Citation: Mohammed, S.D.; Ibrahim, T.H.; Salman, B.F.; Allawi, A.A.; El-Zohairy, A. Structural Behavior of Reactive Powder Concrete under Harmonic Loading. Buildings 2023, 13, 1917. https://doi.org/10.3390/ buildings13081917 Academic Editors: Binsheng (Ben) Zhang and Daxu Zhang Received: 27 June 2023 Revised: 24 July 2023 Accepted: 27 July 2023 Published: 27 July 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). buildings Article Structural Behavior of Reactive Powder Concrete under Harmonic Loading Shatha D. Mohammed 1 , Teghreed H. Ibrahim 1 , Ban F. Salman 1 , Abbas A. Allawi 1 and Ayman El-Zohairy 2, * 1 Department of Civil Engineering, University of Baghdad, Baghdad 17001, Iraq; shatha.dh@coeng.uobaghdad.edu.iq (S.D.M.); tagreed.hassan@coeng.uobaghdad.edu.iq (T.H.I.); ban.fadhil@coeng.uobaghdad.edu.iq (B.F.S.); a.allawi@uobaghdad.edu.iq (A.A.A.) 2 Department of Engineering and Technology, Texas A&M University-Commerce, Commerce, TX 75429, USA * Correspondence: ayman.elzohairy@tamuc.edu; Tel.: +1-903-468-8683 Abstract: Industrial buildings usually are designed to sustain several types of load systems, such as dead, live, and dynamic loads (especially the harmonic load produced by rotary motors). In general, these buildings require high-strength structural elements to carry the applied loads. Moreover, Reactive Powder Concrete (RPC) has been used for this purpose because of its excellent mechanical strength and endurance. Therefore, this study provides an experimental analysis of the structural behaviors of reinforced RPC beams under harmonic loads. The experimental program consisted of testing six simply supported RPC beams with lengths of 1500 mm, widths of 150 mm, and thicknesses of 200 mm under harmonic loading with varied frequencies between 10 and 20 Hz. Different steel fiber ratios of 0%, 0.5%, 0.75%, 1.0%, 1.5%, and 1.75% were provided in the concrete mixes to explore the effect of steel fibers on the dynamic behavior of these beams. Except for the steel fiber volume fraction, all of the examined specimens shared the same material attributes and reinforcing details. The outcomes proved the positive effect of adding steel fibers on the dynamic response under the effect of harmonic loading. The optimum volume fraction of steel fibers was characterized by a percentage of 1.5%. Moreover, the vibration amplitude was more affected by the steel fibers than the support reactions. The inertial force increased as the harmonic loading duration increased. This increase in the inertial force by the load duration was enhanced after adding the steel fibers. However, this enhancement started to decline after increasing the steel fiber content to 1.75%. Keywords: reactive powder concrete; harmonic loading; steel fiber; frequency; load duration effect 1. Introduction The maximum density theory inspired the development of Reactive Powder Concrete (RPC) as an example of ultra-high-strength concrete. Because of its excellent mechani- cal strength and endurance, this form of concrete has been used in many construction projects [1,2]. Many studies have discussed the benefits of RPC over normal-weight con- crete due to its higher compressive and tensile strengths and lower permeability. The coarse material was removed from the concrete to eliminate the weakest link [3]. RPC is usually formed from extremely fine powder materials (cement, sand, quartz powder, and silica fume), steel fibers (optional), and a superplasticizer [4,5]. Conventional aggregate is completely replaced by fine sand, whose particle size is less than 0.6 mm. It was also indicated that the RPC’s compressive stress–strain relation has a linear ascent stage until the ultimate strain, and at that point, the strength drops sharply and an explosive failure occurs suddenly. Hence, it is so difficult to observe a complete descending stage. To solve this problem, steel fibers are usually added to improve ductility [6]. Many studies collectively demonstrated that adding steel fibers to RPC can significantly improve its mechanical properties, including compressive strength, flexural strength, toughness, and Buildings 2023, 13, 1917. https://doi.org/10.3390/buildings13081917 https://www.mdpi.com/journal/buildings