Size and wall effects on compressive strength of concretes

Concrete is a widely used construction material that is obtained by mixing cement, water, aggregates and chemical or mineral additive materials up homogeneously. Cement mainly contains chalk and clay which are common in nature. Still actively referred TS - EN - 197-1 standard categorizes twenty seven different cements into five main types such as CEM I (Portland Cement), CEM II (Portland-Composed Cement), CEM III (Slaggy Cement), CEM IV (Pozzolana Cement) and CEM V (Composed Cement). What is more, aggregates are classified in two groups which are natural aggregates and synthetic aggregates. Natural aggregates are produced using none of the synthetic manufacturing methods except the mechanical processes (e.g. sand, gravel and crushed sand and stone obtained by cutting rocks in crushing machines). Water for concrete production, on the other hand, can be obtained via any drinkable water source that is appropriate for TS-EN-1008 standard without necessitating any additional test method. Mineral additives used in concrete production can be defined as supplementary materials mixed into concrete mixture with relatively low ratios with respect to cement content to change its fresh and hardened properties. Furthermore chemical admixtures also can be used in concrete production to obtain better fresh and hardened concrete properties. Concrete obtained through mix of aforementioned materials hardens as time passes while it has great plasticity in the initial a few hours, which provides this commonly used construction material flexible usage conditions. Concrete is a composite material which has semi-brittle structure. One of the main properties of this important material is compressive strength. Since it has a semi-brittle structure, compressive strength of the concrete is much higher than its tensile strength. Hence, tensile strength of concrete is generally not taken into account in reinforced concrete engineering calculations. On the other hand, there is an approximate relationship between the compressive strength of concrete and its tensile and flexural strengths. For this reason, it is possible to estimate tensile and flexural strengths of a specific concrete if its compressive strength is available. Regarding compressive strength values, concretes can be categorized as normal strength concrete and high strength concrete. Normal strength concretes are most widely used general purpose construction materials nowadays. Their cement contentis relatively low with respect to high strength concretes. Additionally, aggregates used in normal strength concretes have greater radiuses than aggregates used in highstrength concretes. Nevertheless, production costs of normal strength concretes are much lower than production costs of high strength concretes. Concretes having cubic compressive strength changes between 60 and 115 N/mm2 and cylindrical compressive strength stays between 50 and 100 N/mm2 can be classified as high strength concrete considering strength values of other materials used nowadays. Silica fume, high strength cement and superplasticizer materials can be used in production of high strength concretes. Test samples in different sizes reflect different compressive strength properties even they have same geometrical shapes. This fact is called as size effect on compressive strength of concretes. Experimental studies show that compressive strength of larger specimens are lower than the specimens having same geometrical shapes with smaller sizes. These studies also claim that recession in compressive strength of concrete as the size increases falls into relatively slight values above the specific dimensions. Concrete literature entitle this effect as "size effect". Size effect is due to the increased possibility of containing defects in the material when the size increases. The other factor affecting the compressive strength is due to the improper compacting of concrete in the vicinity of walls of a mold. This causes the edge part of specimens to have a different type of concrete comparing with the interior parts. The thickness of this layer took place on the mold-interacted surfaces is approximately a maximum aggregate size. For the small size specimens the effect of this layer on the strength properties is higher than those of the larger ones, because the thickness of this layer is not related to the size of the specimen. This phenomena is known as "wall effect". This thesis presents the effects of the physical dimensions and the mold interacted surface qualities of the cubic shape concrete test specimens, which are obtained via cutting of concrete blocks, on the compressive strength of the concrete itself. Furthermore, influences of the defects, occurred during the cutting process, on the compressive strength of the concrete are also investigated in the scope of the thesis. Six different test samples are prepared using two different types of concrete at the first stage of the study. First type of concrete represents the high-strength concrete where the second type of concrete stands for the normal-strength concrete. Maximum radius of the aggregates used in the preparation of the first type of concrete is 12 mm while it is 22 mm for the second type of concrete. Moreover, first type of concrete is in C50 concrete strength class and the second type of concrete is in C20 concrete grade. Content of cement in the first type of concrete, representing high strength concrete, was 450 kg/m3 where it was only 250 kg/m3 in the second type of concrete. Silica fume, %5 of the cement content, is added into the high strength concrete in order to obtain desired compressive strength in the first type of concrete. Consequently, water/binder ratio in the first type of concrete was 0,33 while this ratio was 0,72 in the second type of concrete. Plywood molds were prepared in different sizes for the test specimens. Molded concrete blocks are preserved under wet burlaps which are kept moisturized by irrigation for seven days. Afterwards, the blocks were held in a sunless environment for thirty days. Ready to use concrete blocks are cut with water cooled marble cutting machines into different sizes. To be able to investigate effects of damage occurred due to cutting process on compressive strength of concrete, test samples 50 mm were prepared using both marble cutting machine and a water jet which minimizes the damages. Finally, comparison study is conducted to expose influence of cutting methods. Concrete might have many defects inside since it is a composite material not having homogenous structure. Hence, it is a fair assumption that we would have defects inside the test samples we prepare during and before the experiments in the scope of this thesis. Natural stones have also heterogeneous structures as concretes. Heterogeneity of natural stones comes from increase or decrease in amount of different mineral components inside the stone or change in dimensions of these components. On the other hand, heterogeneous structure of natural stones are much ordered than structure of concrete. Most important reason for this fact is that natural stoned are not exposed to wall effect during formation phase. Most probably, different test samples taken from same stone give very close results to each other. However, it is not reasonable to come a same conclusion for different specimens taken from different stones belong to same stone family. In this context, subjected thesis also investigates size effects on compressive strength of natural stones which was found in the literature. This thesis presents highly valuable data that show how much defects in prepared test samples during experiments affect compressive strength of concretes as well as the effects of cutting process applied during the preparation of specimens, which is an important issue in the coring of concrete.