**1. Introduction**

Repeated accidental impact is a type of unfavorable load that most building structures are not designed to withstand. Impact loads are a type of short-term dynamic load that exposes the material to unusual and unwanted stresses, which is more effective on brittle materials such as concrete. Repeated impacts occur due to the accidental falling of building materials during the construction period, or other objects during the life span, from higher stories [1–3]. In parking garages, the collision of cars can also be a source of repeated impacts on columns and walls. Other example sources are repeated hits by projectiles or explosive shrapnel in conflict regions [4,5]. The evaluation of the influence of repeated impacts on the material microstructure and the residual performance of the structural members requires an adequate evaluation of the cracking and fracture behavior under such type of loads. Pre-evaluation of material behavior under repeated impacts can be conducted using the ACI 544-2R [6] repeated drop weight impact test, which can be considered as the simplest and cheapest impact test to evaluate the impact performance of concrete [7–9].

**Citation:** Al-Ameri, R.A.; Abid, S.R.; Murali, G.; Ali, S.H.; Özakça, M. Residual Repeated Impact Strength of Concrete Exposed to Elevated Temperatures. *Crystals* **2021**, *11*, 941. https://doi.org/10.3390/cryst11080941

Academic Editors: Cesare Signorini, Antonella Sola, Sumit Chakraborty and Valentina Volpini

Received: 14 July 2021 Accepted: 10 August 2021 Published: 12 August 2021

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This test was used to evaluate the impact performance of several types of concrete mixtures by several recent researchers.

Concrete is a type of cementitious material that is composed mainly of Portland cement and aggregate that are mixed with water to form a moderately low cost construction solution with a sufficient compressive strength [10–13]. Although there have been great advances in the branch of material science and the development of many evolutionary modern types of concrete that possess seriously superior strength and ductility characteristics by incorporating new fiber-reinforced cementitious composites, normal weight, normal strength Portland cement-based concrete is still the most widely used type in reinforced concrete buildings [14,15]. Concrete structures, as with other structure types, are always under the danger of accidental fires that may occur due to electrical issues or inconvenient occupation. With the existence of furniture, fires can reach temperatures as high as 1000 ◦C in a very short period. Yearly, tens of thousands of fires are reported in countries such as the USA and the UK, 40% of which are classified as structural fires [16]. Although concrete is considered a good fire-resistant material, exposure to high temperatures with a steep temperature increase would seriously deteriorate the concrete structural members [15,17]. Due to the low thermal conductivity of concrete and considering the quick temperature increase, high temperature gradients would form between the exposed surfaces, the cores and the opposite unexposed surfaces [18]. Such gradients would induce internal thermal stresses on the microstructural scale, leading to steep material degradation [19,20]. The non-uniform thermal movements (expansion) of the different material parts owing to the non-uniform thermal gradients, and the chemical decomposition of calcium hydroxide beyond 400 ◦C [19,21–23] would initiate the effective degradation of the strength until failure.

Three main types of fire test procedures have been used by researchers to simulate the strength reduction in the material under high temperature exposure, two of which simulate the conditions of the concrete strength during fire exposure for compression and flexural members. In the first of these tests, which is the stressed fire test, the setup simulates the performance of compression members such as columns or compression parts of beams where the member is stressed by approximately 20% to 40% of its strength [24–26]. In this test, the specimens are preloaded to a constant stress level and then heated to the desired level of temperature, at which the temperature is kept constant to assure an approximately zero thermal gradient in the material (temperature saturation to the steady state condition). Then, the load is increased to failure. The second test is usually termed as the unstressed condition, which follows the same procedure of the first test setup but without preloading. This test simulates the conditions of beams and slabs where the concrete is usually under low tensile stresses [25]. On the other hand, the residual unstressed test is the third fire test setup. This test setup simulates the post-fire evaluation of the strength, where the specimens are heated unstressed to the required temperature, saturated at this temperature for a constant period and then cooled to ambient temperature. Finally, the cooled specimens are tested at ambient temperature to examine the residual strength of the concrete after the fire is over. The post-fire exposure strength evaluation is essential to decide whether the building is still functional, requires strength rehabilitation due to being unsuitable or must be demolished. Previous studies [25] showed that if the specimens are cooled in water (as in the case of water distinguishing a fire), the strength deteriorates at faster rates than when cooled under air convection. The volume changes resulting from the re-hydration (with cooling water) of dehydrated calcium silicate after exposure to temperatures higher than 400 ◦C have a destructive influence on the concrete microstructure [26].

Few research works are available in the literature about the evaluation of the concrete impact strength after high temperature exposure, most of which investigate high-strain rate impact tests and blast tests [27–31], while a very limited number of works were found on low-velocity impact tests on reinforced concrete members [18,32]. Most of these works developed numerical analyses to evaluate the dual effect of high temperature and high-strain rate impacts on reinforced concrete and composite structural members [33–35]. Although the combined action of fire exposure and low-velocity repeated impacts is probable in many structures such as parking garages, the authors could not find sufficient literature works that tried to explore the post-fire repeated impact performance of concrete. Mehdipour et al. [18] conducted an experimental study to evaluate the residual mechanical properties of concrete containing recycled rubber as a replacement of coarse aggregate, metakaolin as a partial replacement of Portland cement and steel fibers. Among the investigated mechanical properties, the authors evaluated the residual repeated impact strength using the ACI 544-2R test after exposure to temperatures of 150, 300, 450 and 600 ◦C. The test results showed that high temperature could significantly affect the residual cracking and failure impact numbers, where only one impact could cause the cracking of specimens heated to 600 ◦C. They also reported that a multi-surface cracking was observed for heated specimens instead of the central circular fracture of unheated fibrous specimens.

The current research investigated the impact resistance of normal concrete to lowvelocity repeated impacts after exposure to high temperatures of accidental fires. As reviewed in the previous sections, such scenario is possible along the span life of concrete structures, while the number of studies on this topic is seriously limited. Aiming to fill the gap in knowledge in this area, a research program was initiated starting with the most usual concrete type in reinforced concrete structures, which is normal weight, normal strength concrete.

#### **2. Experimental Work**

As stated previously, the experimental work presented in this study was conducted to be a reference work for future works where the post-fire residual impact performance of fibrous concretes will be assessed. Therefore, a normal plain concrete mixture was adopted in this study to investigate the effect of repeated low-velocity impacts on heated concrete. A cement content of 410 kg/m3 was used with 215 kg/m<sup>3</sup> of water, while the fine and coarse aggregate contents were 787 and 848 kg/m3, respectively. Local crushed gravel and river sand from Wasit Province, Iraq, were used as coarse and fine aggregates. The maximum size of the crushed gravel was 10 mm. The chemical composition and physical properties of the used cement are listed in Table 1, while the gradings of both the sand and gravel are shown in Figure 1. All specimens were cured for 28 days in temperature-controlled water tanks.

**Table 1.** Physical properties of cement.


The impact tests were conducted using the standard ACI 544-2R repeated impact procedure. However, instead of the manual operation of the test, an automatic impact machine was built for this purpose and used in this study to perform the repeated impact tests. The impact machine shown in Figure 2a was built to reduce the efforts and time of the test, where repeated impacts were automatically applied using the standard ACI 544-2R drop weight (4.54 kg) and drop height (457 mm), while the crack initiation and failure were observed using a high-resolution camera, as shown in Figure 2b. The same standard test setup was followed, where a steel ball was placed on the top of the 150 mm-diameter and 64 mm-thick disc specimen, while the drop weight fell directly on the steel ball that transferred it to the center of the specimen's top surface, as shown in Figure 2c. Once the first surface crack appeared, the number of repeated impacts was recorded, which is termed as the cracking number (Ncr). After, the impact was continued until the failure of the specimens, where the failure number (Nf) was recorded.

**Figure 1.** Grading of fine and coarse aggregates.

**Figure 2.** The automatic repeated impact testing machine (**a**) the impact machine; (**b**) crack observation; (**c**) the impact load and holding system.

In addition to the six impact disc specimens, six 100 mm cube specimens and six 100 × 100 × 400 mm prisms were cast from the same concrete patches, as shown in Figure 3. The concrete cubes were used to evaluate the compressive strength, while the prisms were used to evaluate the modulus of rupture. Previous researchers [36] showed that increasing the size of the cube specimen from 100 to 150 mm had no significant effect on the residual compressive strength after exposure to temperatures up to 1200 ◦C, while others [37]

drew a similar conclusion for concrete cylinders having different sizes (diameter × length) of 50 × 100 mm, 100 × 200 mm and 150 × 300 mm. The cylinders were exposed to temperatures up to 800 ◦C. Several previous studies [38–46] adopted 100 mm cubes to evaluate the residual compressive strength of concrete.

**Figure 3.** Single group of disc, cube and prism specimens.

Seven concrete patches were cast and cured in water tanks under the same conditions for 28 days, after which the specimens were left to air dry for a few hours and then dried in an electrical oven at 100 ◦C for 24 hours [15,47–50]. After the specimens were naturally air cooled, six of the seven patches were heated to temperatures of 100, 200, 300, 400, 500 and 600 ◦C using the electrical furnace shown in Figure 4a, while the seventh patch was left as an unheated reference. As shown in Figure 4b, a steel cage was used to reduce the destructive effects of the concrete explosive failure on the internal walls and heaters of the furnace. The heating process followed the heating and cooling procedure shown in Figure 5, where the specimens where heated steadily at a heating rate of approximately 4 ◦C/min until the target temperature. After, the specimens were thermally saturated at this temperature for one hour to assure the thermal steady state condition [51,52]. Finally, the furnace door was opened to allow for the natural air cooling of the specimens, which were left in the laboratory environment and tested the next day.

**Figure 4.** The electrical furnace (**a**) furnace interiors; (**b**) specimens heating.

**Figure 5.** Heating and cooling regime of the electrical furnace.

The heating rate can affect the residual strength of concrete exposed to temperatures exceeding 300 ◦C [53]. Previous researchers used different heating rates to evaluate the residual properties of concrete, which were as low as 0.5 ◦C [54] and 1 ◦C [55] and as high as 30 ◦C [56]. However, heating rates ranging from 2 to 10 ◦C were the most common in the literature [57], where a heating rate of 2 ◦C was adopted by [58,59], while 3 ◦C was adopted by [60,61], and 4 ◦C was adopted by [62,63]. On the other hand, 5 ◦C was used by [64–67], and 6 ◦C and 7 ◦C were used by [38,64,68], while other previous researchers [69–71] used a heating rate of 10 ◦C. In this study, a heating rate within this range, 4 ◦C, was adopted.

#### **3. Results and Discussion**
