1,721,930 research outputs found
Damage to concrete structures
No full textSerious degratadtion mechanisms can severely reduce the service lif of concrete structures : steel reinforcement can corrode, cement matrix can be attacked, and even aggregates can show detrimental processes. Therefore, it is important to understand how damage can occur to concrete structures and to appreciate the time of the actions leasing to damage. Damage to Concrete Structures summarizes the state-of-the-art information on the degradation of concrete structures, and gives a clear comprehensive overveiw of what can go wrong. Offering a logical flow, the chapters are ordered according to the chronological timing of the actions leading to concrete damage. The author explains the different actions or mechanisms in a fundamental manner, without too many physical details, to provide greater clarity and readability. The book describes the different causes of damage to concrete, including inappropriate design, errors during execution, mechanisms occurring during hardening of concrete, and actions or degradation mechanisms during service life (hardened concrete). The degradation mechanisms are illustrated with numerous real-world examples and many drawings and photographs taken of actual structures. Written as a textbook for students as well as a reference for professionals, this easy-to-comprehend book gives readers a deeper understanding of the damage that can occur to concrete during the construction process and service
Modelling of chloride penetration in concrete with artificial cracks
No full textThe durability of concrete is a very important issue in actual concrete research. The service life of concrete structures can severely be reduced by the influence of cracks, caused by early-age thermal cracking due to heat of hydration, or cracking due to shrinkage, impact or overloading in combination with subsequent acceleration of degradation owing to, for example, chloride penetration.
The Magnel Laboratory for Concrete Research, Ghent University, Belgium and the "Politechnica" University of Timisoara, Romania are working on a bilateral project concerning the numerical simulation of the influence of crack formation on chloride penetration in reinforced concrete. In a first step, the chloride penetration in unreinforced concrete specimens with artificial cracks is considered.
In order to simulate cracked concrete, specimens with artificial cracks were manufactured using thin copper plates with a thickness of 0.2 mm or 0.3 mm. These copper plates were positioned inside the fresh concrete for a depth of maximum 20 mm. The chloride penetration into the concrete was realised with the non-steady state migration test developed by Tang (1996). Numerical simulation, based on finite element method, was carried out, leading to a good agreement between the measured and the numerical predicted chloride penetration results
Summary and conclusions
No full textThe State-of-the Art Report of RILEM Technical Committee 228-MPS on Mechanical Properties of Self-Compacting Concrete gathers available information related to mechanical properties and mechanical behaviour of SCC. Due attention is given to the fact that the composition of SCC might be significantly different in different regions. Furthermore, it is not the intention to review the mechanical behaviour of conventional or vibrated concrete. The committee focused on specific mechanical aspects related to self-compacting concrete. All relevant mechanical issues are considered, including compressive strength, stress-strain relationship, tensile and flexural strength, modulus of elasticity, shear strength, effect of elevated temperature, in situ properties, creep, shrinkage, bond properties, and structural behaviour. A chapter on fiber reinforced SCC is included, as well as a chapter on special SCC (light-weight SCC, heavy-weight SCC, preplaced aggregate SCC, special fiber reinforced SCC, and underwater concrete)
Modeling the microstructure change of high performance cement paste at elevated temperatures
No full textIt is commonly understood that the microstructure of cement paste controles the water expulsion from the concrete at high temperature. The pore structure at high temperature has a considerable influence on the spalling behavior. The accumulatieve pore pressure mechanism has become a widely accepted explanation for the explosive spalling in concrete structures. Modeling the physical-chemical changes induced microstructure or micro-cracking evolution of concrete during fire is of great important for engineering practice.
In this paper, the influence of the physicochemical change of paste on the microstructure and on explosive spalling is studied qualitatively and quantitatively. In order to examine the consequence of dehydration or decomposition on the various properties of microstructure, a series of experiments were performed, which include thermal analysis, scanning electron microscopy and mercury intrusion porosimetry on different mixes at high temperatures. A numerical model to describe the change of the microstructure of high performance cement paste during heating is proposed. In the model, the consequences of dehydration or decomposition on the various properties of microstructure are modelled. The simulated degradation of materials with heating is validated and calibrated by experiments. The developed model is applied to investigate the influence of some factors such as the degree of hydration, the water/cement ratio and the limestone contents in high performance concrete. This research shows that computational material science is an effective method to provide insights about material behaviour
Model for heat release of fly ash - Cement pastes
No full textIn the past, isothermal calorimetry has been proposed as an experimental method to predict the heat development of a Portland cement paste using a reaction degree and an apparent activation energy. Herein, it is assumed that the normalised rate of the reaction only depends on the over-all reaction degree of all clinker minerals together. The influence of the curing temperature is modelled with an Arrheniusfunction. Fly ash can interact with the hydration reaction of one or more clinker minerals : some of these reactions will be accelerated through its presence, while others will be delayed. Depending on the types of fly ash and cement, this can lead to a change in the slope of the isothermal heat rate and even to an additonal peak. On the derivative curve of the heat rate over time, up to six inflexion points can be observed. Based upon this, the proposal of considering the heat rate as a superposition of several sub-reactions is investigated. Further research is however needed to investigate whether these reactions can be correlated to real chemical reactions
Self-desiccation and self-desiccation shrinkage of silica fume-cement pastes
Full text linkSelf-desiccation is one common phenomenon of high-performance cementitious materials, which are
characterized by low water/binder (w/b) ratio and high mineral admixture incorporation. As a
consequence, large magnitude of self-desiccation shrinkage, a key factor which influences the
cracking behavior of concrete, develops rapidly in the cement matrix due to the internal relative
humidity (RH) decrease and capillary pressure induced by self-desiccation.
The objective of this study is to evaluate the behavior of self-desiccation and self-desiccation
shrinkage in silica fume (SF) blended cement pasts with low w/b ratio of 0.25. The self-desiccation
process was revealed by the measurement of internal RH of the sealed cement pastes with
conventional method of hygrometer. The shrinkage of the sealed cement pastes was measured by the
corrugated tube method, permitting measurements to start at early age.
Experimental results revealed that SF blending leads to a higher internal RH, indicating slower
self-desiccation process, compared with pure cement paste. Consequently, less self-desiccation
shrinkage was observed in SF blended cement pastes than that in pure cement paste
Introduction and glossary
No full textSelf-compacting concrete (SCC), also known as self-consolidating concrete, was developed in the late 1980's, although earlier "look-alikes" surely exist, though not defined as such. In comparison with conventional concrete, referred to in this report as vibrated concrete (VC), SCC can be considered on the one hand as a new type of high-performance material of a different approach to mix design and rheological characteristics. On the other hand, SCC can be seen as a new approach to casting concrete enabled by adjusted fresh concrete properties. In reality, SCC is a combination of both approaches that has enabled to push the boundaries of concrete technology to a new area. Several methods exists for the mix design of SCC. In various parts of the world, different concepts might be followed for the proportioning of SCC and are referred to as "powder-type SCC", "VMA-type SCC", or "mixed-type SCC". This makes it complicated to present general conclusions and recommendations concerning the mix design and specific characteristics of SCC. Since the introduction of "modern" SCC, RILEM has been very active in producing state-of-the-art reports related to this innovative class of high-performance cementitious material. At first, focus was aimed at describing the mix design and workability of SCC then on mixing and casting of SCC. Later on, other aspects of SCC were targeted, including durability. Also, the American Concrete Institute (ACI) Committee 237 "SCC" is working on producing a comprehensive state-of-the-art report dealing with all aspects of SCC
- …
