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

Cellulosic or wood pulp fibers, like pre-wetted lightweight aggregates and superabsorbent polymers, can be used as internal curing agents in cementitious materials to mitigate autogenous shrinkage. While the internal curing abilities of different types of cellulose fibers have been demonstrated in mortar and concrete, relatively little fundamental research has examined the influence of fiber processing or “pulping” on their efficacy as internal curing agents. This is an important topic because even for fibers derived from the same type of wood, the morphology and composition of its pulp can be altered by processing, and these alterations can have important effects on the fibers’ internal curing capacity. This research examines the effect of variations in pulping process on the internal curing performance of eucalyptus pulp fibers grown in Southeast Asia. Variations in processing produced three fibers – unbleached soda pulp, unbleached kraft pulp, and semi-chemical pulp –which were compared as internal curing agents through standard autogenous shrinkage testing. These data were then compared based on fiber composition and morphology, using results from thermogravimetric analysis (TGA) and scanning electron microscopy, to better understand the complex roles of these factors – as influenced by processing – in providing internal curing to cement-based materials.

First noticed by T. C. Powers, et al in 1948, [22] as beneficial for hydration by supplying water internally, specifiers and contractors in 2012 have grasped how the process of internal curing is implemented, how hydration behaves, and how improvements in mechanical properties, durability, and cost may be beneficial. To meet the time-dependent hydration needs of the concrete, having sufficient water internally available, when, as, and where needed, is vital for achieving optimum characteristic qualities. There is lower life cycle cost with internal curing (IC) and frequently lower first cost. In 2012, the number of projects using internal curing is increasing at an escalating rate, because the process is simple and economically implemented. Pavements, bridges, buildings, and pervious parking lots are being started now in this recession, because specifiers and contractors are saving dollars, as they build longer lasting structures while costs and interest rates are low. Developed initially to reduce autogenous shrinkage in low water-cement ratio and high performance concretes, internal curing has been found to reduce drying shrinkage. Other benefits found include reduced permeability, increased compressive and flexural strengths, less warping, stronger interfacial transition zones, greater durability, and lower carbonation.

Constructing large capacity, monolithically placed water storage tank slabs is a complex proposition. Previously, specifying low-shrinkage concrete mixes and monolithic placement of the slab within a specified time period was the prescribed method, yet shrinkage cracking still occurred. We felt more could be done to improve concrete placing and finishing, reducing shrinkage cracking and enhance durability. An investigation on the use of an Internally Cured Concrete mix on the floor and roof slabs of the Denver Water 10-Million Gallon [MG] (38-Million Liter [ML]) Lone Tree Tank No. 2 that Bates Engineering Inc. was designing was pursued. The tank floor and roof slab are each about 61,000 ft2 (5,700 m2) and would be monolithically placed. Laboratory trial batches performed determined plastic and hardened characteristics of the ICC as compared to traditionally proportioned mix designs. Tests performed in the laboratory included: compressive strength and drying shrinkage (ASTM C 157(1), modified 7-day saturation). An ICC mix was selected based on durability expectations. Results of the floor slab placement were successful and only two shrinkage cracks were observed, 7-day and 28-day compressive strength tests, workability and consistency surpassed expectations. As a result, it was decided to use ICC concrete on the remaining structural components.

Internal curing is an effective method of preventing autogenous shrinkage in early-age concrete. In this approach, water-retentive particles act as internal reservoirs to supply water to the surrounding cement paste matrix. The objective of this study was to determine how the early-age shrinkage behavior of cement-based materials is affected by the addition of saturated jute fiber under sealed conditions. The four cement-paste specimens with water-cement ratio of 0.25, and the individual jute fibers of 6 mm length. The fiber contents by volume were 0, 0.5, 1.0 and 2.0%, respectively. The straw-like structures of the jute fiber were observed in a scanning electron microscope photograph. Additions of 0.5% and 1.0% of jute fiber by volume of cement paste resulted in 12% and 36% reductions in autogenous shrinkage strain at a material age of 8 days, respectively. Additions of 0.5% and 1.0% of jute fiber by volume of cement paste resulted in 3% and 22% reductions in compressive strength at a material age of 8 days, respectively. The material segregation was observed in the cement paste with 2.0% of jute fiber by volume. These results indicate the suitability of jute fiber for an internal curing material.