X-ray CT Investigation of Bond Mechanism in Reinforced SCC Elements of Different Placement Technology.

The effect of different placing methods of the self-compacting concrete (SCC) mix-from the top and from the bottom of the form-on the bond failure mechanism was investigated within the scope of this paper. Existing studies regarding the known mechanisms of bond failure do not consider the bottom-up method of concrete placing, which improves the quality of the concrete microstructure around reinforcing bars. Background tests were performed on panel elements with dimensions of 800 × 480 × 160 mm. Ribbed steel reinforcing bars with a diameter of 16 mm were used in the tests, which were placed horizontally in the forms. A pull-out method was used to investigate the bond strength. X-ray computed tomography (CT) was used as a novel and non-destructive technique that allowed a 3D insight into the bond between the rebar and the concrete after the ultimate bond stress had been reached. The results provided a clear description of the phenomena occurring during the fresh state of concrete in the vicinity of rebars (bleeding, plastic settlement, vertical density variation) and showed their significance for bond mechanisms. Finally, it was demonstrated that placing the mix from the bottom of the form resulted in the same bond failure mechanism for both bars located at the top and the bottom of the panel elements. This was translated into identical bond properties throughout the element with regard to bond stiffness and bond strength. It was found that the described and known mechanisms of bond failure are only an idealized description of the performance of the reinforcing bar-concrete joint. The analysis of the steel-concrete interface (SCI) imaging indicated that, in reality, the forming bond failure mechanisms were a complex process that could be affected by many factors.

Multilayer Casting of Eco-Efficient Self-Compacting Concrete with Reduced Binder Content.

In the study, experiments were performed on two eco-efficient self-compacting concrete mixes of reduced binder content containing supplementary cementitious materials. The behaviour of the eco-efficient self-compacting concrete (SCC) mixture was examined to determine whether it is suitable for multilayer casting. It is recommended that the SCC should be poured in an uninterrupted manner. However, it is not uncommon that contractors are forced to take breaks as a result of delivery delays. Casting the elements in multiple poorly prepared layers may cause the creation of cold joints between them. Two technological variants of the multilayer casting of eco-efficient SCC on beam elements were analysed: pouring the mixture from a minor height on the previously placed layer and placing the subsequent layer on the mechanically disturbed surface of the underlying material. Different delay times were used: 15, 30, 45 and 60 min between the execution of two layers of eco-efficient SCC. The load-bearing capacity of the joint was determined using a splitting tensile strength test on cubic elements. It was observed that, regardless of the mixture and casting variant, the interlayer bond strength decreased as the delay time increased. This effect was less pronounced when the first layer was mechanically disturbed. It was also demonstrated that concrete with reduced binder content is characterized by a lower drop in bond strength between successive layers. Finally, it is noted that the current recommendations and normative guidelines for the multilayer casting of self-compacting concrete should be specified with regard to the time delay allowed for the execution of the next layer in the absence of interference with the previously placed layer. Lack of clarity in this respect may result in the creation of a cold joint and hence a reduction in the load-bearing capacity between layers.

Effect of Placement Technology on the Bond Strength Between Two Layers of Self-Compacting Concrete.

Self-compacting concrete (SCC) should generally be placed continuously, but it is not uncommon for contractors to be forced to use interruptions in concrete works due to delivery delays. The multilayer casting of SCC can cause weak bond conditions in the contact area of subsequent layers. Methods of preventing cold joint or lift line formation for normal concretes are not suitable for self-compacting concretes. This article provides research on the effect of multilayer casting technology on the bond strength between two layers of SCC. Three technological variants of connecting successive layers of SCC mixture on beam elements were analyzed: The free flow of the mixture, dropping the mixture from a greater height, and mechanical disturbance of the first layer. Three delay times were applied: 30, 45, and 60 min between two layers of SCC. In general, the research revealed that, regardless of the multilayer casting variant, the bond strength between two layers decreased as the delay time was extended. The best performance and the lowest drop in bond strength were obtained for samples with a mechanically disturbed first layer, independent of the delay time. This method gave similar results to a reference element made without a break in concreting. It was also demonstrated that current recommendations and standard guidelines for multilayer casting appear to be insufficient for ensuring an adequate bond between layers.

The Impact of Surface Preparation for Self-Compacting, High-Performance, Fiber-Reinforced Concrete Confined with CFRP Using a Cement Matrix.

With the development of concrete technology, the tendency to combine different materials with each other to achieve a greater efficiency and durability of structures can be observed. In the modern construction industry, various materials and techniques are increasingly being combined in order to achieve e.g., an increased resistance to dynamic impacts of a structure, or an increased scope of work of a selected constructional element, which translates into a significant increase in the energy of destruction. Thus, hybrid elements, known as composite ones, are created, which consist of concrete and reinforcements. This study examined the influence of the preparation of the concrete surface on the behavior of high-performance, self-compacting, fiber-reinforced concrete (HPSCFRC), reinforced with carbon fibers (CF) using a cement matrix. In the general lamination processes, this is preformed using epoxy resin. However, epoxy resin is sensitive to relatively low temperatures, and therefore the authors attempted to use a cement matrix in the lamination process. When connecting hardened concrete with a fresh concrete matrix or mixture, the type of the concrete surface is significant. In this research, three types of concrete surfaces e.g., unprepared, sanded and grinded were considered. All of the surfaces were examined using a 3D laser scanner, to determine the Abbott-Firestone profile material share curve. In this research, cylindrical concrete specimens were reinforced with one, two and three layers of laminates. They were then subjected to a uniaxial compressive test. The results of tests showed that the use of cement matrix in the lamination process, due to its low efficiency, should not be applied when reinforcing concrete elements with a high compressive strength. Moreover, the grinded surface of concrete showed the best cooperation with CF reinforcement.

Effect of Placing Self-Compacting Concrete Bottom up on the Quality of Bond Conditions.

A self-compacting concrete (SCC) mixture, due to its special rheological properties, may be placed differently than in traditional variants. The paper presents the results of a study on the effect of a bottom-up placing direction on the quality of bond conditions between steel and self-compacting concrete. Elements with overall dimensions of 160 × 480 × 1600 mm consisting of elementary samples with dimensions of the bond test basic modules were constructed. Ribbed steel rebars with diameters of 16 mm were used and located in formworks perpendicularly to the concrete placing direction. Bond tests were carried out according to the pull-out method. The bottom-up placing contributed to the uniformity of the bond conditions quality in the test elements and an overall improvement of the bond properties, especially in its top part. Given the increasing implementation of alternative methods of mix placing and the promising results, the topic should be further investigated.

Environmental performance of ordinary and new generation concrete structures-a comparative analysis.

The development of concrete technology results in a new generation of cement-based concrete such as high-performance concrete, self-compacting concrete and high-performance, self-compacting concrete. These concretes are characterised by better parameters not only in terms of strength and durability but also rheology of the mixtures. Obtaining such properties requires the adoption of a different composition and proportion of ingredients than ordinary concrete. The greater share of cement in these concretes causes an increase in the energy consumption and emissions (per unit of concrete volume) at the production stage. However, use of new generation concrete allows for a reduction of overall dimensions of a structural element, due to the increased strength parameters. Such a solution may finally result in lower consumption of resources and energy, as well as a decrease of gas emissions. The article presents the results of a comparative environmental analysis of ordinary and new generation concrete structures.