Maria C. G. Juenger

Methods for Calculating Activation Energy for Portland Cement

This paper examines activation energy (E subscript a) calculation methods used to accurately predict the thermal gradients in concrete. A particular emphasis is placed on models characterizing the temperature sensitivity of hydration in cementitious materials. The Arrhenius equation, which requires selecting a specific E subscript a, is the most commonly used method to define the temperature sensitivity of the reaction. The authors use Arrhenius with three different computational methods to determine the E subscript a of different cementitious pastes. These methods include: 1) a single linear approximation, that calculates the reaction rate based on a first-order differential rate equation, 2) an incremental method that calculates the rate incrementally over a specific time period, and 3) an ASTM C 1074-based modified method that uses isothermal calorimetry data instead of compressive strength. The research takes a detailed look at the advantages and disadvantages of the computational methods, as each is applied to a different cementitious paste. From the results, the authors are able to develop a systematic computational method for characterizing E subscript a that accounts for how temperature affects the overall hydration rate in cementitious materials.

Effects of High Alkalinity on Cement Pastes

Adding sodium hydroxide to increase the alkalinity of cement pastes is known to increase the initial rate of hydration, thereby increasing early strength. However, the effects on the microstructure and other properties are unclear. This study examines the influence of NaOH addition on the rate of cement hydration, surface area, and porosity as measured by nitrogen and drying shrinkage. While a 1M NaOH solution increases the rate of reaction prior to 1 day, hydration is retarded at later ages. The surface area is reduced when compared to samples without added NaOH, which is a reflection of a lower volume of small pores. The rate of drying shrinkage appears to be slower when NaOH is added, but the total equilibrium value is unaffected. NaOH also increases the amount of cracking during drying shrinkage.

Modeling Hydration of Cementitious Systems

This article presents the results of an empirical model of concrete hydration. Concrete performance, including strength, susceptibility to delayed ettringite formation, and residual stress development are dependent on early-age temperature development. Concrete temperature prediction during hydration requires an accurate characterization of the concrete adiabatic temperature rise. This article presents the development of a model for predicting the adiabatic temperature development of concrete mixtures based on material properties, mixture proportions, and chemical admixture types and dosages. The model was developed from 204 semi-adiabatic calorimetry results and validated from a separate set of 58 semi-adiabatic tests. The final model provides a useful tool to assess the temperature development of concrete mixtures and thereby facilitate the prevention of thermal cracking and delayed ettringite formation in concrete structures.

Simplified concrete resistivity and rapid chloride permeability test method

A simplified method of measuring concrete resistivity, as an index of permeability, has been developed that is similar to ASTM C1202 or the rapid chloride permeability test (RCPT). It is significantly faster and easier to perform, however In this test, cylinders 100 x 200 mm (4 x 8 in.) were cured in 100% relative humidity and tested using the same solutions, test cells, and rubber gaskets as specified in ASTM C1202. To eliminate the problem of the temperature rise of the sample during the test, only one current reading was taken (after 5 minutes) that could be used to calculate the concrete resistivity. Testing was conducted on various different concrete mixtures after 91 days of moist curing using both the new quicker method and the standard ASTM 0202 method. An empirical correlation between the new method and the standard method demonstrates the validity and promise of the new method.

Evaluation of temperature prediction methods for mass concrete members

Over the years, many different simplified prediction methods have been developed to predict the temperature development within mass concrete members. This paper compares calculated temperature values from three commonly-used concrete temperature prediction methods to actual temperatures in eight different concrete bridge members measured during construction. A simple temperature calculation method, the graphical method of AC! 207.2R, and a numerical heat transfer method (the Schmidt Method) were used to predict peak temperatures. The Schmidt Method performed the best when semi-adiabatic calorimetry results were used in the analysis. Suggestions are made on ways to improve the best technique, which was the Schmidt Method.

Effects of Construction Time and Coarse Aggregate on Bridge Deck Cracking

Temperature changes in bridge decks have long been identified as a significant early-age cracking contributor within the first few days after placement due to changes in ambient conditions and concrete heat of hydration. Development of a method of quantification of how materials and construction methods can influence bridge deck thermal stress was the goal of this project. A concrete mixture test series was then performed to quantify bridge deck concrete material thermal stress behavior with different coefficients of thermal expansion and placement times. Development of thermal stress equal to 75% of the stress at cracking resulted from concrete placed in the morning with a high thermal expansion coefficient. Thermal stresses were also found to be able to be reduced by up to 50% by placing concrete with a lower thermal expansion coefficient at night.

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.

Quantification of Effects of Fly Ash Type on Concrete Early-Age Cracking

The mechanisms that contribute to early-age cracking are complex. Determining the relative importance of each mechanism as well as the combined cracking potential for a given concrete material is essential for the concrete industry to construct structures with a long service life. A method for quantifying the cracking risk of a concrete mixture is presented. The method involves testing for the concrete heat of hydration, setting time free thermal and autogenous movement, restrained stress, and mechanical property development. The concrete uniaxial stress under restrained conditions is measured using a rigid cracking frame. This test setup was used to quantify the effects of using fly ash on the concrete cracking risk using four different fly ashes with varying calcium oxide contents. All fly ashes reduced the cracking risk because of the decrease in the heat of hydration of the cementitious materials and, to a lesser extent, the increased early-age creep.

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.

Statistical Determination of Cracking Probability for Mass Concrete

AbstractThis study addresses the use of a stress-to-strength ratio as a failure criterion for thermal cracking. Restrained cracking frame specimens and accompanying match-cured concrete cylinders were tested to determine the ratio of stress-to-splitting tensile strength at cracking. A stress-to-splitting tensile strength ratio of 0.57 was found to give a 50% probability of cracking and lognormal standard deviation of 0.16 when splitting tensile cylinders sized 100×200  mm (4×8  in.) and 150×150  mm (6×6  in.) rigid cracking frame specimens were used to determine the stress at cracking. Lognormal fits of the cracking stress from 64 cracking frame tests and the tensile strength calculated from the measured compressive strength using three commonly used equations based on compressive strength were developed.