**1. Introduction**

Self-consolidating concrete (SCC), also referred to as self-compacted concrete, is an innovative construction material with favorable rheological behavior that does not require vibration for placing and compaction. It can flow under its weight, filling in formworks, and achieving full compaction, even in the presence of complex-shaped concrete members with highly congested reinforcement [1–4]. Based on these properties, SCC may contribute to a significant improvement of the quality of concrete structures and opens up new fields for the application of concrete. The designation "self-compacting" is based on the fresh concrete properties of this material, which covers the mixture's degree of homogeneity, deformability, and viscosity. The yield point defines the force required to make the concrete flow. The speed of flow of SCC is associated with its plastic viscosity which describes the resistance of SCC to flow under external stresses [5–7]. SCC has become a preferred option for many projects that should satisfy strict fresh stage properties and quality assurance. To ensure stable and robust fresh stage properties, typically, a significant amount of fine

**Citation:** Ahmed, G.H.; Ahmed, H.; Ali, B.; Alyousef, R. Assessment of High Performance Self-Consolidating Concrete through an Experimental and Analytical Multi-Parameter Approach. *Materials* **2021**, *14*, 985. https://doi.org/10.3390/ma14040985

Academic Editor: Alessandro P. Fantilli

Received: 10 December 2020 Accepted: 29 January 2021 Published: 19 February 2021

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**Copyright:** © 2021 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/).

materials has been incorporated into the mixture. In relative to traditional concrete, different durability characteristics can be expected for SCC because it can be produced with various mix compositions and the absence of vibration [8–10]. Due to the relatively short history of SCC in practical applications, there is a significant lack of information about long term performance in real structures. Such a concrete should have a relatively low yield value to ensure high flowability, a moderate viscosity to avoid segregation and bleeding, and must maintain its homogeneity during transportation, placing, and curing [11–13].

High-performance concrete (HPC) is engineered to meet specific needs of a project, including mechanical, durability, or constructability properties. The demand for HPC has been continuously increasing due to its superior mechanical and durability properties [14,15]. When considering the cost of concrete production, HPC is even better than ultra-high performance concrete (UHPC), since heat-curing restricts the applications of the latter and makes it mainly suitable for precast elements, not for ready-mix concrete [16,17]. The development of HPC started in the 1980s, and thereafter the global demand for its consumption has significantly increased over the recent years. HPC can be designed to have high workability and mechanical properties as well as improved durability [18,19]. It has been primarily used in bridges and tall buildings. In general, durability is the most important parameter to increase the service life of any concrete structure [20–22]. Most commonly durability of concrete is affected by sulfate or chloride attack, carbonation, high temperature, and freezing and thawing damage [23,24]. Scanning electron microscopic studies [25,26] show that the pore structure in powder type SCC, including the total pore volume, pore size distribution, and critical pore diameter, is very similar to HPC. Over the past decades, advancements in concrete technology has led to the development of a new generation of concrete (e.g., HPSCC) with significantly better properties in terms of strength, durability features, and rheology of fresh concrete mixtures. In comparison to ordinary concretes, the designing process of HPSCC mix is determined by the increased cement content, superplasticizers, and an additive of reactive materials, i.e., silica fume. HPSCC is thus characterized by its ability to fill a form with congested steel rebars and self-leveling without mechanical compaction and it yields exceptionally high strength and durability [27,28].

Abundant research can be found in the literature on the properties of SCC. Most of the previous works have tested the fresh SCC mixes for common workability tests in order to prove self-consolidation of the concrete. The investigated properties were flowability, deformability and passing-ability, through slump-cone flow, J-Ring, V-funnel, and L-box tests [11,28,29]. The rheological properties of SCC such as yield stress and plastic viscosity [30,31] have also been investigated. Some researchers focused on the mix design and mix proportions [1,8,11]. The influence of mineral admixtures (i.e., silica fume, fly ash, metakaolin, ground granulated blast furnace slag, ladle slag) [32–35] and chemical admixtures (i.e., superplasticizers and viscosity modifying admixtures) [3,9,12] have also been studied on the performance of SCC. Some studies investigated the hydration rate and microstructure of SCC [1,13,17,36]. Researchers have also studied the properties of SCC an HPC with the addition of glass fibers, steel fibers and carbon nanotubes [6,18,37–40]. The stability tests results of SCC, i.e., shrinkage, cracking resistance, and creep are also available in literature [7,15,41,42].

A study reported that the elastic modulus, creep and shrinkage of SCC did not differ significantly from the corresponding properties of normal strength concrete (NSC) [43]. Some of the durability tests, including chloride penetration, water permeability and absorption, gas permeability, carbonation, electrical resistivity, sulfate attack, acid attack, frost resistance, and scaling, have been investigated [17,19,23,44] and more especially the fire resistance, cooling methods, weight loss, and residual mechanical properties of SCC [5,45–47]. Only few studies were found in the literature that investigated HPSCC [2,9,48,49] and its optimization [50–52]; these studies had focused on mechanical properties with either porosity, workability, water penetration, rheological properties, exposure to elevated temperature, or one durability test; but frost or scaling resistance of SCC have rarely been investigated in the literature.

Regarding the novelty of this work, it can be clearly seen in the literature that the HPSCC has been investigated in the past two decades, but its practical application is still limited. This is due to the fact that its consolidated technical performance of HP-SCC (e.g., mechanical strength, durability, and cost) is not fully understood, and there are often insufficient statements concerning its exact overall behavior. Existing research provides information only about the improvements in the properties of HPSCC mixes through the variation in the composition or addition of materials, but it does not inform what will happen to other parameters such as its consolidated economic and engineering performance. In the view of this understanding, this research was designed to present a comprehensive study regarding HPSCC's overall properties and comparing with three common reference concrete types, i.e., normal strength-vibrated concrete (NSVC), highstrength self-compacted concrete (HSSCC), and high-performance highly-viscous concrete (HPVC). The individual comparisons are based on the strength, workability and durability through 14 different types of tests. A new analytical approach has been proposed for a multi-parameter comparison between different types of concrete.

#### **2. Material and Methods**

#### *2.1. Material Properties*

The materials used for the concrete mixes were cement, silica fume, fly ash, fine and coarse aggregates, water, and superplasticizer. The cement was ordinary Portland cement type CEM-I 42.5R (, the micro-silica was MS90, which consisted of very fine SiO2 particles (up to 93.1%). The fly ash was type F, primarily consisting of silica, alumina, iron, and calcium oxides. The chemical and the physical properties of binders are shown in Table 1. The fine aggregate was normal fluvial sand, comprising the average passing percentages shown in Table 2. Fluvial gravel with a nominal maximum particle size of 12.5 mm was used in concrete mixes, and the average grading of 3 samples is shown in Table 2. High-performance superplasticizer concrete admixture Sika Viscocrete–5930 was used for obtaining workable or flowable mix made with a low water to cement ratio. The product was a third-generation superplasticizer with a density of 1.095 kg/L. Regarding the manufactures, cement, aggregates, microsilica, fly ash and superplasticizer were provided by Mass-Kurdistan company (Erbil, Iraq), Kalak quarry Hawler company (Erbil, Iraq), Jordan DCP company (Amman, Jordan), Jordan DCP company (Amman, Jordan), and Sika company (Istanbul, Turkey), respectively.


**Table 1.** Chemical compositions and physical properties of cement, micro-silica, and fly ash.



#### *2.2. Mix Types and Mix Proportions*

Design and selection of the concrete components is the most important step, which subsequently indicates the class and properties of the concrete. The intended concrete class was HPSCC, while three additional reference mixes were selected from 16 trial mixes. The reference mixes were HSSCC, HPVC, and NSVC. The considered four main optimization principles for better concrete production and mix design were workability, strength, cost, and durability. Table 3 can explain that 3 mixes were of the same proportions between cement, sand, and gravel, while NSVC is a conventional normal strength mix. The parameter that changed the HPSCC to self-consolidating concrete was the increased ratio of water, when compared to HPVC, since the binder-to-aggregate ratio was 0.24 for both mixes. Furthermore, the only difference that made HPSCC as high-performance concrete is the admixture type, when compared to HSSCC, as both mixes had the water to binder ratio w/b of 0.35.


**Table 3.** Mix proportions and compositions for the concrete mixes.

Note: C: Cement; S: Sand; G: Gravel; MS: Micro-silica; FA: Fly ash; SP: Superplasticizer; B/A: Binder-to-aggregate ratio; W/B: Water-tobinder ratio; Ad/C: Admixture-to-cement ratio.

#### *2.3. Testing Fresh Concrete Properties*

SCC is characterized by special fresh concrete properties. Many new tests have been developed to measure the SCC's flowability, viscosity, filling ability, passing ability, resistance to segregation, self-leveling, and stability of the mixture. In this project, the conventional slump test, slump flow test, and J-Ring test were performed. The slump test is acceptable to determine the workability of non-flowable concretes having a slump of 15–230 mm when the cone is raised. When concrete is non-plastic or it is not adequately cohesive, the slump test is no more reasonable. The slump test was performed according to ASTM C143 for NSVC and HPVC mixes (see Figure 1).

**Figure 1.** Fresh properties tests (**a**) slump test for vibrated concrete mixes, (**b**) slump flow test for self-consolidating concrete (SCC) mixes, (**c**) a SCC without segregation, and (**d**) restricted slump flow test.

HPVC had low water to binder ratio and more superplasticizer amount, therefore, the mix was very sticky, and needed additional effort for mixing, pouring, and casting. The slump flow test was performed for HPSCC and HSSCC, according to ASTM C1611 [53], to assess the flow rate in the absence of obstructions. During testing the accurate T500 (the time required for the slump flow patty to reach a 500 mm diameter) was recorded and when the concrete flow is stopped, the diameter of the spread at right angles is then measured and the mean is the slump flow (Figure 1). The restricted flow test was also performed according to ASTM C1621 for SCC classes. The J-Ring test represents the reinforcement inside the molds that restricts the flow of the concrete.

#### *2.4. Testing Physical Properties of Hardened Concrete*

Hardened density and absorption tests were performed for the four concrete mixes. The density of concrete was measured for different shapes and sizes and at different ages, in which the dimensions were measured to the accuracy of 0.01 mm, and the weights to 1 g. In the water absorption test, the concrete cubes were oven-dried at 60 ◦C for 48 h and the weights were recorded as oven-dry weights. After the cubes were submerged in water for 48 h, the surfaces were dried to represent saturated surface dry concrete.

#### *2.5. Testing of Mechanical Properties*

Strength tests are the most common for evaluation of different concrete classes; most of them were related to compressive strength by international standards. It is necessary to test as many as possible mechanical properties for special concrete classes, like HPC and SCC. To study the influence of shape and size of the specimens on compressive strength of different strength classes, 100 mm cubes and Ø100 mm cylinders were tested (Figure 2a).

**Figure 2.** Mechanical properties tests (**a**) various size and shape specimens for compressive strength, (**b**) cylinders in splitting tensile strength test, (**c**) flexural strength test of concrete specimens, and (**d**) testing modulus of elasticity for Ø150 mm cylinders.

The age of concrete was also considered, and the tests were performed at 1, 3, 7, 28, 56, 90, and 180 days. Splitting tensile strength was carried out on Ø100 mm cylinders, in which three cylinders were tested for each mix (Figure 2b). Another most common test for evaluating concrete's tensile strength is the modulus of rupture. For this test, three prisms of 75 mm × 75 mm × 350 mm were prepared for each of the mixes and tested with 300 mm clear-span and third-point loading (Figure 2c). The compressive stress-strain relationship of concrete is the most basic constitutive relationship and is necessary for the understanding of structural response of concrete. The compressive stress-strain relationship was tested using Ø150 mm cylinders, that two cylinders for each of the mixes were tested (Figure 2d). Modulus of elasticity was calculated from the compressive stress-strain relationships.

#### *2.6. Durability Tests of the Concrete Mixes*

Heat resistance, direct exposure to the fire, freezing and thawing resistance, and scaling resistance were the tests carried out to assess the durability of the concrete mixes in extreme environments. The resistance of concrete to high temperature is one of the main characteristics of HPC mixes. The age of the concrete cubes of each mix at the time of testing was 36 days, and the maximum temperature of the oven shown in Figure 3 was 1200 ◦C. During exposure to high temperatures, the degree of strength-loss is dependent on the maximum temperature reached, heating/cooling rate, and the exposure duration. The heating rate was 200 ◦C/h up to 600 ◦C, 50 ◦C/h until 700 ◦C and whereas, the cooling rate was 25 ◦C/h. The specimens remained for 7.6 h at a temperature of +600 ◦C, and 2 h in +700 ◦C.

**Figure 3.** Heating of specimens. (**a**,**b**) Heat resistance test for concrete 100 mm cubes. (**c**,**d**) Exposure to direct fire flame test for concrete cubes.

A fire-attack is mostly considered as an accidental action, instead of a degradation process. For understanding the differences between direct fire resistance and oven heating, additional sets of cubes were subjected to direct fire (Figure 3). The test was performed for NSVC and HPSCC, and the average heating rate was 500 ◦C/0.5 h while the cooling rate was 95 ◦C/h. The specimens were exposed to direct fire for 1.70 h at a temperature of +400 ◦C and 0.75 h in +500 ◦C, with the maximum temperature reached, was 520 ◦C. The fire-temperature was regularly measured by a laser thermometer.

In this study, the freeze-thaw test was performed following the same procedure and temperature limitations in ASTM C666 [54], but only for 50 cycles, using 100 mm cubes, as shown in Figure 4a,b. The cubes were submerged in NaCl solution with a concentration of 40 g/L and then tested for loss in weight and strength at 225 days' age so that the possibility of interference of chemical reactions in the microstructure of concrete can be eliminated. The scaling test is used to determine the scaling-resistance of a horizontal concrete-surface exposed to 50 freeze-thaw cycles in the presence of de-icing chemicals. It is intended to evaluate the concrete's surface resistance qualitatively by visual examination as per ASTM C672 [55]. The prepared specimens for the tests were shown in Figure 4c,d of which, an aluminum frame was fixed to concrete specimens by a highly adhesive epoxy. Pans had an inside square dimension of 220 mm, and 25 mm was provided as a dike for the 6 mm depth of the solution.

#### *2.7. Economic Assessment of the Concrete Mixes*

Apart from the technical performance parameters, the cost is also an important factor to optimize the concrete mixes. In this study, the cost of concrete mixes was calculated without VAT (taxes). The data for economic assessment considerably vary between regions. This is because local conditions highly affect the cost of labor, and the market costs for recovered materials, as well as the transportation scenarios. In this study, the most probable case scenario for the city center (Erbil, capital of Kurdistan region in Iraq) was considered to estimate the cost of the concrete mixes. The distance between the concrete plant and the raw materials, namely cement and aggregates was 184 km and 110 km, respectively. Besides, the other raw materials are imported from Turkey.

**Figure 4.** Freeze-thaw testing setup. (**a**,**b**) Freeze-thaw test for concrete cubes. (**c**,**d**) Scaling test for concrete specimens with aluminum frame.
