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Effect of lightweight expanded clay aggregate as partial replacement of coarse aggregate on the mechanical properties of fire-exposed concrete

  • Alaa H. Abdullah EMAIL logo and Shatha D. Mohammed
Published/Copyright: September 11, 2023

Abstract

As aggregate material typically comprises 65–75% of concrete volume and has a significant effect on its mechanical properties, aggregate type considerably affects concrete behavior at high temperatures. In this study, 80 concrete cylinders and 60 cubes were cast to investigate the residual strength of normal concrete that contains lightweight expanded clay aggregate (LECA) with different volumetric replacement ratios (0, 10, 20, and 30%) of the coarse aggregate. After the fire flame exposure effect of steady-state temperatures (300, 400, 500, and 600°C), and a sudden cooling process, the mechanical tests (compressive strength, tensile strength, and modulus of elasticity; Ec), as well as mass loss and thermal conductivity, were carried out on the specimens. The results indicate that increasing the LECA content in the mixture leads to better strength retention after exposure to fire. After exposure to a steady-state temperature of 600°C, the amount of decrease in mass, residual compressive and tensile strengths, and the residual amount of Ec were 7.61, 7.5, 7.16, and 6.24%; 57.1, 66.8, 69.8, and 72.0%; 22.4, 32.7, 41.8, and 48.6%;, and 16.0, 22.3, 23.4, and 24.3%, respectively, for the considered volumetric replacement ratios of 0, 10, 20, and 30%. Also, the values of the thermal conductivity were 1.4889, 1.1667, 1.0912, and 1.0410 W/m K, respectively.

1 Introduction

Fire is one of the disaster events that results in loss of life or damage to property. The duration of an accidental fire in a building is shorter, but the quantity of heat it generates is more intense, causing damage to the structure. In addition to accidental fire, specific buildings are exposed to high temperatures such as jet aircraft takeoff zones, rocket launch pads, nuclear reactors, chimneys, cement factories, coke ovens, hot water and crude oil storage tanks, and metallurgical or chemical industries. The thermal conductivity of concrete is determined by the conductivity of its components, specifically the cement paste and the aggregate. It is well known that the conductivity of concrete decreases with the increase in the temperature [1]. Aggregate materials are generally thermally stable up to 300–350°C. At high temperatures, physical properties, chemical properties, and thermal stability/integrity are the most influential aggregate parameters on the behavior of concrete. However, the mixing ratio, the aggregates’ thermophysical characteristics, and the hydrating cement paste’s properties all impact the concrete’s thermal properties [2,3].

Even though concrete is more durable than steel, it deteriorates in terms of mechanical properties when exposed to high temperatures [4]. Several researchers studied the response and behavior of reinforced concrete structures under different load and fire condition states [58].

LECA is an abbreviation for lightweight expanded clay aggregate. It is a product of porous ceramic that exhibits a uniform pore structure consisting of small, sealed cells and a compact, robust outer layer. The production process involves the utilization of fundamental components such as clay minerals, which are subjected to high-temperature treatment in rotary kilns. Upon completion of the preparation, molding, and firing process of the raw material, it undergoes a notable volumetric expansion due to temperature exposure ranging from 1,100 to 1,200°C. The internal cellular structure of LECA grains, which contains thousands of air-filled voids, provides thermal and acoustic insulation. Expanded clay aggregates are employed in a variety of industries due to their technical properties and several advantages over other industrial raw materials. Because of its good insulating features, LECA was added to the concrete mixture to increase its properties [9]. LECA’s chemical composition primarily comprises of SiO2, Al2O3, Fe2O3, CaO, and certain alkalis like Na2O and K2O [10]. The analysis reveals that the SiO2 content in the overall composition exhibited variations ranging from 53.3 to 70%, while Al2O3 exhibited fluctuations ranging from 15.05 to 27%. Similarly, Fe2O3 demonstrated fluctuations ranging from 1 to 14.3%, and CaO exhibited variations ranging from 0.2 to 3.92% [11]. The thermal conductivity of the material falls within the range of 0.097–0.123 W/m K [1225]. The wide Applicability of LECA is shown in Table 1.

Table 1

Wide applicability of LECA [9]

Wide applicability Average density* (kg/m3) LECA gradation (mm)
Prefabricated panels and slabs, lightweight block, LECA lightweight concrete (LWC), light filler, agriculture and aquaculture. LECA mortar and water purification system 510 (0–4)
Drainage, sewage system, landscaping, agriculture and aquaculture, and weight filler concrete 320 (4–10)
Lightweight concrete, prefabricated panels and aquaculture, lightweight block, and ornamentation 250 (10–25)
Floor and roof sloping, road construction, and lightweight filler 270 (0–25)

*Average density allowable tolerance is ±50 kg/m3.

Compared to normal strength concrete (NSC), LWC has much better resistance to high temperatures and fire. This is one of the important advantages of LWC. Because concrete is a mix of components with varied thermal properties, moisture, and porosity-dependent properties, the mechanical properties and load-carrying capacity of concrete parts may change substantially during the fire. Unlike NSC, LWC can resist fire better because its lightweight particles include pores that may be used to relieve the pressure created by the fire. LWC is less damaged at high temperatures in both the hot and residual stages, hence buildings composed of LWC may fare better in fires [26].

Used in its unsaturated nature, lightweight aggregate (LWA) proved to be more resistant than its saturated counterpart. In certain cases, particularly at higher temperatures, the pre-saturation nature of LWA that is widely utilized in practice may cause spalling of such lightweight matrices. Volcanic eruption or combustion produces LWAs like ceramist, pumice, and expanded clay. As a consequence, they have high heat resistance and low heat conductivity. As a result, concrete made with such aggregates should have better mechanical properties at high temperatures than normal aggregate concrete [27].

The thermal performance of lightweight aggregate concrete (LWAC) is better in contrast to ordinary concrete, considerably decreasing the buildings’ energy consumption [28,29]. Because the spaces in the LWAs assist the release of vapor and lower the tension caused by the evaporated water, LWAC has greater resilience to fire when compared to normal-weight concrete [30].

The bond produced between the cement pastes and the aggregates is severely degraded at temperatures over 300°C [31]. The addition of silica fume to LWAC may strengthen the bond between the LWAs and cement paste, increase concrete compressive strength, and decrease cement content; silica fume may also minimize porosity inside the LWAC structure.

LECA was improved to be a significant partial or full replacement for normal aggregate in the construction of several structural elements that were subjected to different types of loading (static or dynamic) and different types of disaster just as fire flame effect [3235].

This study aims to investigate concrete behavior with various amounts (0, 10, 20, and 30%) of coarse aggregate replaced by LECA after being subjected to different levels of fire flame effect (300, 400, 500, and 600°C). The behavior is evaluated by the concrete’s remaining mechanical properties (compressive strength, tensile strength, and modulus of elasticity; Ec), mass loss, and thermal conductivity.

2 Experimental work

2.1 Constitutive materials

2.1.1 Cement

For casting all samples ordinary Portland cement (CEM I 42.5R) produced in Iraq by Mass brand was used in the concrete mix. The physical and chemical test results are shown in Tables 2 and 3, respectively. The test results agreed with Iraqi Specification No. 5/2019 [36].

Table 2

Cement physical composition

Physical properties Test results Iraqi specification limit no. 5/2019
Setting time (Vicat’s method)
Initial setting time (h:min) 2:12 ≥45 min
Final setting time (h:min) 4:50 ≤10 h
Expansion (autoclave method) 0.01% ≤0.8%
Specific surface area (Blaine’s method) (m2/kg) 365 ≥250
Compressive strength (MPa)
For 2 days 16.22 ≥10 MPa
For 28 days 48.80 ≥42.5 MPa
Table 3

Cement chemical composition

Chemical composition Content % Iraqi specification limit no. 5/2019
MgO 3.33 ≤5%
L.O.I 2.02 ≤4%
SO3 2.01 ≤2.8%
C3A 6.37
I.R. 0.64 ≤1.5%

2.1.2 Fine aggregate

Natural sand was used for casting all samples with a maximum size of 4.75 mm. The test results are shown in Table 4 according to the limits of Iraqi Specification No. 45/1984 and its modifications [37]. According to the results of the sieve analysis, the adopted sand is classed as Zone 2.

Table 4

Fine aggregate properties

Tests Cumulative passing (%) Iraqi specification limit no. 45/1984 and its modifications
Zone 1 Zone 2 Zone 3 Zone 4
Sieve size (mm) 10.0 100 100 100 100 100
4.75 97.2 90–100 90–100 90–100 95–100
2.36 81.16 60–95 75–100 85–100 95–100
1.18 65.36 30–70 55–90 75–100 90–100
0.60 55.64 15–34 35–59 60–79 80–100
0.30 27.32 5–20 8–30 12–40 15–50
0.15 3.16 0–10 0–10 0–10 0–15
Sulfate contained (SO3) % 0.39 0.5 (max.)
Absorption% 3.1
Specific gravity 2.67

2.1.3 Coarse aggregate

Crushed gravel siliceous aggregate of 10 mm maximum size was used in the concrete mixes. The graded crushed gravel was washed and dispersed in the air till the saturated dry surface condition was satisfied. The results of the test are shown in Table 5 according to the limits of Iraqi specification no. 45/1984 and its modifications [37].

Table 5

Coarse aggregate properties

Tests % Passing by weight Iraqi specification limit no. 45/1984 and its modification
5–40 5–20 5–14
Sieve Size (mm) 75 100 100 Not limited Not limited
63 100 Not limited Not limited Not limited
37.5 100 95–100 100 Not limited
20 100 35–70 95–100 100
14 100 Not limited Not limited 90–100
10 83.4 10–40 30–60 50–85
5 2.48 0–5 0–10 0–10
2.36 0.36 Not limited Not limited Not limited
Sulfate contained (SO3) % 0.06 0.1 (max.)
Absorption% 0.65
Specific gravity 2.66

2.1.4 LECA

LECA with diameters between 4 and 10 mm as shown in Figure 1 was used. Tables 6 and 7 show the test results of LECA properties and grading, respectively.

Figure 1 
                     LECA.
Figure 1

LECA.

Table 6

LECA properties

Properties Experimental value
Density (kg/m3) 320
Specific gravity 0.55
Water absorption% 21.6
Fineness modulus 3
Table 7

LECA grading

Sieve size (mm) % Passing by weight
37.5 100
20 100
14 100
10 95.36
5 2.18
2.36 0.6

2.1.5 Silica fume

A very fine pozzolanic material (microsilica), product of Conmix, had adopted to produce self-compacted concrete (SCC) as an additive (pozzolanic material). The chemical and physical compositions of the silica are conformed to the ASTM C1240 [38], and the limitations are described in Tables 8 and 9.

Table 8

Chemical properties of microsilica fume

Oxide composition Abbreviation Oxide content (%) Requirement of ASTM C1240 specification
Silica SiO2 95.62 85.0 (min)
Sodium oxide Na2O 0.19
Iron oxide Fe2O3 0.048
Alumina Al2O3 0.034
Lime CaO 1.44
Magnesia MgO 0.38
Potassium oxide K2O 1.12
Sulfate SO3 0.51
Phosphorus pentoxide P2O5 0.21
Titanium dioxide TiO2 0.0081
Loss on ignition L.O.I. 3.74 6.0 (max)
Table 9

Physical properties of microsilica fume

Physical properties Results of MS Requirement of ASTM C1240 Specification
Color Grey to medium grey
Strength Active Index with Portland cement at 7 days, min. percent of control 129 ≥105
Specific surface area (m2/kg) 22,000 ≥15,000
Percentage of retained on 45 μm (No. 325), max. (%) 8 ≤10

2.1.6 Superplasticizer

Sika’s ViscoCrete-5930 was the commercial name of the superplasticizer utilized in the concrete mixes to induce SCC for all specimens in this study. It is a high-performance superplasticizer additive for concrete that is based on an aqueous solution of modified polycarboxylate and agrees with ASTM-C-494 [39], types G and F, as well as BS EN 934 part 2 [40]. For optimal water reduction, the superplasticizer is added to the gaging water or poured it into the concrete mixer simultaneously with the water and mixed for at least 60 s. Table 10 shows the technical properties of ViscoCrete-5930 at 5–35°C.

Table 10

Technical properties of ViscoCrete-5930

1 Color Turbid
2 Specific gravity 1.095 ± 0.02
3 Dosage 0.8–2% liter by weight
4 Freezing point ≈30°C
5 Cleaning Washed with water
6 Form Viscous liquid
7 Health and safety Not classified as hazardous material
8 Fire Non-flammable
9 Air entrainment Typically, less than 2% additional air is entrained

2.2 Tested specimens

This study involved casting and testing 60 cubes and 80 cylinders. The mix proportions are shown in Table 11. Cubes of 150 × 150 × 150 mm were used to measure compressive strength according to B.S.1881: part 116 [41]. Cylinders of 150 × 300 mm were used to measure splitting tensile strength and Ec following ASTM C496/C496M-17 [42] and ASTM C469/C469M-14 [43], respectively. Cubes of 100 × 100 × 100 mm were used to measure thermal conductivity. The curing process of the specimens is illustrated in Figure 2.

Table 11

Details of considered mixes

Mix type Mix proportion (kg/m3) SP (L/m3) SF* (%) fcu** (MPa)
Water Cement Sand Gravel LECA
M0 185 441 750 950 0 4 2 60.1
M10 185 441 750 855 18.8 4 2 57.0
M20 185 441 750 760 37.6 4 2 53.0
M30 185 441 750 665 56.4 4 2 48.9

*Replacement by weight of cement.

**These values are the average of three control specimens.

Figure 2 
                  Specimen curing process.
Figure 2

Specimen curing process.

3 Burning and cooling

The specimens were burned by a direct fire flame under the same conditions (a steady-state temperature of 300, 400, 500, and 600°C, and a 1 h exposure time). ASTM E-119 Standard [44] was adopted for temperature–time relation requirements. The specimens were put inside a gas furnace, which has a fire chamber, steel framework, and steel columns (outer dimensions of 3.50 m in length, 2.00 m in width, and 0.80 m in height) with 20 gas burners, 8 for each pair of opposing longitudinal sides and 2 for each transverse side, connected to 10 methane gas canisters in the sidewalls at the base to deliver heat during the burning test (i.e., one methane gas canister for every two gas burners). Two blowers supplied furnace air during the fire test to maintain fire intensity and heat distribution as shown in Figure 3. The top cover is detachable which permits specimens to be put within the furnace. A digital thermometer reader (DT-612) with sensor wire type K was used to check the temperature of the specimen and furnace space over time. The fire flame was turned off after the burning stage, and the specimens were suddenly cooled by spraying them with water.

Figure 3 
               Specimens burning.
Figure 3

Specimens burning.

4 Results and discussion

4.1 Mass loss

The amount of mass loss for each considered concrete mix is shown in Table 12. As expected, all four mixes lose more weight as the steady-state temperature increases. The results show that the mass loss decreases when the amount of LECA content increases, as illustrated in Figure 4. This may be due to the increase in the effects of spalling [45,46,47], whose risk and intensity decrease as the content of LWA increases.

Table 12

Percentage of mass loss

Temp. (°C) Mix type
M0 M10 M20 M30
Density (kg/m3) Mass loss (%) Density (kg/m3) Mass loss (%) Density (kg/m3) Mass loss (%) Density (kg/m3) Mass loss (%)
30 2,326 2,267 2,207 2,148
300 2,215 4.77 2,175 4.06 2,126 3.67 2,078 3.26
400 2,203 5.29 2,152 5.07 2,102 4.76 2,051 4.52
500 2,185 6.06 2,135 5.82 2,084 5.57 2,035 5.26
600 2,149 7.61 2,097 7.50 2,049 7.16 2,014 6.24
Figure 4 
                  Specimens mass loss after fire exposure.
Figure 4

Specimens mass loss after fire exposure.

4.2 Compressive strength

The compressive strength of hardened concrete is a commonly known mechanical property that exhibits characteristics that can be associated with its strength. The results of this test were based on the average of three cubes at the test age for each adopted concrete mix. Table 13 shows the specimens’ residual compressive strength. The remaining compressive strength decreases more as the burning steady-state temperatures increase as shown in Figure 5. However, as the amount of LECA in the mix increased, the retention factor rose. The retention factor is the ratio of the remaining compressive strength of specimens after being exposed to a fire flame to the specimens’ compressive strength kept at room temperature. Concrete compressive strength drops sharply when exposed to high temperatures (after 300°C), that belongs to several reasons. One reason is the dehydration of concrete by losing free water, interlayer water, and chemically combined water which are essential for the strength and bonding of the concrete matrix. Another reason is the thermal expansion of concrete and its constituents, which causes internal stresses and cracks that weaken the concrete structure. A third reason is the chemical decomposition of some concrete components, such as calcium hydroxide and calcium silicate hydrate, which reduces the cementitious properties of the concrete [48,49]. The rate and extent of strength loss depend on the type and composition of concrete, as well as the heating rate and duration. Concrete that contains LECA has lower thermal conductivity and higher porosity. LECA’s porous structure provides a hideout for the evaporating water, resulting in less cracking and damage inside the concrete and it can better resist heat transfer and spalling [50], which may explain why increasing the LECA content in concrete leads to better strength retention.

Table 13

Residual compressive strength

Temp. (°C) Mix type
M0 M10 M20 M30
fcu (MPa) Residual (%) fcu (MPa) Residual (%) fcu (MPa) Residual (%) fcu (MPa) Residual (%)
30 60.1 57.0 53.0 48.9
300 51.0 84.9 50.0 87.7 47.8 90.2 44.6 91.2
400 46.5 77.4 46.8 82.1 45.5 85.8 42.6 87.1
500 41.2 68.6 43.5 76.3 41.8 78.9 39.0 79.8
600 34.3 57.1 38.1 66.8 37.0 69.8 35.2 72.0
Figure 5 
                  Compressive strength retention after fire exposure.
Figure 5

Compressive strength retention after fire exposure.

4.3 Tensile strength

Cylindrical specimens of 150 × 300 mm, according to ASTM C496/C496M-17 [42], were subjected to a splitting tensile test before and after being exposed to a fire flame. Table 14 depicts the tensile strength retention of the burned specimens. It is clear from Table 14 and Figure 6 that the specimen’s tensile strength reduces considerably as the burning steady-state temperatures increase. However, increasing the content of LECA in concrete leads to better strength retention. Thermal stress-induced cracking is one of the primary effects of exposure to high temperatures [50,51]. With a rise in the specimen’s maximum temperature, these cracks tend to get wider and deeper as shown in Figure 7 and 8. It is understood that the porosity of mixtures that contain LECA will reduce thermal stresses and thus fewer cracks will occur, resulting in greater strength retention.

Table 14

Residual splitting tensile strength

Temp. (°C) Mix type
M0 M10 M20 M30
f t (MPa) Residual (%) f t (MPa) Residual (%) f t (MPa) Residual (%) f t (MPa) Residual (%)
30 4.46 4.16 3.88 3.56
300 2.36 52.9 2.64 63.5 2.78 71.6 2.76 77.5
400 1.78 39.9 2.12 51.0 2.35 60.6 2.41 67.7
500 1.39 31.2 1.72 41.3 1.89 48.7 1.90 53.4
600 1.00 22.4 1.36 32.7 1.62 41.8 1.73 48.6
Figure 6 
                  Tensile strength retention after fire exposure.
Figure 6

Tensile strength retention after fire exposure.

Figure 7 
                  Cracks pattern of the burned specimens at steady-state temperature of 500°C.
Figure 7

Cracks pattern of the burned specimens at steady-state temperature of 500°C.

Figure 8 
                  Concrete surface texture of the specimens after fire exposure.
Figure 8

Concrete surface texture of the specimens after fire exposure.

4.4 Static Ec

The Ec stands out among the concrete’s important mechanical properties. Following ASTM specification (C469/C469M-14) [43], it could be obtained by a compressive test on the cylinders of concrete. For hardened concrete, it is the stress-to-strain ratio changes in the elastic range. It can be known as the secant in the stress–strain curve at the point congruous to 40% from the ultimate strength [52]. The experiment results indicate that the Ec decreases significantly when exposed to high fire temperatures. LECA contains thousands of small voids in the clay, forming a honeycomb structure that gives LECA its low density, high porosity, and low thermal conductivity. These properties mean that LECA can insulate the concrete from heat transfer, prevent spalling and cracking due to thermal stress, and retain its mechanical strength and thermal insulation properties better than normal aggregate under fire exposure [53]. So, mixtures containing LECA keep their stiffness better due to less heat degradation. The variations in the specimens’ residual Ec are depicted in Table 15 and Figure 9.

Table 15

Residual Ec

Temp. (°C) Mix type
M0 M10 M20 M30
Ec (MPa) Residual (%) Ec (MPa) Residual (%) Ec (MPa) Residual (%) Ec (MPa) Residual (%)
30 31,606 27,896 25,687 23,453
300 18,056 57.1 17,744 63.6 17,163 66.8 16,228 69.2
400 14,030 44.4 14,242 51.1 13,894 54.1 13,222 56.4
500 9,208 29.1 10,109 36.2 9,657 37.6 9,151 39.0
600 5,061 16.0 6,222 22.3 6,023 23.4 5,704 24.3
Figure 9 
                  Ec retention after fire exposure.
Figure 9

Ec retention after fire exposure.

4.5 Thermal conductivity

Concrete’s thermal conductivity is influenced by the thermal properties of its phases, particularly the paste, and aggregates. As aggregates mainly account for 70% of the concrete volume, the incorporation of low thermal conductivity aggregates may lead to a considerable improvement in the concrete’s thermal insulation capabilities [54,55]. Figure 10 shows the test of the thermal conductivity and Table 16 shows the thermal conductivity experimental results for all mixes. The experiment findings showed that the concrete thermal conductivity reduces by increasing the content of the LECA and hence enhances its thermal insulation. The enhanced thermal insulation capabilities are mostly due to the poor thermal conductivity of the air confined in the porous structure of the LECA [56,57].

Figure 10 
                  Thermal conductivity test.
Figure 10

Thermal conductivity test.

Table 16

Experimental results of the thermal conductivity

Mix type Thermal conductivity (W/m K)
M0 1.4889
M10 1.1667
M20 1.0912
M30 1.0410

5 Conclusion

The following are the major findings:

  1. The density, compressive strength, tensile strength, and Ec of concrete are all decreased when LECA is used as a partial volumetric replacement for the coarse aggregate, and the amount of reduction in these properties increases with the increase in the content of LECA.

  2. Adding LECA as a partial volumetric replacement for the coarse aggregate reduces the mass loss due to the fire flame effect for all the considered burning steady-state temperatures.

  3. Concrete’s residual compressive strength decreases with burning steady-state temperature, whereas greater LECA content led to better strength retention. Where the residual compressive strength at 600°C was 57.1 and 72.0% for the replacement ratios 0 and 30%, respectively.

  4. The tensile strength reduces significantly as the specimen’s burning steady-state temperature increases. However, increasing the LECA content in concrete leads to better strength retention.

  5. The Ec reduces at high temperatures. The concrete that contains LECA was more resistant to damage, and increasing the LECA content in concrete leads to better behavior.

  6. Also, by increasing the LECA content, the thermal conductivity of the concrete decreases, thus increasing the thermal insulation of the concrete.

  7. Deeper investigation is recommended including the mesoscopic scale of LECA that is used in concrete to resist fire exposure.

Acknowledgements

The author is grateful to the staff in the Civil Department at the Engineering College, Baghdad University, as well as the staff in the Construction Tests Laboratory at the Kut Technical Institute.

  1. Funding information: The authors state no funding is involved.

  2. Author contributions: All authors have accepted responsibility for the entire content of this manuscript and approved its submission.

  3. Conflict of interest: The authors state no conflict of interest.

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Received: 2023-05-24
Revised: 2023-08-15
Accepted: 2023-08-16
Published Online: 2023-09-11

© 2023 the author(s), published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

Articles in the same Issue

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  2. The mechanical properties of lightweight (volcanic pumice) concrete containing fibers with exposure to high temperatures
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