Experimentally investigating comparison between the behavior of fibrous concrete slabs with steel stiffeners and reinforced concrete slabs under dynamic–static loads

Al Zahid Ali Adnan; Al Kulabi Ahmed Kamil

doi:10.1515/eng-2022-0018

Article Open Access

Experimentally investigating comparison between the behavior of fibrous concrete slabs with steel stiffeners and reinforced concrete slabs under dynamic–static loads

Al Zahid Ali Adnan and Al Kulabi Ahmed Kamil

Published/Copyright: March 10, 2022

Published by

Become an author with De Gruyter Brill

Submit Manuscript Author Information

From the journal Open Engineering Volume 12 Issue 1

Abstract

Three scaled concrete slabs were cast and tested till failure under drop-weight impacts and static loads, sequentially. Two of the three slabs contained 0.5% steel fiber while the third slab was constructed by using only steel bar reinforcement (i.e., ordinary reinforced concrete). The obtained data from the testing program were used to distinguish between the performance of traditional reinforced concrete and steel fiber concrete slabs under impacting and static weights and to establish a data base for the upcoming studies pertaining to this research area. The test results and observations showed that the extension of steel fiber to the ingredients of concrete was efficient in decreasing crack widths and softening local damage under dynamic and static loads. On the other hand, reinforcement was more effective in increasing slab capacity than steel stiffeners, consequently different failure modes were observed.

Keywords: impact load; flexural capacity; steel fiber; steel reinforcement; steel stiffeners; deflection

1 Introduction

How fibrous concrete or reinforced concrete will behave under static loading after applying impact loads and what are the differences between them and reinforced concrete slabs under impact and subsequent static loads is the research area that is not obviously understood; anyway, investigation in this area is still inspired by numerous engineering applications. Examples comprise concrete structures designed to withstand casual loading scenarios, such as impact of a falling rock, collision of a car with buildings, offshores, and/or bridges, and facilities used in a high-risky applications like nuclear and military facilities. Steel fibers were needed since cementitious materials are brittle, and addition of steel fibers to them improved a number of concrete properties, such as ductility, strength, and durability, and made them the most suitable concrete for many engineering applications with unique requirements [1,2]. Hence, an improved type of concrete was obtained; this concrete type could provide a 50 MPa strength compared with normal weight concrete, which provides less than 50 MPa strength [3]. In addition, the developed concrete is more durable and can provide more strength of tensile when it is compared with the common concrete [4,5]. As a result of the improved properties mentioned in the previous paragraph, fibrous concrete is the most favorite type for engineering applications that may encounter dynamic loads whether man-made or natural dynamic loads. Man-made dynamic loads include generated loads from the explosions of blasts, rockets, etc. while natural dynamic loads comprise created loads from natural sources such as tornados, winds, and earthquakes [6,7]. Hence, to get a better understanding and to assess the performance of conventional reinforced concrete and fibrous concrete under successive effects of impact and static loads, three concrete slabs with dimensions of 80 × 80 × 4 cm 3 were cast and loaded. The slabs, T4SS1 and T4SS2, had 0.5% steel fiber with no reinforcement bars and had steel stiffeners with thicknesses of 1 and 2 mm, respectively, while the slab T4R had no steel fiber, but it had one layer of 6 mm steel bar reinforcement in both directions. Moreover, the three slabs were impacted from a height of 120 cm by one drop-weight impact only. Thus, all slabs had a similarity in all influencing parameters on their behaviour under dynamic and static weights; slab dimensions, slab thickness, height of dropping weight, number of impacts, and amount of static weight except steel fiber, steel stiffeners, and reinforcement existence were the various parameters between the two sets of slabs. Needless to say that the reason for matching the influencing parameters is to figure out the key role all steel fiber and reinforcement in resisting dropping-weigh impacts and static weights.

2 Materials and methods

A summary about material properties, material types and sources, and method of testing is included in this section.

2.1 Material properties of the fibrous concrete slabs

The three concrete slabs were cast by using the following materials: cement, fine aggregate, silica fume, steel fiber, glenium, steel reinforcement, and molds made from medium density fiberboard wood. The types and properties of the utilized ingredients are summarized in this section as follows: Cement: the cement type that is put to use in this study is ordinary Portland cement. Fine aggregate: all the particles with large sizes were removed to produce dense concrete, and the remaining particles were with sizes ranging between 600–150 μ m micrometers. Silica fume: a pozzolanic material was used to work as filler and binder in concrete mixes [8]. The type of the added silica fume to the concrete of this research is Sika-Fume S92D. Figure 1 shows the type of the used silica fume. Steel Fiber: the type of added steel fibers into concrete mix of this research is WSF 0213; the source of this type is China, and it meets the specifications of ISO 9001/2008. The properties of the added steel fibers comprise brass-coated surface, straight, a tensile strength of 2,300 MPa, an average length of 13 mm, 0.2 mm in diameter, an aspect ratio of 65. As mentioned before, steel fibers were added to improve concrete ductility and other properties [9]. Figure 2 illustrates the added steel fibers to the concrete mix. Glenium: Glenium 54 was used in the prepared concrete mix to enhance the performance of concrete by reducing water and by getting concrete workability improved [10]. Figure 3 shows the glenium type used in this research. Epoxy: It was needed to anchor the steel plates on the concrete slabs. The type of epoxy used is ET-HP with the following properties: bond strength for 2 days is 14 MPa, while the compressive yield strength of the used epoxy according to the test method (ASTM D 695) is 63. 25 and 92.32 MPa for 1 and 7 days, respectively. Steel reinforcement and steel plates: tensile strength and yield strength of 1 mm steel plate, 2 mm steel plate, and steel reinforcement are listed in Table 1. Wood: MDF or medium density fiber was used to make a mold with plane, clear faces as illustrated in Figure 4.

Figure 1

Silica fume.

Figure 2

Steel fiber.

Figure 3

Glenium.

Table 1

Yield strength and tensile strength

Slab	Yield strength (MPa)	Tensile strength (MPa)
T4SS1	301	361
T4SS2	366	470
T4R	554	608

Figure 4

MDF wood molds.

2.2 Impact testing apparatus

The three slabs impacted by using the drop-weight apparatus are illustrated in Figure 5. This equipment was fabricated locally by Al Zahid [11]. I and rectangular steel sections were needed to manufacture this impacting device. Other supplements were required to enable the equipment from doing its job and to obtain the essential data such as electronic infrared sensor, impacting weight, electronic ultrasonic sensor, and electronic accelerometer sensor. A steel ball with a weight of 5 kg was used as a drop-weight to generate impacts on the concrete slabs. The electronic accelerometer sensor was added to the equipment to provide the author with the acceleration of the dropping weight through recording the developed impacting force by the dropping weight with time. The electronic ultrasonic sensor was needed to specify the amount of lateral displacement while they are impacting with time. The electronic infrared sensor was essential to measure the displacement of the slabs with time.

Figure 5

Test apparatus for impact with sensors.

2.3 Method of carrying out the test of impact loading

The three concrete slabs were put on the dropping-weight equipment, and then the steel ball, impacting weight, was allowed to freely drop under the influence of its own weight to impact the slabs at the center from their top faces. The dropping height was 120 cm, and it is the same impacting height for all slabs. Next, the electronic accelerometer sensor was set up on the dropping-weight to measure the generated impacting force with the corresponding time. The electronic ultrasonic sensor was installed in a way where the developed lateral displacement of the slabs is recorded, and the electronic infrared sensor was fixed at the center of the bottom face of the slabs to provide data referring deflection with time.

2.4 Methodology of testing static load

The hydraulic machine shown in Figure 6 was used for testing flexural strength of the three slabs after being impacted. The reason of testing flexural strength of the slabs after testing their impact resistance is that the published studies before this study did not define precisely the properties and behavior of normal weight concrete and fibrous concrete under the sequential effect of dynamic and static loading. For example, Aravind et al. [12] conducted an exploratory investigation regarding the flexural resistance of fibrous reinforced concrete beams having dimensions of 1.5 × 1.5 × 0.18 m 3 . Besides, Jia et al. [13] performed a laboratory research referring the behavior of fibrous concrete under the influence of impact loading by applying blast loading to two-way-reinforced concrete slabs. Therefore, the same three slabs were loaded statically after being applied to impact loading to figure out how much the service loads of these slabs would be affected after being exposed to explosions or any other source generating impacts on slabs. For the case of static loading, the slabs were placed on the testing apparatus by setting up a frame on the device with simple supports that were destined by welding a roller on the frame as shown in Figure 6. Before loading the slabs, a 15 × 15 cm 2 rigid cube was centered at the slabs’ top face to spread out the developed loads by the hydraulic machine into two directions of the slabs, achieving two-way slabs under concentrated monotonic increasing load condition.

Figure 6

The hydraulic static testing equipment.

3 Results and discussions

The experimental results of compression, dropping-weight, and static tests are given in this section. In addition, the results are discussed depending on obtained data and observations.

3.1 Compression test results

For each slab, three cubes were sampled and tested by Al Zahid [11] to specify the compressive strength of fibrous concrete. The average of three cubes of each slab with the corresponding age of testing is included in Table 2. Besides, the failure shape of the concrete cubes with and without steel fibers is shown in Figure 7a and b, respectively.

Table 2

Average amounts of three cubes’ compressive strength

Slab	Compressive strength (MPa)	Age (Day)
T4SS1	85.76	58
T4SS2	84.31	61
T4R	92.11	64

Figure 7

Failure shape of concrete cubes (a) with steel fiber, (b) without steel fiber.

3.2 Dropping-weight test results

Each slab of the three slabs was impacted from a 120 cm height and for one time. The impacting strength of each slab with the corresponding deflection is included in Table 3.

Table 3

Load-deflection results for impact test

Slab	Impact load (kN)	Deflection (mm)
T4SS1	6.18	2.35
T4SS2	6.18	1.76
T4R	7.79	3.89

Figure 8

Crack pattern of the slab T4SS1 under impact loading.

The average crack width of the slabs is included in Table 4, and the crack patterns and distributions of the slabs: T4SS1, T4SS2, and T4R are shown in Figures 8, 9, and 10, respectively.

Table 4

Average crack widths of slabs under drop-weight loading

Slab	Average crack width (mm)
T4SS1	0.026
T4SS2	0.020
T4R	0.066

Figure 9

Crack pattern of the slab T4SS2 under impact loading.

Figure 10

Crack pattern of the slab T4R under impact loading.

As it is clear from the results in Tables 3 and 4, the slab with steel bars, T4R, provided the highest resistance to the impact loading and at the same time deflected most than the other two slabs with steel fibers: T4SS1 and T4SS2. From the other side, the slab, T4R, had the widest crack width, which was about 0.066 mm.

3.3 Static loading test results

The three slabs were loaded statically after subjecting them to the drop-weight loading. The applied static load was progressively raised up by each 0.5 kN until reaching failure load, and the deflection development was recorded for each load increment. The maximum static capacity in kilo newton of each slab with the corresponding deflection in millimeter is included in Table 5.

Table 5

Load-deflection results for static load

Slab	Static load (kN)	Deflection (mm)
T4SS1	17.5	3.72
T4SS2	18	4.44
T4R	35	25.31

As it is obvious from the results in Table 3, T4R slab resisted most static loading and deflected most than the other two slabs. Next, the slab with 1 mm steel stiffener, T4SS1, provided less flexural capacity and deflected less than the slab with 2 mm steel stiffener, T4SS2. Figure 11 shows the slabs’ progression curves of load-deflection.

Figure 11

Slabs’ load-deflection curves.

The developed cracks because of static loading of all slabs were marked and compared as done before for the cracks of drop-weight loading. To differentiate between drop-weight and flexural cracks, red and blue markers were used to signal the drop-weight and static cracks, respectively. Statically loading the slab T4SS1 caused more widening the cracks of drop-weight loading on the bottom face of the slab and extending them till reaching the edges of the slab. Also, new cracks showed up on the slab’s bottom and top faces beginning from the point of slab loading as shown in Figures 12 and 13. The average crack width of the bottom face of the slab is 2.245 mm.

Figure 12

Crack pattern under impact-flexural loads of the T4SS1 slab bottom face.

Figure 13

Crack pattern under impact-flexural loads of the T4SS1 slab top face.

Figure 14 shows crack pattern and distribution created as a result of loading the slab T4SS2 dynamically and statically. Only the bottom face of the slab had cracks; no cracks appeared on the slab’s top face. The same matter happened here where the cracks of dynamic loading were more expanded because of the consequent static loading, and new cracks showed up, starting from the center of slab (point of loading). The average width of cracks of the slab is 1.4 mm.

Figure 14

Crack pattern under impact-flexural loads of the T4SS2 slab bottom face.

Applying static load to the slab T4R caused generating a lot of new cracks on the slab’s bottom face, deriving their way from the center toward the edges of the slab. Besides, new circular cracks showed up on the top face of the slab, surrounding the outer four corners of the solid cube shown in Figure 6. The average crack width of the bottom face of the slab is 1.9 mm. Figures 15 and 16 show the crack pattern and distribution on the slab’s top and bottom faces, respectively.

Figure 15

Crack pattern under impact-flexural loads of the top face of the slab T4R.

Figure 16

Crack pattern under impact-flexural loads of the bottom face of the slab T4R.

4 Results and Discussions

In this section, discussions are made depending on the observations and test results. This includes discussing the results of testing two concrete slabs with steel fiber and steel stiffener and testing one concrete slab with zero steel fiber content and with steel bar reinforcement. As it is noticed from the previous sections, the samples responded differently to the applied loads; consequently various outputs were observed regarding: capacity of impact loading, deflection, capacity of static loading, failure mode, and number and width of cracks. All of these parameters are discussed in detail in the following paragraphs. Loading capacity, the slab, T4R, showed a higher initial resistance to the impact loading than the other two slabs with steel fiber and stiffeners. This slab provided a resistance to the applied impacts more than the resistance of the slabs, T4SS1 and T4SS2, by about 26%. The same concept is applicable regarding flexural loading resistance where T4R slab withstood more static loading than T4SS1 and T4SS2 by approximately 100 and 94.4%, respectively. Moreover, the slabs with steel fiber and stiffeners provided the same loading capacity under impacts and almost the same capacity under static loading where T4SS2 slab withstood a little bit higher static loads than T4SS2 by nearly 2.9%. The other remarkable thing is that steel stiffeners during dynamic loading did not take off from the slabs, but at the time of static loading, they debonded. Figure 17 compares the resistance capacities of the slabs to the impact and flexural loading.

Figure 17

Impact and static loading capacity of the slabs.

Deflection, steel fiber content, and steel stiffener were the most affecting parameters on controlling the amount of deflection. For impact loading case, the slab, T4SS2, was the minimal deflected slab among the other slabs. In contrast, the slab with steel bar reinforcement, T4R, was the most deflected slab. T4R slab deflected more than the slab T4SS2 by about 121%. This means that the absence of steel fiber in the slab T4R changed the property of concrete material from a ductile to a brittle material, and steel fiber is distributed in a smeared way among the ingredients of the mix, and at the same time it is very different from the distribution of steel bar reinforcement. Therefore, steel fibers in concrete play the key role in absorbing energy before reaching to fracture. Concerning the importance of adding stiffeners to the samples, increasing steel stiffener thickness by 1 mm decreased the amount of deflection by 25. 11% when comparing deflection of the slabs T4SS1 and T4SS2. Identically, for static loading case, T4R slab deflected more than the slab T4SS2 by 470%. And, T4SS2 slab deflected more than the slab T4SS1 by 19.4% where stiffeners were debonded. Figure 18 shows and compares the amount of deflection of the three slabs and under the effect of impact and static loading. Crack width and number, the steel fiber role here is slowing down the progression of cracks by altering the property of concrete from a brittle to ductile concrete [14,15]. Therefore, T4R slab was strongly affected by impacts, and from laboratory observations cracks were rapidly increased and spread out through the slab’s bottom face. In addition, more circular cracks showed up on the slab’s top face. When comparing the slabs T4R and T4SS2, the average crack width of the slab T4R is more than the average crack width of the T4SS2 by 230%. Also, increasing steel stiffeners by 1 mm caused reducing the average width of cracks by 23.1% when the average crack widths of the slabs T4SS1 and T4SS2 are compared. Referring crack width under static loading, the average crack width was increased, and the crack number was increased as well. The average width of cracks of T4R under static loading was more than the crack width of T4SS2 by 35.17%. Besides, the average crack width of the slab T4SS2 was less than the average crack width of the slab T4SS1 by 37.64%, which illustrates the necessity of adding steel stiffener to the concrete slabs. Additionally, T4SS1 had cracks on the top face at the time of static loading while T4SS2 did not have. Figure 19 shows and compares the amount of deflection of the slabs generated as a result of the dynamic and static loading.

Figure 18

Deflection of the slabs under impact and static loading.

Figure 19

Average crack width of the slabs under the effect of loads of impact and static.

Failure mode, after reviewing a number of published studies, many parameters were found to specify the failure mode of concrete slabs like concrete type whether conventional concrete or fibrous concrete, type of loading whether static or dynamic, loading position, amount of loading, etc. For impact loading case, Jia et al. [13] indicated that failure modes of concrete slabs rely on two main parameters: loading amount or in other words weight of loading and position of loading or hitting point of slabs whether at center or at one of the edges of the slabs. Besides, they found that damage degree of slabs would increase by increasing the amount of weight of explosive or by moving the impacting point from the center toward the edges of the slab, consequently failure mode of slabs would vary progressively from flexural to flexural-shear failure mode. Although both impacting weight and hitting point parameters were the same for all slabs, different failure modes were noticed due to changing other parameters among slabs like steel fiber content, steel stiffener containing, thickness of steel stiffener, and steel bar reinforcement. Hence, T4SS1 and T4SS2 slabs were found to fail in flexural, and T4R slab failed in flexural and punching.

5 Conclusion

Danger rates on buildings’ residents have been increasing due to increasing collapse cases of buildings all around the world as a result of the natural disasters and terrorism attacks. Thus, researchers who are specialized in concrete are inspired to more enhance and investigate the properties of concrete in case to design safe structures. Therefore, three scaled concrete slabs were cast and tested by applying impact and static loads consecutively. Two of the three slabs had 0.5% steel fiber and had steel stiffeners as well; one slab had 1 mm steel stiffener and the other had 2 mm steel stiffener while the third slab had no steel fiber, but it had steel bar reinforcement instead. Test outputs proved the necessity of adding steel fiber to concrete mixes to improve concrete ductility, consequently more energy is absorbed, which resulted in decreasing deflection and crack width and number when compared with tested concrete with steel bar reinforcement. On the other hand, steel bar reinforcement was found to be more beneficial in increasing the resistance of concrete to the applied loads. Moreover, steel stiffeners were essential in improving slabs resistance to the loads and in decreasing deflection and slab’s crack number and width.

Conflict of interest: Authors state no conflict of interest.

References

[1] Chen JJ, Ng PL, Li LG, Kwan AKH. Modern building materials, structures and technologies. MBMST 2016. Vol. 172. Amsterdam: Elsevier; 2017. p. 165–71. Search in Google Scholar

[2] Vinayagam P. Experimental investigation on high performance concrete using silica fume and superplasticizer. Int J Comput Commun Eng. 2012;1:168–71. 10.7763/IJCCE.2012.V1.44Search in Google Scholar

[3] Al-Jubory NH. Mechanical properties of reactive powder concrete (RPC) with mineral admixture. Al-Rafadain Eng J. 2013;21:92–101. 10.33899/rengj.2013.79579Search in Google Scholar

[4] Singh S, Ritu. Natural reactive powder concrete (Rpc) - in form of a super-plasticized portland cement mixture with silica fume, steel fibers and ground fine quartz - a review. Int J Tech Res (IJTR). 2016;5:217–22. Search in Google Scholar

[5] Wu Z, Shi C, Wu L. Effects of steel fiber content and shape on mechanical properties of ultra-high performance concrete. Vol. 103. Amsterdam: Elsevier; 2016. p. 8–14. 10.1016/j.conbuildmat.2015.11.028Search in Google Scholar

[6] Koccaz Z, Sutcu F, Torunbalci N. Architectural and structural design for blast resistance buildings. In: The 14th World Conference on Earthquake Engineering October 12–17. Beijing, China; 2008.Search in Google Scholar

[7] Patel PB, Thakur LS, Maroliya MK, Patel BA. Shear and impact behavior of reactive powder concrete with varying powder content. Vol. 1. Birla Vishvakarma Mahavidyalaya, Vallabh Vidyanagar, Anand: Kalpa publications in CE; 2017. p. 307–14. 10.29007/z5zxSearch in Google Scholar

[8] Rasol MA. Effect of silica fume on concrete properties and advantages for Kurdistan region, Iraq. Int J Sci Eng. Res. 2015;6:170–3. Search in Google Scholar

[9] Baarimah AO, Mohsin SMS. Behaviour of reinforced concrete slabs with steel fibers. United Kingdom: IOP; 2017. p. 271–012099. 10.1088/1757-899X/271/1/012099Search in Google Scholar

[10] Maroliya MK, Modhera CD Influence of type of superplasticizers on workability and compressive strength of reactive powder concrete. Int J Adv Eng Technol. 2010;I(II):123–30. Search in Google Scholar

[11] AL Zahid MAA. Impact resistance of cambered reactive powder concrete slabs with steel stiffeners. PhD Eng. thesis, Babylon, Iraq: Babylon University; 2016. Search in Google Scholar

[12] Aravind M, Senthil KV, Manikandan G. Flexural behavior of high strength reactive powder concrete. SSRG Int J Civil Eng. 2017;ICRTCETM-2017:350–4. Search in Google Scholar

[13] Jia H, Yu L, Wu G. Damage assessment of two-way bending RC slabs subjected to blast loadings. Sci World J. 2014;2014:1–12. 10.1155/2014/718702Search in Google Scholar PubMed PubMed Central

[14] Behbahani HP, Nematollahi B, Farasatpour M. Steel fiber reinforced concrete: a review. ICSECM. 2011;(ICSECM2011):1–12.Search in Google Scholar

[15] Dahake AG, Charkha KS. Effect of steel fibers on strength of concrete. J Eng Sci Manag Edu. 2016;9(I):45–51. Search in Google Scholar

Received: 2021-08-08

Revised: 2021-09-27

Accepted: 2021-09-27

Published Online: 2022-03-10

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

Articles in the same Issue

https://doi.org/10.1515/eng-2022-0018

Keywords for this article

impact load; flexural capacity; steel fiber; steel reinforcement; steel stiffeners; deflection

Creative Commons

BY 4.0