Towards tailored cement-based materials with ground calcium carbonates

Proportioning the dosage of Ground Calcium Carbonates (GCC) in cementitious materials, beyond current normative levels, is one of the most promising ways towards sustainability of mortar and concrete technology. Performance parameters such as the water/binder ratio do not represent the very significant benefits in terms of mechanical performances, of clinker replacement by GCC in mixes with water/cement ratios in the range of that for normal concretes used in construction (0.4 to 0.6). Alternative parameters such as the water/fines ratio proved to be reliable indicator of performances for Ultra High-Performance Fiber Reinforced Concretes. This concept needs to be further extended to normal concretes, on a scientific basis. With this aim in view, mortar mixes with a similar water/cement ratio of 0.5 and progressive replacement of sand by GCC with water/fines ratios as low as 0.2 were studied. The packing density of the cement and GCC was determined by means of wet packing measurements using the mixing power method. The Compressible Interaction Packing Model from Fennis was generalized to multiple polydisperse components and used to predict the packing density of the mixes. The rheology at fresh state of the mixes was put into perspective with the packing density, water film thickness and sand packing factor to define reliable indicators of performances at fresh states. Finally, the compressive strength of the mixes at 1 and 28 days was related to the efficiency coefficient of the GCC used.

Monitoring and modelling of construction materials during hardening

Marco Viviani

During the past decade, the science of concrete behavior has made much progress and many useful results are now incorporated in technical codes such as those governing mix design, quality of mix and post pouring verification. The stage of Very Early Age (VEA) concrete begins with pouring and ends with hardening. Behavior at this stage influences safety and durability of structures. Concrete is a cast-in-place hardening material. Other hardening materials, such as epoxy mortars have recently raised interest. Techniques used for concrete can potentially be applied to these new composite materials. The overall objective of this thesis is to develop a methodology for predicting mechanical properties and for separating important phenomenon in hardening materials, such as commercial concrete and mortar, to be ultimately applied in real time and in situ. A literature survey has shown a lack of knowledge in the areas of measurement techniques at VEA, prediction of compressive strength, separation of the autogenous deformation and values for the thermal expansion coefficient. This lack of knowledge is especially clear for cement-based materials hardening at varying temperatures. Few VEA data are available and few tools are capable of monitoring VEA parameters. A new device has been constructed to monitor deformations and help identify phenomenological contributions to the total deformation. The design of this device takes into account aspects such as usability in the field and reliability in practice. More than 80 embedded fiber optic sensors and 100 specimens were used for development and testing. Several special sensors have been designed and tested so that their features are adapted to the characteristics of the hardening material at VEA. An important part of this work has been the development of procedures where VEA data are interpreted and reduced in order to determine the parameters that describe the evolution of the degree of reaction, as well as mechanical properties, and allow for the decoupling of values for the total deformation into parts that are related to different physical phenomena. Concluding, this methodology allows accurate prediction of the compressive strength evolution in term of degree of reaction using VEA data carried out within the first 72 hours of the hardening material life. The methodology also leads to straightforward determination of values for the thermal expansion coefficient and the separation of values for autogenous and drying deformation. Furthermore, pilot testing on epoxy mortar demonstrates that the methodology is not limited to cement-based materials Finally, the results of this thesis have the potential to make further contributions to scientific research and to strategies for quality control and safety during construction.

EPFL2005

New Connection System for Confined Concrete Columns and Beams. II: Theoretical Modeling

The concept of confined concrete has been widely accepted throughout the structural engineering field, such as the spiral, steel tube, or carbon fiber composite shell confined concrete, etc. In the present study, this concept has not only been used in the structural columns, but also in joints. In this connection system, the steel tube is interrupted at the floor, the concrete in the connection zone is confined by a stiffening ring with multiple lateral hoops, and the reinforced concrete beams are continuously arranged through the joint. The structural behavior of this new connection system in axial compression tests and reversed cyclic loading tests is presented in a companion paper. In the present study, the bearing strength of composite columns and the joint in axial compression is obtained based on the stress and strain analyses. The critical volume fraction of transverse stirrups of the composite column and the effective confining radius in the stiffening ring are proposed based on the thick cylinder model with the assumption of plane stress state. By using these solutions, it is favorable to obtain the stress distribution in confining concrete and the bearing strength of the confined concrete. For the reversed cyclic loading tests, the Clough hysteresis model is used to simulate the hysteresis loops of the specimens without considering the stiffness degradation in the unloading process. The results of the theoretical modeling are generally in good agreement with the experimental observations.

2008

A meso-mechanical model for concrete under dynamic tensile and compressive loading

Fabrice Gatuingt, Jean-François Molinari, Leonardo Snozzi

We present a computational model, which combines interface debonding and frictional contact, in order to investigate the response of concrete specimens subjected to dynamic tensile and compressive loading. Concrete is modeled using a meso-mechanical approach in which aggregates and mortar are represented explicitly, thus allowing all material parameters to be physically identified. The material phases are considered to behave elastically, while initiation, coalescence and propagation of cracks are modeled by dynamically inserted cohesive elements. The impenetrability condition is enforced by a contact algorithm that resorts to the classical law of Coulomb friction. We show that the proposed model is able to capture the general increase in strength with increasing rate of loading and the tension/compression asymmetry. Moreover, we simulate compression with lateral confinement showing that the model reproduces the increase in peak strength with increasing confinement level. We also quantify the increase in the ratio between dissipated frictional energy and dissipated fracture energy as the confining pressure is augmented. Our results demonstrate the fundamental importance of capturing frictional mechanisms, which appear to dissipate a similar amount of energy when compared to cracking under compressive loading.

Springer Verlag2012

Monitoring and modelling of construction materials during hardening

Marco Viviani

EPFL2005