Infl uence of elevated temperatures up to 100 °C on the mechanical properties of concrete

Before planning the experiments, a literature review was carried out which aimed to identify the most important characteristics that play a role on the mechanical behavior of concrete subjected to elevated temperatures. The last part of the thesis deals with the development of an overall material model to predict the mechanical behavior of concrete subject to elevated temperatures up to 100 °C.

Problem statement and objectives of the investigation

The new material model estimates the time, temperature and moisture dependence of the concrete's compressive strength, tensile strength, modulus of elasticity, creep and shrinkage. This thesis therefore provides specific knowledge about the influence of temperature and moisture content on the mechanical behavior of concrete as well as a tool to calculate these influences.

Structure of the document

Structure of hardened Portland cement paste

Henceforth, the essential characteristics of the composition and structure of hardened cement paste are summarized below. These classifications are based on the interaction between the internal surfaces of the hardened cement paste and water.

Figure 2.1: Volume proportions in relation to w/c-ratio in fully hydrated hardened cement paste [40]

Moisture storage in the concrete microstructure

The amount of stored water in the concrete microstructure is characterized by the material's sorption isotherm. The IBP method corresponds to the mathematical implementation of this cement paste model and takes into account the hysteresis effect of the sorption isotherms.

Figure 2.5: Schematic representation of a sorption isotherm for hygroscopic porous materials [82].

Hydrothermal effects on the microstructure of concrete

For the development of the method called the paint pore method (IBP-method), a new model for cement paste based on investigations of the CSH structure carried out by Stark et al. 47] (see also Rostásy et al. [131]) could not evidence any reaction between free Ca(OH)2 of cement paste and SiO2 of siliceous aggregates when subjected to siliceous concretes at 95 °C and 95% RH.

Figure 2.7: Solubility in water of calcium hydroxide, amorphous silicone dioxide, and quartz

Moisture transport in concrete

Associated mechanisms

Certain forms of moisture transport dominate depending on the frequency range of the pore sizes, the material's temperature level, and as shown in fig. In the first phase, water enters the microstructure of the material not as steam, but as isolated molecules that are adsorbed by the pore surfaces.

Figure 2.9: Water movement in various stages in the wetting of a porous material [127]

Describing moisture transport in porous materials

In the micromechanical approach, heat and mass transfer processes are calculated on a microscopic scale, which requires the use of a microphysical model of the porous system. According to Whitaker, the driving potential of a flow of liquid water is the pressure of the liquid, and the effect of thermodiffusion can be neglected.

Description of concrete drying based on diffusion

The influence of the relative humidity on the diffusion coefficient f(h) is approximated by an s-shaped function defined by three parameters as illustrated in the left diagram in fig. The parameter α0 represents the ratio between the minimum and maximum value of the diffusion coefficient, and characterizes the spread of the decrease in the diffusion coefficient.

Figure 2.10: Influence of the pore relative humidity and the temperature on the diffusion coefficient of concrete according to Bažant and Najjar [11]

Influence of temperature on the concrete mechanical properties

Experimental investigations on concrete submitted to

Based on the collected data, no clear trend can be discerned regarding the influence of temperature on the tensile strength. The studies compare the creep and shrinkage behavior of the concrete samples at elevated temperature with samples tested at ambient temperature.

Fig. 2.11 presents the results of the collected data on compressive strength. The diagram is composed by a total of 216 measurements taken from the experiments conducted by Behnood and Ziari [20], Bingöl and Gül [22], Campbell-Allen and Desai [36], Castill

Theoretical approaches towards the conception of an

As an initial assumption, the effects or hydrothermal reactions on the strength and stiffness of the concrete will be neglected. The gain on the significance of the deformation behavior of concrete that the water transport processes undergo due to temperature influence can only be captured by a model that can directly consider the effects of moisture content and moisture gradients.

Figure 2.15: Schematic description of the time-development of the influence of temperature on the strength and stiffness of concrete

Summary

Source Materials

Using only Portland cement and silicate aggregates as source materials limits the compass of the investigation. Nevertheless, the applicability and potential reach of the material model are discussed at length in Section 6.2.3.

Mixtures

The use of admixtures was necessary to set a consistency of the fresh concrete that could ensure appropriate workability at low values of w/c ratio. The fresh concrete characteristics and the cube compressive strengths of the hardened concrete are given in Table 3.3.

Figure 3.1: Particle size distribution of the aggregates from the investigated concretes and standard grading curves according to DIN 1045-2 [N1]

Production scheme

The preparation of the concrete was carried out using a pan mixer with a nominal capacity of 250 dm3. This storage condition allowed the concrete elements to reach high levels of hydration before the production of the samples could proceed.

Figure 3.2: Geometry of the prepared samples (dimensions in mm)

Experimental procedure

Conditioning of the samples
Determination of the relative humidity and water content
Evaluation of the concrete microstructure
Determination of strength and stiffness
Determination of creep and shrinkage

The sensors were numbered according to their position relative to the central axis of the concrete samples. Based on these comparisons, indications of the influence of temperature on the concrete microstructure can be assessed qualitatively.

Fig. 3.4 provides an overview of the conditioning of the samples previous to the tests and during testing.

Summary

Concretes drying at room temperature

For a given concrete mix, the lower the relative humidity of the environment, the greater the water loss of the concrete samples. Furthermore, by a given relative humidity of the environment, the lower the w/c ratio of the concrete samples the slower the development of the water loss in time.

Fig. 4.2 shows the measurements of the moisture distribution in the concrete samples made with the mixture MRC and stored at 20 °C with 85 % and 65 % RH

Concretes drying at elevated temperatures

Moreover, increasing temperature leads to an increase in the relative humidity in the concrete pores, which promotes the expansion of water from the concrete microstructure. Analogous to the results of the measurements carried out at 20 °C (see Fig. 4.3), the concrete with higher capillary porosity dries faster.

Figure 4.4: Time development of water stored in concrete MRC in relation to the water stored before the beginning of drying for temperatures of 20, 60 and 80 °C and different ambient relative humidities

Temperature changes

On-line measurements show that there is no change in the relative humidity of the concrete pores due to temperature changes. 4.8, increasing the temperature by 20 °C always led to an increase in the relative humidity of the concrete pores.

Measurements related to concrete microstructure

Mercury intrusion porosimetry

The curve of concrete MLC represented by the dashed line lies below the curves of the concretes MRC and MHC in the ranges of micro- and meso-capillary pores. However, the differences in the microstructure of the concretes MRC and MHC are less easy to identify.

Fig. 4.10 provides information about the influence of temperature and moisture content on the microstructure of the concrete MRC after being dried at 20 °C and 95 % RH and then heated up to 60 °C

Gas permeability

The lower the w/c ratio, the denser the concrete microstructure and therefore lower permeability. Lower permeability coefficients are related to densification and higher permeability coefficients to damage of the concrete microstructure.

Table 4.1: Permeability coefficients of the concretes at reference conditions

Measurements of concrete strength and stiffness

Compressive strength

This diagram shows the influence of temperature on the compressive strength of concrete, taking into account the history of the moisture content of the concrete before heating. In the legend of the diagram, the layers are named according to the average relative humidity the samples reached before heating.

Tensile strength

The relative values of tensile strength show an increase of the strength within the entire range of relative humidities reached in the samples. The concrete achieved an increase of about 40 % of the tensile strength by an average internal relative humidity in the range of 40 to 50.

Modulus of elasticity

Contrary to the results of strength tests, a dependence of the concrete microstructure on the elastic modulus can be established. After the abrupt change of the elastic modulus, the recovery is greater as the porosity (w/c ratio) of the concrete mix is higher.

Measurements of concrete creep and shrinkage

Influence of water content

Both creep and shrinkage are the higher, the higher the water content of the concrete and, more specifically, the faster the concrete loses water with the environment [98]. In the legend of the diagram, the layers are named according to the conditioning of the samples before the measurements were taken.

Influence of concrete microstructure

As a result, these measured values can be considered consistent if the unexpected behavior is also visible in creep pattern measurements. As already discussed in chapter 4.1 (see Figures 4.3 and 4.7), the smaller the porosity, the slower the concrete drying process.

Figure 4.22: Time development of the shrinkage deformation of the concretes MLC, MRC and MHC tested at 20 °C / 65 % RH and previously conditioned at 20 °C / 85 % RH

Influence of concrete temperature

The measured strains of the concrete samples tested at 40 and 70 °C decreased until approximately one day after the start of the test. During the first day after loading, the samples experienced a small temperature increase which may have influenced the development of the creep strains.

Figure 4.24: Time development of the shrinkage deformation of the concrete MRC tested at 65 % RH and 20, 40 and 70 °C and previously conditioned at 20 °C / 65 % RH

Summary and conclusions

Effects of drying

As discussed in chapter 4.3.3, water at a relative humidity in the concrete pores of 100% contributes to the load-bearing capacity of the concrete. The recovery of the elastic modulus is completed at a relative humidity of about 78% where it reaches its maximum corresponding to 86% of the stiffness the concrete had at reference humidity and temperature conditions.

Fig. 5.1 presents the results of the tests of compressive and tensile strength conducted on the concretes MLC, MRC and MHC at 20 °C and different mean relative humidities.

Combined effects of temperature and moisture content

By following the same approach used for the formulation of the equations to describe the influence of thermal incompatibility on the strength of concrete, cf. Unlike the formulations that have been presented to describe the influence of thermal incompatibility on the strength of concrete in which a second-order polynomial has been selected, according to the measurements a linear relationship between temperature and the reduction of the modulus of elasticity can be assumed (see Chapter 4.3.3).

Figure 5.3: Relative compressive strength (top) and tensile strength (bottom) of concrete MRC heated at different moisture contents

Time dependent deformations

The vertical axis on the left indicates deformations due to shrinkage, and the vertical axis on the right indicates the average relative humidity of the concrete pores. In addition, Kbc1 depends on the concrete microstructure, which is described by an S-shaped function regulated by the w/c ratio of the concrete mixture.

Figure 5.5: Shrinkage deformation and mean relative humidity of the samples from the concrete MRC previously conditioned at 20 °C and 95 % RH during the creep tests at 20 °C and 65 % RH

Elevated temperatures

5.27, with increasing temperature, the factor related to changes in the relative humidity of the concrete pores with drying shrinkage deformation decreases. Following the formulations presented previously, creep at 20 °C can be modeled based on the development of the average relative humidity of the concrete pores during the loading period.

Figure 5.7: Drying shrinkage calculated with the mean relative humidity measured in the concrete pores in comparison with the measured values for the concrete MRC (top) and MHC (bottom) on companion specimens during the creep tests conducted at 70 °C and 6

Summary and conclusions

Drying at reference temperature

According to the model of Bažant and Najjar, the higher the diffusion coefficient, the higher the relative humidity of the concrete pores. The dependence of the diffusion coefficient on the relative humidity of the environment as defined by Eq.

Figure 6.1: Comparisons between the measured values of concrete MRC drying at 20 °C and 85 % as well as 65 % RH and the calculated values using the model from Bažant and Najjar [11]

Drying at elevated temperatures

The model follows the development of the average relative humidity in the concrete pores as it was measured very accurately for both concretes. At elevated temperature, the concrete MLC and MHC behave similarly because the concrete MLC obtains more energy for the diffusion process than the concrete MHC due to the fact that the activation energy of the concrete MLC is higher.

Table 6.2: Quotients between the activation energy of diffusion and the universal gas constant calibrated according to the experimental results

Effect of temperature changes

As the temperature rises, the water molecules coming out of the gel pores will not be able to increase the relative humidity of the capillary pores in the same way as concrete with lower w/c ratios, because the amount of gel pores is limited at such high w/c ratios. The proposed formulation of the hygrothermal coefficient of concrete does not include any time influence and therefore assumes that the changes in the relative humidity of the concrete pores occur instantaneously after the temperature variation occurs.

Figure 6.5: Changes of the relative humidity in the concrete pores due to a temperature change

Modelling the concrete mechanical properties

Assumptions
Implementation
Coverage and capabilities
Comparing the model results with measurements

In table 6.3, the model's general coverage area is indicated in the left column. For the creep tests performed at 20 °C, the model follows the trend of the measurements.

Table 6.3: Coverage of the new material model

Summary and conclusions

Based on the measurements, a formulation was developed to calculate the influence of a temperature change on the relative humidity of concrete pores. This section contains the results of relative humidity measurements in concrete pores.