University of Architecture, Civil Engineering and Geodesy

Gypsum-cement-pozzolan binders (GCPB) offer the potential to reduce the environmental footprint of construction, particularly through the use of recycled gypsum obtained from construction waste. GCPBs are characterised by rapid setting and hardening, as well as increased water resistance compared to gypsum compositions. The structure formation processes in GCPBs exhibit several specific features and the use of recycled gypsum may introduce additional particularities. The present study aims to investigate the behavior of GCPBs with improved properties, in which the gypsum component is recycled from gypsum plasterboard waste, the cement content does not exceed 35 %, and natural zeolite, used as a pozzolanic additive, is applied without any additional processing. The research focuses on the structure formation process, strength development kinetics, and water resistance of two binder compositions with different cement-to-zeolite ratios (1:1 and 1:0,6), cured under both air-dry and underwater conditions. Standardized testing methods for binders were employed, complemented by X-ray diffraction (XRD) and differential thermal analysis (DTA). The results indicate that the structure formation and system stability depend strongly on both the composition and curing conditions. It has been established that the process of structure formation depends on the composition and aging conditions: the 1:1 ratio of cement and zeolite is more favorable for achieving stability of the system over time even under water and leads to higher strength (up to 30 MPa). Based on the construction and technical indicators of the research compositions, gypsum-cement-pozzolan binders based on recycled gypsum can be used without restrictions in environments with increased humidity for plaster and masonry mortars, masonry units and thin-walled composite elements.

The paper summarizes the basic requirements and possibilities for the implementation of structurally integrated waterproofing systems (SIHS). Based on a critical analysis of experimental results, the essential requirements for the classification of premises, determination of the composition of concrete, selection of sealing additives, treatment of joints and cracks, etc. are summarized. A methodology for the design of this type of waterproofing systems in the implementation of buildings and facilities in our country is presented. Requirements for monitoring the implemented SIHS and determining the need for follow-up measures are presented.

The paper examines the main changes in the draft European standard prEN 1992-1-2:2021E for the design of reinforced concrete structures subjected to fire. A brief overview and analysis of some basic assumptions for the calculation of reinforced concrete elements in a fire situation are made.

The Level of Information (LOI) in Building Information Modeling (BIM) serves as a critical framework for defining the depth and precision of non-graphical data associated with BIM elements. Recent studies highlight its transformative impact: 75% of AEC professionals report that effective LOI management reduces errors and rework by 30%, significantly enhancing project efficiency (Emerald Insight, 2022). LOI is structured into progressive tiers (LOI 100 to LOI 500), ranging from basic descriptions to detailed as-built specifications. Its synergy with the Level of Development (LOD) ensures seamless collaboration across multidisciplinary teams. For instance, the Celsius project in Sweden—a 12,000-square-meter office and laboratory complex—replaced traditional 2D drawings with a fully BIM-driven approach, cutting interdisciplinary conflicts by 40% and earning the 2020 buildingSMART Award for innovation. Yet challenges persist. The sheer volume of data in complex projects, requiring up to 400,000 unique datasets (Vital Consulting, 2023) can lead to overload, while interoperability gaps between software platforms hinder seamless integration. Standardization under frameworks like ISO 19650 and emerging technologies like digital twins and AI are paving the way forward. Notably, 60% of leading firms now prioritize IoT integration for real-time BIM updates (Novatr, 2023). In essence, LOI transcends mere data management—it fosters transparency, mitigates risks, and drives productivity. As the industry evolves, balancing information richness with usability will be paramount. With continuous upskilling and technological adoption, LOI is poised to redefine construction excellence, turning BIM into a cornerstone of sustainable, efficient project delivery. The future belongs to those who master the art of informed collaboration.

Despite the elucidated antioxidant mechanisms of tea polyphenols (TP) in bitumen, their influence on the rheological behavior of SBS (styrene–butadiene–styrene)-modified bitumen remains unclear. To address this gap, TP (0%, 3%, 6%, 9%, and 12%) was incorporated into 4.5% SBS-modified bitumen to prepare five binders, which were then subjected to thermo-oxidative aging, yielding ten samples. Subsequently, a series of rheological tests, including frequency sweep, multiple stress creep and recovery, linear amplitude sweep, and low-temperature relaxation, were conducted to analyze their behavior. Furthermore, high- and low-temperature rheological models were developed for all binders. The results indicate that TP increases the complex modulus and reduces the phase angle of SBS-modified bitumen, thereby enhancing its elastic response, with the optimum effect achieved at a 3% dosage. At this content, under the unaged condition, the SBS/TP composite-modified bitumen also exhibits the greatest fatigue life. Moreover, TP enhances the high-temperature elastic recovery and deformation resistance of binders, as evidenced by increased recovery rate and reduced non-recoverable creep compliance and creep compliance. Notably, TP compromises the low-temperature stress relaxation capability of SBS-modified bitumen, as indicated by higher max stress, shear stress ratio, relaxation time, and model parameter A, along with a lower model parameter n.

Ultra-high-performance geopolymer concrete is a low-carbon and environmentally friendly cementitious composite formed through alkali activation of industrial by-products and incorporation of ultrafine fillers and fiber reinforcement, exhibiting high compressive strength and excellent durability. In this study, ultra-high-strength geopolymer concretes (UHSGM) were prepared using ground granulated blast furnace slag (GGBFS), fly ash (FA), and silica fume (SF) as binders. Three GGBFS-to-FA mass ratios, namely 4:1, 1:1, and 1:4, were adopted. The SF content was varied at 0%, 5%, 10%, 20%, and 30% by mass of the precursor. In addition, three types of steel fiber were incorporated at a constant volume fraction of 2%. Two curing conditions, namely standard curing and steam curing, were adopted. The influences of mixture proportions, with or without silica fume and steel fiber, on the flowability, compressive strength, and flexural performance of UHSGM were systematically evaluated. The results indicated that an increase in GGBFS content led to a reduction in flowability but an enhancement in the compressive strength of UHSGM. The influence of SF on flowability and compressive strength was strongly dependent on the GGBFS-to-FA ratio. The highest compressive strength of the geopolymer reached 157.0 MPa at a GGBFS-to-FA ratio of 4:1 with 5% SF, which is comparable to that of ultra-high-performance concrete. An increase in GGBFS content and the incorporation of steel fibers enhanced the flexural strength of UHSGM, whereas high-temperature curing led to a reduction in flexural strength. An increase in SF had a negative effect on the deflection capacity and toughness of UHSGM, regardless of curing conditions. Further investigation is required to optimize the overall performance of UHSGM.

This study aims to propose a practical indicator that enables quick and reliable evaluation of the relationship between influent characteristics and energy performance and recovery in municipal wastewater treatment plants (WWTPs). A composite Wastewater Strength Indicator (WWSI) was developed, integrating wastewater dilution and pollutant load into a single metric. Theoretical correlations were established through mathematical estimation and verified using case studies of six WWTPs in Bulgaria based on operational data from 2020–2022. WWSI correlates strongly with both specific energy consumption (kWh/kg COD removed) and electrical energy recovery rate. WWTPs with a WWSI below 0.25 perform unsatisfactory, exhibiting specific energy consumption levels above 2.0 kWh/kg removed COD, whereas those with a WWSI above 0.35 demonstrate higher energy efficiency, with specific consumption below 1.0 kWh/kg removed COD. The treatment of low-strength wastewater leads to inherent energy inefficiencies that are difficult to overcome through sludge digestion and cogeneration alone. Despite sludge calorific values ranging from 11.5 to 19.4 MJ/kg, the energy recovery potential in the studied WWTPs remained below 35%, confirming that energy neutrality is challenging for diluted wastewater. A conversion coefficient of 0.039 kWh/MJ was introduced to facilitate rapid estimation of potential electrical recovery from sludge calorific values. The proposed WWSI provides a simple yet effective tool for benchmarking WWTPs and supports future upgrades toward energy-neutral wastewater management.

The wide use of continuously inhomogeneous engineering materials in various load-bearing structures puts high requirements with respect to their fracture behavior. In many applications fracture represents the most often mode of structural failure. In view of this, exploring fracture behavior under various loading conditions is highly needed for guaranteeing the load-bearing capacity, safety and proper functioning of engineering structures. Developing of analyses which encompass more factors influencing fracture behavior of continuously inhomogeneous structures is very important direction in fracture mechanics. One of the extreme loadings acting on engineering structures is the impact loading which may be generated by objects falling on the structure. It happens frequently that impact loading acts in combination with other influences on the structure like, for instance, temperature field. Therefore, this paper deals with analysis of the effect of temperature filed on the lengthwise fracture in a continuously inhomogeneous beam structure subjected to impact loading by an object falling on the structure. The beam has non-linear elastic behavior. The fracture under impact loading and temperature field is analyzed by considering the strain energy release rate. The analysis is verified by applying the integral J. The change of the fracture behavior due to temperature field and the parameters of the impact loading is studied.

While urban land use planning receives much attention in land management in Bulgaria, agricultural land use planning has been significantly neglected. Today, more than ever, effective and efficient land use planning is needed to address the global and local challenges, such as protecting prime agricultural lands for food production, achieving sustainable productivity gains, improving water use efficiency, reducing greenhouse gas emissions, etc. The objective of the paper is to review the international experience in rural and agricultural land use planning and its theoretical foundations, to identify and analyze the gaps in Bulgarian legislation in this area, and to develop some scientific ideas for including agricultural land planning into Bulgarian Regulatory Framework and its further implementation in practice. The need for agricultural territory planning in Bulgaria is emphasized. Intra-zoning of agricultural land on a holistic basis is proposed as a useful planning method for balancing agricultural development with the environmental protection, while maintaining both land productivity and the quality of natural resources. Information from strategic planning documents and literature resources from the USA, Canada, Australia, European and other countries is presented and analyzed. Issues of building permits, land categorization systems and assessment of land suitability for growing agricultural crops are studied.