Veerle Boel

Bond Behaviour and Shear Capacity of Self-Compacting Concrete

In this paper the bond mechanism of steel reinforcement to concrete and the shear capacity are examined. Tests have been conducted on conventional vibrated concrete (CVC) and self-compacting concrete (SCC). The results from pull-out tests on 200 mm cube specimens show that for the same compressive strength the maximum bond stress for SCC is as high or higher than for CVC and this for all tested diameters (8, 12 or 16 mm). The bond stress increases with increasing bar diameter. The specimens were loaded at constant rate and during testing the slip of the bars and the applied load were recorded. The four-point loading tests point out a slightly decreased shear capacity of SCC in respect to CVC with the same compressive strength. The shear capacity decreases with increasing shear span-to-depth ratio a/d (2 to 3) for all the tested concrete types. During the testing the maximum applied load was recorded and the crack and failure mechanism were observed.

Chloride penetration in self compacting concrete by cyclic immersion

Durability and more specifically chloride penetration, is of major importance for reinforced concrete structures. As the conception of self compacting concrete (SCC) is totally different from traditional concrete (TC), some changes in durability behaviour might occur. In order to locate and prevent possible problems, the chloride penetration in SCC has to be investigated. An extended experimental programme was set up: the chloride penetration of 16 self compacting concrete mixtures and 4 traditional concrete mixtures was determined. The test is performed on cylindrical specimens with a diameter of 230mm and a height of 70mm. These specimens are alternately immersed in a solution containing chlorides and exposed to air. One cycle takes approximately I hour. After 6, 12, 18, 24, 30 and 36 weeks the specimens are broken and the penetration depth is determined. For the self compacting concrete, four types of cement and three types of filler (fly ash and two types of limestone filler with a different grading curve) are used and the influence of the amount of powder, water, the water/cement ratio and the cement/powder ratio is studied.

Chloride penetration and carbonation in self-compacting concrete

In this research program, both the steady-state and the non-steady-state migration test are used to determine the chloride diffusion coefficient D (m2/s) of 8 different self-compacting concrete mixes and 1 reference, traditionally vibrated, concrete mix. Simultaneously, the carbonation behaviour of those mixes was investigated. Here fore, the carbonation depth was measured at regular points in time according to NT BUILD 357 (1989) and a carbonation constant A (mm/√year) was deduced. Concerning the chloride diffusion coefficient, test results revealed that the determination of the steady-state migration coefficient according to NT BUILD 355 (1997) is far from easy and question marks could be placed beside the corresponding diffusion coefficient but an explanation for the observed anomalies could not be found yet. The non-steady-state diffusion coefficient according to NT BUILD 492 (1999) was used in order to rank the different concrete mixtures. The carbonation constant could best be measured using an inflated CO2-concentration, resulting in a more linear behaviour of the carbonation depth in function of time. Besides, using this carbonation constant, results reveal that the concrete mixtures could be ranked in the same way as they were by the non-steady-state chloride diffusion coefficient.

Selecting a Suitable Specimen Shape with Low Constraint Value for Determination of Fracture Parameters of Cementitious Composites

The stress intensity factor and the T-stress describing the near-crack-tip fields for selected specimen shapes of a test geometry based on wedge splitting and three point bending tests with several variants of boundary conditions are computed using finite element software ANSYS. The test configuration in question is expected to be a convenient alternative to classical fracture tests (especially the tensile ones) for investigation of the quasi-brittle fracture of building materials, when low constraint is requested. These specimens are investigated within the framework of two-parameter fracture mechanics; near-crack-tip stress field parameters are determined and compared with those of the wedge splitting test due to their shape similarity. The sensitivity of the values of these parameters to the boundary conditions is also shown. Suitable choice of the shape of the specimens is discussed.

Vibrated Concrete vs. Self-Compacting Concrete: Comparison of Fracture Mechanics Properties

This study focuses on the fracture mechanics aspect of self-compacting concrete, compared to vibrated concrete. The most commonly used experiments to investigate the toughness and cracking behaviour of concrete are the three-point bending test (3PBT) on small, notched beams, and the wedge-splitting test (WST) on cubic samples with guiding groove and starter notch. From the resulting P-CMOD curves (applied load versus crack mouth opening displacement), different fracture parameters, such as fracture energy and fracture toughness, can be extracted. Moreover, using inverse analysis, the σ-w relationship (tensile stress versus crack width) can be derived. This paper lists the results of a series of tests on samples, made of VC, SCC of equal strength, and SCC with identical w/c factor. Subsequently, a comparison of the mechanical characteristics is made, revealing important differences regarding several fracture parameters.

Experimental Investigation of the Influence of the Bond Conditions on the Shear Bond Strength between Steel and Self-Compacting Concrete Using Push-Out Tests

Steel-concrete joints are often provided with welded shear studs. However, stress concentrations are induced in the structure due to the welding. Moreover, a reduction in toughness and ductility of the steel and a decreased fatigue endurance of the construction is observed. In this paper the shear bond strength between steel and ultra-high performance concrete (UHPC) without mechanical shear connectors is evaluated through push-out tests. The test samples consist of two sandblasted steel plates with a thickness of 10 mm and a concrete core. The connection between steel and concrete is obtained by a 2-component epoxy resin. Test samples with a smooth adhesive layer are compared with those with an epoxy layer, which is applied with a toothed paddle and/or gritted with small aggregates. In this research, specimens prepared with river gravel, crushed stone, and steel grit are compared and also two different epoxy resins are used. During the tests, the ultimate shear force is recorded as well as the slip between steel and concrete. All test specimens exhibited a concrete-adhesive or concrete failure. Furthermore, test results show that the use of a more fluid epoxy resin improves the anchorage of the gritted aggregates in the adhesive layer, resulting in higher shear bond stresses. No significant difference is found between specimens, gritted with river gravel or crushed stone. Applying the adhesive layer with the toothed paddle in horizontal direction slightly improves the bond behaviour. Finally, the experimental results of the test members with a smooth epoxy layer without gritted aggregates, provide test data for a fracture mechanics approach, which uses a 2D numerical model of the test specimen, composed of steel, epoxy resin, and concrete.

Crack Width Analysis of Reinforced Self-Compacting Concrete Beams

Self compacting concrete is a concrete mixture specifically designed not to require external energy for compaction. This property results in many advantages for precast as well as ready-mix concrete applications. Especially, dense reinforcements or slender elements can be achieved. However, in current design codes this concrete is treated as traditional concrete although the mix composition is substantially different. Due to a decrease in coarse aggregates, combined with a higher amount of chemical and mineral admixtures, the overall mechanical behavior may differ from that of traditional concrete even when the compressive strength of both mixtures are equal. This is especially visible in the crack formation in the tensile zone of concrete beams. This paper presents results of an analysis of crack formation, distribution and width on reinforced concrete beams with varying reinforcement ratios. Differences in crack properties, favoring self-compacted over traditional concrete are found for all considered reinforcement ratios, although the results are less pronounced for the higher ratios. The results may allow a favorable serviceability limit state criteria for this material.

Design of Cellular Materials and Mesostructures with Improved Structural and Thermal Performances

Honeycomb mesostructures and other types of cellular material such as wood, coral, and cancellous bone have properties that make them suitable for use in many structural engineering applications. This includes not only superior mechanical behavior and lightweight high-strength characteristics, but also better thermal conductivity, electrical resistivity, etc. A good understanding of these structures and materials can help structural engineers to design lightweight building systems with, for example, improved stability combined with improved thermal and acoustic insulation. Moreover, advances in additive manufacturing methods capable of producing these cellular structures also add to the motivation. In this paper, the application of cellular materials for construction applications in the building sector is illustrated and FEM analysis is used to examine several types of mesostructures. Furthermore, we focus on optimizing the equivalent thermal conductivity and stiffness of structures with a relative density of 0.5. Special attention is given to situations for which the required properties are not necessarily equal in all directions. Results show that using multidisciplinary topology optimization methods, the structural and thermal performances of these structures can be efficiently optimized.

Comparative Study on the Fatigue Behaviour of SCC and VC

Continuous cyclic loading on concrete constructions involves a progressive cracking mechanism, leading to significant changes of the material properties during the lifetime of the structure. Gradually, irreversible damage is inflicted and the carrying capacity is affected, which may cause structural collapse at a stress or strain level much lower than in case of a single static load. This so-called fatigue phenomenon is well-documented in literature for traditional, vibrated concrete (VC), but this is not the case for self-compacting concrete (SCC). Given the fact that this latter concrete type is already used worldwide in many types of structures, including cyclically loaded ones, a good knowledge and understanding of the static and fatigue material behaviour is crucial. Up till now, it is unsure whether SCC performs better, worse, or equally under fatigue loading conditions. Therefore, in this study, destructive four-point bending tests are performed on large beams, made from VC and SCC, both statically and cyclically (at different loading rates). A comparison of the deflection, strain, crack pattern and crack width evolution of the different concrete types is made. The results reveal some significant differences regarding concrete strain and crack width development during the cyclic tests.

Chapter 2 - Mechanical properties

The State-of-the-Art Report of RILEM Technical Committee 228-MPS on Mechanical properties of Self-Compacting Concrete (SCC) summarizes and extensive body of information related to mechanical properties and mechanical behaviour of SCC. Due attention is given to the fact that the composition of SCC varies significantly. A wide range of mechanical properties are considered, including compressive strength, stress-strain relationship, tensile and flexural strengths, modulus of elasticity, shear strength, effect of elevated temperature, such as fire spalling and residual properties after fire, in-situ properties, creep, shrinkage, bond properties, and structural behaviour. A chapter on fibre-reinforced SCC is included, as well as a chapter on specialty SCC, such as light-weight SCC, heavy-weight SCC, preplaced aggregate SCC, special fibre reinforced SCC, and underwater concrete.