A STUDY ON TENSILE STRENGTH OF ULTRA HIGH STRENGTH CONCRETE

Abstract

Tensile strength of concrete is the most neglected property of concrete, which is always obtained through indirect methods. Interestingly no other property has to face this dilemma because proper and direct tests are available for their measurement. On the other hand every fracture in concrete is tension fracture. In one way or the other, whenever tensile capacity of concrete is reached, cracks start growing and propagating within the mass of concrete, hence it can be safely stated that, “every crack is a tension crack”. Though some direct tests are available in the literature, but they are not easy to perform. The first aim of this study is to devise a new direct test for tensile strength of concrete. For true tensile strength, instead of relying upon conventional in-direct methods, a new easy to perform direct tension method is devised and tensile strength is measured using this new technique. For this purpose a new dumbbell shaped mold is fabricated which has special characteristic of being used twice, i.e. for casting of specimen and then again for load application. This new test is found successful for measuring direct tensile strength of concrete. Concrete having strength more than 90 MPa (13,000 psi) is termed as UHSC (ultra high strength concrete). The second aim of this study is to produce ultra high strength concrete using constituents available in Pakistan. Three different types of aggregates are tried in order to achieve this goal and finally heavy mineral aggregates containing about 30 to 40 % of chromium metal are found suitable for production of required strength. It is generally believed that tensile strength of concrete is about 10% of its compressive strength and cylinder strength is about 80% of its cube strength. Based on experimental results different co-relations for concrete strengths are postulated between compressive- tensile strengths, direct-indirect tensile strengths and cube-cylinder compressive strengths. These relations show that conventional 10% concept regarding tensile strength and 80% regarding cylinder-cube strengths is not true for ultra high strength concrete. For ultra high strength concrete its tensile strength varies from 7.4% to 7.85% of its cylinder strength, whereas its cylinder strength is 85% of its cube strength. viiiThe field of development of hydration stresses within concrete during the period of its strength development is under-investigated, which has been carefully diagnosed in this study and a model for finding actual compressive strength of concrete is also proposed. Now it is the turn of future researchers to focus their attention on this topic and find true compressive strength of concrete. Dense packing is one of the many factors influencing concrete strength. Some complex packing density models are available which become more and more complex as the number of concrete constituents is increased. Lengthy models are generalized in the form of geometric series in this study and a simple model is suggested which can be used for any number of concrete ingredients. Size effect is considered as basic property of all structures, but at the same time reverse size effect is also reported by various researchers as well as by the author. Different causes of size effect are explored and it is believed that size effect is basically the result of strong stress/ strain gradients and care must be exercised before incorporating it into the concrete codes. Direct tension test developed in this study proved very successful and is capable of furnishing true uniaxial tensile strength of concrete. Griping and eccentricity problems are not faced during testing and above all test is easy to perform. Through continuing research molds can be further improved to make them more useful for academic understanding of concrete properties. Moreover, it is also proved that ultra high strength concrete can be produced in Pakistan using locally available materials.