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Article

Influence of Hydroxypropyl Methylcellulose Dosage on the Mechanical Properties of 3D Printable Mortars with and without Fiber Reinforcement

by
Çağlar Yalçınkaya
Department of Civil Engineering, Faculty of Engineering, Dokuz Eylül University, İzmir 35160, Turkey
Buildings 2022, 12(3), 360; https://doi.org/10.3390/buildings12030360
Submission received: 8 February 2022 / Revised: 12 March 2022 / Accepted: 14 March 2022 / Published: 16 March 2022
(This article belongs to the Special Issue Innovative Manufacturing Technologies in Architecture and Structural Engineering)
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Abstract

:
Hydroxypropyl Methylcellulose (HPMC) is one of the most frequently used viscosity modifying admixtures in 3D printable cement-based materials. In this study, the effects of HPMC dosage on the mechanical properties of 3D printable cement-based mortars were investigated. For this purpose, mortar mixtures with and without micro steel fibers containing three different HPMC dosages (0%, 0.15%, and 0.30% by weight of cement) were produced. Reliant on the HPMC dosage, heat flow and cumulative heat curves were obtained. At the end of 7 and 28 days of standard curing, flexural, compressive, and shear bond strengths, as well as flexural toughness, were measured. Additionally, porosity values were obtained on molded, single-layer, and three-layer printed specimens. The results showed that the increase in HPMC dosage prolonged the setting times and decreased the heat release. Moreover, the porosity values increased with an increase in the HPMC dosage and the number of printed layers. All mechanical properties were drastically decreased with the use of HPMC. The decrements were more significant at the first 0.15% HPMC dosage and the shear bond strengths. Prolonging the curing period from 7 to 28 days did not lead to meaningful recovery in the mechanical properties. The negative effects of HPMC on flexural and shear bond performances were more pronounced in fiber-reinforced mortars.

1. Introduction

Additive manufacturing (AM) is increasingly attracting a great deal of attention among researchers in the field of construction materials science due to its promising prospect as a better alternative to conventional concreting methods [1]. According to ASTM F2792 [2], AM, commonly known as 3D printing, is a process of constructing 3D physical objects through a layer-by-layer stacking technique based on pre-defined 3D model data. Currently, extrusion-based 3D concrete printing (3DCP) dominates the implementation of AM in the construction industry [3,4]. Previous studies have demonstrated that this innovative way of building can decrease construction-related wastes, labor costs, and the duration of the project and accordingly improve the safety of construction sites [4]. Overall, eliminating the need for formwork provides greater flexibility and freedom of architectural design [5].
The elimination of formwork in extrusion-based 3DCP means that printable mixtures must be tailored to exhibit unique rheological characteristics. In the study on the fundamental rheological properties of printable mixtures, Roussel [6] concluded that printable mixtures must find a balance between having low slumps for successful pumping and extrusion and high thixotropy for stability and buildability. Furthermore, extruded mixtures must possess sufficient green strength to resist deformation from their weight and the layer being stacked above [7]. Accordingly, a considerable number of previous studies on printable mixtures have reported on the combined usage of superplasticizers and viscosity modifying admixtures (VMA) as a way of successfully producing printable mixtures [8,9,10,11,12,13].
The use of superplasticizer in printable mixtures serves the purpose of improving extrudability of these composites by reducing both yield stress and plastic viscosity, whereas VMA is used to control the yield stress and plastic viscosity, thus improving cohesion, viscosity, and shape stability after extrusion that significantly improves their buildability [14]. Hydroxypropyl methylcellulose (HPMC) is the most widely used cellulose derivative VMA in the production of 3DCP in most literature studies. Although its usage is exceedingly popular in 3DCP, HPMC may adversely affect the mechanical and microstructure properties of cement-based materials depending on its dosage, methoxyl content, molecular weight [15,16]. In their study, Chaves et al. [17] investigated the effect of HPMC on hydration and microstructure of Portland cement paste; they reported that the hydration of cement pastes was retarded because of HPMC addition. Furthermore, increasing the dosage of HPMC resulted in more air voids, which led to a considerable reduction in the 28-day compressive of these composites. Similarly, Chen et al. [10], in their study on the effect of HPMC on the extrudability of limestone and calcined clay-based cementitious composites for extrusion-based 3DCP, reported that exceeding the optimum dosage of HPMC negatively affected the rheological and mechanical properties of these composites. Since HPMC is used together with superplasticizer in 3DCP, Ma et al. [15] researched the effect of HPMC on the rheology of cement paste produced with the addition of a polycarboxylate-based superplasticizer, they observed that HPMC adversely affected the dispersion ability of superplasticizer by impeding its adsorption of the surface of cement grains. Che et al. [18] used HPMC and a water-reducing admixture in the mix design to modify the flowability and buildability of the 3D printable mortars. The researchers reported that 3D printed specimens had a higher air-void content and a larger air-void aspect ratio than the mold-cast specimens, resulting in lower mechanical properties. Xiao et al. [19] evaluated the microstructure of HPMC-based printable mortars. They reported based on the computed tomography scan results that the lower strength of the printed mortars compared to that of the mold-cast ones was mainly due to the extra pore formation between the printed layers. Chen et al. [20] analyzed the air void system of 3D printed mortars containing HPMC. They revealed that the large air voids (1000–6000 μm) were formed mainly at the interface region, and the majority of these had irregular and elongated shapes. The amount of HPMC having a significant effect on the air void structure, therefore, plays a critical role in the mechanical properties.
It is seen that the use of HPMC in the development of cement-based materials for 3DCP technology continues to increase. However, the HPMC dosage in 3D printable mortars has been mostly investigated to meet the rheological requirements, and its side effects on the mechanical properties have been studied in mold-cast specimens rather than printed ones. Additionally, in the case of micro steel fibers, a frequently preferred reinforcement strategy in 3DCP technology, performance changes relative to HPMC dosage have not been systematically investigated. In this study, the effects of HPMC dosage on the compressive strength, flexural strength, flexural toughness, shear-bond strength were investigated on both fiber-free and fiber-reinforced 3D printed cement mortars. Moreover, hydration heat, setting time, and porosity values were evaluated.

2. Materials and Methods

2.1. Materials and Mix Proportions

The Portland cement (CEM I 42.5 R) used was produced by Soma Cement Company in Turkey. The particle size distribution of Portland cement is shown in Figure 1. The chemical compositions determined by X-ray fluorescence and selected properties of Portland cement are given in Table 1. River sand with a maximum size of 2 mm was used. The specific gravity, water absorption capacity, and fineness modulus of river sand were 2.59, 1.98%, and 1.4, respectively. To adjust the contradictory requirements of 3D printable mortars at a fresh state, a polycarboxylate ether-based (PCE) superplasticizer and HPMC-based VMA were used together in the formulations. The generalized molecular structure and physical properties of HPMC are given in Figure 2 and Table 2, respectively. To evaluate the effect of HPMC dosage on the mechanical properties of fiber-reinforced 3D printable mortars, brass-coated straight micro steel fibers of a length of 13 mm and a diameter of 0.2 mm were incorporated. Micro steel fibers had reported tensile strength of 2750 MPa and Young’s modulus of 200 GPa.
First, it was produced an HPMC-free control mixture with a compressive strength of 60 ± 5 MPa, since previous studies showed that there could be dramatic decreases in the mechanical properties of cementitious systems because of a high amount of VMA addition [21,22,23]. An extrusion-based printability test was performed on the mortar mixtures while adjusting the HPMC dosages. During the tests, a mortar injection gun set was used to represent an extrusion-based printing. In this study, the mixtures containing three different dosages of HPMC were developed to determine the effect of HPMC-based VMA dosage on the mechanical properties of 3D printable mortars. These were produced by adding 0%, 0.15%, and 0.30% HPMC by weight of cement. These were denoted as VMA-0, VMA-0.15, and VMA-0.30, respectively. To determine the effects of HPMC dosage on the mechanical properties of fiber-reinforced 3D printable mortars, 0.5% micro steel fibers by volume were incorporated into the mortar mixtures. Mixtures with micro steel fiber reinforcement were coded as VMA-0 (F), VMA-0.15 (F), and VMA-0.30 (F), respectively. Mix proportions are given in Table 3. The mixtures have a water to cement ratio of 0.33, and an aggregate to cement ratio of 1.67. Note that the volume needed for micro steel fiber inclusion was deducted from the volume of the sand to keep constant paste volume among the mixtures.
Mortar mixtures were prepared using a Hobart-type mixer. First, Portland cement and river sand were mixed. Then, the water + superplasticizer solution was added to the dry ingredients. After obtaining a fluid mortar, HPMC was incorporated. The addition of HPMC changed the consistency of mortars suddenly from fluid to stiff by increasing their viscosities. Then, in the case of fiber-reinforced mortars, micro steel fibers were gradually added to the mixture. Finally, high-speed mixing was initiated. The whole mixing procedure took 10 min (Figure 3).

2.2. Workability and Setting Time Measurements

Fresh mortars were assessed immediately at the end of the mixing procedure by performing a standard flow table test following ASTM 1437-20 [24]. The initial and final setting times were measured per TS EN 480-2 [25] using an automatic Vicat apparatus.

2.3. Hydration Heat Measurement

To compare the hydration heat development of the mortars depending on HPMC dosage, the semi-adiabatic calorimetry technique was employed in accordance with NT BUILD 388 [26]. All mortar ingredients and test equipment were stored at 20 °C for 24 h before the calorimetric measurements. After the mixing procedure given in Figure 3, fresh mortars were poured into cylinder molds (Ø150 × 300 mm). The mold was covered with a lid, and the temperature sensor was inserted into the sample. After placing the mold in the insulated box, the box lid was closed. Then, the temperature data measured from the sample and ambient temperature were recorded at five-minute intervals by a data logger. Note that during the test, the ambient temperature was kept at 20 ± 1 °C. The heat flows of the mortars were monitored over 48 h. The heat flows were calculated based on the heat capacity and activation energy of the mortar samples by taking account of the heat losses through the insulated box with the help of calorimetry software.

2.4. Preparation of the Specimens

The printed specimens were prepared by extrusion of the mortars using an injection gun set (Figure 4). The injection tool has a cylindrical container (Ø65 × 300 mm). The output of the nozzle is 42 mm wide and 15 mm high. In the case of 3D printable mortars containing HPMC, specimens were printed in three layers using a mortar injection gun at an approximate printing speed of 5 mm/s. Four specimens were printed from the same batch. Eight printed beams were produced for each mixture (Figure 5). First, the base layers of four specimens were extruded. Then, the consecutive layers were printed following the same procedure. The time interval of two adjacent layers was chosen 5 min to reflect a printing scenario of small objects, e.g., column, and post-tensioned girder parts, having a path length of 1500 mm, considering the printing speed. The beams consisting of three layers and two interlayer regions were kept under 20 ± 1°C and 50 ± 5% relative humidity for 24 h. After that, standard water curing was initiated for periods of 7 and 28 days.
At the end of the curing period, the printed specimens were cut into small pieces for flexural, compressive, and shear bond strength, as illustrated in Figure 6. Due to the manual printing applied by hand, which is less precise than automated 3D concrete printing, consecutive layers can be slightly (1–2 mm) out of alignment. Therefore, to eliminate the possible defects around the lateral face of the consecutive layers and to make the lateral surfaces flat enough, specimens having dimensions of 35 mm × 40 mm × 160 mm were extracted from the printed specimen by cutting (Figure 6). Some of these specimens were then cut into smaller prisms for the other tests. On the other hand, HPMC-free control mixtures were unprintable because of high flowability. These mixtures that do not have any shape stability were extruded into the steel molds having dimensions of 40 mm × 40 mm × 160 mm with the help of an injection gun in three layers by applying a time interval of 5 min. In this way, it produced specimens comparable with HPMC-bearing printable ones in terms of the time interval between the layers and the fiber alignment under extrusion. These specimens were cut into the required dimensions by following the same specimen preparation procedures at the end of the curing periods.
To determine the standard compressive strength of the mold-cast specimens, HPMC-free mortars as well as HPMC-bearing printable mortars were poured into the steel molds having dimensions of 40 mm × 40 mm × 160 mm monolithically. Then, the casting surfaces were troweled gently. After curing periods, specimens having dimensions of 35 mm × 35 mm × 40 mm were extracted from the mold-cast specimens by cutting.

2.5. Determination of Porosity

The porosity of mold-cast, single-layer, and three-layer printed specimens, was measured by implementing the cold-water saturation method described in ASTM C-642 [27] as in the study of Safiuddin and Hearn [28]. First, the specimens were cured for 28 days. Then, the aforementioned specimen preparation steps were applied. The mold-cast and three-layer printed specimens were 35 mm × 40 mm × 160 mm, whereas single-layer printed specimens have dimensions of ~35 mm × 15 mm × 160 mm. The specimens were first oven-dried at 100 °C for 24 h. After removing specimens from the oven, they were allowed to cool in dry air to a temperature of 20 °C, and the oven-dry mass (Md) was determined. To determine the saturated surface-dry mass in the saturation method used, the specimens were immersed in water at 20 °C for 48 h. Then, the surface moisture was removed with a towel, and the mass of the saturated surface-dry specimen (Ms) was determined. Following this, the mass of the saturated specimen in water (Mb) was measured using the buoyancy method. The following Equation (1) has been used to calculate the permeable porosity percentage (P):
P(%) = ((Ms − Md)/(Ms − Mb)) × 100

2.6. Mechanical Tests

The mechanical properties of the mixtures depending on HPMC dosage were evaluated with and without fiber reinforcement. The mechanical properties consisting of compressive, flexural, and shear bond strength were determined after 7 and 28 days of standard curing. Four specimens were tested for each mixture and curing period. The mechanical strength tests were conducted on mold-cast and extruded specimens following the specifications of TS EN 196-1 [29] with some modifications in the specimen dimensions.
The compression test was conducted using an ELE testing machine with a capacity of 3000 kN. It was conducted on specimens having the dimensions of 35 mm × 35 mm × 40 mm. The extruded specimens were loaded perpendicular to the printing direction from the surfaces of 35 mm × 35 mm at a loading rate of 2400 N/s, as illustrated in Figure 7a. The flexural performance of the extruded specimens was evaluated by implementing a three-point bending test. To obtain load-deflection graphs and toughness values, a displacement-controlled electromechanical testing machine with a capacity of 100 kN was used. The simply supported specimens with dimensions of 35 mm × 40 mm × 160 mm were loaded from the mid-span at a displacement rate of 1 mm/min, perpendicular to the printing direction as shown in Figure 7b. The free span between supports is equal to 130 mm. The flexural load-deflection curves were drawn using the result closest to the average mechanical performance. A practical method for measuring shear bond strength was implemented, as in the study of Alchaar and Al-Tamimi [30]. The shear-bond test was conducted using the electromechanical testing machine aforementioned. The specimens with dimensions of 35 mm × 40 mm × 60 mm (Figure 7c) were loaded from the mid-layer at a rate of 100 N/s while the top and bottom layers were supported by metallic prisms from their bottom surfaces, as shown in Figure 7d. The load that resulted in the separation at one of the mid-layer interfaces was divided by the interlayer area of 35 mm × 60 mm to calculate shear bond strength.

3. Results and Discussion

3.1. Workability and Setting Time

Trial castings were conducted to achieve proper formulation of mortars that ensure printability. Accordingly, a control mixture that does not contain HPMC and two printable mixtures produced by incorporating two different HPMC dosages into the control mixture were chosen in the context of the experimental program. The flow diameters and typical slump-flow spread views of the mixtures are given in Figure 8. All HPMC-bearing mixtures had almost zero-slump considering the self-flow diameters measured before dropping the flow table 25 times. It was observed that mixtures with a slump-flow value between 140 and 200 mm provide shape stability and buildability as well as extrudability. While the plain control mixture without HPMC (VMA-0) did not have a printable nature due to its remarkably high slump-flow value (280 mm), 0.15% and 0.30% HPMC-bearing mixtures (coded as VMA-0.15 and VMA-0.30, in sequence) exhibited good printability by owing to the slump flow values of 185 and 155 mm, respectively (Figure 8). Tay and Tan [31] investigated the relation between slump-flow values measured following ASTM C 1437 and maximum height printed before collapsing. They showed that a slump flow value between 150 and 190 mm gives a smooth surface and high buildability. Therefore, slump-flow values of HPMC-bearing plain mixtures fall between the desired range. On the other hand, the inclusion of micro steel fibers reduced the slump flow values by increasing the inter-particle friction. VMA-0.15 (F) and VMA-0.30 (F) mixtures showed 150 mm, and 140 mm slump-flow values, respectively. These mixtures were easily extruded using the mortar injection gun, as in the case of their fiber-free equivalent mixtures.
The initial and final setting times of fiber-free plain mixtures depending on HPMC dosage are given in Table 4. Initial and final setting times were prolonged by increasing the HPMC dosage in mortars. This can be attributed to the surrounding cement particles by the long-chain polymer and the retention of free water required for cement hydration by HPMC molecules [17,32,33]. It is obvious that the initial setting time was more sensitive to HPMC dosage compared to the final setting time. Moreover, a noticeably short time interval was measured between the initial setting and final setting of the HPMC-bearing mixtures as in the study by Chen et al. [34].

3.2. Hydration Heat

The heat flow and cumulative heat curves of cement mortars measured for the first 48 h are given in Figure 9. The given heat values were normalized by the total weight of Portland cement. The test data started to record nearly 30 min after water addition. Therefore, the first peak, which could not be fully recorded, is particularly due to the hydration of C3A, which upon dissolution, reacts with Ca2+ and SO4− ions present in the liquid phase to form ettringite (AFt) [35]. Following this first peak, there was a dormant period before the main peak. Then, the main heat flow started to evolve because of C3S hydration. The main peak of the heat flow curve was weakened in the case of 0.3% HPMC. The peak of heat flow curves was reached at 15.0 h, 15.8 h, and 17.7 h for VMA-0, VMA-0.15, and VMA-0.30 mixtures, respectively. The cumulative heat releases of VMA-0, VMA-0.15, and VMA-0.30 mixtures were 160.8, 152.6, and 148.9 J/g in sequence. The use of HPMC reduced the total heat released. It was shown that O–H groups of HPMC may reduce Ca2+ concentration by combining with Ca2+, thereby having a retardation effect at an early age [36]. Moreover, a shoulder formation depending on the depletion of sulfate and C3A dissolution [37] can be seen in the deceleration period of the VMA-0 mixture, while HPMC-bearing mixtures did not notably reflect this shoulder. Similar hydration delay phenomena were reported in the previous studies on normal and 3D printable mortars [38,39].

3.3. Evaluation of Porosity

The average porosity values are presented in Table 5 for different types of specimens. Four specimens were tested for each combination. The measured values are comparable with those of previous studies [40,41]. When mold-cast specimens are compared, it is obvious that porosity values were increased by increasing HPMC dosage. This can be attributed to an increase in entrained and entrapped air contents because of the enhanced viscosity of the mortars in the presence of HPMC. Besides, the use of fibers contributed slightly to the increase in porosity by reducing workability. When it comes to single-layer printed specimens, the porosity values were increased by around 2–2.5% compared with mold-cast specimens. Moreover, the use of fibers contributed to the increase in porosity by increasing inner friction under extrusion force. Contrary to these findings, Rahul et al. [42] reported a slight reduction in porosity due to the extrusion process leading to higher compaction and packing density. These contradictory findings can be attributed to the different types of extruders used. In this study, the piston-type extrusion was applied, while most 3D-concrete printers are equipped with a mortar pump with a screw-type extruder. In the case of three-layer printed specimens, the measured porosity values were around 3.9–4.7% higher than in single-layer printed ones. Moreover, an increase in HPMC dosage and the use of fiber reinforcement gave cause for slightly higher porosity values. The cross-section of a three-layer specimen printed by extrusion of the VMA-0.30 (F) mixture having the lowest flow diameter is presented in Figure 10. Each layer has a notable porosity consisting of mostly entrapped air voids and macropores. This type of porosity can be observed particularly around steel fibers. Additionally, the narrow interspace combined with some air voids at the interlayer zone can be distinguished. This local porosity occurred between the layers; therefore, it can be shown as the reason for the permeable porosity increment in three-layer printed specimens compared with the single-layer ones (Table 5). The amount of local porosity observed between the layers depends on the surface roughness after extrusion, initial yield stress, thixotropy of the mixture, the time interval between the consecutive layers, stiffness of the layers, and surface moisture [43,44,45,46].

3.4. Compressive Strength of Mold-Cast Specimens

To evaluate the strength level of the mortar mixtures, the mold-cast specimens having the dimensions of 35 mm × 35 mm × 40 mm were tested. Note that these specimens were poured into the molds monolithically and prepared by cutting mold-cast prisms having standard dimensions of 40 mm × 40 mm × 160 mm. The results are presented in Figure 11. The use of HPMC in the formulation led to a sharp decrease in the compressive strengths, as reported in the previous studies [39,47,48,49]. The decrement was more obvious at the early stage of the curing period. The reductions in compressive strength of plain mortars were found to be 37.3% and 41.3% for 0.15% and 0.30% HPMC ratios at 7-day, respectively. These reduction percentages were decreased by approximately 5% when it came to the curing period of 28 days. In the case of fiber-reinforced mortars, compressive strength decrements were 41% and 42.7% for 0.15% and 0.30% HPMC ratios at 7-day, respectively. These decrement ratios were weakened by approximately 8% when it came to the curing period of 28 days. When these experimental data are examined, it is seen that the use of HPMC brought slightly higher strength loss in fiber-reinforced mortars than in plain ones. In the mold-cast specimens, compressive strength loss because of HPMC use can be attributed to an increment air void ratio due to the reduction in workability as well as the retardation effect of HPMC molecules on cement hydration [50,51,52].

3.5. Compressive Strength of Extruded Specimens

Compressive strengths of extruded mortars with and without fiber reinforcement are presented in Figure 12. Since the HPMC-free mixtures did not have shape stability and buildability, these were extruded into the molds in three layers by applying the same time interval (5 min) as the printed ones. Comparing Figure 11 and Figure 12, it can be seen that extrusion into the mold in three layers or monolithically casting did not affect the compressive strength level of HPMC-free mixtures considerably. In other words, there was no cold-joint formation-induced reduction in the bond properties in the case of HPMC-free mixtures. In the case of using 0.15% HPMC in fiber-free mixtures, the compressive strengths decreased by 58% and 50% at the end of the 7-day and 28-day curing periods in sequence (Figure 12a). When the HPMC ratio was increased to 0.30%, the compressive strength loss increased remarkably. The strength losses were 63% and 53% after 7-day and 28-day curing periods, respectively. Regarding fiber-reinforced mixtures, 0.15% HPMC usage led to 59% and 49% strength losses for 7 and 28 days of curing (Figure 12b). However, the increment of HPMC dosage from 0.15% to 0.30% did not further contribute to strength losses and caused 59% and 51% strength reductions for curing periods of 7 and 28 days, respectively. Both mold-cast and printed specimens exhibited a semi-explosive failure mode that is evaluated as satisfactory. There was no debonding of the printed layers under the loading direction perpendicular to the interfaces, which agrees with previous studies [53,54,55]. To clarify the contribution of weak interlayer bond properties to the decrease in compressive strength, however, more comprehensive experimental tests, as well as numerical modeling, are required [56,57,58]. Consequently, the negative effect of HPMC usage could be fully compensated by neither prolonged curing time nor fiber reinforcement. This is not only due to the weak interlayer areas in the specimens but also the reduction of compressive strength on a mixture basis in the presence of HPMC (Figure 11). Even though the fresh state properties such as rheological parameters, buildability, and green strength can be enhanced by increasing the HPMC dosage [59,60,61], an inverse relation between HPMC dosage and compressive strength paralleled previous findings [62,63,64].

3.6. Flexural Performance of Extruded Specimens

The representative flexural load–mid-span deflection curves are given in Figure 13. The loading was ended at the deflection value of 4 mm (one-tenth of the specimen height). The fiber-free mixtures exhibited a brittle failure after the first crack. HPMC usage led to a sharp reduction in the peak loads reached by the fiber-free specimens. In the case of fiber reinforcement, on the other hand, after the first crack, the load was prevented from decreasing abruptly to zero, and a deflection softening behavior was obtained. The failure modes were characterized by the formation of a single flexure crack initiated from the midsection of the bottom layer and propagating to the loading pin. In the case of fiber-reinforced specimens, a more complex crack path was observed in line with the previous studies [65,66]. This can be attributed to not only the crack bridging effect of the discontinuous short fibers but also the weaker mechanical properties of the layer interfaces than that of the layer body, triggering a change in the direction of the crack front [67]. Note that because of the alignment of the fibers parallel to the printing direction, most of the fibers cannot pass through the interface between printed layers [66]. Pham et al. [68] reported that the volume fraction of straight micro steel fibers less than 0.75% did not lead to the deflection hardening behavior because of an insufficient number of fibers. In contrast, Arunothayan et al. [69] showed that it was possible to obtain deflection hardening behavior in the presence of 2% of 13 mm long micro steel fibers and silica fume in the formulation. With the increase of HPMC dosage, the sudden load drop in the load-deflection curves after the first crack became apparent, and the deflection softening behavior was weakened. In other words, the use of HPMC seems to reduce the crack bridging ability of the fibers. Increasing the curing period enhanced the crack-bridging ability of the fibers in the HPMC-free mixture markedly while it did not contribute to the behavior of HPMC-bearing ones. This can be attributed to a reduction in the bond properties between steel fiber and cementitious matrix and the existence of weaker interlayer areas as an HPMC side effect. In Figure 10, it can be realized that most of the fibers have a macropore around them at the cross-section. It was reported that HPMC usage could increase porosity around the interfacial transition zone [70]. Moreover, HPMC may adversely affect the micromechanical properties of the matrix [43]. To clarify the effect of HPMC on fiber-matrix bond properties, further studies need to be carried out. After the curing period of 7 days, the flexural toughness values of fiber-reinforced mixtures were 8.7, 5.3, and 5.2 times that of plain mixtures lacking fiber for 0%, 0.15%, and 0.30% HPMC content in sequence (Figure 14). These values were 13.3, 7.9, and 6.2 times at the end of the curing period of 28 days, respectively. The contribution of fiber reinforcement to flexural toughness was enhanced through prolonged curing, whereas it was adversely affected by increasing HPMC dosage.
Flexural strengths of the extruded specimens depending on HPMC dosage, fiber reinforcement, and curing time are presented in Figure 15. The flexural strengths obtained are comparable with those of previous studies [71,72,73]. It is seen that fiber reinforcement enhanced the strengths only in the HPMC-free matrix noticeably (~30%). The use of HPMC for obtaining printable mixtures reduced flexural strengths remarkably. The reductions in flexural strength of fiber-free mortars were found to be 42.6% and 64% for 0.15% and 0.30% HPMC ratios at 7-day, respectively. These reduction percentages were 46.5% and 52.3% in sequence when the curing period was prolonged to 28 days. On the other hand, the strength decrements of fiber-reinforced mortars were 56.2% and 74.3% for 0.15% and 0.30% HPMC ratios at 7-day in sequence. These decrement ratios were 56.8% and 59.6% in sequence following the curing period of 28 days. A more significant enhancement in the performance with prolonged curing was reported for 3D printed polyethylene fiber-reinforced concrete showing deflection hardening behavior under flexure [74]. These findings can be attributed to weakened bond properties between steel fiber and matrix and the existence of weaker layer interfaces as a side effect of the use of HPMC. Apparently, the prolonged curing time is essential in reducing the negative effects of HPMC, especially for a dosage of 0.30%. Due to these drawbacks, the use of silica fume and other supplementary cementitious materials with high fineness for adjusting fresh state requirements instead of HPMC attracts research attention [75].

3.7. Shear Bond Strength of Extruded Specimens

The shear bond strength between the printed layers is critical in a structure designed to be influenced by earthquake loads. Thus, an inherently weak interlayer bond, in addition to the difficulty of integrating shear reinforcement, has proven to be among the main challenges of 3DCP technology. Shear bond strengths of the specimens with and without fiber reinforcement are given in Figure 16. Note that the strengths of non-printable HPMC-free mixtures were measured on the specimens extruded into the molds in three layers by applying the 5 min delay time between the layers. In both fiber-reinforced and fiber-free printed specimens, the interlayer shear failure culminated in a sudden separation of the layers was observed. The measured strengths are comparable with those of previous studies [30,76,77]. The use of HPMC to produce printable mortars triggered a sharp decrease in the shear bond strengths. In the case of 0.15% HPMC use in the mixtures without fiber reinforcement, the shear bond strengths were reduced by 72% and 68% at the end of the 7-day and 28-day curing periods in sequence. In the case of 0.30% HPMC, the strength losses were 77% after 7-day and 28-day curing periods (Figure 16a). When the fibers were included, the shear bond performance of HPMC-free specimens was enhanced, while the strengths of HPMC-bearing printed ones were weakened slightly (Figure 16b). The reductions in shear bond strengths of fiber-reinforced mixtures due to HPMC usage were approximately 5% higher than in the fiber-free case at all curing ages. Even if an enhancement in shear strength of the mortars through fiber reinforcement is anticipated, the alignment of the fibers parallel to the printing direction inhibited the enhancement by disabling fiber to transfer the stress between the layers and to bridge the micro-cracks under shear loading, as shown in Figure 17. It was reported that the fibers around the nozzle walls could be aligned more toward the printing direction than the fibers located in the middle section [78]. This further hampers the contribution of the micro steel fibers around the layer interface to the shear bond strength. Moreover, it was concluded previously that the local porosity values at the layer interface were increased by increasing HPMC content and fibers (Table 5). The use of HPMC may not only cause an increase in the interface porosity but also reduce the surface moisture of extruded filaments via a water retention mechanism [38]. Sanjayan et al. [46] reported that the tensile bond properties between the printed layers were weakened due to the reduction in the surface moisture depending on the time interval between two adjacent layers. On the other hand, the stiffness of the printed layer, which plays a significant role in determining the contact area between consecutive layers [79], was enhanced by increasing the HPMC dosage. It can be concluded that the use of the HPMC in dosage more than needed for buildability requirements may adversely affect the shear-bond properties. Moreover, the negative effects could not be compensated either by implementing fiber reinforcement or prolonging curing time.

3.8. Discussion of the Mechanisms behind the Side Effects of HPMC

A great number of previous studies have relied on the utilization of HPMC as a way of improving shape stability and buildability for the development of 3D printable mortars [34,59,60,61]. It has been determined in this study that the utilization of HPMC adversely affects the hardened mechanical properties of 3D printed mortars. The side effects of HPMC can be attributed to five main mechanisms: the retardation effect on cement hydration, the increase of entrapped air voids, the increase in interlayer porosity due to higher stiffness of the mixtures, the reduction in surface moisture of the layer, and the reduction in bond properties between fiber and matrix. In this study, a delay in hydration was observed by calorimetric measurements (Figure 9). Furthermore, setting times were prolonged with an increase in the HPMC dosage, confirming the retardation of hydration by the use of HPMC (Table 4). The porosity measurements revealed that increasing the HPMC dosage triggered an increment of porosity of both mold-cast and single layer extruded specimens (Table 5). Moreover, further increment of porosity in three-layer printed specimens was observed. This indicates a local porosity increment at the interlayer region. This interlayer porosity can be attributed to the HPMC-induced stiffness increment that reduces the deformation of the substrate after depositing the top layer, thereby reducing the interacted contact area between the deposited layers. The stiffness increment is also shown as the main mechanism underlying the lower bond strength obtained in the case of longer time intervals (but before the initial setting) between extruded layers [79]. A reduction in the surface moisture of extruded layers by the drying time, on the other hand, has an obvious negative effect on the interlayer bond strength [46]. A similar mechanism can be valid in the case of a moisture reduction at the surface due to the water retention property of HPMC [38]. However, surface moisture measurements were not taken in this study. The weaker reinforcing effect of steel fiber in the presence of a higher HPMC dosage (Figure 13) can be attributed to a reduction in fiber-matrix bond strength because of reduced micromechanical properties of the matrix and a porous interfacial transition zone [43,70]. Single-fiber pullout tests, therefore, should be performed related to the HPMC dosage of the matrix in future studies.
To avoid the aforementioned adverse effects of HPMC, alternative materials capable of enhancing the viscosity, shape stability, green strength, and thus buildability, while having no adverse effect on the hardened mechanical properties, can be used. Pozzolanic materials with a high specific surface area, such as silica fume and different types of clays, can be used in the formulations as a thickener and for green strength to achieve less porosity than in HPMC-based printable concretes [8,75,80,81]. On the other hand, nano-sized admixtures (nano-silica and nano-clays) are more effective in increasing the static yield stress and the rate of thixotropic buildup compared to micro-sized materials such as silica fume and metakaolin [82]. The nanoparticles such as nano-clay, nano-silica, and nano-titanium dioxide can accelerate hydration and green strength development [83]. By using micro and nano-sized particles, fiber-matrix bond properties can be enhanced as well [84]. These alternative materials, as well as optimized mix proportions, should be comprehensively investigated to eliminate or reduce the use of HPMC and so enhance the engineering properties of 3D printable cementitious materials. Moreover, some alternative methods can be implemented to compensate for the reductions in interlayer bond strength. It has been reported that the use of superabsorbent polymers can improve interlayer adhesion by providing extra water to the anhydrous cementitious materials after the surface moisture has been reduced by evaporation [85]. Additionally, applying secondary materials such as cement paste and bonding agents to the printed layer surface during the deposition process can lead to the enhancement of the bond properties [86,87]. This method, however, needs additional equipment for 3D printers and so may bring higher costs compared to the solutions based on a mixture design.

4. Conclusions

In this study, the effects of different dosages of HPMC (0%, 0.15%, and 0.30%), one of the most widely used chemical admixtures to develop mixtures appropriate for 3DCP, on the fresh state and hardened state properties of the mixtures were investigated. The following major conclusions can be drawn within the limitations of this study:
  • The mixtures with a slump-flow value (ASTM C 1437) between 140 and 200 mm exhibited shape stability and buildability as well as extrudability. An HPMC dosage of up to 0.3% of cement weight could be employed in both fiber-free and fiber-reinforced mortars for achieving the printability criteria.
  • The setting times were prolonged with an increase in the HPMC dosage. The initial setting time was more sensitive to HPMC dosage compared to the final setting time. Additionally, the calorimetric investigations revealed that the use of HPMC reduced the total heat released while the main peak of the heat flow curve was decreased in the case of 0.3% HPMC.
  • Increasing the HPMC dosage caused more porosity. The negative effect of HPMC was more pronounced on the printed specimens compared with mold-cast ones. Moreover, the synergetic effect of fiber reinforcement and increment of HPMC dosage on the porosity was observed. An increment in the number of printed layers further increased the measured porosity. These findings revealed that porosity in a single layer and on the interface between the printed layers was increased as a side effect of HPMC usage.
  • Compared with the HPMC-free mortars extruded into the mold, compressive strengths of the printed fiber-free specimens decreased by 50% and 53% for 0.15% and 0.30% HPMC ratios in sequence after a curing period of 28 days. In the case of fiber-reinforced mortars, these reductions were 49% and 51%, respectively. The reductions depending on HPMC dosage were more pronounced during a short curing period (7 days). Nevertheless, the negative effect of HPMC usage could be fully compensated by neither prolonged curing nor fiber reinforcement.
  • The use of HPMC for obtaining printable mixtures weakened the flexural performance notably. The reductions in flexural strength of fiber-free mortars were found to be 46.5% and 52.3% for 0.15% and 0.30% HPMC ratios at 28-day, respectively. In the presence of fiber reinforcement, the decrement ratios were 56.8% and 59.6% in sequence. This indicates the existence of weakened fiber-matrix bond properties and layer interfaces as a side effect of HPMC. Furthermore, the negative effect of HPMC usage was more distinct on flexural toughness, due specifically to adversely affected post-peak behavior.
  • As a side effect of HPMC use, the greatest decrease was recorded at shear bond strengths. The shear bond strengths were reduced by 68% and 77% in the cases of 0.15% and 0.30% HPMC ratios at 28-day, respectively. Incorporating the fibers enhanced the shear-bond strength of HPMC-free specimens whereas slightly reduced the strength of HPMC-bearing printable mixtures. HPMC-induced reductions in shear bond strength of fiber-reinforced mixtures were approximately 5% higher than in the fiber-free case. Under extrusion forces, the alignment of the fibers in parallel to the printing direction hampered the contribution of fibers to the shear-bond capacity of printed specimens.
  • In brief, the mechanical properties were severely affected by the use of HPMC. The negative effects of HPMC were more pronounced at the first 0.15% dosage. The prolonged curing period (28 days) did not lead to meaningful recovery in the mechanical properties obtained from 7-day. The side effect of HPMC on flexural and shear bond performances was more significant in the case of fiber-reinforced mortars.
Further optimizing the mix proportions seems to be essential to cut down on the dosage of HPMC, thereby reducing the side effects observed in this study. The results point out the necessity of investigations on using alternative materials (silica fume, nano-sized particles, etc.) that can eliminate the use of HPMC or reduce the dosage required to develop 3D printable cement-based composites. Moreover, to better ascertain the reduction in flexural performance, the changes in steel fiber-cementitious matrix bond characteristics depending on the HPMC dosage should be investigated by carrying out single-fiber pullout tests and microstructural investigations.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The author declares no conflict of interest.

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Figure 1. The particle size distribution of Portland cement.
Figure 1. The particle size distribution of Portland cement.
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Figure 2. The generalized molecular structure of HPMC.
Figure 2. The generalized molecular structure of HPMC.
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Figure 3. Mixing procedure of 3D printable mortars.
Figure 3. Mixing procedure of 3D printable mortars.
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Figure 4. Mortar injection gun (a), the dimensions of the nozzle (b), and extrusion process (c).
Figure 4. Mortar injection gun (a), the dimensions of the nozzle (b), and extrusion process (c).
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Figure 5. Typical appearance of freshly printed mortar beams.
Figure 5. Typical appearance of freshly printed mortar beams.
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Figure 6. The specimen preparation for mechanical tests.
Figure 6. The specimen preparation for mechanical tests.
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Figure 7. Compressive (a), flexural (b), shear bond (c) strength test specimens, and shear bond testing (d).
Figure 7. Compressive (a), flexural (b), shear bond (c) strength test specimens, and shear bond testing (d).
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Figure 8. The flow diameters (a) and slump-flow spread views (b).
Figure 8. The flow diameters (a) and slump-flow spread views (b).
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Figure 9. Heat flow (a) and cumulative heat (b) curves of cement mortars.
Figure 9. Heat flow (a) and cumulative heat (b) curves of cement mortars.
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Figure 10. Typical porosity observed at the cross-section of printed specimens (VMA-30 (F)).
Figure 10. Typical porosity observed at the cross-section of printed specimens (VMA-30 (F)).
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Figure 11. Compressive strength of the mold-cast specimens.
Figure 11. Compressive strength of the mold-cast specimens.
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Figure 12. Compressive strength of the extruded specimens for fiber-free (a) and fiber-reinforced (b) mixtures.
Figure 12. Compressive strength of the extruded specimens for fiber-free (a) and fiber-reinforced (b) mixtures.
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Figure 13. Load–deflection curves of the extruded specimens after 7 (a) and 28 (b) days of curing.
Figure 13. Load–deflection curves of the extruded specimens after 7 (a) and 28 (b) days of curing.
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Figure 14. Flexural toughness of the extruded specimens for fiber-free (a) and fiber-reinforced (b) mixtures.
Figure 14. Flexural toughness of the extruded specimens for fiber-free (a) and fiber-reinforced (b) mixtures.
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Figure 15. Flexural strengths of the extruded specimens for fiber-free (a) and fiber-reinforced (b) mixtures.
Figure 15. Flexural strengths of the extruded specimens for fiber-free (a) and fiber-reinforced (b) mixtures.
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Figure 16. Shear bond strength of the extruded specimens for fiber-free (a) and fiber-reinforced (b) mixtures.
Figure 16. Shear bond strength of the extruded specimens for fiber-free (a) and fiber-reinforced (b) mixtures.
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Figure 17. The fiber alignment parallel to the printing direction (a), ineffective fibers on the layer interface of the printed specimens subjected to the shear bond test (b).
Figure 17. The fiber alignment parallel to the printing direction (a), ineffective fibers on the layer interface of the printed specimens subjected to the shear bond test (b).
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Table
Table 1. Chemical compositions and selected physical properties of Portland cement.
Table 1. Chemical compositions and selected physical properties of Portland cement.
Chemical Composition (wt.%)Portland Cement
CaO63.71
SiO219.79
Al2O34.78
Fe2O33.39
MgO1.78
K2O0.78
SO32.84
Cl−0.0089
Free CaO1.80
Loss on Ignition2.09
Insoluble Residue0.30
Physical Properties
Specific Surface (m2/kg)395
Specific Gravity3.10
Table
Table 2. Physical properties of HPMC.
Table 2. Physical properties of HPMC.
PropertiesHPMC
AppearanceWhitish powder
Viscosity (mPa.s) 140.000–50.000
Water content (%)<5
Particles passing the 150 µm sieve (%)>95
Specific Gravity1.285
1 2% solution on a dry basis, Brookfield RVT at 20 °C.
Table
Table 3. Mix proportions.
Table 3. Mix proportions.
Materials (kg/m3)VMA-0VMA-0.15VMA-0.30VMA-0 (F)VMA-0.15 (F)VMA-0.30 (F)
Water250250250250250250
CEM I 42.5 R750750750750750750
River sand 125612541252124412411238
Micro steel fiber---35.8535.8535.85
VMA (HPMC)-1.1252.25-1.1252.25
Superplasticizer3.23.23.23.23.23.2
Design Parameters
VMA (%) 100.150.3000.150.30
Water-binder ratio0.330.330.330.330.330.33
Paste volume (%)525252525252
Steel fiber (%)---0.50.50.5
1 by weight of Portland cement.
Table
Table 4. Initial and final setting times.
Table 4. Initial and final setting times.
MixturesInitial Setting Time (h:min)Final Setting Time (h:min)
VMA-03:534:25
VMA-0.154:374:42
VMA-0.304:454:50
Table
Table 5. Porosity values.
Table 5. Porosity values.
Porosity (%)
MixturesMold-Cast SpecimensSingle-Layer
Printed Specimens		Three-Layer
Printed Specimens
VMA-06.8 ± 0.5--
VMA-0.159.4 ± 0.810.9 ± 1.514.8 ± 1.8
VMA-0.3010.5 ± 0.812.1 ± 2.216.0 ± 2.1
VMA-0 (F)7.0 ± 0.8--
VMA-0.15 (F)9.8 ± 1.212.2 ± 1.816.7 ± 2.0
VMA-0.30 (F)11.2 ± 1.113.8 ± 2.118.5 ± 2.3
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Yalçınkaya, Ç. Influence of Hydroxypropyl Methylcellulose Dosage on the Mechanical Properties of 3D Printable Mortars with and without Fiber Reinforcement. Buildings 2022, 12, 360. https://doi.org/10.3390/buildings12030360

AMA Style

Yalçınkaya Ç. Influence of Hydroxypropyl Methylcellulose Dosage on the Mechanical Properties of 3D Printable Mortars with and without Fiber Reinforcement. Buildings. 2022; 12(3):360. https://doi.org/10.3390/buildings12030360

Chicago/Turabian Style

Yalçınkaya, Çağlar. 2022. "Influence of Hydroxypropyl Methylcellulose Dosage on the Mechanical Properties of 3D Printable Mortars with and without Fiber Reinforcement" Buildings 12, no. 3: 360. https://doi.org/10.3390/buildings12030360

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