R. Douglas Hooton

Effects of Temperature, Chemical, and Mineral Admixtures on the Electrical Conductivity of Concrete

ASTM C1202 has become a very common test method for prequalification purposes and for performance-based specifications in North America. Although the test neither directly determines the permeability or chloride resistance, it has often been shown to have good correlation to those properties since electrical conductivity is also related to the porosity and connectivity of the pore structure. The prevalence of the test is largely based on its ease of execution and its wide acceptance and use by many state and provincial DOTs. More recently, ASTM subcommittee C09.66 has discussed replacing the above test method with a more rapid method measuring conductivity. Several factors affect the conductivity of concrete, mixture design, inclusion of chemical and mineral admixtures, the temperature during testing and the age or maturity at test time. Research was carried out to investigate the magnitude of these variables on measured conductivity. Conductivity was measured using the same equipment as the ASTM C1202 method with changes in the magnitude and duration of the applied voltage as well as the solutions used in the test cell chamber. Conductivity was measured every three hours starting at one day after casting until seven days and weekly until 28 days. Conductivity was found to decrease with hydration as expected. It was determined that mixture design and temperature have significant effects on measured conductivity while chemical admixtures have less influence with the exception of corrosion inhibitors. The developed test method presents potential as a tool for prequalification and quality control that can be directly related to maturity and durability.

Methods for Evaluating Fly Ash for Use in Highway Concrete

This report presents recommended changes to coal fly ash specifications and test protocols contained in AASHTO Standard Specifications for Transportation Materials and Methods of Sampling and Testing (AASHTO M 295). These changes include modifications to the test methods currently specified for evaluating acceptability of fly ash for use in highway concrete as well as the introduction of new test methods for enhancing such evaluations. The modified specifications and test protocols will guide materials engineers and fly ash producers in evaluating fly ash and assuring that highway concrete is enhanced, and not deleteriously affected, by replacing a portion of the cement in the concrete mixture with fly ash. The information contained in the report will be of immediate interest to state materials engineers and others involved in specifying and evaluating concrete mixtures for use in highway pavements and structures.

Achieving Concrete Durability in Chloride Exposures

Obtaining durability in concrete structures over a long service life in chloride exposures requires knowledge of the concrete properties, relevant transport processes, depths of cover as well as minimization of cracking and construction defects. For example, imperfect curing can result in depth-dependent effects of the concrete cover’s resistance to chloride ingress. Several service life models with various levels of sophistication exist for prediction of time-to-corrosion of concrete structures exposed to chlorides. The model inputs have uncertainty associated with them such as boundary conditions (level of saturation and temperature), cover depths, diffusion coefficients, time-dependent changes, and rates of buildup of chlorides at the surface. The performance test methods used to obtain predictive model inputs as well as how models handle these properties have a dramatic impact on predicted service lives. Very few models deal with the influence of cracks or the fact that concrete in the cover zone will almost certainly have a higher diffusion coefficient than the bulk concrete as the result of imperfect curing or compaction. While many models account for variability in input properties, they will never be able to account for extremes in construction defects. Therefore, to ensure the reliability of service life predictions and to attain a concrete structure that achieves its predicted potential, designers, contractors and suppliers need to work together, using proper inspection, to ensure proper detailing, minimize defects, and adopt adequate, yet achievable, curing procedures. As well, concrete structures are often exposed to other destructive elements in addition to chlorides (eg. freezing or ASR) and this adds another level of complexity since regardless of cause, cracks will accelerate the ingress of chlorides. These issues are discussed along with the need to use performance-based specifications together with predictive models.