Adhesion and interaction of inorganic binder systems with a biodegradable polymer based on polylactic acid

Adhesion and interaction of inorganic binder systems with a biodegradable polymer based on polylactic acid Radoslav Gandel1 * , Nestor Plastun1, Petr Cmiel2, and Katerina Matyskova1 1VSB – Technical University of Ostrava, Faculty of Civil Engineering, Department of Building Materials and Diagnostics of Structures, Ludvika Podeste 1875/17, 708 00 Ostrava-Poruba, Czech Republic 2TESTSTAV, spol. s r.o., Františka Lyska 1599/6, 700 30 Ostrava – Belsky Les, Czech Republic Abstract. The interaction of common building materials with materials with diametrically different physical-mechanical properties represents one of the main problems in the creation of new types of composites. Some types of polymers have proven themselves in the past in the form of dispersed reinforcement. However, the overuse of plastics and the problem of their disposal now provides an open door for other alternative ideas. This work deals with the observation of the adhesion of a biodegradable polylactic acid-based polymer with composites such as conventional concrete, highperformance concrete and alkali-activated system under three-point bending test. Results show that adhesion and mechanical performance depend strongly on both the matrix and reinforcement type. Ordinary Portland cement concrete (OPCC) specimens demonstrated the best bond with polylactic acid (PLA) and the highest flexural strength gains, particularly with ribbed PLA (60 %). In contrast, high-performance concrete (HPC) and alkali-activated material (AAM) showed reduced adhesion, with flexural strength decreasing by up to 20 %. 1 Introduction The interaction of different building materials is the fundamental basis for the optimal use of their full potential in construction practice. However, the different properties of the different types of materials complicate this considerably. Thermal expansion [1] is one of the key properties ensuring the long-term interaction of the individual components of a composite exposed to external influences. A typical example is the most widely used structural composite, reinforced concrete, combining the best properties of both materials (concrete and steel), or some of its hybrid variants [2-4]. The thermal expansion of concrete and steel is approximately the same, which allows reinforced concrete to react at normal temperatures to external conditions in a somewhat homogeneous manner. In addition, the deformation properties of steel complement the deficiencies of concrete and the deformation properties of concrete in turn complement those of steel in the necessary areas. Efforts to create a composite material with similar properties to reinforced concrete, but with the * Corresponding author: ra[email protected] E3S Web of Conferences 641, 01024 (2025) https://doi.org/10.1051/e3sconf/202564101024 Young Scientist 2025 © The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (https://creativecommons.org/licenses/by/4.0/). removal of some of its shortcomings, such as the environmental footprint [5], the corrosion of steel [6-8] or the high weight, are still relevant. Researches such as [9-11] are mainly concerned with the replacement of dispersed steel fibers in concrete, while an alternative to concrete reinforcement is not easy to find. One option could be, for example, a concrete composite with a custom, 3D printed, recyclable, biodegradable, polylactic acid-based polymer (PLA), which is the subject of this article. Specifically, it focuses on the adhesion (another of the basic properties of structural composites) between a smooth-surfaced polymer reinforcement and a ribbed surface in compositions with ordinary portland cement-based concrete, high-performance concrete, and alkali-activated material in the context of flexural strength testing. One of the key aspects of this paper is the interfacial transition zone (ITZ) between the PLA reinforcement and the surrounding matrix, which significantly affects the mechanical behavior of the composite. The quality of this phase interface controls the load transfer efficiency and crack propagation resistance. Just as the surface structure of the aggregate in concrete influences the ITZ [12], the surface structure of the reinforcement element will also play an important role in this case. Mechanical improvement in the adhesion of the PLA reinforcement element with the concrete matrix can be achieved by the shape of the reinforcement element, as shown in [13], where the flexural strength was significantly improved by the addition of hook-ended PLA fibres compared to straight shaped fibres. 2 Materials and Methods For the experimental program, portland cement concrete (OPCC), high-performance concrete (HPC) and alkali-activated material (AAM) mixtures from previous researches were selected, the compositions of which are shown in Table 1. Table 1. Composition of selected mixtures. Input materials Weight [kg∙m-3] OPCC HPC AAM Cement CEM I 42.5 R, Hranice 450 - - Cement CEM I 52.5 R, Hranice - 650 - GGBFS, 420 m2/kg - - 450 Water 240 150 172 Aggregate 0/4 Tovacov 1230 890 920 Aggregate 4/8 Litice 550 570 670 Polycarboxylate plasticizer 3 20 - Plasticizer based on polycarboxylate and polyphosphonate - 10 - Plasticizer based on modified naphthalene polymers - - 9 Limestone, finely ground - 80 - Silica fume - 70 - Sodium silicate, Ms = 2.0 - - 56.3 50 % solution of KOH - - 42 The test specimens were 40x40x160 mm prisms made of materials listed in Table 1 with PLA polymer reinforcement, which were applied to the test prisms during concreting without additional adhesive layer. The PLA reinforcement element was placed on the bottom of the mould before concreting. This method was chosen on the basis of avoiding loosening of the protruding element during vertical placement in the mould during concreting of the E3S Web of Conferences 641, 01024 (2025) https://doi.org/10.1051/e3sconf/202564101024 Young Scientist 2025 2 specimens. The specimens were stored in the mould for 2 days and after demoulding were wrapped in foil to prevent evaporation of water. The composite prism specimens were tested 28 days after concreting for three-point bending according to standard ČSN EN 196-1 Methods of testing cement - Part 1: Determination of strength [14], with the reinforcing element placed in a test press on the underside of the prism (on support rollers) to simulate the loading of steel reinforcement in reinforced concrete stressed for bending. The dimensions and surface relief of the applied reinforcement elements can be seen in Fig. 1. Fig. 1. Geometry of the reinforcement elements. 3 Results of experimental program The following bar charts show the results of the three-point bending tests carried out in accordance with standard ČSN EN 196-1 Methods of testing cement - Part 1: Determination of strength [14], with 3 specimens tested in each series. Fig. 2 shows the 28-day flexural strength and the bar chart in Fig. 3 shows the ratio of the flexural strength of the reinforced specimens (fc_reinf) to that of the reference (non-reinforced) specimens (fc_ref). Fig. 2. Flexural strength after 28 days (on the left). Failure type of OPCC specimen (top) and AAM specimen (bottom) (on the right). 0 2 4 6 8 10 12 14 OPC_Plain OPC_Ribbed OPC_Ref HPC_Plain HPC_Ribbed HPC_Ref AAM_Plain AAM_Ribbed AAM_Ref Flexural strength [MPa] E3S Web of Conferences 641, 01024 (2025) https://doi.org/10.1051/e3sconf/202564101024 Young Scientist 2025 3 Fig. 3. The ratio of the flexural strength of the reinforced and the reference specimen. From the graphs in Figures 2 and 4, the following conclusions can be drawn: a significant improvement in flexural strength was achieved only for the OPCC mixture - for the specimens with the reinforced element with a plain surface, the flexural strength increased by approximately 14 % compared to the reference unreinforced specimens, and for the specimens reinforced with the ribbed element by almost 63 %. For both the HPC and AAM mixtures, there was only a slight increase in flexural strength, and only for the specimens with the plained-surfaced reinforcing element - for HPC the increase was approximately 4 %, for AAM only approximately 0.8 %. For the specimens with ribbed reinforcement, a decrease in flexural strength was even observed, by almost 20% for both mixtures. Fig. 3 shows one of the types of failure that occurred during the flexural strength tests: in most of the cases of beams with plain surface reinforcement, the concrete part of the specimen failed without failure of the plastic reinforcement, while the adhesion was still maintained. Another case was the failure of the concrete part of the specimen without breaking the plastic part, while the adhesion was disturbed - the same in the case of specimen with plain reinforcement. The last case (especially in the case of HPC specimens with ribbed reinforcement) was failure of the specimens in the entire cross-section, including the plastic reinforcement, while adhesion was still preserved. However, compared to the reference unreinforced specimens, a decrease in flexural strength was observed for these specimens, similar to that observed for the AAM specimens with ribbed reinforcement. 4 Summary and conclusions Based on the results presented in the previous chapter, the following conclusions can be drawn: 1. Not only the type of reinforcement element but also the type of concrete of the specimen had a significant effect on the adhesion of the materials, with the best adhesion being achieved by specimens based on portland cement. For the cementless specimens, in several cases (not only for plain but also for ribbed reinforcement elements), there was a significant reduction in adhesion, even before the actual flexural strength test, 0,6 0,7 0,8 0,9 1 1,1 1,2 1,3 1,4 1,5 1,6 1,7 OPC_Plain OPC_Ribbed HPC_Plain HPC_Ribbed AAM_Plain AAM_Ribbed fc_reinf / fc_ref [-] E3S Web of Conferences 641, 01024 (2025) https://doi.org/10.1051/e3sconf/202564101024 Young Scientist 2025 4 indicating that probably the higher pH of the AAM or the chemical composition negatively affects its adhesion with PLA. 2. Plain reinforcement in all cases showed an improvement in the flexural strength of the specimens. The OPCC specimens showed the highest increase, approximately 14 % compared to the reference ones, while the increase was negligible for HPC and AAM, with approximately 4 % for HPC and 1% for AAM. 3. For the ribbed reinforcement specimens, where an overall better adhesion was observed than for the plain reinforcement, an unexpected phenomenon occurred where the OPCC specimens showed an increase in flexural strength of more than 60 %, while the HPC and AAM specimens showed a decrease of about 20 % compared to the reference ones. In the future, in addition to focusing on the influence of the concrete composition on the adhesion with PLA, it is proposed to compare it with other types of reinforcement. Data presented in this study are available on [15] and [16]. 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