This paper presents a brief review of the theory and construction practice with concrete mixtures that contain more than 50% fly ash by mass of the cementitious material. The mechanisms by which the incorporation of high volume of the ash in concrete reduces the water demand, improves the workability, minimizes cracking due to thermal and drying shrinkage, and enhances durability to reinforcement corrosion, sulfate attack, and alkalile-silica expansion are discussed. This technology can play a huge role in meeting the large demand for infrastructure in a sustainable manner for countries like China and India.

http://web.pdx.edu/~cgriffin/references/Mehta_2004_HVFA_Concrete.pdf

High performance, high-volume fly ash concrete for sustainable development

Three designs for the internal release of sealants, adhesives, and waterproofing chemicals into concrete to reduce permeability

The primary greenhouse gas is carbon dioxide and, during the 20th century, its concentration in the environment has risen by 50 percent. Carbon dioxide is a major by-product in the manufacturing of the two most important materials of construction: portland cement and steel. Therefore, the construction industry needs to determine how future infrastructural needs can be met without further increases in the production of cement and steel. Conservation of these materials through enhancing the durability of structures is one of the ways by which the construction industry can become a part of the solution to the problem of sustainable

/pdf/building-durable-structures-in-the-21st-century-65gnpb1uw7.pdf

Building Durable Structures in the 21st Century

Quantifying the effects of crack width, tortuosity, and roughness on water permeability of cracked mortars

Tensile stress can develop in concrete when rest raint prevents the concrete from shrinking freely in response to drying, chemical reaction, or temperature reduction. When these tensile stresses exceed the tensile strength of the concrete, visible cracking may be expected to occur. While several test methods have been developed to assess a material's potential for early-age shrinkage cracking, this paper describes the use of the ‘ring-test’ for assessing the performance of a fiber reinforced concrete. An equation is presented that enables the residual stress that develops in the concrete ring specimen to be estimated. This expression is used to approximate the stress that is transferred across a crack. It is shown that for conventional fiber reinforced concrete (i.e., the mixtures described in this paper) the pre-peak mechanical properties (elastic modulus, splitting tensile strength and free shrinkage) are not influenced greatly by the addition of fibers. Rather, fibers appear to influence shrinkage-cracking behavior only after the crack begins to open. To better understand the role of fibers in mitigating early-age shrinkage cracking passive acoustic emission testing was used. Acoustic emission measurements indicate microcracking due to the heterogenous nature of the concrete and the presence of moisture gradients. The acoustic emission results highlight how a single crack begins to form into a visible crack. Finally, an approach is presented for estimating crack-width based on the strain data measured from the ring test. The role of the specimen geometry is discussed for comparing the ring test with field applications.

Quantifying shrinkage cracking in fiber reinforced concrete using the ring test

Many concrete bridge decks develop transverse cracks soon after construction. The aims of this project were to investigate the causes of such cracking and to specify methods to reduce its incidence. Literature, research reports, and current practices concerning transverse cracking in bridge decks were reviewed to learn the extent of and the perceived causes of cracking. All departments of transportation in the United States and many foreign transportation departments were surveyed to learn the extent of deck cracking, and standard design and construction practices. The survey response revealed that transverse bridge deck cracking is a severe problem, since about half the bridges in the United States develop early cracking. Construction methods vary widely, as do techniques used in attempts to prevent cracking. The Portland-Columbia Bridge was instrumented and monitored during its redecking. The comprehensive recorded data provide important insight into early bridge behavior and cracking, influenced by thermal loading and concrete shrinkage. Systems of equations to predict shrinkage and thermal stresses in a composite bridge were developed. Behavior predicted by these equations compared favorably with actual measured behavior of the Portland-Columbia Bridge. The equations allow bridge designers to predict thermal and shrinkage stresses that may develop in their designs. Designers can evaluate and compare the thermal and shrinkage stresses of various designs. Analytical studies using the derived equations evaluated the influence of various material and geometry factors on deck stresses and cracking. The analyses examined the effects of various concrete properties, including modulus of elasticity, creep, drying shrinkage, and coefficient of thermal expansion. Girders were either steel or concrete, small or large. Simply-supported and continuous spans were analyzed. The effects of deck reinforcement were also studied. Three different temperature conditions and two different deck drying shrinkage profile conditions were analyzed. About 18,000 combinations of system and conditions were analyzed. The analytical study determined the material properties and geometries most likely to cause transverse deck cracking. These are deck restraint, the concrete modulus of elasticity and creep, and shrinkage and thermal strains. Because decks are restrained primarily by their supporting girders, and span length limits girder stiffness, often little can be done to reduce restraint. However, concrete properties have a large effect on stresses, and concrete properties can be easily changed. Construction techniques can also be easily improved to reduce early thermal strains and shrinkage. A test method to predict cracking tendency of concrete mixes was developed. This project identified and ranked the factors or combinations of factors that contribute to transverse cracking of newly constructed bridge decks.

P D Krauss

Papers

Transverse cracking in newly constructed bridge decks