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

2.1 Cement

A locally available cement was used for the lightweight concrete mixes. The cement was obtained from Badoosh manufacture in Nineveh Governorate of Iraq. The specific surface area estimated from Blain’s method was 290 m²/kg, and the setting time from Vicat’s method showed an initial setting time of 109 min, and a final setting time of 3.25 h.

The natural river sand used as fine aggregate was supplied from Kanhash region in Mosul. The grading limits are according to ASTM C33-02 [14]. The properties of the fine and coarse aggregate are shown in Table 1. Coarse aggregate with max aggregate size of 12.5 mm obtained from Mosul city was used. This gravel is compatible with Iraqi specification No. 45/2010 [15].

2.2 VP

VP was used in this study. It is a light gray colored coarse aggregate, which floats on water. The density of pumice is 835 kg/m³ (Figure 1, Table 2).

Figure 1

Lightweight aggregate.

2.3 Silica fume (SiF)

SiF is specified under ASTM C1240. The specific surface area of SiF used in this research was between 17,000 and 23,000 kg/m³, as shown in Figure 2. SiO₂ when firing at 1,000°C for 2 h was greater than 99.8%, while the weight loss when drying at 1,000°C for 2 h was less than 2%. In addition, the pH was less than 1.5 and the retained SiF on a 40 µm sieve was <0.04% with a moisture content of 0.82%. The chemical composition of SiF is shown in Table 3.

Figure 2

Silica fume.

2.4 PPF, carbon fibers (CF), and steel fibers (StF)

PPF, CF, and StF were used in the concrete mixes in this study, all of which have a high chemical resistance and low electrical conductivity (Figure 3).

Figure 3

Fiber types used in this study.

The mechanical and physical parameters of each type of fiber utilized are listed in Table 4.

2.5 Superplasticizer (SP)

SP is “Naphthalene sulfonate based set retarder, high range water reducers, and superplasticizer admixture used to improve the workability by giving rheoplastic property.” This SP (Type G) according to ASTM C494 [16] is shown in Table 5.

The permissible range for SP is 1–2% of cement weight, as shown in Table 5, and in our study, 1.5% was within this range. While maintaining the same slump as the control mix, the inclusion of SP decreased the water to cement ratio, because improved workability results in a deluge of lightweight pumice particles, which isolate these materials upwards. In order to maintain the slump value, the water cement ratio was fixed at 0.4.

3.1 Slump test

Workability was measured using ASTM C143 [17]. Workability is described as “The property of freshly mixed or mortar which determines the ease and homogeneity with which it can be mixed, placed, consolidated, and finished.”

3.2 Fresh density test

The weight of the empty mold is compared to the weight of the filled mold, and the difference between the weights is used to determine the fresh density of the concrete (unit weight). ASTM C138 [18] was used to determine the fresh density. It was calculated by dividing the net weight of freshly mixed concrete by the volume of concrete made at the time of casting.

4.1 Compressive strength test

For measuring the compressive strength of lightweight aggregate concrete, research specimens of 100 mm × 100 mm × 100 mm cubes were used according to BS 1881: part 116 [19,20], average of three cubes was used to determine the compressive strength for each test.

4.2 Flexural strength test

A flexural strength test (modulus of rupture) was performed using concrete prisms (100 mm × 100 mm × 400 mm) according to ASTM C78-02 [21]. The flexural strength was measured using a capacity testing equipment with a capacity of 30,000 kg.

4.3 Water absorption test

An average of three 100 mm × 100 mm × 100 mm cubes were utilized to determine the percentage of water absorption, and the technique was carried out in accordance with ASTM C642 [22].

4.4 Oven dry density test

According to ASTM C642 [22], the dry density of the hardened lightweight aggregate concrete was achieved.

5.1 Heating and cooling procedure

To begin, the specimens were heated using electrical furnaces at varied temperatures of 100, 250, 350, and 450°C at 28 days. Because of the furnace’s high heating rate, each temperature was held for 1.5 h to attain the steady burning state. All of the specimens used in this study were cured for 28 days in water before being utilized to test the effect of exposure to higher temperature. The specimens remained at the target temperature for 90 min after achieving it. To guarantee that the samples achieve the surface temperature indicated by the thermometer, the temperature was measured and applied at a heating rate of 15°C/min until the samples reached the maximum thermometer temperature on the surface.

6.1 Hardened properties of lightweight concrete at ambient temperature

The general trend is a decrease in compressive strength with increasing replacement ratios of VP rather than fine material. Besides, as indicated in Figure 4, it is much reduced now that coarse aggregate is being replaced at a higher rate.

Figure 4

Compressive strength variation for trail mixes.

SiF is an extra binder that improves the characteristics of concrete. Because of its fineness and its ability to fill voids between the particles, it basically acts as a filler, similar to how sand fills the spaces between coarse aggregate particles and cement grains fill the spaces between fine aggregate particles.

The addition of fibers to C14 and C16 fiber mixtures increased the material’s energy absorption capacity and toughness. There is a significant improvement in post-cracking behaviors in concrete containing fibers. When compared to concrete without fibers, fiber-reinforced concrete is significantly tougher and more impact resistant. The failure of fiber-reinforced concrete specimens is primarily due to fiber pull-out or deboning, according to tests. A fiber-reinforced concrete specimen, unlike conventional concrete, does not fail within a short period when the first fracture begins. The compressive strength, which is determined by the area under the load, is increased as a result. The impacts of varying percentages of VPA, PPF, CF, and StF on all trail mixes at 7 and 28 days are shown in Figure 5. The compressive strength of concrete mixtures was increased by adding SP, SiF, and various types of fibers, as illustrated in Figure 5. Compressive strength is improved when PPF and CF are added, 21.4 and 20.8 MPa, respectively. StF, on the other hand, has the highest compressive strength (22.44 MPa).

Figure 5

Compressive strength for different mixes.

The addition of SP, SiF, PPF, CF, and StF to LWPC enhanced the flexural strength. When compared to the standard LWPC combination, the use of 1.5% SP increased the compressive and flexural strength by almost 15 and 0%, respectively. The addition of SP lowered the water/cement ratio, while also making the material more workable. However, the bond between the cement and the lightweight aggregate particles is significantly weaker than the bond between the cement and the fine particles (sand). The compressive strength was slightly changed as a result. The lightweight aggregate’s many cavities cause the upper portion of the samples to shrink, quickening the separation of the lower portion and causing cracks.

With a 10% substitution of SiF with cement, flexural strength was increased by roughly 11.3%. The addition of fiber to the mix resulted in a significant increase. However, when 0.025% PPF (C14), 1% CF (C15), and 1% StF (C16) were added to the mix (C11), the flexural strength increased by a percentage difference of 39%, 63%, and 83%, respectively, as shown in Figure 6.

Figure 6

Flexural strength for different mixes.

The addition of fibers improved the flexural strength of the lightweight concrete, with StF providing the best results due to the fact that StF has the best durability. The major goal of using used fiber is to increase the stiffness and absorbency of the material. This also improves the concrete’s flexural properties. Compared to LWPC without fiber, fiber-reinforced LWPC is much tougher and more resistant to impact; therefore, there was a considerable improvement in the post-cracking behavior of the concretes containing fibers.

In general, adding SP, SiF, PPF, CF, and StF to LWPC decreased the water absorption, as seen in Figure 7. Although there were small gaps in the lightweight debris itself (mixes C11 and C12), the latter behavior may be attributed to the fact that a better plasticizer helped to reduce the water content and thus reduce the resulting voids after solidification. The SiFs and fiber types may have also played a role in matrix formation. There is a good link between aggregates and cement particles, and as a result, absorption is slightly reduced.

Figure 7

Water absorption for concrete mixes at 28 days.

6.2 Hardened properties of LWPC at elevated temperature

Figure 8(a–g) shows the residual compressive strength of the LWPC mix after being subjected to various temperature gradients. The effect of temperature on LWPC samples has been reported to result in a decrease in compressive strength. The strength decreases due to a succession of complex physical and chemical variables, which will be addressed in this section. During the direct test, Figure 8(a–g) shows the decline in compressive strength as the exposure temperature rises. At three distinct temperatures, the findings were 250, 350, and 450°C. The concrete mixtures that have been given directly test the compressive strength approach to carrying water-cooled concrete mixtures with a little variation, and their compressive strength is lower than that of cooled concrete mixtures with air. Internal stresses may have been created by thermal shock and the models’ high heat tolerance is one of the causes of the low compressive strength.

Figure 8

(a–g) Residual compressive strength of mixes with different temperatures.

6.3 Air-cooled specimens

Cases involving water- and air-cooling must be studied. The majority of the buildings in Mosul, Iraq, were in risk of burning because of recent events in 2018. Some of them were put out by water, while others were left to cool off in the air. Therefore, research was required to demonstrate the extent to which the fire affected the buildings in each scenario. Air-cooled specimens showed an increase in residual compressive strength for all concrete mixtures at 250°C. It was 114, 105, 115, 112, 110, 102, and 105% for C1, C11, C12, C13, C14, C15, and C16 mixtures, respectively. At this temperature, the increase in compressive strength can be explained by one of the three hypotheses.

The initial assumption is that the composition is elemental. Because a component of the link between its groups and water has been lost, the surface energies that lead to the rise and improvement of strength may have increased. This rise, according to the second viewpoint, is related to the rehydration of the paste due to the permeability of water in the pores. The third concept depicts the enhanced strength due to the evaporation of free water at this temperature. Because the cement gel layers are close together, the hydro-cement paste would be structured in a fashion that leads upward.

For LWPC, at 350°C, C1, C11, C12, C13, C14, C15, and C16 mixes had relative residual compressive strengths of 122, 110, 117, 118, 114, 105, and 106%, respectively. Each mixture’s compressive strength increased at a different rate, ranging from 5 to 22%. When we evaluate the results at 200, 400, and 23°C, we discover that the concrete that has been exposed to high temperatures and then allowed to cool naturally is the best instance. C1, C11, C12, C13, C14, C15, and C16 mixes had relative residual compressive strengths of 90, 100, 101, 102, 108, 98, and 95% at 450°C, respectively. The C16 mixes lose less strength when compared to the C11 mix, as shown in Figure 8(a–g).

6.4 Water-cooled specimens

Figure 8(a–g) shows the decrease in compressive strength as a function of exposure temperature after cooling with water (water spraying). It is easy to see how water-cooled concrete mixtures work.

The air-cooled specimens had developed some strength at this temperature, as previously mentioned, but the situation was different for the water-cooled specimens. There is no increase in residual compressive strength at 250°C. The relative residual strength ranged from 10 to 40% at 250, 350, and 450°C. Due to thermal shocks and internal strains, water-cooled samples have less compressive strength than air-cooled samples, resulting in reduced compressive strength. Thermal shock was induced by spraying water on heated concrete specimens, and it is possible that cracks occurred as a result of tensile stress in the specimens, resulting in a significant drop in compressive strength as shown in Figure 9.

Figure 9

Visual inspection test of air- and water-cooling affects.

6.5 Weight loss

Due to the hot conditions, each cubic specimen was weighed before and after each temperature cycle, and the percentage weight loss was calculated. The most common cause of weight change was drying of the cement paste. The loss of weight allows the drying of concrete to be measured after each heating. Table 9 lists the results of weight loss. It was discovered that when the temperature of exposure increased, the weight loss of all concrete mixes increased. For C1, C11, C12, C13, C14, C15, and C16 mixtures, the temperature started at 100, 250, 350, and 450°C, respectively.

Dehydration of cement paste, as well as drying of LWA, was one of the main causes of weight change since they retain a lot of water, and their loss of total humidity owing to the increase in high temperatures results in a loss of weight in general. The weight loss results that were provided showed that as the temperature of exposure increased, weight loss increased for all concrete mixes. The evaporation of capillary water, followed by the departure of adsorbed (gel water) and interlayer water, was the primary cause of the high rate of weight loss.

The evaporation of capillary water, followed by the departure of adsorbed (gel water) and interlayer water, was the primary cause of the rapid rate of weight loss up to a temperature of 450°C. The release of chemically mixed water, which is a component of cement hydrate products and is the hardest to evaporate, is related to the moisture loss at this temperature, which results in a somewhat reduced increase rate in mass loss up to 450°C. So, it is expected that the mass loss rate will decline. Further mass loss occurred at 450°C because of the thermal breakdown of hydration product constituents. These factors were further supported by thermogravimetric analysis performed by ASTM C642 [22]. The analysis of cement paste consists of three components: loss of water, dehydration of pentlandite, and decarbonization of carbon (CaCO₃).