Shrinkage of self-compacting concrete. A comparative analysis

Self-compacting concrete (SCC) is a concrete type that does not require vibration for placing and compacting. SCC possesses special technical features and properties that recommend its application in many jobs. Nevertheless, in some situations, it has been observed an inadequate behaviour of the material at early ages due to shrinkage. The existing shrinkage prediction models were developed for standard concrete. In this paper three SCC mixtures, with different compressive strength, are studied in terms of autogenous and total shrinkage. The results are compared with the Eurocode 2 model. For the studied mixtures it was found that this model underestimates the autogenous shrinkage, while the total shrinkage is generally overestimated.Fundacao para a Ciencia e a Tecnologia (FCT), Portugal [UID/MULTI/00308/2013]info:eu-repo/semantics/publishedVersio

Optimization of the MgO-SiO₂ binding system for fiber-cement production with cellulosic reinforcing elements

A range of MgO and SiO2 blends mixed with water are analyzed to develop clinker-free fiber-cement products reinforced with cellulosic fibers. The target is the development of a binder which is not chemically aggressive to the fibers, but which develops high mechanical strength Mechanical performance of the materials developed is not only influenced by magnesium silicate hydrate (M-S-H) gel content, but is more related to the void content within the paste due to unreacted water, meaning that the gel-space ratio concept is valuable in describing the compressive strengths of these materials. A higher MgO content in the mix formulation leads to M-S-H gels with increased Mg/Si ratio. The Mg/Si ratio also increases over time for each mix, indicated by changes in the gel structure as reaction is not yet complete after 28 days. SEM shows a heterogeneous microstructure which also has regions of high Si content. The 60 wt%MgO-40 wt%SiO2 system is chosen as the optimal formulation since it is the least alkaline binder with high mechanical strength. Bending tests on pastes reinforced with cellulosic pulps prove the efficiency of this binder, which preserves the reinforcing capacity of the fibers much better than Portland cement pastes after 200 cycles of accelerated ageing

Impregnation and encapsulation of lightweight aggregates for self-healing concrete

This study investigated a technique of impregnating potential self-healing agents into lightweight aggregates (LWA) and the self-healing performance of concrete mixed with the impregnated LWA. Lightweight aggregates with a diameter range of 4–8 mm were impregnated with a sodium silicate solution as a potential self-healing agent. Concrete specimens containing the impregnated LWA and control specimens were pre-cracked up to 300 μm crack width at 7 days. Flexural strength recovery and reduction in water sorptivity were examined. After 28 days healing in water, the specimens containing the impregnated LWA showed ∼80% recovery of the pre-cracking strength, which accounts more than five times of the control specimens’ recovery. The capillary water absorption was also significantly improved; the specimens healed with the impregnated LWA showed a 50% reduction in the sorptivity index compared with the control cracked specimens and a very similar response to the control uncracked specimens. The contribution of sodium silicate in producing more calcium silicate hydrate gel was confirmed by characterisation the healing products using X-ray diffraction, Fourier transform spectroscopy, and scanning electron microscopy.Yousef Jameel Foundation through Cambridge Commonwealth, European & International Trust, Engineering and Physical Sciences Research Council (Project Ref. EP/K026631/1 – ‘‘Materials for Life”

Evaluation and Observation of Autogenous Healing Ability of Bond Cracks along Rebar

Micro cracks occurring in concrete around tensile rebar is well known latent damage phenomenon. These micro cracks develop, and can be detected after reaching the surface of the concrete. Detection of these cracks before they are fully formed is preferable, but observing the whole crack structure is difficult. Another problem is repairing micro cracks under the concrete surface. The autogenous ability of bond cracks along rebar was evaluated using the air permeability test. Air permeability coefficients were measured before and after tensile loading, and experimental air permeability coefficients became larger near cracks along rebar as a result of tensile loading. Recuring for 28 days after tensile loading made the air permeability coefficients smaller, but this restriction only occurred during water recuring. Observation of crack patterns helped the understanding of change in the air permeability coefficients. Several small cracks along rebar were observed after tensile loading, and most cracks along rebar were not found after water recuring. On the other hand, the crack pattern did not change after air recuring. These results indicate that bond cracks along rebar can be closed by autogenous healing, and cause the air permeability coefficients

INFLUENCE OF FLY ASH REPLACEMENT RATIO ON COMPOSITIONS OF C-S-H GEL

ABSTRACT This study reports the effect of fly ash replacement ratio on composition of C-S-H gel. The compositions of C-S-H gel were estimated by combination of Rietveld/XRD analysis, selective dissolution and ignition loss. Experiment results confirm the decreasing of CaO/SiO 2 ratio as the increasing replacement ratio of fly ash. Bonding water in hydrated gel decreased as CaO/SiO 2 ratio increased. Such changing of compositions of C-S-H gel affected its density