Reducing Volume Change-Induced Cracking of Concrete: Field Implementation and Evaluation of Crack-Reduction Technologies
Research Project Statement:
Cracking has been a problem for as long as concrete has been used as a construction material. Cracks reduce durability, affect appearance, may represent a major structural problem, and are costly to repair. Volume change in concrete represents a major contributor to cracking, and the control of cracking induced by volume changes in plastic and hardened concrete has been identified as an Industry Critical Technology (ICT) by the Strategic Development Council (SDC) of the American Concrete Institute. This proposal describes the field implementation and evaluation phase of an effort to enable the application of crack reduction technologies on the widest possible scale.
Cracking in concrete structures has many causes and can be initiated in both plastic and hardened concrete. Prior to setting, plastic shrinkage cracks can occur due to the evaporation of water from the surface at a rate that exceeds its replacement by bleed water, and settlement cracks can occur as the concrete continues to consolidate around fixed objects, such as reinforcing bars. In hardened concrete, cracks can occur due to load, restrained drying shrinkage, and differential temperature changes. Slabs-on-grade are subject to curling due to differences in moisture or temperature through the depth of the slab, which can, in turn, lead to cracking as load is applied.
Strategies to limit or prevent cracking have seen various levels of success. These include limiting evaporation or initiating early curing to limit plastic shrinkage cracking; lowering concrete slump and increasing cover over the reinforcing steel to limit settlement cracking; initiating rapid curing and increasing the curing period to reduce both plastic shrinkage cracking and long-term drying shrinkage cracking; instituting temperature control of the concrete, as well as applying the concepts of mass concrete construction, to limit thermal cracking; and increasing air content, reducing cementitious paste content, minimizing finishing, and limiting concrete strength to reduce drying shrinkage cracking. The success achieved by these methods has been demonstrated in projects such as those involving the construction of low-cracking high-performance concrete (LC-HPC) bridge decks as part of a long-term pooled-fund study by the University of Kansas, 19 state DOTs, and the Federal Highway Administration. The crack densities of the LC-HPC decks have been consistently lower than that of matching “control” decks, which were constructed using more conventional approaches.
Despite the relative success of these and other methods, structures do not, in general, achieve zero cracking, and the need to consistently limit cracking in concrete structures remains an Industry Critical Technology. This is especially true for structures such as bridge and parking decks and industrial floors, but it is also important for general concrete construction, including most slabs-on-grade because of the widespread impact of cracking on durability and aesthetics.
In addition to the more traditional strategies, such as applied on the LC-HPC bridge decks, additional technologies offer the potential to significantly reduce cracking in concrete structures. These include the use of shrinkage reducing admixtures, steel and synthetic fibers, and internal curing, shown to be especially effective when combined with slag cement or slag cement and silica fume as a partial replacements for portland cement, along with the application of alternative binder compositions (supplemental cementitious materials) and construction techniques, including concrete temperature control and careful selection of concrete materials and workability. The latter have seen some success in reducing the effects of curling on floors. The use of these technologies, alone and in combination with the more traditional techniques, have not been fully vetted in full-scale applications.
While the mechanisms that cause concrete cracking are fairly well understood, the proposed effort will be designed to use an appropriate combination of technologies to minimize the potential that these mechanisms will simultaneously raise the probability of cracking.
As a key portion of the effort to pursue this Industry Critical Technology to reduce cracking in concrete structures, lab and field evaluations of crack-reduction strategies are proposed, with emphasis on shrinkage reducing admixtures, fibers, internal curing of concrete, and alternate binders, along with careful application of construction techniques designed to minimize cracking. These technologies have been selected because they are relatively mature and have demonstrated a significant degree of success. They have, however, only rarely been evaluated in controlled field applications. The evaluations will involve the incorporation of these technologies in full-scale construction projects, including bridge decks, parking structures, and industrial floors. The efforts will include laboratory and on-site material testing, and work will be performed in partnership with state departments of transportation, owners of parking structures and industrial/commercial facilities, and suppliers of the technologies. Individual installations will be surveyed for three years after construction.
With this background, the objective of the proposed effort is to implement cost-effective combinations of materials and construction techniques to minimize cracking in highly crack-susceptible structures, especially bridge decks, parking structures, and industrial/commercial floors, and compare the performance of structures constructed using these techniques with that of structures constructed without of crack-reducing technologies. Project steps include the following:
1. Develop detailed plans to construct crack-critical structures incorporating best practices in materials, construction procedures, and design. This task will involve both conventional and new technology, with emphasis on shrinkage reducing admixtures, fibers, internal curing of concrete, and alternate binders.
2. Work with state DOTs, owners, material suppliers, designers, contractors, and inspectors to modify materials, designs, specifications, contracting procedures, and construction techniques to obtain concrete slabs, decks, and floors that exhibit minimal cracking.
3. Select and schedule structures to be constructed. Working with owners, material suppliers, and other stakeholders select the “best practices” to be used for each project, including prequalification tests and evaluation of the technologies both in the lab and in the field. Wherever possible, similar structures that do not incorporate all of the selected best practices will be selected to serve as control structures to determine the effectiveness of the techniques.
4. Perform detailed crack surveys of the structures six months, one year, two years, and three years after construction. The surveys will be performed using techniques developed at the University of Kansas for bridge decks that involve identifying, measuring, and quantifying observable cracks and will be enhanced for slabs-on grade to include measurements to evaluate the impact of temperature and humidity gradients on curling.
5. Correlate the cracking and curling measured in Task 4 with material properties, environmental and site conditions, construction techniques, and design specifications, and compare with data for existing structures. Actual costs and cost estimates for new construction and maintenance over the life of the structures will be compared with the potential benefits.
6. Document the findings in progress reports, regular presentations to SDC, and a final project report.
7. Develop an outreach and training program to assist the industry in implementing the findings of the study. The program will develop workshops and web-based tools, such and easy-to-use programs for concrete mixture proportioning, for use by practitioners.
Owners expend significant effort and resources on the construction of durable reinforced concrete structures. Existing data indicates that specific modifications to construction procedures, materials, and design details will significantly reduce the degree of cracking, thus improving the durability, serviceability, and appearance of the structures. The goals of the proposed effort are to implement cost-effective combinations of materials and construction techniques to minimize cracking in highly crack-susceptible structures and to allow the effectiveness of the techniques to be evaluated in full-scale applications. To this end, the project will provide a mechanism for developing an expanded comprehensive strategy for minimizing cracking in concrete structures.