acknowledges the China Scholarship Council Grant, China Postdoctoral Science Foundation (Certification Number: 2023M733759) and National Natural Science Foundation of China (Certification Number:52304106).
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Fengzhen He: Conceptualization, Methodology, Investigation, Data analysis, Validation, Writing - Original Draft, Visualization. Luigi Biolzi: Conceptualization, Methodology, Investigation, Validation, Writing - Review & Editing, Supervision. Valter Carvelli: Conceptualization, Investigation, Data analysis, Validation, Writing - Review & Editing, Supervision. Xiaowei Feng: Writing - Review & Editing.
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He, F., Biolzi, L., Carvelli, V. et al. A review on the mechanical characteristics of thermally damaged steel and polypropylene hybrid fiber-reinforced concretes. Archiv.Civ.Mech.Eng 24, 69 (2024). https://doi.org/10.1007/s43452-024-00880-2
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DOI: https://doi.org/10.1007/s43452-024-00880-2