Fiber Interfacial Transition Zone Concept for Steel Fiber-Reinforced Concrete by SEM Observation

Fiber-reinforced concrete (FRC), which has become quite popular in recent years, improves many of concrete’s mechanical properties. It uses fibers discretely and is utilized in different structures. This paper proposes, between steel fibers and concrete, a fiber interfacial transition zone (FITZ) which is the most vulnerable part of steel FRC (SFRC) because it has a high cracking and microcracking potential due to fiber-concrete separation. In the prepared specimens, steel fibers were added to concrete in hooked and twisted forms, the SFRC microstructure was studied in both cases under a scanning electron microscope (SEM), and the related images were compared as secondary electron (SE) images. The SEM analysis showed highly precise images of the cracks and their microstructures in the FITZ and lab results show that the newly defined FITZ illustrates the cracking patterns well for both fiber types. Because twisted fibers have cracking angles and larger contact surfaces, the concrete-fiber bond is increased and the related crack widths decrease considerably. A comparison of the crack widths showed that those in the FITZ of specimens with twisted fibers decreased by a factor of approximately seven compared to those with hooked fibers.

Investigation Of Water Leakage Through Direct Tension Cracks In Reinforced Concrete Panels

In this study the leakage of pressurized water through direct tension cracks in reinforced concrete (RC) panels is investigated experimentally. The results of the experiment can contribute to the field from different aspects since not many experimental studies of this kind have been reported in the literature. In this experimental program, three RC panels are subjected to direct tension force in order to form a major crack. The leakage test is then performed on the cracked specimens and the leakage of water through the crack is measured for different crack widths. The collected data consists of reinforcement strains, crack widths and the leakage rate through the cracks. Related theoretical models are compared against the experiment results. These results indicate that the leakage of water through a crack depend on the pressure of the water, cube of the crack width and crack width gradient.

Investigation Of Water Leakage Through Direct Tension Cracks In Reinforced Concrete Panels

In this study the leakage of pressurized water through direct tension cracks in reinforced concrete (RC) panels is investigated experimentally. The results of the experiment can contribute to the field from different aspects since not many experimental studies of this kind have been reported in the literature. In this experimental program, three RC panels are subjected to direct tension force in order to form a major crack. The leakage test is then performed on the cracked specimens and the leakage of water through the crack is measured for different crack widths. The collected data consists of reinforcement strains, crack widths and the leakage rate through the cracks. Related theoretical models are compared against the experiment results. These results indicate that the leakage of water through a crack depend on the pressure of the water, cube of the crack width and crack width gradient.

Case Study—An Extreme Example of Soil–Structure Interaction and the Damage Caused by Works on Foundation Strengthening

This paper describes the works on foundation strengthening of the towers of the Cathedral of St. Theresa of Avila in Subotica and the damages caused by these works. Strengthening was performed by means of jacked-in piles and deep soil injection. The construction of the Cathedral began in 1773 and it lasted for several decades with frequent interruptions and changes to the project. The present appearance of the facade was created in 1912. According to historic data, several years after construction, the cracks appeared on the front facade. With time, these cracks became more pronounced, and in 2015, when the remediation project started, the total width of major cracks reached about 15 cm. The first contemporary attempt to repair the towers was made in 2017 by inserting piles beneath the foundations. These works were interrupted due to increased settlements and the appearance of new cracks. In the second attempt, the strengthening was performed by deep injection of soil with expansive resins. During these works, settlements and damages intensified even more, causing the works to be halted in 2018. Analysis of the whole structure and revaluation of all the results, obtained from continuous monitoring of settlements and crack widths from the previous period, led to the new remediation proposal. The imperative was to retain the original appearance of the Cathedral facades while performing the total reconstruction of the upper sections of the front facade. This implies that the overall weight of the reconstructed parts is to be decreased, while the strength is to be increased. Strong structural connections are planned, both among the two towers, and between the towers and the nave. These clear structural solutions will lead to reduced stresses within the existing brick walls, reduced contact soil pressures and ceasing of increased settlements and tilting of the Cathedral towers.

Applicability of Existing Crack Controlling Criteria for Structures with Large Concrete Cover Thickness

Widely used crack width calculation models and allowable crack width limits have changed from time to time and differ from region to region. It can be identified that some crack width calculation models consist with limitations for parameters like cover thickness. The current Norwegian requirement for cover thickness is larger than these limitations. The applicability of existing crack width calculation models and the allowable crack width limits must be verified for structures with large cover thickness. The background of crack width calculation models in Eurocode, Model Code 2010, Japanese code, American code and British code have been examined. By comparing the experimental crack widths with the predictions of the aforementioned models, the existing codes can be identified as requiring modification. Considering the durability aspect, it can be identified a long-term study proving that the allowable crack width can be increased with the increase in cover thickness. When considering the aesthetic aspect, the authors suggest categorizing the structures based on their prestige level and deciding the allowable crack widths accordingly. The paper proposes potential solutions for future research on how to improve both crack width calculation methods and allowable crack width limits to be used effectively in structures with large cover thickness.

Rehabilitation of concrete one-way slabs using precast prestressed concrete prisms

The rehabilitation of structures is a complex process that involves many considerations. This paper presents results of a feasibility study on the use of precast prestressed concrete prisms (PCPs) as a near-surface mounted reinforcement for rehabilitation of cracked one-way slabs. The PCPs were cast with high-strength concrete and were concentrically prestressed by one 9 mm diameter stress-relieved seven-wire steel strand. Six identical one-way slabs with a length of 3000 mm were tested. They were loaded until cracking, then fully unloaded and rehabilitated with four PCPs before loading them to failure. Deflected profiles and crack widths of the slabs before and after rehabilitation were compared in this research. This rehabilitation method reduced the magnitude of deflection and crack width of the samples. The cracks induced in the first phase of loading before PCP rehabilitation appeared fully closed after the PCP rehabilitation until five times the original cracking loads of the slabs.

Evaluation of Methodologies for Assessing Self-Healing Performance of Concrete with Mineral Expansive Agents: An Interlaboratory Study

Self-healing concrete has the potential to optimise traditional design approaches; however, commercial uptake requires the ability to harmonize against standardized frameworks. Within EU SARCOS COST Action, different interlaboratory tests were executed on different self-healing techniques. This paper reports on the evaluation of the effectiveness of proposed experimental methodologies suited for self-healing concrete with expansive mineral additions. Concrete prisms and discs with MgO-based healing agents were produced and precracked. Water absorption and water flow tests were executed over a healing period spanning 6 months to assess the sealing efficiency, and the crack width reduction with time was monitored. High variability was reported for both reference (REF) and healing-addition (ADD) series affecting the reproducibility of cracking. However, within each lab, the crack width creation was repeatable. ADD reported larger crack widths. The latter influenced the observed healing making direct comparisons across labs prone to errors. Water absorption tests highlighted were susceptible to application errors. Concurrently, the potential of water flow tests as a facile method for assessment of healing performance was shown across all labs. Overall, the importance of repeatability and reproducibility of testing methods is highlighted in providing a sound basis for incorporation of self-healing concepts in practical applications.