Bahtiyar Ünver

Fracture resistance of tooth fragment reattachment: effects of different preparation techniques and adhesive materials

The purpose of this study was to evaluate and compare the bond strengths of experimentally fractured human tooth fragments reattached with different adhesive materials and retentive techniques in vitro. Uncomplicated crown fractures were obtained on intact human mandibular permanent incisors by applying perpendicular load to the buccal aspect of tooth crowns. Fractured teeth were randomly assigned into one of three reattachment protocols: (i) Simple reattachment, (ii) Overcontour preparation, and (iii) Internal dentin groove. The first and second groups were divided into 10 subgroups, and the third group into five subgroups (n = 10 per group) with respect to five different adhesive systems (Prime&Bond NT, Adper Single Bond II, Adper Prompt L-Pop, Clearfil S(3) Bond, G Bond) used with or without a hybrid resin composite (Z250). Restored teeth were subjected to thermal cycling, and subsequently to the same loading protocol used for fracturing intact teeth. Fracture strength after reattachment procedures was recorded as a percentage of the original fracture strength. Both type of adhesive material and placement of an intermediate layer of resin composite affected the fracture resistance (P < 0.05). The highest fracture strength recovery was obtained using the internal dentin groove technique (54 +/- 0.58%, P < 0.05), followed by the overcontour and simple reattachment protocols (49 +/- 0.58% and 32 +/- 0.82%, respectively, P < 0.05). Ultramorphological evaluation of bonded specimens revealed voids and microcracks along the adhesive interface, which might contribute to postadhesive failure.

A new equation for estimating the maximum surface settlement above tunnels excavated in soft ground

Nowadays, due to urbanization and population increase, need for metro tunnels, has been considerably increased in urban areas. Common characterization of urban area tunnels is that they are excavated in very shallow depths and soft ground. In such excavations, main challenge for tunneling is low bearing capacity and easy deformation characteristic of the ground. Tunnel face instability and the potential surface settlement are the most hazardous factors that should be considered in all tunneling methods applied in urban areas. Incorrect estimation of the maximum surface settlement value can lead to irreparable damages to the buildings and other nearby structures. There are several published relationships concerned with field measurements and analytical solutions to estimate the amount of the maximum surface settlement value due to tunneling. These relationships are not precise for calculating the aimed values. Therefore, providing accurate equations for estimation of these values is certainly useful. First purpose of this study is to determine the effective parameters such as geotechnical factors (cohesion, internal friction angle, density, Young’s modulus and Poisson’s ratio), and engineering factors (tunnel depth, tunnel diameter and face support pressure) on the maximum surface settlement value. In this study, three metro project constructions namely Istanbul, Tehran, Mashhad in the Middle East were chosen. FLAC3D (Itasca Consulting Group 2002) was used for detailed numerical analysis. The second aim is to present better equations in estimating the maximum surface settlement-based actual data set from several tunnel projects and numerical modeling. The results from the new estimation equation are compared with results of empirical and field observations. The maximum surface settlement values obtained from the new equation have good agreement with the actual results for three different metro case studies.

Effects of a fluoride-containing casein phosphopeptide- amorphous calcium phosphate complex on the shear bond strength of orthodontic brackets

The purpose of this study was to investigate the effects of enamel pre-treatment with a new fluoride-containing casein phosphopeptide-amorphous calcium phosphate (CPP-ACP) complex on the shear bond strength (SBS) of brackets bonded with etch-and-rinse or self-etching adhesive systems. The material comprised 66 extracted human premolars randomly divided into six equal groups with respect to the enamel pre-treatment and adhesive system employed: 1. No pre-treatment and brackets bonded with the etch-and-rinse adhesive system (Transbond XT). 2. Pre-treatment with fluoride-containing CPP-ACP paste (MI Paste Plus) and Transbond XT. 3. Pre-treatment with non-fluoride CPP-ACP paste (MI Paste) and Transbond XT.4. No pre-treatment and brackets bonded with the self-etching adhesive system (Transbond Plus). 5 and 6. Enamel pre-treated as for groups 2 and 3, respectively, and the Transbond Plus. Bonded specimens were subjected to thermal cycling (×1000) before SBS testing. The residual adhesive on the enamel surface was evaluated after debonding with the adhesive remnant index (ARI). Data evaluation was made using one-way analysis of variance and Tukey test for SBS results, and Kruskal-Wallis test for ARI results. The results showed that enamel pre-treatment with either fluoride or non-fluoride CPP-ACP paste had no significant effect on the SBS of the self-etching adhesive system (P > 0.05). Enamel pre-treatment with non-fluoride CPP-ACP in group 3 significantly reduced the SBS of the etch-and-rinse adhesive (P < 0.001), while pre-treatment with fluoride-containing CPP-ACP paste (groups 2 and 5) did not affect debonding values (P > 0.05). The fluoride-containing CPP-ACP did not compromise the SBS of brackets bonded with the tested etch-and-rinse and self-etching systems, but its non-fluoride version significantly decreased the SBS of the etch-and-rinse adhesive system.

Application of heat transfer analysis for frozen food storage caverns

Supply of high quality preserved food products has always been very important. Storage of food in underground caverns is superior over food storage tanks constructed at surface from an energy conservation point of view. There are a lot of underground openings excavated in tuff used for the storage of fruits and vegetables at the Cappadocia Region of Turkey. However, there has not been any attempt to use these underground openings for deep frozen food storage and related heat transfer details. The main purpose of this work was to investigate underground food storage openings in the Cappadocia Region and their potential use for frozen food storage. Details of heat transfer around openings for various geometrics in three-dimensional have been modelled by using a finite element software package called MARC. For the sake of comparison, heat transfer details around a similar opening excavated in granite were also determined. A comparison of underground caverns excavated in tuff and granite with a surface deep freeze storage tank was also made by both taking construction and operational costs into account. Construction of an underground deep freeze storage cavern excavated in tuff would cost one half of a same size surface tank whereas, energy loss due to heat transfer is three times lower in the favour of underground storage cavern opened in tuff.

Investigation of surface and subsurface displacements due to multiple tunnels excavation in urban area

Underground structures are currently widely used and are built as urbanism develops. The interactions between perpendicularly crossing and parallel tunnels in the Tehran region are investigated by using a full three-dimensional (3D) finite difference analysis with elastic-plastic material models. Special attention is paid to the effect of subsequent tunneling on the support system, i.e., the shotcrete lining and rock bolts of the existing tunnel. Eventually, as the tunnels are excavated at certain levels, the interaction between the tunnels will certainly have a significant influence on both stress distribution and consequently deformations. Since multilayer tunneling is a three-dimensional phenomenon in nature, 3D numerical solutions must be utilized for analyzing effect of perpendicularly crossing tunnels at various levels. As Tohid twin tunnels and Line 7 pass beneath the Line 4 metro tunnel, changes in stress distribution, deformations, and surface settlements are studied for various conditions and the results are presented in this paper. Consequently, it is shown that there is a significant interaction between tunnels that necessitate certain preventive measures to maintain a stable tunneling operation.

A Geostatistical Study of Tertiary Coal Fields in Turkey

The majority of lignite and sub-bituminous coal resources known in Turkey are of Tertiary age. Therefore Tertiary coals in Turkey are of great importance in terms of coal potential and production. This paper presents a study of spatial distributions of calorific value, ash content and moisture content over Tertiary fields. For this purpose data collected from 187 coal fields in Turkey are used. The experimental variograms of three quality attributes are computed from these data and models are fitted. These models are used in estimation of spatial distributions of the corresponding variables by ordinary kriging. In addition, the probabilities of calorific value being less than 1,600 kCal/kg at estimation nodes are computed by using Direct- Sequential Simulation. Finally, additional sampling locations are proposed especially for the fields in which further information are required.

Analysis of Roof Caving Characteristics at a Coal Mine by Using Full Scale 3D Numerical Modeling

Regular and efficient caving of roof strata behind is essential in maintaining a trouble free operation in underground longwall mining when especially mined area is left for caving. As the face advances, roof strata should be regularly caved forming a goaf as homogenous as possible. In case of having an uncaved roof behind the coal face, load on the face increases dramatically leading to serious fall of roof conditions. Therefore it is of paramout importance to have the roof regularly caved behind a longwall face. This paper presents the problems encountered at a coal mine in Turkey due to high face pressures and subsequent flow of roof at the face roof junction. The height of the fully mechanised longwall face is 4.5 m. Sliding of face coal and later the fall of roof strata in front of the shields created serious stability and safety problems in the mine. Stopes opened in the roof had to be filled by usign forepoling, foam and concrete. This rescue operation had to be completed securely before starting of the cutting operation at the longwall face. Obviously rate of production of the longwall face has been severly declined during this period. There were a couple of reasons for having such a difficult condition in the mine. The longwall panel was located near to a syncline axis leading to high tectonic stresses. There were lots of small faults through the working face. Longwall face was extremely loaded by a very strong limestone layer having a thickness of up to 80 m located at 120 m above the coal seam. Although the strata between the limestone layer and the coal seam has a readily caving characteristics, the limestone caved at long intervals causing high face pressures due to its cantilever beam effect. Moreover during caving of the limestone, severe dynamic loads are experienced in the vicinity of longwall face deteriorating stability conditions. Therefore it was decided to model the effect of limestone layer’s behaviour by means of numerical modelling. A full scale model was created in accordance with all geometrical conditions and operational parameters by using FLAC3D software. The face advance is also simulated on the model. Stress and deformation state of the coal face, surrounding rock and especially the problematic limestone layer are analysed. To solve the problem, a blasting pattern is selected to weaken the limestone layer by using drill holes opened from the surface. This paper presents the numerical modelling results in relation to selection of the best blasthole geometry to decrease loading on the face and hence maintain a safe, efficient and stable longwall operation.