Investigation on the mechanisms govergning the robustness of self-compacting concrete at paste level

In spite of the many advantages, the use of self-compacting concrete (SCC) is currently widely limited to application in precast factories and situations in which external vibration would cause large difficulties. One of the main limitations is the higher sensitivity to small variations in mix proportions, material characteristics and procedures, also referred to as the lower robustness of SCC compared to vibrated concrete. This paper investigates the mechanisms governing the robustness at paste level. Phenomenological aspects are examined for a series of paste mixtures varying in water film thickness and superplasticizer-to-powder ratio. The impact of small variations in the water content on the early-age structural buildup and the robustness of the paste rheology is investigated using rotational and oscillating rheometry

Shape factors of self-compacting concrete specimens subjected to uniaxial loading

One of the problems encountered when comparing the mechanical properties of self-compacting concrete (SCC) is the use of different specimen sizes all over the world. For vibrated concrete (VC), conversion factors are defined to convert the obtained compressive strength on one specimen type to another. In order to investigate the applicability of these factors for SCC, a total of 2 VC and 10 SCC mixtures were selected varying in cement type, cement content, water-to-cement ratio and water-to-powder ratio. Beside cubes with sides of 100 mm, 150 mm and 200 mm, cylinders with a diameter of 100 mm and diameter 150 mm were cast and cores with a diameter of 100 mm, 80 mm and 50 mm were drilled. A significant difference of about 10% in the shape factors between SCC and VC has been found. Mix design parameters, such as the fraction of powder, the cement-to-powder ratio and the water-to-cement ratio, seem to have little influence on the obtained shape factors

The positive and negative influences of VMA's on the robustness of fresh self-compacting concrete

Over time, several mix design metliods liave been developed to obtain a selfcompacting concrete (SCC) with suitable fresh and hardened concrete properties. The very fluid concrete with no need for external compaction is achieved by using a higher powder content and the use of chemical admixtures. Segregation and bleeding are prevented by either an appropriate yield stress, a high plastic viscosity, or intermediate values of both (assuming the Bingham model). To avoid rejection of batches, it is important for an SCC to have a sufficient robustness: the ability to withstand small variations in the mix proportions, material properties, and methods of casting. This experimental study examined the effect of two VMA's with different working mechanisms on the robustness of SCC mixtures. The influence of attapulgite clay, diutan gum, and no VMA addition on the robustness of the slump flow, V-funnel time, sieve stability, and rheological parameters of a SCC mixture stabilized by a relatively high yield stress and a SCC mixture stabilized by a high plastic viscosity is studied in this experimental program.Structural EngineeringCivil Engineering and Geoscience

Influence of the composition of powder-type SCC on conversion factors for compressive strength

Depending on practical considerations and standards, different types of test specimens are used for the determination of the uniaxial compressive strength of concrete. Appropriate conversion factors, also referred to as shape factors, are needed to convert the compressive strength as determined on one type of specimen to the equivalent compressive strength of another type of specimen. As such shape factors for vibrated concrete are influenced by the composition of the concrete, questions arise concerning the factors that affect the shape factors for powder-type self-compacting concrete (SCC). This paper describes the findings of an extensive research programme performed by three laboratories on the influencing parameters of the shape factors for SCC. Concrete composition parameters such as powder content, cement/powder ratio, water/cement ratio and the type of coarse aggregates, cement, filler and plasticiser appear to have little effect on the experimentally obtained shape factors. Any possible influence on compressive strength was overshadowed by experimental scatter

The combined influence of paste volume and volumetric water-to- powder ratio on robustness of fresh self-compacting concrete

In order to avoid durability problems caused by an inadequate consolidation of concrete, self-compacting concrete (SCC) has been developed. The mix design of SCC aims at balancing a minimum flowability allowing air bubbles to escape and a maximum flowability in order to avoid segregation. Because of the higher demands on mix design and additional requirements related to casting, SCC mixtures are in general more sensitive to small variations in its mix composition compared to conventional vibrated concrete. Besides improving the robustness of SCC with admixtures like Viscosity-Modifying Agents (VMAs), it is also important to find out why certain mixtures are more robust than others. This paper investigates the influence of the paste volume and the waterto- powder ratio (volumetric) on the robustness of fresh SCC mixtures. Nine SCC mixtures with a paste volume of 350, 375, and 400 l/m^ and a volumetric water-to-powder ratio of 0.75, 0.90, and 1.05 were subjected to a variation of ±8 l/m^ water. The robustness of the produced mixtures was quantified measuring the slump flow, V-funnel time, L-box ratio, and sieve stability.Structural EngineeringCivil Engineering and Geoscience

Conversion factors for the compressive strength of Self Compacting Concrete (SCC)

Different shapes and dimensions of test specimens are used for the determination of the uniaxial compressive strength of concrete. In order to compare strength results conversion formulas are needed. Those formulas are well-known for vibrated concrete (VC), whereas this is not the case for self-compacting concrete (SCC). In the paper, strength ratios of SCC are compared to those for VC as suggested by different standards. In total 24 powder-type SCC mixtures and 2 VC mixtures have been prepared and studied in order to analyze the effect on the strength ratio of several parameters, including the type of cement. The influence of the water-to-cement ratio (W/C), the cement-to-powder ratio (C/P) and the powder content (P) is investigated as well. The same mix procedure is applied in the three laboratories. In the mixtures SCC P520, SCC P560, SCC ref and SCC P640, the powder content varies, while C/P and W/C are kept constant. The effect is studied by replacing gravel by crushed limestone or crushed porphyry. The influence of the water-to-cement ratio, the cement-to-powder ratio, the powder content, the type of cement, the filler, the superplasticizer and the aggregate, on the compressive strength conversion factors seems to be insignificant or too scattered to be considered

Investigation on the Mechanisms Governing the Robustness of Self-Compacting Concrete at Paste Level

In spite of the many advantages, the use of self-compacting concrete(SCC) is currently widely limited to application in precast factories and sihiationsin which external vibration would cause large difficulties. One of the mainlimitations is the higher sensitivity to small variations in mix proportions, material characteristics and procedures, also referred to as the lower robustness of SCC compared to vibrated concrete. This paper investigates the mechanisms governing the robustness at paste level. Phenomenological aspects are examined for a series of paste mixtures varying in water film thickness and superplasticizer-to-powder ratio. The impact of small variations in the water content on the early-age structural buildup and the robustness of the paste rheology is investigated using rotational and oscillating rheometiy.Steel & Composite Structure

Modification of existing shape factor models for self-compacting concrete strength by means of Bayesian updating techniques

In this paper, the existing formulas for the shape factors for uniaxial compressive strength of vibrated concrete, in which the influence of the shape, size and damage due to drilling has been included, are adapted for powder-type self-compacting concrete. An extensive test program was executed and Bayesian updating techniques were applied in order to calculate posterior distributions of the parameters involved in the conversion formulas for shape factors. Using this method, differences between the shape factor formulas of vibrated and self-compacting concrete could be analysed despite the large variability in the experimentally obtained test results. The methodology developed in this paper for the Bayesian updating of shape factor formulas can also be applied to other material properties of self-compacting concrete and even other materials, especially in cases where a limited number of samples are available or in cases where a considerable variability in the test data is observed