Anton K. Schindler

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.

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.

Properties of Self-Consolidating Concrete for Prestressed Members

This paper evaluates self-consolidating concrete (SCC) mixtures for use in prestressed concrete applications. In the laboratory, 21 SCC mixtures are created with varying water-to-cementitious materials ratios, sand-to-total aggregate ratios, and cementitious material combinations (e.g., Type III cement, Class C fly ash, ground-granular blast-furnace slag, silica fume). Prestress transfer compressive strengths of between 5470 and 9530 psi (38 and 66 MPa) are shown for the SCC mixtures. The authors note that the moduli of elasticity of the SCC mixtures show reasonable agreement with the elastic stiffness assumed to be present in the design of conventional slump concrete structures. Also noted is that long-term drying shrinkage strain for all the SCC mixtures show approximately the same or less shrinkage strain than those measured for the control mixtures. Moreover, long-term drying shrinkage is not significantly affected by a change in sand-to-total aggregate ratio. And, at later ages of 56 and 112 days, measured drying shrinkage corresponds reasonably well to drying shrinkage predicted by the American Concrete Institute (ACI) Committee 209 procedure.

Importance of Concrete Temperature Control During Concrete Pavement Construction in Hot Weather Conditions

The development of high concrete temperatures could cause a number of effects that have been shown to be detrimental to long-term concrete performance. High concrete temperatures increase the rate of hydration, thermal stresses, the tendency for drying shrinkage cracking, and permeability and decrease long-term concrete strengths and durability as a result of cracking. Data from the Texas Rigid Pavement database were analyzed to reveal whether there are increased numbers of failures as the air temperature at placement increases. It was shown that this was the case for both major coarse aggregate types: limestone and siliceous river gravel. The results of the analysis emphasize the importance of concrete temperature control during concrete pavement construction in hot weather conditions. Most states specify a maximum concrete temperature at placement to mitigate the detrimental effects of placement during hot weather. The specified limit remains the same irrespective of the type of mineral or chemical admixtures used. To produce specifications that encourage contractor innovation and the use of improved materials, modern specifications should account for these materials to ensure improved concrete performance under all placement conditions. To provide improved performance for sections paved under hot weather conditions, it is proposed that the continuously reinforced concrete pavement reinforcement standards be redesigned to provide steel quantities for specific use during hot weather conditions and that an end-result specification that limits the maximum in-place concrete temperature during hydration be implemented.

Concrete pavement temperature prediction and case studies with the FHWA HIPERPAV models

High performance concrete paving (HIPERPAV) is a concrete paving software product sponsored by the Federal Highway Administration. The objective of this paper is to present recent improvements made to the temperature prediction model, and to illustrate that this model can be used to predict the in-place temperature development in portland cement and fast-setting hydraulic cement concrete paving applications. The concrete temperature prediction model consists of a transient one-dimensional finite-difference model, which includes the heat of hydration of the cementitious materials and the heat transfer mechanisms of thermal conduction, convection (including evaporative cooling), solar radiation, and irradiation. A new model is introduced to account for the effect of evaporative cooling, which may occur on the concrete surface. To validate the temperature model, the concrete temperatures measured in the field were compared to the concrete temperatures predicted with the temperature model. The HIPERPAV temperature model produced accurate predictions of the in-place temperature development of hydrating concrete.

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.