International Concrete Abstracts Portal

This CD-ROM consists of ten papers that were presented by ACI Committees 236 and 188, at the ACI Fall 2009 Convention in New Orleans, LA, in November 2009. The papers cover durability models, early age models, virtual testing and mechanical behavior models. Note: The individual papers are also available. Please click on the following link to view the papers available, or call 248.848.3800 to order. SP-266

A simulation algorithm was developed for modeling the dense packing of large assemblies of particulate materials (in the order of millions). These assemblies represent the real aggregate systems of portland cement concrete. Two variations of the algorithm are proposed: Sequential Packing Model and Particles Suspension Model. A developed multi-cell packing procedure as well as fine adjustment of the algorithm’s parameters were useful to optimize the computational resources (i.e., to realize the trade-off between the memory and packing time). Some options to speed up the algorithm and to pack very large volumes of spherical entities (up to 10 millions) are discussed. The described procedure resulted in a quick method for packing of large assemblies of particulate materials. The influence of model variables on the degree of packing and the corresponding distribution of particles was analyzed. Based on the simulation results, different particle size distributions of particulate materials are correlated to their packing degree. The developed algorithm generates and visualizes dense packings corresponding to concrete aggregates. These packings show a good agreement with the standard requirements and available research data. The results of the research can be applied to the optimal proportioning of concrete mixtures.

Stresses develop in portland cement concrete pavement at early ages when volume changes associated with hydration reactions, moisture loss, and temperature variations are restrained. Saw-cuts are placed in concrete pavements to provide a weakened plane that enables cracks to form as intended, thereby relieving developed residual stresses. Although the idea of creating a weakened plane by saw-cutting is relatively straight forward, practically determining the timing and depth of saw-cut can be complicated in field construction. This study uses a finite element model (FEMMASSE) to evaluate influence of saw-cut timing on cracking behavior of concrete pavements. The model considers the influence of ambient temperature, cooling effect of wind, and time of casting. It is shown that the saw-cutting time window was reduced as ambient temperature was increased. Higher wind speeds influence the saw-cutting time window to a lesser degree at high ambient temperatures than they do at lower ambient temperatures. It was also shown that the time of casting influences the saw-cutting time window and it needs to be considered in estimating the saw-cutting time window especially at high ambient temperatures.

The alkali aggregate reaction (AAR) is affecting numerous civil engineering structures and is responsible for unrecoverable expansion and cracking which can affect their functional capacity. In order to control the safety level and the maintenance cost of its hydraulic dams, Electricité de France (EDF) has to get a better understanding and a better prediction of the expansion phenomena. In this context, EDF is developing a numerical modelling based on the finite element method in order to assess the mechanical behavior of degraded structures. Obtaining a good prediction of expansive phenomena requires the identification and realistic modelling of the underlying physical, chemical and mechanical phenomena. The model takes into account the mechanical damage, the creep of concrete and the stress induced by the formation of AAR gel. Coupling between the different phenomena (creep, AAR and anisotropic damage) are taken into account through a rheological modelling. First , experimental results obtained on concrete cylinders and beams affected by AAR are simulated to verify whether the model can describe the behavior of degraded structures.

Early-age cracking can reduce the service life of reinforced concrete structures by providing a path for the ingress of moisture. This cracking is caused by a complex interaction among concrete material properties, construction methods, and the environment, especially during the early age curing period.  In order to prevent early age cracking, the concrete mixture and construction methods must be complementary and chosen with care. Early age concrete simulations can be used to minimize the risk of cracking by optimizing the materials and construction techniques for the local environmental conditions. These simulations are rarely performed however, because of the great expense and time needed to quantify the early age concrete mechanical properties (modulus, tensile strength, creep, coefficient of thermal expansion, etc.). Recent breakthroughs in material science and concrete technology have enabled the development of needed early-age concrete material property models.   An early age temperature development and thermal stress simulation tool named ConcreteWorks was recently completed that allows engineers and contractors to quickly optimize concrete construction with reduced laboratory testing. ConcreteWorks includes several material behavior models that were developed to eliminate the need for expensive, specialized testing. This paper presents the development of ConcreteWorks, along with examples of its application on recently completed construction projects. These case studies illustrate how materials science modeling techniques can be simplified for the end user needs.