Development of a novel bio-inspired cement-based composite material to improve the fire resistance of engineering structures

Construction and Building Materials 225 (2019) 99–111

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Development of a novel bio-inspired cement-based composite material to improve the fire resistance of engineering structures Tong Zhang a,b, Yao Zhang a,b, Ziqi Xiao a,b, Zhenglong Yang c,d, Hehua Zhu a,b, J. Woody Ju e, Zhiguo Yan a,b,⇑ a

State Key Laboratory for Disaster Reduction in Civil Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, China Department of Geotechnical Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, China c Key Laboratory of Advanced Civil Engineering Materials of Ministry of Education, Tongji University, 4800 Cao’an Road, Shanghai 201804, China d Department of Polymeric Materials, School of Materials Science and Engineering, Tongji University, 4800 Cao’an Road, Shanghai 201804, China e Department of Civil and Environmental Engineering, University of California, Los Angeles, CA 90095, USA b

h i g h l i g h t s  A novel bio-inspired cement-based composite system is proposed.  The self-retardant mechanism of APP-PER-EN composite is introduced in detail.  The thermal behavior of APP-PER-EN composite is characterized.  One-side heating tests are conducted to reveal the interaction of novel system.

a r t i c l e

i n f o

Article history: Received 11 December 2018 Received in revised form 16 May 2019 Accepted 16 July 2019

Keywords: Cement-based composite Self-retardant Bio-inspired material Fire resistance

a b s t r a c t The mechanical properties of concrete can degrade significantly under high temperature, posing a threat to the safety of structures. In this work, a novel bio-inspired cement-based composite material that can improve the fire resistance of engineering structures is put forward. This bio-inspired cementitious system, which functions in a manner similar to that of sweat glands and skin in the human body, consists of APP-PER-EN composite, synthetic reinforcing fibers, and concrete binder. The APP-PER-EN composite is mainly composed of ammonium polyphosphate, pentaerythritol, and melamine, with low-density polyethylene as the base material, which is prepared by melt compounding in a twin-screw extruder followed by grain cutting molding technique. At the micro level, multiple analysis methods are used, including Xray diffraction, Fourier transform infrared ray, thermo-gravimetric analysis, differential scanning calorimeter, and scanning electron microscopy, to characterize the thermal and flame retardant behavior of this novel composite. At the macro level, the one-side heating tests indicate that the thermally triggered fire-resistant system can exhibit significant thermal insulation effect through melting, connecting, foaming, and overflowing. Therefore, the novel composite system is expected to exhibit a promising prospect in the fire resistance of engineering structures. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Generally, concrete exhibits better fire resistance than steel. However, tremendous fire disasters can seriously degrade the mechanical properties of concrete because of high temperatures (up to 1200 °C in tunnels) [1–3]. The deterioration of mechanical properties of concrete at elevated temperatures, which has been comprehensively investigated since the 1950s [4], can pose a serious threat to the safety of engineering structures. At the macro

⇑ Corresponding author. E-mail address: [email protected] (Z. Yan). https://doi.org/10.1016/j.conbuildmat.2019.07.121 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

level, the fire damage during or after a high-temperature exposure can be estimated through the decline of macro-mechanical indexes, such as Young’s modulus, shear modulus, tensile strength and compressive strength [5]. At the micro level, the degradation of concrete may be attributed to the decomposition of cement paste, aggregate cracking and the decline of mechanical properties of the aggregate/matrix interface [6–8]. In order to reduce the loss caused by fire incidents, several passive fire protection (PFP) methods have been developed to protect the concrete structure from a fire in the previous decades. Current PFPs can be broadly summarized as follows:

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(I) Building a fire barrier near the surface of structure components, such as installing fire prevention board or ceramic fiber mats. Particularly, fiber-metal laminates (FMLs), which are hybrid composite materials built up from interlacing layers of thin metals and fiber reinforced adhesives, have been highlighted due to the optimum mechanical properties [9– 11]. (II) Incorporating fiber systems are adopted in concrete for releasing internal evaporative pressure and bridging cracks caused by high thermal loadings, such as polypropylene (PP) fibers, basalt textile or steel fibers [12–16]. It is evident that the addition of steel fibers can restrain the propagation of micro-cracks in concrete under high temperature. Besides, incorporating PP fibers are effective in providing channels for the release of internal pressure via melting at high temperatures and connecting internal pores [17]. In particular, PP fibers are helpful in lightening the explosive spalling of high-strength concrete, which can be partly explained by the introduction of additional interfacial transition zones (ITZ) between the fibers and the cement paste [18]. (III) Various intumescent organic coating systems based on aluminosilicate source and fire-protecting mortars have been a common solution. Suitable aluminosilicate source admixtures include by-products, such as silica fume, fly ash and phase change material, with fly ash which being more commonly used in high volume applications [19–25]. The high porosity and specific surface of the added by-products increase the mortar insulating capacity and tendency to retain water [26–27]. Although the aforementioned methods can to some extent enhance the fire resistance of structures, they are all passive modes of protection and have several disadvantages in practical application: (a) Adverse effect on structural maintenance. For both system (I) and (III), the covering of the surface of the component hinders the direct observation of crack propagation and leakage water in the underground structure, which can be a great challenge to structure safety. Besides, the coating system may also affect the appearance of the surface to which they are attached, and the concrete structure can be subjected to water absorption and retention [11]. (b) The increased additional economic burden. Although the brushing of fireproof coating appears to be more feasible, the coating falls off easily when the moisture is high, leading to regularly repainting every year which can be wasteful and tedious. (c) Negative impact on structural stress. For system (III), in terms of structural behavior, the coating systems contribute to the weight of a structure without adding to its stiffness or strength because of the plasticity of foaming material. Moreover, the addition of PP fibers can delay spalling to some degree, but no significant improvement in heat insulation has been observed under high temperatures. Therefore, more effective approaches are necessary to improve the fire resistance of engineering structures. This paper proposes an active fire protection (AFP) method achieved through a novel bio-inspired self-retardant cementitious composite system. In comparison with the traditional passive fire protection (PFP) methods, advantages of the novel system are summarized as follows: Firstly, replacing the traditional passive insulation, the selfretardant composite layer can respond actively to the fire in a manner similar to the perspire process of skin issue through four stages (details in Section 2). Besides, the mechanism of AFP method can

be seen as a hybrid effect of absorption, dissipation, and insulation of heat. And the flame retardant effect of the novel system has been demonstrated at both micro level and macro level (details in Section 4 and 5). Furthermore, the novel composite system is expected to achieve the synergistic effect of fire protection and carrying capacity. In actual applications, the composite system can be used to replace the normal protective concrete layer to cover the steel bars. Additionally, it is convenient for crack detection and structural maintenance.

2. Mechanism of the bio-inspired self-retardant cementitious composite system Evaporative heat dissipation is an effective approach for the human body to maintain temperature balance. Sweating is the most general and effective form of heat dissipation, typically when the ambient temperature increases or activity intensifies. Consequently, when people work in a high-temperature environment, they usually sweat profusely. The normal sweating process under high temperatures can be typically divided into four parts [28– 31]: (a) the eccrine sweat gland secretes sweat under a high temperature; (b) sweat spreads to the skin surface by the sweat duct; (c) sweat accumulation forms a thin layer on the skin surface; and (d) sweat evaporates and takes away the heat. Fig. 1(a) illustrates the sweating process in detail. Based on the heat self-regulation mechanism of the sweat gland, a novel bio-inspired self-retardant cementitious composite is designed to improve the fire resistance of engineering structures. Fig. 1(b) illustrates the concept of the newly developed selfretardant composite system, which is composed of the retardant APP-PER-EN composite, synthetic reinforcing fibers and concrete binder. In engineering applications, the concrete protective layer for steel bars in concrete components can be replaced by this self-retardant cementitious composite. Under fire condition, the self-retardant function is achieved through the following stages: Stage I: Melting. Between 100 and 160 °C, both synthetic fibers (such as polypropylene or polyethylene fibers) and the selfretardant APP-PER-EN system absorb a substantial amount of heat via melting during the stage. Stage II: Connecting. When the temperature is beyond 170 °C, micro-channels caused by the melting fibers form. Meanwhile, micro-cracks can also be induced by the thermal load. Furthermore, the connection between micro-cracks and micro-channels can facilitate the outflow of the retardant agent. Apparently, at this stage, a large number of connecting channels are formed, which are similar to sweat ducts. Stage III: Foaming and overflowing. Above 300 °C, the thermally triggered APP-PER-EN composite system starts to expand and foam due to comprehensive internal esterification and chain formation reactions among ammonium polyphosphate, pentaerythritol, and melamine [32–34]. As a result, the foaming retardant material, characterized by intumescent structure, fills the internal cracks and functions as thermal resistance. Furthermore, the retardant can overflow to the heated surface through micro-cracks and gradually forms a fire insulation layer driven by some inflammable gases [35]. Stage IV: Formation of the thermal insulation layer. The fire insulation layer covering the surface of the self-retardant composite layer is essential in blocking the heat transfer into the internal concrete. Meanwhile, at a certain thickness, the self-retardant composite layer forms a ‘‘honeycomb” structure, which helps further reduce the thermal conductivity. In analogy to the function of the human sweat glands and skin, a novel self-retardant composite system is designed to overcome

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(a) Sweat

Sweat gland Skin ssue

(b) Under normal condion

Stage 4: Formaon of thermal insulaon layer

Sustain the working load together

Reconstrucon Thermal insulaon layer

Under fire condion Stage 1: Melng

Reinforced concrete layer The novel composite

Synthec fibers

Stage 3: Overflowing Trigger point

Self-retardant layer T Stage 2: Connecng

The overflowed flameretardant material

Micro channels Fig. 1. Mechanism of self-retardant sweat-gland functioned cementitious composite system with application to underground tunnels.

the disadvantages of traditional solutions. Compared with traditional methods, this novel material system achieves a real ‘‘active mode” of fire resistance. In addition, this new method focuses on utilizing the synergism of concrete, self-retardant agent, and hybrid fibers to achieve self-retardant structures. In actual projects, the self-retardant composite system can be used to replace the normal protective concrete layer. During the normal operation period, the self-retardant composite layer sustains the working load as one part of the component. After a fire disaster, the selfretardant layer will become a porous material, which can be easily removed and reconstructed. In this paper, the constituents of bio-inspired composite materials and corresponding thermal properties obtained from the thermogravimetric analysis are described in detail. In addition, one-side

heating tests are conducted to make an initial validation of the effectiveness of the self-retardant cementitious composite. Additionally, two types of mixing inclusions and two types of geometries are taken into consideration to attain a desired fire-resistant effect.

3. Experimental preparation 3.1. Materials The raw material for flame-retardant APP-PER-EN composite system is commercially obtained and used without further purification. As the acid source, ammonium polyphosphate (APP) with an average degree of polymerization n > 1000 is supplied by Hangzhou Flame Retardants Chemical Company. Organic chemically-grade pentaerythritol (PER) functioned as a carbon agent is purchased from Shanghai Chemical Reagent Company. As the foaming agent, analytically-

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Table 1 Material composition of additive constitutes in APP-PER-EN composite system. Specimen number

S1 S2 S3 S4

Material composition of constitutes (Quality ratio) Fiber

Particle

None APP-PER-EN system I: EN: APP:PER = 2:2:1 APP-PER-EN system II: SA: EN: APP: PER = 2:1.2:1.2:0.6 Polypropylene (PP) fiber

None None None APP-PER-EN system II

grade melamine (EN) is obtained from Shanghai Source Industrial Company, and low-density polyethylene (LDPE) as the base material is supplied by Suzhou Oleochemicals Company. Additionally, commercially stearic acid (SA) purchased from Suzhou Bohui Chemical Company is added in APP-PER-EN composite system II as the lubricant agent. All detailed information about the specific quality ratio of each constituent is listed in Table 1. The raw material for double-layer concrete specimen contains cement (ordinary Portland cement 42.5, density: 3.08 g/cm3, fineness: 2.7%), sand (fineness modulus: 3.4, density: 2.54 g/cm3, mud content: 0.5%), aggregate (size gradation: 5–15 mm, density: 2.62 g/cm3, acicular content: 1.5%) and river water with specific quality ratio as listed in Table 2. 3.2. The preparation of flame-retardant composite The novel self-retardant composite is mainly composed of APP, EN, and PER with LDPE as the basic material. Firstly, the constitutes, containing APP, PER, EN, and SA (only for APP-PER-EN composite II), are sampled and weighed according to the specific material composition ratio shown in Table 1 and mixed for 5 min

in a high-speed mixer to acquire uniform distribution. Afterward, LDPE is introduced and another 5-minute mixing is conducted to guarantee the blending homogeneity of APP-PER-EN mixture. Noticed that due to the confidentiality of materials/compositions which is filed as technology disclosure [36], detailed information on the specific ratio of constitutes used in the composite system will not be discussed here. During the polymerization process, LDPE is firstly introduced in the twin-screw extruder as a ‘‘washing material” until the color of extruded material to be pure white. Subsequently, with the suitable temperature set-up for each section of the twin-screw extruder ranged from 100 to 170 °C, the mixture of all constitutes including LDPE is put into the feeding port. After experiencing melting and mixing processes, the extruded composite material will mold through cooling water groove driven by grain cutting machine. Through regulating the speed of grain cutting equipment, the length and diameter of fibers/particles can be appropriately adjusted. Fig. 2 provides more detailed information on the twin-screw extruder and the corresponding machine used in processing raw materials. The APP-PER-EN system I is firstly designed to be fiber-shaped because the connectivity of micro-tunnels can easily form through the melting of fibers. Additionally, the rice-shaped particle is also applied to achieve a higher mixing proportion with the retardant layer. To facilitate the production process, a part of LDPE is replaced by stearic acid (SA) in APP-PER-EN system II. The diameter of the riceshaped particle is approximately 1–2 mm, while the length of fiber is approximately 15–20 mm with a diameter of 0.8–1.0 mm.

3.3. Analysis and characterization methods 3.3.1. X-ray diffraction (XRD) To better understand the microcosmic structure and analyze the chemical components of the composite, both APP-PER-EN system I and II are analyzed by XPert Pro MRD diffractometer (PANalytical, Netherlands) with Cu Ka radiation (k = 0.154056 nm) operating at pressure/current 40 kV/35 mA. The diffraction profile is detected using a locked couple 2h scan from 10 to 90°.

Table 2 Basic parameters of dimension and material proportions for experimental samples. Sample number

S1 S2 S3 S4

Specimen size (l  w  h; mm)

Concrete layer

Retardant layer

600  400  60 600  400  30 600  400  30 600  400  30

None 600  400  30 600  400  30 600  400  30

Material proportions of concrete layer (Quality ratio)

Cement/River sand/Aggregate/ Water = 1:2.3:2.5:0.5

Volume fraction of addition in retardant layer (%) Fiber

Particle

None 2.5% 2.5% 0.5%

None None None 4.5%

Cooling molding groove

Feeding port

Grain cung Blow dryer Water pump Vacuum pump

Twin-screw electric machinery

Temperature parameters input

Fig. 2. The experimental set-up for preparation the APP-PER-EN composite using twin-screw extruder followed by grain cutting molding technique.

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T. Zhang et al. / Construction and Building Materials 225 (2019) 99–111 3.3.2. Fourier transform infrared ray (FTIR) The composite of APP-PER-EN system I and its burned charring layer in the form of powder are planished to be sampled with KBr respectively. The changes of groups in the system are analyzed through FTIR. Fourier-infrared spectra of samples are recorded by a Nicolet 380 infrared spectrum instrument (Thermo Fisher, USA) within the range of 4000–400 cm 1 using a resolution of 4 cm 1, and the number of scanning is eight.

3.3.3. Thermogravimetric analysis and differential scanning calorimetry The thermal properties and reactions of the flame-retardant composite are assessed by thermo-gravimetric analysis (TGA) and differential scanning calorimetry (DSC). Both TGA and DSC measurements of samples (approximate 5 mg) are performed on a simultaneous thermal analyzer (NETZSCH, Germany) in a continuous nitrogen atmosphere at a heating rate of 20 °C/min over the temperature range from 20 to 800 °C. And the curves of TGA and DSC are disposed by TA Universal Analysis software.

3.3.4. Scanning electric microscope (SEM) The distribution of thermal pathways formed by the formation of the retardant composite after exposure to high temperature and their morphological structures in the concrete blinder are observed and analyzed by EM-3200 SEM (KYKY, China).

(a)

3.4. One-side heating test 3.4.1. Preparation of double-layer samples One-side heating tests are conducted on double-layer samples that are 600-mm long, 400-mm wide, and 60-mm thick. In general, the thickness of the covering layer of steel bar in tunnels is approximately 15–45 mm [37]. Therefore, doublelayer samples with a 30-mm thick self-retardant layer (consisted of cement paste, river sand, aggregate, water, and APP-PER-EN composite), and a 30-mm thick normal concrete layer (without self-retardant composite) are adopted for the test as illustrated in Fig. 3. Note that the composite of APP-PER-EN system is introduced in the concrete blinder which is the same as the one used in the normal concrete layer. Therefore, the physical and mechanical properties of the double layers, such as compressive strength, bonding performance, and Young’s modulus can be basically guaranteed. In addition, the basic parameters of dimension and material proportions for experimental samples are listed in Table 2. To illustrate the effect of the fiber-shaped self-retardant material system, the maximum mixing proportion is firstly adopted for APP-PER-EN I and II, which is determined through several times of mixing. In addition, the reference specimen, made of the pure normal concrete layer, is denoted as S1. Samples made of the retardant layer mixed with APP-PER-EN I and II fibers are named S2 and S3, respectively. These samples are prepared by pouring the evenly mixed concrete mixture into plastic molds using laminated pouring method. The interval time between

(b) Flame retardant co on Embedded metal handle

Double-layer sample

K-type thermocouple

Furnace body Control part

Copper powder

Flame retardant co on

Top view

(c)

A

K-type thermocouple for concrete surface

(d)

Heang source

A

400

Furnace body

400

Needle-like thermocouple for furnace

600

Self-retardant layer

300

400

K-type thermocouple for middle interface Normal concrete layer The same concrete binder used

400 600

600

Side view

A-A view

Unit: mm

Fig. 3. Graphical description on the set-up for one-side heating test: (a) dimensions of the furnace; (b) top-view photo of the device; (c) side view of test set-up and distribution of K-type thermocouples; and (d) A-A view of the furnace and distribution of internal heating sources.

Table 3 Physical and mechanical characteristics of experimental samples. Sample number

Cement dosage (kg/m3)

28-day compressive strength (MPa)

Unit mass of concrete (kg/m3)

Slump value (mm)

S1 S2 S3 S4

612 606 604 597

61.2 ± 1.2 57.9 ± 1.5 54.2 ± 1.3 56.8 ± 3.1

2337 2305 2289 2283

96 85 76 54

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the pouring of two layers is 4 h to guarantee the bonding performance. In order to acquire temperature data in the middle interface of the double-layer, a 30-mm long steel bar is inserted at the center of the retardant layer and removed after 24 h of curing. All test samples are cured for 28 days in a laboratory with controlled temperature (20 ± 2 °C) and humidity (95% RH). Physical and mechanical characteristics of experimental specimens, including the cement dosage in kg/m3, the compressive strength, a unit mass of concrete and the slump value, are listed in Table 3.

copper powder is used to guarantee close contact between thermocouples and samples, while flame retardant cotton is employed surrounding the test sample to reduce the heat loss as demonstrated in Fig. 3(b).

4. Thermal characterizations of the flame-retardant composite 4.1. XRD analysis

3.4.2. Experimental set-up for the heating test For better observing the trigger process and internal reactions of the novel composite, high-temperature heating is applied through an electric furnace with the maximum heated temperature of 500 °C. The whole experimental set-up can be roughly divided into three parts, namely control part, furnace body and doublelayer sample with the dimension of each part illustrated in Fig. 3(c) and (d). Through pre-setting of temperature in the control part, the heating curve raises linearly to 300 °C with the maximum heating rate of 30 °C/min, and steadily increases to 500 °C after about 60 min of heating. Subsequently, the temperature inside the electric furnace maintains at around 500 °C until the end of the test. More detailed information about temperature curves will be discussed in Section 5.1, with the measured temperature curves for electric furnace shown in Fig. 8. For each double-layer sample, the whole one-side heating process lasts for nearly 3 h. The double-layer sample is placed covering the furnace mouth while the high temperature is applied to heat only one side of the sample (the self-retardant layer) as presented in Fig. 3(a) and (c). When it comes to data acquisition method, the temperature on the concrete surface (exposed to room temperature) and the middle interface between double layers can be easily obtained by inserting K-type thermocouples in the reserved hole left during the preparation of samples. Furthermore, a needle-like K-type thermocouple with the length of 200 mm is applied to monitor the real-time temperature inside the furnace shown by Fig. 3(c). Additionally, the

(a)

4.2. FTIR analysis Fig. 5(a) and (b) compares the spectra of APP-PER-EN system I with that of its charring layer after exposure to high temperature. The intense stretching vibration at 2800–2975 cm 1 for carbonhydrogen bonds (CAH), a weak stretching vibration absorption at 1640 cm 1 for carbon duplet bond (C@C) in LDPE and a distinctive

(a) Ammonium polyphosphate (APP) Melamine (EN) Low density polyethylene Ammonium phosphate

400 300 200

100 95

Transmittance (%)

500

Intensity (counts)

The XRD analysis of both APP-PER-EN composite system I and II is presented in Fig. 4. The tested result displays that low-watersolubility and preferable decentralization can be achieved through the containing of great quantities of phosphates and high polymerization degree (n > 1000). Referring to correlative literature [38], the flame-retardant composite with the addition of APP and PER has a good adhesive attraction, possesses high heat stability and forms steady intumescent charring layer under a flame.

90 85 80

100 75

0 10

20

30

40

50

60

70

70 4000

80

3500

Degree (2 theta)

3000

2500

N-H

CH

C=C

C=N

2000

1500

P=O 1000

500

Wavenumbers (cm-1)

(b)

(b)

100

500

Intensity (counts)

300 200

95

Transmittance (%)

Ammonium polyphosphate (APP) Melamine (EN) Stearic acid (SA) Low density polyethylene Ammonium phosphate (NH4PO3)

400

90 85 80

100 75

0 10

20

30

40

50

60

70

80

Degree (2 theta) Fig. 4. XRD analysis of composites (a) APP-PER-EN system I and (b) APP-PER-EN system II.

70 4000

3500

3000

N-H

O-H

-CH

C=C

C-O-C

P=O

2500

2000

1500

P-H 1000

500

-1

Wavenumbers (cm ) Fig. 5. FTIR spectra of (a) APP-PER-EN composite system I and (b) its burned charring layer.

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-7

(a) 100

-6

0

-4 -3 PER EN APP LDPE SA

-50

DTG (%/oC)

TG (%)

-5

PER EN APP LDPE SA

50

-2 -1 0

-100 0

100

200

300

400

500

600

700

800

Temperature (oC)

(b) 5

DSC (mW/mg)

0

-5

EXO

-10

PER EN APP LDPE SA

-15

-20 0

100

200

300

400

500

600

700

800

Temperature (oC) Fig. 6. (a) TG, DTG and (b) DSC curves of basic components for pentaerythritol (PER), melamine (MEL), ammonium polyphosphate (APP), low-density polyethylene (LDPE) and stearic acid (SA).

adsorption at around 1140 cm 1 for P@O bond in APP (PO34 groups) are similar for both APP-PER-EN systems. The distinctive absorption at 1500 cm 1 for stretching vibration of carbonyl (C@N) shown in Fig. 5(a) disappears in the charring layer presented in Fig. 5(b). However, the amino groups (ANH) of EN and APP in flame-retardant composite greatly strengthen their chain structure, because more amino groups of pyrophosphoric acid and polyphosphoric acid in residue substance show weak absorption at 3400 cm 1 in Fig. 5(b). In addition, the distinctive double absorption at 1250–1050 cm 1 for stretching vibration of ether bond (CAOAC) of flame-retardant composite appears in the charring layer. The change above is due to the decomposition of the organic part on flame-retardants and the chain formation in APP-PER-EN composite system.

4.3. Thermal analysis (TGA and DSC) of retardant constituents Thermo-gravimetric analysis has been widely used by most researchers to study the thermal degradation of composites. How-

ever, for the complicated composite system, the main problem in quantitative TGA experiments is the overlap of peaks in differential thermo-gravimetric curves [39]. To avoid the overlapping phenomenon, it is critical to conduct the TG for each basic retardant component firstly. Therefore, TGA of all constituents in the novel flame-retardant composite is conducted, and the result of TGA, DTG (derivative) and DSC curves are presented in Fig. 6. In addition, essential temperature points with 5 wt% mass loss (T5%), maximum decomposition temperature (T50%), and remaining residues at 800 °C of TG curves are summarized in Table 4. Ammonium polyphosphate, as the carbonization catalyst and acid source, starts to decompose at 287.1 °C, liberating phosphoric acid and amine gas at the same time. The acid, produced in reaction (a) of Scheme 1, triggers a series of reactions starting with the dehydration of carbonized compound and the subsequent chain formation [40]. Furthermore, when the temperature exceeds 320 °C, phosphoric acid dehydrated to form pyrophosphate and polyphosphate, which causes several endothermic peaks during 320–600 °C. Pentaerythritol, the carbonization agent in the APP-PER-EN system, undergoes a double-stage thermal degradation. An endothermic peak (240.7–306.3 °C) and a small endothermic peak (319.5– 381.4 °C) are attributed to the transformation of the crystal structure and melting of pentaerythritol, respectively. Furthermore, it should be noted that the overlapped temperature range, approximately 320–380 °C, for the decomposition of both pentaerythritol and ammonium polyphosphate is conducive to the chain formation between ammonium polyphosphate and pentaerythritol. Melamine, as the foaming agent, partly sublimated and decomposed at 278.4–343.9 °C, releasing some nonflammable gases, such as NH3 and CO2, which are useful in forming honeycomb chain structure layers over the substrate [41]. As presented in Table 3, the similar melting point, reaction, and decomposition temperature between pentaerythritol and melamine guarantee good synergism in the self-retardant system. Stearic acid, which functions as a lubricant in the chain formation of the system, has a single peak at 278.5 °C. In addition, the decomposition of stearic acid and the foaming of APP-PER-EN system are practically in the same temperature range. Low-density polyethylene, serving as the base material for the APP-PER-EN system, presents a single-stage mass loss during 370.2–537.3 °C. In addition, CO2 and steam, which are released from stearic acid and polyethylene, are helpful in diluting the concentration of inflammable gas and further forming an appropriate protective layer with the chain residue [42].

4.4. Thermal analysis (TGA and DSC) of APP-PER-EN composite system The interaction and chain formation of APP-PER-EN system function as key points in the flame-retardant structure. As shown in Fig. 7, the curves for TGA and DSC of the flame-retardant system can be generally divided into four stages. Stage I: For the stability stage (30–150 °C), the heat weight loss of the retardant system, with both APP-PER-EN systems I and II, are lower than 5% under this stage. The main lost components are

Table 4 Summary of TGA result in a continuous nitrogen environment. Characteristic points a

T5% (°C) T50%b (°C) T95%c (°C) Residual weight at 800 °C (%) a b c

APP

PER

EN

SA

LDPE

APP-PER-EN system I

APP-PER-EN system II

287.1 596.7 — 18.79

240.7 295.7 321.5 —

278.4 326.2 343.9 —

212.3 266.2 288.6 —

410.8 464.1 486.6 —

273.2 472.5 — 9.74

208.8 459.6 — 8.51

T5% namely the onset temperature, defined as the temperature at which 5% mass loss occurs. T50% namely the maximum decomposition temperature, defined as the temperature at which 50% mass loss occurs. T95% namely the complete decomposition temperature, defined as the temperature at which 95% mass loss occurs.

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Scheme 1. Internal chain formation reactions of APP-PER-EN composite system [34,42–45].

(a)

DTG DSC (%/oC) (mW/mg) 0

TG (%) 100

EXO 80

TG DTG DSC

60 40

-1.5 -5 -1.0

-0.5

-10

20 0.0

-15

0 0

100

200

300

400

500

600

700

800

Temperature (oC)

(b)

DTG DSC (%/oC) (mW/mg) 0

TG (%) 100

EXO 80

TG DTG DSC

60 40

-1.5 -10 -1.0

-0.5

-20

20 0.0

-30

0 0

100

200

300

400

500

600

700

800

Temperature (oC) Fig. 7. TG, DTG and DSC curves of (a) APP-PER-EN system I and (b) APP-PER-EN system II.

volatilized solvents and small molecules of polymer in a free state [40]. The small peak at approximately 120 °C in DSC curves, denoting the melting and softening processes of resin, which absorbed an amount of heat. Stage II: During the formation stage of the chai layer (150– 350 °C), APP-PER-EN system II experiences a contributing endothermic drop in weight of 30.16% at 263 °C. In contrast, the first mass loss peak in APP-PER-EN system I is considerably lower with only 9.01% at 260 °C at this stage. The decomposition and reaction of melamine and pentaerythritol (reaction (b) and (c) in Scheme 1) account for the first peak in weight loss, which leads to the exclusion of large quantities of CO2 and causes the formation of a carbon-like foam layer over the substrate. Stage III: At the formation stage of the organic layer (350– 600 °C), the weight loss reaches as high as 79.32% and 58.59% in the two flame-retardant systems, respectively. As the carbonization catalyst, ammonium polyphosphate liberates phosphoric acid which triggers a series of decomposition and esterification reactions—the dominant process of APP-PER-EN system [42–43] represented by reaction (d) and (e) in Scheme 1. As a result, an intumescent composition swells up with the formation of a foam structure until a porous foam organic layer formed [44–45]. Additionally, stearic acid as lubricant and binder in APP-PER-EN system II significantly promotes the cross-linking reaction and chain formation processes. Stage IV: During the chain loss stage (600–800 °C), both APPPER-EN systems I and II present weight losses lower than 5%, and the residual weights at 800 °C are 8.51 and 9.74%, respectively. Further heat decomposition of the charring layer mainly causes heat absorption and heat weight loss during the aforementioned stages [41]. In addition, it should be noted that the specific ratios of this APP-PER-EN system have been carefully designed in order to optimize its fire-resisting performance. The temperature range of four stages is in accordance with typical changes in mechanical proper-

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ties of concrete structures at high temperatures (details in Section 6). Besides, TGA and DSC analysis confirm that the retardant system works efficiently within the temperature range of approximately 200–600 °C. Therefore, the thermal stability of components satisfies the requirement of cement-based material, and the adopted ratios of basic ingredients are reasonable. 5. Experimental validation 5.1. Thermal analysis of heating curves To better demonstrate the interaction mechanism and flameretardant effect of the novel composite system, one-side heating tests of double-layer composite samples are conducted under evaluated temperature. Fig. 8 shows the temperature curves of doublelayer samples during the one-side heating tests, while the temperature measuring positions referring to each heating curve presented in Fig. 3(c). As shown in Fig. 8(a), for the reference normal concrete sample (S1), the growth of temperature rising rate slows down in the middle layer at approximately 100 °C, which is caused by the gasification process of water inside the sample. This phenomenon is more obvious on the surface of the concrete layer belonging to S1. Compared with the reference sample, the addition of APP-PER-EN composite reduces the internal temperature of double-layer samples. For sample S2, the maximum temperature on the concrete surface is 37 °C lower than that of S1. In addition, the maximum tempera-

ture in the middle layer of S1 is 64 °C higher than that of S2. A similar trend is observed in Fig. 8(c), which indicates that the addition of stearic acid may not contribute to accelerating the heat transfer. However, according to the observation made in the trialmanufacture process, stearic acid improves the physical properties of APP-PER-EN system to some extent. In addition, SA functions as a lubricant, which is critical to the manufacture of the novel composite. To satisfy the requirement of good workability, the mixing proportion of the fiber-shaped APP-PER-EN system is limited. In order to further investigate the thermal insulation in the case of high content of APP-PER-EN II system, rice-shaped one is also incorporated into the concrete sample, named S4. In addition, a small amount of PP fibers (volume fraction of 0.2%) is added to promote the connectivity of micro-tunnels. The maximum mixing ratio significantly increases to as much as twice higher than that of S2 and S3 because of the change in geometry. From Fig. 8(d), it can be observed that temperatures in the middle layer and on the concrete surface are significantly lower than those of S1, S2, and S3 after exposure to 500 °C for 3 h. In addition, S4 has a considerably lower temperature rising rate than the other samples. The maximum temperatures in the middle layer and on the concrete surface are 109 and 92 °C respectively, about a third of those of S1. Fig. 8 displays the physical changes of the sample surface exposed to high temperature, which are in accordance with temperature curves. Several uplift cracks occur on the surface of S1 due to the unstable temperature field, with non-violent

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a

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5.2. Morphology analysis of the structure

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sloughing-off spalling (according to the four-category grouped method of spalling by Khoury [46]) as highlighted in blue color. In comparison, no observed spalling occurs on the heated surface belonging to S2 and S3, of which the integrity of fire safety structure design is guaranteed. Furthermore, the thermal insulation layer is partially formed on the heated surface, covering by flame-retardant material produced by the chain formation of APP-PER-EN composite. When it comes to S4, an obvious insulation layer covering throughout the sample surface contributes to further reduce the thermal conductivity as shown in Fig. 8(d).

Owing to the convenience and easier operation of the optical microscope (OM) method, a magnification of 500–2000 OM is first adopted for better characterizing the features of APP-PER-EN composite after experiencing high temperatures. Fig. 9 presents the optical micrographs of the internal pore structures in the selfretardant layer after exposure to high temperature for sample S2, S3, and S4. As demonstrated by Fig. 9(a) and (b), quantities of new thermal pathways are formed in S2 after exposure to 500 °C for 3 h, which are produced by the melting of fibers and foaming of APP-PER-EN composite, respectively. In addition, pores left by self-retardant APP-PER-EN system are surrounded by a number of new microfissures in concrete blinders in Fig. 9(b), indicating that the meshed pathway network can be realized in the charring part. Furthermore, nearly no retardant material left after the one-side heating test, which is partly explained by the limited mixing ratio of the fiber-shaped composite to ensure the integrity of double-layer

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Heated surface

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Fig. 11. Multi-layer structure of the self-retardant layer after one-side heating test.

samples. In comparison, the addition of rice-shaped APP-PER-EN II composite can not only promote the thermal network formation, but also dramatically increase the mixing ratio of retardant material in S4. As shown in Fig. 9(c), only part of the rice-shaped composite is triggered by high temperature in charring part, which indirectly proves the better flame-retardant effect leading to lower temperature field in S4. Besides, those untriggered rice-shaped composite, in melting and intact part, can be seen as a safety reserve for fire with greater heat radiation. In order to gain more clearly features of pore structure in S4, an alternative analysis made through SEM is shown in Fig. 10. The micro-fissures formed by foaming of the rice-shaped composite are helpful in further reduce the thermal conductivity of the concrete structure. After the chain formation, the combination of porous concrete matrix and overflowed retardant materials can be considered as a thermal insulation layer. Therefore, after experiencing heating treatment, the finally formed multi-layer selfretardant structure can be roughly divided into four parts, namely heated surface (combined with the thermal insulation layer), charring part, melting part, and intact part, as illustrated in Fig. 11.

6. Discussions 6.1. Coherence between APP-PER-EN composite and OPC concrete The thermal behavior of this APP-PER-EN system has been carefully studied in order to optimize its fire-resisting performance, which demonstrates good coherence between the composite system and ordinary Portland cement (OPC) concrete. It is acknowledged that the common changes in OPC concrete during heating contained multiple physical and chemical processes. Similarly, the novel composite also experiences complicated reactions which are in agreement with concrete structures. In accordance with Section 4.4: Stage I: For OPC concrete structure, free or evaporable water and a small part of the bound water may escape between 30 and 105 °C. During this stage, transient thermal creep (TTC) accommodates the thermal strain incompatibilities between the paste and the aggregates, which ensures the Portland cement does not break down at relatively low temperature (<100 °C) as it theoretically should [27]. Stage II: Generally, the free water completely vanishes at 120 °C. In the temperature range of 110–170 °C, the water of calcium carbonate hydrates is also lost. Meanwhile, the melting and liquefaction for both synthetic polypropylene fibers and APPPER-EN composite contribute to absorbing a substantial amount of heat and connecting natural pores/channels in the concrete structure.

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Stage III: The decomposition of calcium silicate hydrates occurs within the temperature range 180–300 °C, which leads to the loss of bound water. Besides, some flint aggregates dehydrate, coupled with the strength loss of siliceous concrete [46]. During this stage, the thermally triggered composite system starts to foam, filling the internal micro-pores with retardant material. In the same time, the foaming process will also help to introduce additional interfacial transition zones (ITZ) between the composite and the cement paste, which leads to further reduction of thermal conductivity. Furthermore, for high-performance concrete, the increased permeability can also help to prevent spalling, which may lead to the reinforcing steel bars exposed to an excessive temperature in reinforced concrete structure [17–18]. Stage IV: Subsequently, coupled with the foaming of the APPPER-EN composite, retardant material can overflow through micro-pores to the heated surface, forming a ‘‘honeycomb” insulation layer gradually. Once the temperature of concrete reached 600 °C when the dehydration of the calcium hydroxide and a-b quartz expansive inversion (sand) may appear, the concrete can be treated as structurally useless due to the seriously degrade of the mechanical properties. It should be noticed that the mentioned temperatures are for the APP-PER-EN composite and concrete material instead of the fire source. From the analysis above, it can be concluded that during each temperature stage when OPC concrete experiences physicochemical processes, the novel composite system has corresponding response to slow down the temperature rising rate. Therefore, the coherence of thermal behavior between APP-PEREN composite and OPC concrete optimizes their advantages and improves the fire resisting performance of structures. 6.2. Further effort on the novel composite system In this paper, a bio-inspired novel self-retardant composite system is proposed. Furthermore, to make the solution an effective one, more attention still needs to be paid on: (a) According to the mechanism of the novel cement-based composite system, the self-retardant layer can sustain the working load as part of the component, which reflects the synergistic action between load-bearing and fire-resistant in actual application. However, considering the probable negative influence on mechanical properties for selfretardant layer, preliminary mechanical compressive experiments are conducted on standard cubic samples (dimension of 10  10  10 mm3) with the same mixed proportion belonging to S1 and the self-retardant layer of S2, S3, and S4. Totally four groups are employed, namely NC-S1, FC-S2, FC-S3, and PC-S4, while each group contains twelve samples including four parallel evaluated temperatures of 20, 100, 200, and 400 °C with a set of three specimens for each condition. During the heating process, the samples are thoroughly exposed to the evaluated temperature for 3 h, which is referred to the PIARC Proposal on the Design Criteria for Resistance to Fire for Road Tunnel Structures [47]. Afterwards, a microcomputer controlled electro-hydraulic serving testing machine is employed to acquire the related mechanical properties. The experimental result of maximum compressive strength and corresponding strain are exhibited in Fig. 12(a). Based on the test result, we can figure out that the embedded of APP-PER-EN system in the concrete has some negative effect on compressive strength at ambient temperature. However, the negative influence gradually weakens with the increase of temperature, especially at 300 °C the self-retardant layer demonstrates a higher

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compressive strength than the normal concrete. Furthermore, by comparing the differences among strains corresponding to peak strength, the incorporation of APP-PER-EN system can actually promote the ductility of the concrete structure to some degree, which is helpful in reducing explosive spalling under the high temperature. Furthermore, some modified measures, such as incorporating high modulus synthetic fibers or appropriately increase the strength grade of concrete, can be taken for balancing the possible negative influence on mechanical properties for self-retardant layer. Referring to similar researches [3,17,48–54], the negative mechanical impact of embedding PP fibers can be modified by adding suitable content of higher young modulus steel fibers. More importantly, the application of this strength balancing method ranges from hybrid fiber reinforced concrete (HFRC) of 60 MPa [49] to ultra-high-performance concrete (UHPC) of 150 MPa [50] with different PP fiber contents, as concluded in Fig. 12(b). Based on the above analysis, concerning to mechanical properties, more efforts still need to be made on following issues. On one hand, optimizing APP-PER-EN system by adding some inorganic mineral with better physical characteristics, aiming at higher young modulus and higher strength. On the other hand, improving both physical and mechanical properties of concrete embedded with APP-PER-EN composite at room temperature by incorporating high modulus synthetic fibers.

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(b) The bond performance between the self-retardant layer and reinforced concrete layer during or after being exposed to high temperatures also requires determination. It is expected that the interfacial strength should be strong enough to sustain its weight in order to avoid corner/ sloughing-off spalling during a fire accident. (c) The thickness of the self-retardant layer used in this study is 30 mm, which is generally utilized in tunnels. However, various thicknesses and multiple fire conditions, concerning to real fire condition or standard fire tests represented by ISO 834 curve and H-C curve, should be investigated to determine their effect on the thermal insulation. (d) The permeability of the self-retardant layer should be tested before its engineering application, especially for underground structures, because it is critical to the corrosion protection for reinforcing steel bars.

7. Conclusions From this study, some concluding remarks are made as follows: (a) Following the function of human sweat glands, a novel selfretardant composite system is developed to achieve a real active fire protection mode for the concrete structure. This novel system with APP-PER-EN composite as main chain formation component has the potential to provide heat insulation and improve the fire resistance of engineering structures. (b) The APP-PER-EN composite, prepared by melt compounding in a twin-screw extruder followed by grain cutting molding technique, demonstrates a four-stage complicated interaction and chain formation under high temperature based on thermal analysis. Consequently, the mechanism of the concrete incorporated with self-retardant composite at evaluated temperature can also be explained by a four-stage process: melting, connecting, foaming and overflowing, which finally leading to the formation of the insulation layer on the heated surface. (c) At the micro level, the multiple physical and chemical processes of APP-PER-EN composite are revealed through XRD, TGA, and DSC. Besides, the foaming chain structure in charring layer after burning is characterized by FTIR. Based on the aforementioned characterizations, comprehensive internal chain formation reactions of the novel APP-PER-EN composite are figured out. (d) One-side heating tests are conducted to demonstrate the effect of embedding novel self-retardant composite in the concrete structure. Test results indicate that the selfretardant layer with either fiber-shaped or rice-shaped composite presents great fire-resistant effect, while the riceshaped APP-PER-EN composite can be added into the concrete with a higher proportion. (e) At the macro level, morphology analysis of the structure through SEM also reveals the action mechanism of the self-retardant composite. In addition, after exposure to high temperature, the finally formed self-retardant structure presented a multi-layer phenomenon, and the untriggered composite can be seen as a safety reserve for greater heat radiation.

PP fiber content (kg/m3) Fig. 12. Illustrations on mechanical properties of embedding APP-PER-EN composite in concrete blinder: (a) compressive strength at elevated temperatures (NC– normal concrete, FC–concrete embedded with self-retardant fiber, PC–concrete embedded with self-retardant particle); (b) effect of steel fiber volume fraction on UHPC and HFRC with different PP fiber contents (ST–steel fiber) [49–50].

Declaration of Competing Interest The authors declared that there is no conflict of interest.

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