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Nowa koncepcja wykonywania wałowanych warstw ścieralnych nawierzchni o dużym obciążeniu ruchem (HVA)

Murawski Wiktor

Drogownictwo

2019

nr 2

43--49

W artykule przedstawiono nowy rodzaj warstwy ścieralnej z przeznaczeniem do wykonania nawierzchni dróg o ruchu ciężkim. Mieszanka oznaczona symbolem HVA, charakteryzuje się obniżoną zawartością asfaltu w stosunku do dotychczas stosowanych w Niemczech warstw typu zagęszczanego z betonu asfaltowego i mastyksu grysowego SMA, które odpowiadają wymaganiom przepisów (TL Asphalt–StB i ZTV Asphalt-StB). {Mieszanka HVA stanowi rezultat wieloletnich badań warstwy ścieralnej z betonu asfaltowego, zawierającej jedynie 5,4% asfaltu B80, natomiast uzyskany wskaźnik zagęszczenia warstwy jest znacznie powyżej 100%.}

The paper presents a new type waring course intended for construction of heavy traffic load pavement. The mixture, marked with HVA symbol, has a lower asphalt content than asphalt concrete and stone mastic asphalt SMA used in Germany, which comply with the regulations (TL Asphalt-StB and ZTV Asphalt-StB). The HVA mix is the result of many years of testing of asphalt wearing course made of asphalt concrete containing only 5,4% B80 asphalt, while the compression rate is well above 100%.

Podstawowe zagadnienia przyczepności stali i betonów w elementach żelbetowych

Pędziwiatr J.

Prace Naukowe Instytutu Budownictwa Politechniki Wrocławskiej. Monografie

2007

Vol. 88, nr 37

1--290

Typowe konstrukcje żelbetowe są tak projektowane, aby zapewnić współpracę stali zbrojeniowej i betonu przy przenoszeniu działających obciążeń. Po zarysowaniu transfer sił ze stali na beton bazuje na tak zwanej wtórnej przyczepności, której głównymi czynnikami są zjawiska związane z mechanicznym zazębieniem żeberek zbrojenia o beton oraz tarciem towarzyszącym przemieszczaniu się pręta. Zjawisko to ma kluczowe znaczenie w analizach odksztalcalności zarysowanych konstrukcji żelbetowych. Jest ono wszechstronnie badane w świecie od wielu lat. Ostatnimi czasy rozwój nowych technologii, dotyczących zarówno betonu jak i zbrojenia znacząco poszerzył obszar prowadzonych prac eksperymentalnych i analiz teoretycznych. W literaturze światowej dostępna jest duża liczba różnorakich publikacji na temat przyczepności, ale nie dotyczy to niestety Polski. Książka ta ma na celu między innymi wypełnienie tej luki. Analizy przyczepności można prowadzić na trzech poziomach ogólności - żeberka, pręta zbrojeniowego oraz elementu konstrukcji. Badania związane z pierwszym poziomem są dobrze rozwinięte i dostarczają zadowalające rezultaty, które dotyczą mechanizmów niszczenia przyczepności i wpływu na niej różnorodnych czynników. Brak jest natomiast satysfakcjonujących wyników badań, odnoszących się do problemu przyczepności w skali całego pręta i elementu konstrukcyjnego. Próby transponowania wyników badań i teorii, odnoszących się do bardzo krótkich elementów na te wyższe poziomy analizy, okazywały się nieudane. W książce przedstawiono nową autorską koncepcję teoretyczną, dotyczącą drugiego poziomu analizy przyczepności. Jej podstawą są funkcje przyczepności opisujące przebieg naprężeń przyczepności wzdłuż osi pręta w funkcji zarówno położenia przekroju jak i wartości naprężeń w zbrojeniu. Zaproponowane rozwiązanie teoretyczne pozwala uwzględniać i szczegółowo analizować te zagadnienia, które do tej pory umykały możliwości uwzględnienia. Cenną zaletą opracowanej koncepcji jest możliwość dokonania względnie łatwego, a przy tym spójnego przejścia w analizach z poziomu drugiego na trzeci. Prace eksperymentalne weryfikujące modele teoretyczne były prowadzone w Instytucie Budownictwa Politechniki Wrocławskiej w latach od 1996 do 2002. Wykorzystano w nich indywidualnie zaprojektowane elementy do badań, dostosowane do ogólnej koncepcji teoretycznej. Pozwoliło to na precyzyjny i pełny pomiar odkształceń w zbrojeniu i otaczającym betonie, a specyficzny kształt elementów umożliwiał wykorzystanie optycznych technik pomiarowych, co przekładało się na możliwość prowadzenia obserwacji zjawisk zachodzących na dużych powierzchniach.

A typical reinforced concrete structure is designed under the assumption that concrete and steel bars interact to carry loads. As long as there are no cracks in the tension zone, interaction is based on the so-called primary, perfect bond. This means that in any cross section, the strain value in a steel bar is the same as in adjacent concrete, i.e., ss = ea. At those levels of loading the bond relies on adhesion forces. After cracking the situation is much more complicated. Steel strains are much greater than concrete ones and there is a slip between a steel bar and surrounding concrete, which breaks an adhesion. The transfer of tensile forces is mainly based on the bearing of lugs between steel ribs and friction of steel-concrete surface. As a cracking state is typical of concrete members with tensile zone, such a bond mechanism is more important than primary one. The bond between concrete and steel bars is of fundamental importance to deformation characteristics of cracked concrete structures. It has been extensively studied for many years, especially, starting from the early 1970's. In bond testing, it is very important to choose a proper specimen. In most experiments, members of a very short embedded length are used, according to RILEM recommendations. This allows us to assume that bond stress is constant and one can only measure an acting force and a relative slip between steel bar and surrounding concrete. Results of such experiments are presented in the form of function r= r(A). Experiments on specimens with a very short transfer length are relatively cheap and simple. These allow us to examine many interesting parameters, e.g., concrete strength, bar rib patterns, confinement effect and loading history. Testing should explain the influence of those variables on the bond strength, the average bond stress along the development length and the initial bond stiffness. The best results have been obtained in testing the bond strength. Bond strength is a maximum value of bond stress which can be obtained in an embedded bar without failure. There are two patterns of this failure. The first one consists in pulling out a bar from concrete member and the second is connected with a concrete cover splitting. The first type of bond strength has much higher value while the second is more apt to possible failure mode in a real concrete structure. Concrete tension strength appears to be the main factor governing the bond strength. Its increase causes an increase of bond strength. More detailed experiments and studies led to a conclusion that also concrete compression strength must be taken into account. This is particularly important for high strength concretes, in which correlation between tension and compression strengths differs from an ordinary concrete. In high-performance concrete dosage of silica fume, superplasticizer and fibres has strong influence on bond strength. As far as experimental data are concerned, lots of theoretical models were presented. The simplest of them are based on elastic cracked model of concrete. There also exist more sophisticated models such as elastic-cohesive or elastic-plastic-cohesive ones. Deformed bars have different patterns of ribs. Since concrete resistance between lugs is the source of bond strength, the rib characteristics are very important. The main parameters are: rib height, rib sparing and an angle between lug surface and a bar axis. For practical use, a global parameter related to rib area fR is used. All tests confirm that an increase of fR leads to higher values of the bond strength. This is advantageous for the sake of a limit crack width, for higher initial bond stiffness and higher average bond stresses imply smaller crack spacing and crack width. A bar geometry with large fR leads to a considerable contribution of concrete between cracks and thus the rotation capacity of plastic hinges is significantly reduced. This is disadvantageous for indeterministic concrete structures. Moreover, also bendability properties of bars become worse. Confinement phenomenon is due to a concrete cover, transverse bars or transverse stresses acting on a member. A positive influence of a concrete coveris best known. If a cover is higher than three times a steel bar, the pull-out failure is only possible and thus the bond strength is much bigger than during splitting failure. A similar effect can be obtained if stirrups are spread close enough and are under stress. In some cases external pressure, for example, from bearing, improves bond conditions. It is worth noticing that all these phenomena have positive influence mostly on the bond strength. Tests on members with a very short embedded length cannot give an explanation of their weight on average bond stress or an initial stiffness. There are few tests on cyclic or long term loading. In fatigue tests on specimen with a short development length, bond tendends to diminish. If there were slip-control tests, a bond stress would decrease to its residual value. This depended on a friction between steel and concrete. In cases of force-controlled tests, slip values were increasing and when they exceeded a value comparable to ribs spacing, failure occurred. Such results are very negative from the point of view of member serviceability, but in some tests where embedded lengths were higher than twenty times a steel bar there was not any failure. Results of long term test differ from one another and it is difficult to say if there exists something that we can call bond creep. Some other parameters such as effect of casting position, maturing, and group bars and bar position, extremely low or high temperatures can be taken into account during tests on short members. They mainly provide information about the obtained bond strength but results of those studies seem to be inadequate to concrete structure analysis. In engineering practice and codes, they are unnoticed. There are also tests where a specimen span is similar to a typical spacing of cracks. Such tests are called double pullout. In that case axially situated bar is specially instrumented to make measurement of steel strains possible. A typical steel bar is sawn into two parts. Then, each half-bar is milled to provide a channel to install strain gauges and later put together again. During tests, an acting force and strains are measured. Bond stresses are calculated as a change of steel strains between two succeeding gauges. Slips were calculated by integration of steel strains. Sometimes also strains in concrete were measured. Such experiments are much more complicated and expensive, but obtained results, very often, have a very large scatter. The most interesting conclusion can be summarized to a thesis that bond stresses are not only a function of slips but also a distance from a cracked cross section. The tests described can be considered as connected with the first level of bond problems. The second one is called bar scale and at that level the reinforcement bar and surrounding concrete are treated as continuum. The third level is a member scale. These levels are very important from a practical point of view. At the second level the problem of crack width and spacing can be analyzed on the basis of bond theories. In a member scale the effect of concrete strains remaining in tension zone between cracks, called tension stiffening, is very important for proper determination of the whole member stiffness. In literature some authors attempt to present their ideas of describing the cracking processes and stiffening changes after cracking in terms of bond concepts. Practically all of them are limited to theoretical studies based on experimental data taken from the first level. Since these data are from a specimen of a very short embedded length, the results are rather of little importance from practical point of view. They overlook a lot of phenomena, which take place in real concrete structures. One of the most important differences is the presence of internal cracks in a real member. Internal cracks arise close to steel bars and develop towards a member edge. Until they are inside a member they are not visible but they change steel strain distribution along a bar and they have an influence on bond. They can reach an edge and they are the so called "secondary" cracks. The phenomenon of internal cracks is answerable for great scatter of observed crack width and spacing. Analysis of internal crack and bond can explain in a convincing way the phenomenon of irregular concrete strain distribution in uniaxial tension reinforced cross section. Theoretical assumptions and a kind of specimen for tests are very strictly connected. In theoretical studies an assumption that bond stress is a function of both distance from a cracked cross section and steel slip was made. Moreover, two bond functions were determined to describe bond phenomenon. The first one is suitable if a load level is very close to cracking and, in a member, there still exists a cross section where strains in a steel bar and surrounding concrete are the same. The second function is used when more than one crack exists in a specimen. Theoretical studies allow us to describe some phenomena which were omitted in earlier theories. Especially, this concerns occurrence of internal cracks, their development and turning into secondary cracks. Theoretical model is able to explain non-uniform concrete strain distribution in perpendicular cross sections. Theoretical studies can be applied not only for members under axial tension, but also for beams and structures under eccentric tension. Experimental research was performed at the Institute of Civil Engineering Wroclaw University of Technology from the beginning of the 1990s to 2001. The research schedule and types of tested elements were chosen to obey the theoretical concepts. The experiments were carried out on members under axial tension, bending and eccentric tension. Specimens were designed and prepared to allow direct observation of internal cracks and measurements of steel and concrete strains. The use of steel bars moulded to their half diameters as reinforcements was the key solution. Then they were placed in such a way that their even surfaces faced external concrete surface. Such specimens enable direct measurement of strains in a steel bar thanks to strain gauges glued to the bar surface. It was also easy to measure strains in surrounding concrete in many cross sections and variations. In some cases, elastooptical surfaces were glued to concrete. The coating position was changing - it covered either 14 or '/< of a specimen even surface. The visualization enables one to watch continuously the deformation in interesting regions. Experimental research gave a lot of information about steel and concrete strain distribution, formation of bond stresses, internal cracks and so on. Experimental data confirm theoretical prediction and allows us to make the model more precise. Specific kind of specimens allows us to spread knowledge of bond phenomenon and explaining some sophisticated problems. On the other hand, they are still far from real concrete structures and specimens used for classic testing of beam deflection or width of cracks. It seems reasonable to design a testing procedure in which proposed "bond" specimens and classical ones will be tested together under the same conditions. This will allow a significant improvement in bond research and practical application of them.