Title: An overview of the models for time dependent behavior of concrete
Original Title: Pregled modela vremenski ovisnog ponašanja betona
Authors: Ante Džolan
DOI: https://doi.org/10.5592/CO/ZT.2017.21
First page: 147 Last page: 154
Publish date: 18.9.2017 Publish date online: 18.9.2017
Abstract: Using of the concrete as building material in its present form began at the end of the 19th century. However, the compressive strength of the concrete is considerably higher than the tensile strength and the concrete is reinforced that the reinforcement takes the stresses in the tensile zone of concrete. In the tensile zone of reinforced concrete, concrete cracking occurs when tensile stresses have reached the value of tensile strength of concrete. In order to avoid cracking or limiting their width, the idea of pre-stressing concrete appears. By prestressing, compresive pressures are applied to the concrete before loading the concrete, thereby it is provided compresive pressure distribution at the height of the loaded concrete section, avoiding the occurrence of cracks. The first attempts prestressing haven’t been succeed because it was unknown phenomenon of creep and shrinkage concrete. Namely, there are stresses in concrete that are caused with creep and shrinkage and that can cause cracking of concrete before loading of concrete. Creep and shrinkage represent time-dependent deformations, where creep, unlike shrinkage, also depends of the load. Deformations caused by creep and shrinkage may also be several times greater than deformations caused by the action of short-term static loads, so it is meaningless to study the behavior of the structures by neglecting the effects of creep and shrinkage of concrete.
Sažetak: Beton kao građevni materijal, u današnjem obliku, počeo se koristiti krajem 19. stoljeća. Međutim, tlačna čvrstoća betona znatno je veća od vlačne, te se beton armira kako bi armatura preuzela naprezanja u vlačnoj zoni betona. U vlačnoj zoni armiranog betona dolazi do otvaranja pukotina u betonu kada su vlačna naprezanja dostigla vrijednost vlačne čvrstoće betona. Kako bi se izbjegla pojava pukotina ili ograničila njihova širina, osmišljeno je prednapinanje betona. Prednapinjanjem se u beton unose tlačna naprezanja prije njegova opterećivanja, te se time omogućuje tlačna raspodjela naprezanja po visini opterećenog betonskog presjeka, čime se izbjegava pojava pukotina. Prvi pokušaji prednapinjanja su propadali zbog nepoznavanja fenomena puzanja i skupljanja betona. Naime, u betonu se uslijed puzanja i skupljanja pojavljuju naprezanja koja mogu dovesti do otvaranja pukotine prije opterećivanja betona. Puzanje i skupljanje predstavljaju vremenski ovisne deformacije, gdje puzanje, za razliku od skupljanja, ovisi i o opterećenju. Deformacije izazvane puzanjem i skupljanjem mogu biti i nekoliko puta veće od deformacija izazvanih djelovanjem kratkotrajnih statičkih opterećenja, tako da je besmisleno proučavati ponašanje konstrukcija zanemarujući efekte puzanja i skupljanja betona.