Kevin J. Folliard

HEAT OF HYDRATION MODELS FOR CEMENTITIOUS MATERIALS

Models are used to characterize the behavior of concrete exposed to in-place conditions. These models need to include methods to quantify the heat of hydration of cementitious materials. This article presents the formulation of a general hydration model for cementitious materials. The authors note that the degree of hydration characterizes the formation of hydration products as hydration progresses over time, and each concrete mixture has a unique degree of hydration development. The authors used semi-adiabatic calorimeter tests on 13 different concrete mixtures and with heat of hydration data from 20 different cement types to provide a convenient, indirect means of characterizing the formation of hydration products by measuring the heat released during hydration. Their hydration model incorporates the effect of following variables: cement chemical composition, cement fineness, supplementary cementing materials (Class F fly ash, Class C fly ash, and ground-granulated blast-furnace (GGBF) slag cement), mixture proportions, and concrete properties (density, thermal conductivity, and specific heat). The authors conclude that this model provides a reasonable and accurate representation of the heat of hydration development under different curing temperatures.

Performance Limits for Evaluating Supplementary Cementing Materials Using Accelerated Mortar Bar Test

The accelerated mortar bar test (AMBT) was originally developed for the purpose of identifying alkali-silica reactive aggregates, but has been widely used to evaluate the preventive action of supplementary cementing materials (SCM). Indeed, a modified version of the AMBT for testing the effectiveness of pozzolans and slag for controlling expansion due to alkali-silica reaction (ASR) was recently developed and published as an ASTM standard test method (ASTM C 1567). In this paper, results from accelerated mortar bar tests on reactive aggregate-SCM combinations are compared with the performance of the same combination of materials in concrete structures, field-exposed concrete blocks, and laboratory expansion tests on concrete prisms (ASTM C 1293). It is concluded that the use of a 14-day expansion limit of 0.10% in the AMBT produces an outcome that agrees well with the performance of concrete in the laboratory or under field conditions. Combinations of reactive aggregates and SCM that pass this limit when tested in mortar have a very low risk of resulting in damage when used in concrete. Furthermore, the minimum level of SCM required to control expansion with a given reactive aggregate can be determined using the 14-day expansion limit and the result is in good agreement with the amount of SCM required to prevent cracking in concrete. Extending the duration of the test (for example to 28 days) is overly conservative and results in estimates of much higher levels of SCM (by 1.5 times on average) to control expansion than that actually required in concrete.

Use of Alternative Materials to Reduce Shrinkage Cracking in Bridge Decks

According to a survey conducted in 1996, respondents in several state departments of transportation indicated that more than 100,000 bridge decks in the U.S. have suffered from early age transverse cracking, a crack pattern that typically arises due to drying shrinkage. Concrete material properties are treated as a means through which to improve the resistance restrained drying shrinkage cracking. Various test methods are discussed as they relate to determining the resistance of a material to shrinkage cracking. Materials-based methods of controlling drying shrinkage are presented. The materials discussed include fibers, shrinkage-compensating concrete, shrinkage-reducing admixtures, and extensible concrete. It was determined in small laboratory specimens, and confirmed in large-scale bridge deck specimens, that several of the alternative mixtures adequately reduced restrained drying-shrinkage cracking.

Effectiveness of Lithium-Based Products in Concrete Made with Canadian Natural Aggregates Susceptible to Alkali-Silica Reactivity

To evaluate the effectiveness of lithium-based products to counteract alkali-silica reaction (ASR), a total of 87 concrete mixtures were made incorporating 12 reactive aggregates of various types and degrees of ASR, and using various dosages of LiNO 3 and Li glass in combination or not with supplementary cementing materials (SCM). The concrete prisms were tested at 38 °C (100 °F), according to CSA A23.2-14A or ASTM C 1293, and also at 60 °C (140 °F) to evaluate the possibility of accelerating the testing procedure. Using LiNO 3 at the [Li]/[Na + KJ ratio of 0.74 recommended by the manufacturer, satisfied the 2-year 0.04% expansion limit criterion (CSA A23.2-28A) at 38 °C (100 °F) for six aggregates; three aggregates required a ratio between 0.74 and 1.11, while a ratio 1.11 was not effective with the other three aggregates. The lithium glass was not effective. The ternary silica fume/slag cement tested was effective and the fly ashes and slag as well provided they were in sufficient quantities and they had a proper composition. Most LiNO 3 -SCM combinations did not show significant synergetic effect. The required LiNO 3 dosage is not related to the aggregate reactivity. The 6-month expansion at 60 °C (140 °F) correlated well with the 2-year expansion at 38 °C (100 °F) for the control and SCM mixtures, but not for the LiNO 3 mixtures.

Apparent Diffusivity Model for Concrete Containing Supplementary Cementitious Materials

Concrete’s resistance to chloride diffusion is one of the primary factors governing the concrete structure service life and life-cycle costs. This paper presents a new model developed for estimating the apparent concrete diffusivity based on the mixture proportions, cementitious materials used, and concrete age. The model includes the effects of supplementary cementitious material types commonly found in other service life models such as fly ash, ground-granulated blast-furnace slag, and silica fume. Also included are ultra-fine fly ash and metakaolin, which were not available in previous service life models. For validation of the model, chloride profiles have been measured on concrete blocks exposed daily to seawater for 25 years at the Treat Island, ME concrete exposure site. Concrete mixtures tested as part of the validation dataset contained up to 80% ground-granulated blast-furnace slag, 25% fly ash, or 20% silica fume, and were compared against the predicted values and are presented in this paper.

Evaluation of Autogenous Deformation of Concrete at Early Ages

Autogenous shrinkage, significant primarily in concretes with a low water-cementitious material ratio (w/cm), has received more attention in recent years due to increasing use of high-performance concretes (HPCs). In this study, autogenous shrinkage was quantified in both unrestrained and restrained concrete. The specimens were sealed and kept at a constant isothermal temperature of 20°C (68°F) to prevent deformation due to temperature change or moisture loss. Various materials were evaluated to compare their effectiveness in reducing autogenous deformation and stress development, including saturated lightweight aggregates, shrinkage-reducing admixtures, and a shrinkage-compensating additive (based on calcium sulfoaluminate). The data obtained also provides insight into mechanisms behind autogenous shrinkage and the resulting stress development in restrained members and quantify effects of methods used to reduce autogenous shrinkage and resultant stresses.

New Model for Estimating Apparent Activation Energy of Cementitious Systems

This paper will discuss that some predictive models for concrete temperature development and strength depend on the application of the Arrhenius equation to characterize the progress of hydration, which in turn requires an apparent activation energy E(a) value to define the temperature sensitivity of the hydration reactions. Testing to determine E(a) can be very time-consuming and expensive. It would be useful to have a model that estimates E(a) for a given concrete mixture. To be broadly applied, such a model must account for the variable chemistry of cementitious systems. This paper describes the development of a model for E(a) using multi-variate statistics analysis of experimental hydration data from 116 cementitious mixtures. The model was also validated by an independent set of hydration data from six different cementitious mixtures. The model accounts for the effects of cement chemistry, supplementary cementitious materials, and chemical admixtures.

Characterizing Alkali-Silica Reactivity of Aggregates Using ASTM C 1293, ASTM C 1260, and Their Modifications

Identifying the susceptibility of an aggregate to the alkali-silica reaction (ASR) before using it in concrete is one of the most efficient practices for preventing damage. ASTM C 1260 (Mortar-Bar Test) and ASTM C 1293 (Concrete-Prism Test) have been used to identify ASR-susceptible aggregates. ASTM C 1260 modifications consisted of performing the test using different NaOH solution normalities, namely, 1N, 0.75N, 0.5N, and 0.25N. ASTM C 1293 modifications consisted of increasing the storage temperature from 38°C to 60°C and storing the concrete prisms in 1N NaOH solution at 38°C. The evaluation, comparison, and results for these two tests and their suggested modifications are presented. Data indicate that the levels of test results suggested in the nonmandatory information in the appendix to ASTM C 1260 as “potentially deleterious” are achieved by many aggregates with a good service record. Results suggest that ASTM C 1260 should be used in combination with ASTM C 1293. The nonmandatory information in the appendix to ASTM C 1293 on levels of expansion that should be regarded as potentially deleterious is less likely to be exceeded by aggregates with good service records than in the case of those in the appendix to ASTM C 1260. Changes in the solution normality of the ASTM C 1260 test could be used to evaluate the effect of the alkali content on ASR but do not alleviate the severity of the test for the aggregates investigated. ASTM C 1293 could be effectively accelerated to generate results within 3 months instead of 1 year by increasing the testing temperature from 38°C to 60°C.

Hydration Study of Cementitious Materials using Semi-Adiabatic Calorimetry

Accurate characterization of the temperature rise in a concrete element requires an estimate of the adiabatic temperature rise of the concrete mixture. Semi-adiabatic calorimetry is commonly used to provide an estimate of the heat generation characteristics of a concrete mixture because of the relative simplicity of the test. This study examines the sources of variability in semi-adiabatic calorimetry, and an estimate of the confidence limits of the test is calculated. Then, twenty concrete mixtures are investigated using semi-adiabatic calorimetry. Activation energy values are calculated for each mixture using isothermal calorimetry. The adiabatic temperature rise is then calculated. The following mixture properties are investigated: cement type, cementitious content, water/cementitious material ratio, coarse aggregate type (siliceous river gravel and limestone), mixture placement temperature, and the effects of selected supplementary cementing materials. The following factors were the most important to reduce the adiabatic temperature rise: reduced cement content, use of a lower-heat cement, such as a Type V cement type, reduced aggregate specific heat, and substitution of cement with Class F fly ash.

EFFECTS OF CURING CONDITIONS ON STRENGTH DEVELOPMENT OF CONTROLLED LOW-STRENGTH MATERIAL

This research investigated the effects of a curing regime on the compressive and splitting tensile strength of controlled low-strength material (CLSM). Six CLSM mixtures representing a range of materials and mixture proportions were included in the study. Three curing temperatures were used, and for each curing temperature, 2 different relative humidity conditions were used to examine the influence of temperature and humidity on strength gain at ages of 7, 28, and 91 days. The effects of temperature and humidity were found to largely depend on the constituent materials and mixture proportioning. In particular, reactivity of fly ash was found to be critical in influencing the strength of CLSM mixtures, especially when an ASTM Class C fly ash was used. Some CLSM mixtures containing Class C fly ash exhibited a significant increase in strength when cured under the highest temperature conditions. Although the dependence of cementitious materials containing fly ash on temperature is well known, the increases in strength of CLSM mixtures containing a high-calcium fly ash far exceeded the strength increases typically observed for conventional concrete.