International Concrete Abstracts Portal

This CD consists of 14 papers presented at the ACI Fall Convention, Toronto, Canada, October 2012, and sponsored by ACI Committees 130, Sustainability of Concrete; 213, Lightweight Aggregate and Concrete; and 231, Concrete Properties at Early Ages.These papers cover the following general topics: impact on sustainability, mixture proportioning, internal curing methods and their implementation, hydration impacts, volume change effects, mechanical properties, cracking tendency, durability aspects, life-cycle cost analysis, and case studies that document the use of internal curing in full-scale production applications. 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-290

Over the last fifteen years there has been growing interest in using internally cured concrete. While the original intention of using internal curing was to reduce autogenous shrinkage, it has been observed that the internally cured concretes have additional benefits. For example, previous research has shown that internally cured concrete has lower water absorption than comparable conventional (plain) concrete mixtures. This paper presents results of chloride transport experiments performed using a conventional (plain) concrete mixture and an internally cured concrete mixture. Chloride transport performance was evaluated using a series of experimental techniques including: 1) resistivity, 2) rapid chloride penetration (RCP), 3) rapid chloride migration (the Nord Test), 4) migration cell testing (STADIUM cells) and 5) chloride ponding and profiling. The results indicate that internally cured concretes have similar or superior performance to plain concrete. Several testing artifacts are noted associated with the pre-wetted lightweight aggregate that overestimate the transport measures for the internally cured concrete. The experimental results suggest that by reducing the chloride transport rate the use of internally cured concrete can result in structures with improved durability (due to the time it takes chloride ions to cause corrosion at the reinforcing steel).

Internal curing by means of superabsorbent polymers (SAP) is an efficient method for providing additional curing water in high performance concrete with low w/c. In order to fully use the potential of internal curing reservoirs, the water needs to be supplied possibly uniformly in the whole volume of hydrating cement paste and moreover at rates sufficiently high to compensate for the self-desiccation. At the same time, it is of importance to predict how the internal curing process will influence the overall material behavior at the macroscopic level. In this work, the investigation of the aforementioned phenomena is performed at two scales using poromechanical modeling. First, a mechanistic model of cementitious material is applied for the analysis of internal curing at the meso-level to describe water transport from the reservoirs to the surrounding cement paste. The meso-level simulations confirm that curing water can be practically uniformly and instantaneously distributed within the volume of the surrounding paste at the early stages of hydration. Second, based on this information, a source term due to internal curing is introduced at the macro-level, enabling description of the influence of the SAP on such phenomena as self-desiccation and autogenous shrinkage. This approach provides a very good agreement with the experimental data.

Highway bridges and parking structures, subject to coupled effects of mechanical loads and corrosion, often show early signs of distress such as concrete cracking and rebar corrosion leading to reduced structural performance and shortened service life. One solution to this problem is to use low-shrinkage low-permeability high-performance concrete (HPC) for bridge decks exposed to de-icing salts and severe loading conditions. A new HPC was formulated to achieve low shrinkage and low permeability, high early-strength, and 28-day compressive strength over 60 MPa (8,700 psi). Its mechanical performance and durability were tested both in the lab and field under severe test conditions, including restrained shrinkage, cycling loading, freezing and thawing cycles, and application of de-icing salts. Models were developed and calibrated to predict structural performance and service life of concrete bridge decks under severe exposure conditions. Prediction models indicate that bridge decks designed with low-shrinkage HPC can achieve a service life up to 100 years. Compared to normal concrete decks, short-t t-to-medium span bridge decks using low-shrinkage HPC could be built at a comparable initial construction cost, but at less than 35% of the life-cycle cost.

One strategy for achieving excellent long-term performance of concrete bridge decks is to combine low permeability with minimal early-age cracking. Low permeability can be achieved through the use of concretes with low water-cement ratios; however, topical curing techniques are usually insufficient to maximize hydration and minimize autogenous shrinkage effects. This autogenous shrinkage causes stresses in restrained concrete, which can lead to deleterious early-age cracking. Curing effectiveness can be enhanced through the implementation of prewetted lightweight fine aggregates. Internal curing is provided as the aggregate water gradually desorbs into the surrounding paste. A study of the early-age behavior of internally cured concrete is described in this paper. Internal curing was provided by means of expanded shale, clay, and slate lightweight fine aggregates. Ten mixtures with water-cement ratios of 0.42, 0.36, and 0.30 were investigated. Compressive and tensile strengths of the internally cured concretes were similar to or slightly greater than the strengths of their non-internally cured counterparts, and concrete stiffness decreased as expected in the internally cured mixtures. Autogenous shrinkage strains and stresses were found to increase as the water-cement ratio decreases. However, the autogenous effects were reduced or eliminated in the internally cured concretes.