International Concrete Abstracts Portal

This ACI Special Publication (ACI SP-253) CD-ROM contains 25 papers from the Fifth ACI/CANMET/IBRACON International Conference on High-Performance Concrete Structures and Materials that was held in Manaus, Amazon State (AM), Brazil, June 18-20, 2008. Topics include Durability, Self-Compactability, Curing, Retarders, and Abrasion Resistance. The Fifth Conference is a result of the collaboration of ACI and CANMET along with Sao Paulo University, Federal University of Goias, Amazon State University, and Brazilian Concrete Institute (IBRACON).

The shape and texture of coarse aggregate affect the properties of fresh and hardened concrete. However, there is limited information about the effects of aggregate characteristics on self-compactability, which can be relevant in countries like Argentina, where a large variety of aggregates is available even in the same region. This paper analyzes the effect of shape and texture of coarse aggregate on self-compacting concrete (SCC) properties. Three different types of coarse aggregate were selected: granitic crushed stone, quartzitic crushed stone, and a siliceous river gravel. The granitic crushed stone presents irregular shape, rough texture, and low absorption. The quartzitic crushed stone also has irregular shape and rough texture, but it has high water absorption. Finally, the natural siliceous river gravel is composed by strong particles with smooth surface and low porosity. SCC was prepared using each type of coarse aggregate with 19 mm maximum size and similar particle size distribution. Slump-flow, V-funnel, and J-Ring tests were carried out. Effects of the changing volume of batched and mixing energy were also studied. The homogeneity of the coarse aggregate distribution was evaluated along vertical cuts on 1 m in height tubes and along a U-tube with 1.80 m height, measuring the ultrasonic pulse velocity and the unit weight in different sections. In addition, the compressive strength and modulus of elasticity were determined. From the results, was concluded that, as expected, the shape and texture of coarse aggregate modify the flowability, but also the mixing energy can strongly affect the plastic viscosity of SCC.

The development of a program for the design and construction of a facility for the final disposal of intermediate-level radioactive wastes in the Argentine Republic is responsibility of the Atomic Energy Commission (CNEA). The proposed model is the near-surface monolithic repository similar to that in operation in El Cabril, Spain. The design of this type of repository is based on the use of multiple, independent, and redundant barriers. Since the vault and cover are made of reinforced concrete and they are major components of the engineered barriers, the durability of these concrete structures is an important aspect for the facilities’ integrity. This work presents a laboratory and field investigation performed during the last 6 years on reinforced concrete specimens made with high-performance concrete, in order to predict the service life of the intermediate-level radioactive waste disposal vaults from data obtained from electrochemical techniques. On the other hand, the development of corrosion sensors that allow on-line measurements of reinforcing steel corrosion potential and corrosion current density, incoming oxygen flow that reaches the metal surface, concrete electrical resistivity, chloride concentration, and internal concrete temperature is shown. These sensors, properly embedded in a prototype of the vault, allow the monitoring of the corrosion process of the reinforcing steel embedded in the structure. All the information obtained from the sensors is being used for the final design of the container to achieve a service life greater than the foreseen durability for this type of facilities (more than 300 years).

The heat of hydration in massive concrete structures can raise the temperature to a level where thermal cracks can pose a problem. Hardening retarders are admixtures that lower the rate of hydration, distribute the heat release over time, and lower maximum temperature in concrete. Such admixtures will inherently lead to lower early strength, but should lead to comparable 28-day strength to reference concrete. Relative large amounts of urea works, in particular when the additional retardation of setting (not hardening) is counteracted by the set accelerator calcium nitrate. However, these dosages are high, and urea will also slowly decompose to ammonia that may limit the urea application to outdoor use, if any. The latest potential admixtures are combinations of minor amounts of strong setting retarders like organic acids (0.1-0.3 %) with the setting accelerator calcium nitrate (1-3%), where a true synergy between the two leads to hardening retardation.

Concrete curing is an essential stage in construction in order to obtain an adequate degree of hydration of cement. Frequently, this step is not carried out correctly, but an adequate structure performance depends to a great extent on curing. Nowadays, blended cement concrete (BCC) is used widely in Argentina, despite the lack of long-standing BCC structures constructed to demonstrate adequate servicelife performance. In this paper, the modification of BCC pore structure due to curing treatment is analyzed by evaluation of transport properties in concretes with different w/c and maximum coarse aggregate size. To simulate diverse weather conditions to which the same concrete type may be exposed according to geographical location, different curing conditions were applied during 28 consecutive days. Transport properties evaluated were water permeability under pressure, capillary suction, and chloride ingress rate. Total porosity and 24-h water absorption were also determined. According to the results obtained, a notable influence of curing conditions on the pore structure of BCC was observed, emphasizing the importance of a proper curing on the service life of BCC structures.