Experimental Investigation on the Structural Performance of Single Span Hollow Core Slab under Successive Impact Loading

In Lebanon and many other countries where structures are vulnerable to impact loads caused by accidental rock falls due to landslides, specifically bridges with hollow core slab, it is mandatory to develop safe and efficient design procedures to design such types of structures to withstand extreme cases of loading. The structural response of concrete members subjected to low velocity high falling weight raised the interest of researchers in the previous years. The effect of impact due to landslide falling rocks on reinforced concrete (RC) slabs has been investigated by many researchers, while very few studied the effect of impact loading on pre-stressed structures, noting that a recent study was conducted at Beirut Arab University which compared the dynamic behavior of reinforced concrete and post-tensioned slabs under impact loading from a 605 kg impactor freely dropped from a height of 20 m. Hollow core slabs are widely used in bridges and precast structures. Thus, studying their behavior due to such hazards becomes inevitable. This study focuses on these types of slabs. For a better understanding of the behavior, a full scale experimental program consists of testing a single span hollow core slab. The specimen has 6000 mm × 1200 mm × 200 mm dimensions with a 100 mm cast in a place topping slab. Successive free fall drops cases from 14 m height will be investigated on the prescribed slab having a span of 6000 m. This series of impacts will be held by hitting the single span hollow core slab at three different locations: center, edge, and near the support. The data from the testing program were used to assess the structural response in terms of experimental observations, maximum impact and inertia forces, structural damage/failure: type and pattern, acceleration response, and structural design recommendations. This research showed that the hollow core slab has a different dynamic behavior compared to the post tensioned and reinforced concrete slabs mentioned in the literature review section.

Analysis of damages and defects of the repair and operational unit of the compressor station

The need to maintain the technical condition of buildings with a significant service life determines the regularity of their inspection. The paper presents the analysis of the results of the inspection of the building of the repair and operational block of the compressor station. The analysis of the design documentation and the results of the measurement work showed that the construction is a one-story basementless building with dimensions of 54.62x18.74x5.9 m. The performed visual examination showed the presence of damages and defects of the third and fourth categories, which requires certain actions to eliminate them. Visual examination showed the need for instrumental examination. In particular, the need to determine the strength of brickwork and the strength of concrete in structures. The analysis of the results of the instrumental examination showed: the results of testing the wall masonry by the shock pulse method using the ONIKS 2.5 device showed that the compressive strength grade of silicate and ceramic bricks of the walls of the repair and maintenance block corresponds to M100, and the grade of masonry mortar is M50; The results of concrete testing by the non-destructive method of strength control in accordance with GOST 22690-88 with the DigiShmidt 2000 device showed that the class of concrete in terms of compressive strength in reinforced concrete slabs of the covering of the repair and operational block corresponds to B20, and the class of concrete in reinforced concrete beams of the covering corresponds to B25. The verification calculation showed that the strength of the reinforced concrete pavement of the repair and maintenance unit under the full design load acting at the time of the survey was ensured. The analysis showed that the technical condition of the maintenance and operation unit is of limited serviceability. The work provides recommendations for eliminating defects and damages.

ANALYSIS FOR IMPLEMENTING GREEN ROOFS ON REINFORCED CONCRETE SLABS. A CASE STUDY IN THE CITY OF TUXTLA GUTIERREZ, CHIAPAS, MEXICO

En la búsqueda de asumir métodos que brinden un mejor confort en el sistema de vivienda, éstos requieren de mejores prestaciones estructurales en las edificaciones actuales. Las prácticas constructivas en la región se han ido desprestigiando con el paso del tiempo, concibiendo edificaciones vulnerables, que dejan expuesto a quienes las habitan. El objetivo de esta investigación fue analizar de forma integral las características técnicas, constructivas y de diseño de losas de concreto armado, en su implementación como estructura de soporte para techos verdes en la ciudad de Tuxtla Gutiérrez, Chiapas, México. Para realizar el análisis se consideraron las propiedades intrínsecas del miembro de soporte (losas), el estado actual en las que estas se encuentran y su diseño bajo regímenes de durabilidad. Tras realizar la investigación se encontró que los espesores de la losa y las cuantías de acero son menores para claros promedio, según lo estipulado por las normas técnicas correspondientes. Los daños por corrosión del acero son esperados y se encuentran bajo un régimen de diseño inadecuado, aunado a la deficiencia durante el proceso constructivo. Las construcciones analizadas y desarrolladas por procesos de construcción formal o de autoconstrucción evidencian patologías similares. La falta de centros de capacitación para los propietarios y los obreros que adopten los métodos de autoconstrucción, así como el poco rigor de las autoridades en el cumplimiento de los reglamentos de construcción se han convertido en una brecha del conocimiento clave. Finalmente, es poco probable la implementación de techos verdes en estas estructuras sin antes realizar cambios significativos en toda la construcción.

DEFORMABILITY AND CRACK RESISTANCE OF AIRFIELD SLABS

The results of experimental studies of deformability and crack resistance of models of airfield slabs made of reinforced concrete and steel fiber concrete are presented. Two series of plates were tested ‒ three models of reinforced concrete and three models with steel fiber added to the concrete mixture in amount of 1% of the total volume of the product. The load was applied in small steps, the instrument readings were recorded twice at each step, and the crack opening width was measured starting from the moment of the first crack formation. Dial gauges and deflectometers were used as measuring instruments. According to the normative documents acting in Ukraine, one of two possible loading schemes was considered ‒ with the loading by the concentrated force applied on the cantilever part of a plate. The plate models were tested on a specially made stand which consisted of four supporting struts connected in pairs by beams. The airfield slab was supported by the beams. The load was applied along the width of the plate in steps ‒ 0.05 of the destructive load, along two concentrated vertical strips. Each degree of load ended with a five-minute dwell time, at the beginning and end of which readings were taken on the measuring instruments. The deformations at the same levels were measured with dial gauges. The process of crack formation was observed with a Brinell tube in the places of the greatest crack opening. It follows from the obtained results that the process of cracking in the fiber concrete slab begins at higher loads than in the reinforced concrete slab. The final and initial crack opening widths of all cracks in the fiber concrete slab are significantly lower than in the reinforced concrete slab. The deformations in steel-fiber concrete slabs during the application of load in the cantilever part, both for compressed and stretched fibers are higher than in reinforced concrete slabs. At the initial stages of load application in the cantilevered part of the slabs, the deflections increase in a linear relationship. The curves get non-linear character for airfield slabs made of reinforced concrete when the load reaches the level of 10÷25 kN, for steel-fiber-concrete slabs ‒ 15÷30 kN. In reinforced concrete slabs, the non-linearity starts a little earlier and is expressed more clearly. Experimental studies show that dispersed reinforcement of airfield slabs with steel fiber leads to their higher crack resistance.

NUMERICAL STUDY OF THE PUNCHING SHEAR MECHANISM FOR THIN AND THICK REINFORCED CONCRETE SLABS

The assessment of the punching shear capacity for reinforced concrete slabs, carried out according to the regulatorydocuments of a number of countries, leads to significantly various results. At the same time, the results of thecalculated forecast may have great differences from the experimental data. A great influence on the accuracy of the resultsof the calculated forecast is exerted by the thickness of the examined slabs, as well as the value of longitudinal reinforcement.These parameters determine the features of the mechanisms of destruction of slabs in case of the punching shearmechanism, as indicated by individual interpretations of the results of experimental studies. In order to determine thefeatures of the punching shear mechanism of reinforced concrete slabs of various thicknesses, numerical studies of theprocess of cracking and destruction of slabs of different thicknesses have been performed. Differences in the mechanismof formation and development of cracks in thin and thick slabs are revealed. The paper shows that the behavior of thinand thick slabs has qualitative distinctions at the initial stages of formation and development of the cracks leading todestruction. The authors have also shown the difference between stress-strain state of thick and thin slabs before destruction.In conclusion, it was established that the influence of longitudinal reinforcement on the strength during punching inthick slabs is much less than in thin ones.When evaluating the punching shear capacity of reinforced concrete slabs, the regulatory documents of different countries give significantly different results. In this case, the calculation results may differ significantly from the experimental data. The deterioration of the thickness of the calculated slabs, as well as the value of the longitudinal reinforcement has a great influence on the accuracy of the calculation results. These parameters determine the features of the destruction mechanisms of slabs under punching. This fact is indicated by some interpretations of the results of experimental studies. In order to establish the peculiarities of the punching shear mechanism of reinforced concrete slabs of different thicknesses, a numerical investigation of the cracking and destruction of slabs of different thicknesses have been performed. Differences in the mechanism of formation and development of cracks in thin and thick slabs have been revealed. The paper shows that the behavior of thin and thick slabs has qualitative differences at the initial stages of the cracks formation and development that leads to destruction. The difference between stress-strain state of thick and thin slabs before breaking have been shown. It was found that the effect of longitudinal reinforcement on the punching shear strength in thick slabs is much less than in thin ones.

Determination of features of composite steel and concrete slab behavior under fire condition

Methods for calculating the fire resistance of steel-reinforced concrete slabs made using profiled steel sheets under the influence of a standard temperature regime for more than 120 minutes are considered and analyzed. Research has been carried out to determine the heating parameters and the stress-strain state of steel-reinforced concrete slabs made using profiled steel sheets under fire conditions for more than 120 minutes. The results of this study allow to obtain indicators of temperature distribution for assessing the fire resistance of such structures for fire resistance classes above REI 120. Accordingly, the results obtained are a scientific basis for improving the existing method for calculating the fire resistance of steel-reinforced concrete slabs made using profiled steel sheets. The temperature distribution in the cross-section of structures was obtained using a general theoretical approach to solving the problem of heat conduction using the finite element method. Using the obtained temperature distributions, the parameters of the stress-strain state were determined based on the method of limiting states. To carry out the calculations, appropriate mathematical models were created that describe the effect of the standard temperature regime of a fire, to determine the temperature distribution at every minute in the sections of steel-reinforced concrete slabs with profiled steel sheets. A method is proposed for dividing the section into zones to take into account the decrease in the indicators of the mechanical properties of concrete and steel. A simplified method for the design assessment of steel-reinforced concrete slabs made using profiled steel sheets is proposed, which is consistent with the current EU standards and can be effectively used to analyze their fire resistance when establishing their compliance with the fire resistance class REI 120 and higher.