Evaluation of Strength and Modulus of Elasticity of Polymer-Modified Cement Concrete (PCC) Under Thermal Impact Within a Defined Service Temperature Range
Authors
:
Alexander Flohr, Catharina Rohde, Savitha Devarajamohalla Narayana, Andrea Osburg
The chapter delves into the evaluation of the strength and modulus of elasticity of polymer-modified cement concrete (PCC) under thermal impact within a defined service temperature range of -20°C to 60°C. It begins by tracing the historical development and increasing importance of PCC in both repair and construction. The experimental program involves preparing and conditioning mortar specimens under alternating thermal conditions, using three different thermoplastic polymers: styrene-acrylate copolymer (SA), ethylene-vinyl acetate copolymer (EVA), and styrene butadiene rubber copolymer (SBR). The study focuses on determining key mechanical properties such as flexural and compressive strength, Young’s modulus, and dynamic modulus of elasticity at different ages and temperatures. The results reveal significant changes in these properties due to temperature variations, with some unexpected behaviors observed, particularly at lower temperatures. The findings highlight the need for further investigation into the temperature impact on PCC and the development of accurate prediction models. This research contributes to the understanding of PCC behavior under real-world conditions and aids in the creation of robust prediction models for its mechanical properties.
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Abstract
Polymer-modified cement mortars (PCM) and concretes (PCC) are mainly used in concrete repair and restoration exhibiting improved durability, suitable chemical resistance, and beneficial adhesion strength compared to unmodified cementitious materials. Due to these favorable properties, the material is increasingly implemented in construction. Commonly, the modifiers applied to cementitious binders consist of thermoplastic polymers, which feature a change in the deformation behavior under the influence of different temperatures. Despite the distinct temperature-dependent properties of the polymers, the load-dependent deformation behavior of PCM and PCC was barely investigated within a service temperature range. To make statements about the effect of polymers on the load bearing and elastic deformation behavior of PCM and PCC, the engineering properties of the material have to be experimentally assessed under thermal conditioning. Accordingly, the compressive and flexural strength as well as dynamic and static modulus of elasticity of seven different PCM mixtures were characterized while the specimens were exposed to service temperatures of −20 ℃, 20 ℃, and 60 ℃. After the specimens were thermally conditioned in a climate chamber, the samples were transferred to the equally conditioned test machine and tested in the proposed temperature scope. The experimental results reveal influential changes in all tested mechanical attributes for the modified system within the applied service temperature range compared to an unmodified reference. This knowledge is essential to further investigate the temperature impact on the material and develop appropriate prediction models for the application of polymer-modified cementitious materials in construction and the integration in design guidelines.
1 Introduction
Firstly, patented in the 1920s [1], the development of and research on polymer-modified cementitious materials (PCC) increasingly intensified in the 1960s and 1970s [2]. Since then, the compound material was mainly used in repair and restoration but became eventually more important also as construction material due to specific mechanical properties compared with ordinary cement mortar and concrete. Alongside with Portland cement and aggregates, the modified mortar or concrete incorporates usually thermoplastic polymers. These polymers have the underlying characteristic to be sensitive to temperature. While higher temperatures can cause a decrease of strength and deformation resistance, lower temperatures can foster a brittle state of the polymer. Whereas the difference in temperature causing to change the material from a brittle to plastic state often lies within a small temperature range of approximately 40 ℃ to 70 ℃. Due to the necessity of application of polymer-modified mortar and concrete for refurbishment matters and the wish to be able to estimate the behavior of the material within the supposed service temperature range, multiple studies were conducted exposing the material to high-temperature conditions above 200 ℃ [3‐8]. Some studies included elevated temperatures but do not address typical mechanical behavior such as flexural and compressive strength or the determination of the modulus of elasticity [9‐13]. And a manageable number of studies were concerned with polymer-modified cementitious material exposed to cold climates [14]. Hence, the typical mechanical engineering properties of polymer-modified mortar and concrete under the thermal impact within a defined service temperature range between −20 ℃ to 60 ℃ need to be experimentally evaluated to further investigate the thermally implied impact and to develop fitting prediction models.
2 Experimental Program
To obtain adequate results, the experimental program followed consecutive steps, which were performed under alternating thermal conditions as it can be seen in Fig. 1. At a temperature of 20 ℃, the material was prepared and mixed to adequate mortar specimens, fresh mortar properties such as consistency, air void content, and fresh bulk density were determined as well as the mortar specimens were stored according to the corresponding standards. While the specimens were conditioned and then hardened mortar properties were obtained at temperatures of −20 ℃, 20 ℃, and 60 ℃.
Fig. 1.
Flow diagram of the experimental program
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2.1 Materials
Cement. All experiments were conducted using a Portland cement (CEM I 42.5 R). The product exhibited a density of 3.11 g/cm³, an average particle size of 14.29 µm, and a specific surface (Blaine) of 3580 cm²/g. The chemical composition can be seen in Table 1.
Table 1.
Chemical components of the used Portland cement in weight percentage
Chemical component
CaO
SiO2
Al2O3
Fe2O3
SO3
MgO
K2O
Na2O
LOI
Percentage [%]
64.1
19.5
5.0
2.9
3.2
1.5
1.0
0.2
2.2
Polymers. To modify the mortar, three different thermoplastic polymer dispersions were used: styrene-acrylate copolymer (SA), ethylene-vinyl acetate copolymer (EVA) and styrene butadiene rubber copolymer (SBR). All three polymer dispersions contained water as liquid phase and are commercially available as well as specifically suitable for the application in mortars. The material characteristics of the polymer dispersions are summarized in Table 2.
Table 2.
Material properties of the polymer dispersions
Polymer
SA
EVA
SBR
Main constituents
styrene, acrylic acid ester
ethylene, vinyl acetate
styrene, butadiene
Solid particle content [%]
Density [g/cm3]
Particle size range [µm]
Mean particle size [µm]
pH value at 20 ℃ [−]
Dynamic viscosity [mPas]
50.6
1.03 ± 0.02
0.04 – 2.11
0.15
8.18
50 − 200 (25 ℃)
53.7
1.07
0.52 – 7.08
1.34
3.24
400 (23 ℃)
50.8
1.02
0.08 – 0.21
0.13
8.04
30 – 150 (23 ℃)
MFT [°C]
Tg [°C]
Tm [°C]
33
22
380
0
−6
330
12
16
130
2.2 Specimen Preparation
According to DIN EN 196-1, a sand-to-cement ratio (s/c) of 3.0 was predefined so that 450 g cement and 1350 g sand were applied to each mixture. With respect to a w/c-ratio of 0.40 and the water given due to the liquid phase of the polymer dispersions, the mix design was determined as it can be seen in Table 3. The mixing process in compliance with DIN EN 196-1 was performed utilizing a mixer with automated mix program preset. The designation of the samples is based on the following principle. REF is the abbreviation of the reference sample whilst PCM is the one for polymer-modified mortars. The following numbers 1 to 3 mark the particular modifying polymer and the last numbers the polymer content in percent based on cement weight (p/c).
Table 3.
Mix design for the mortars
Sample ID
w/c
p/c
s/c
Water [g]
Polymer [g]
Cement [g]
Sand [g]
PCM_Ref
0.4
0.00
3.0
180.0
0.0
450
1350
PCM_SA05
0.4
0.05
3.0
158.0
44.5
450
1350
PCM_SA15
0.4
0.15
3.0
114.1
133.5
450
1350
PCM_EVA05
0.4
0.05
3.0
160.6
41.9
450
1350
PCM_EVA15
0.4
0.15
3.0
121.8
125.7
450
1350
PCM_SBR05
0.4
0.05
3.0
158.2
44.3
450
1350
PCM_SBR15
0.4
0.15
3.0
114.6
132.9
450
1350
2.3 Specimen Conditioning
The specimens prepared for the testing of flexural and compressive strength as well as stabilized secant modulus (Young’s modulus) and dynamic modulus of elasticity were stored according to the national annex of DIN EN 12390-2. After preparation, the molded mortar specimens were protected from loss of moisture and stored for 24 h at a temperature of 20 ℃. Then, the specimens were demolded and cured for six days in water at a temperature of 20 ℃. Following the seventh day of preparation, the mortar specimens were stored in a draught-free environment at 20 ℃ and 65% relative humidity. At the day of testing, the specimens were subjected to a temperature regime of −20 ℃, 20 ℃, or 60 ℃. Prior to the analyses at −20 ℃ or 60 ℃, the specimens were thermally conditioned utilizing a climate chamber. To guarantee a uniform temperature distribution throughout the complete specimens, the samples were cooled or heated for 2 h until the core of the sample reached the target temperature of −20 ℃ or 60 ℃, respectively.
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2.4 Test Procedures
Fresh Mortar Properties. The fresh mortar properties can be appropriately represented by determining the consistency using the slump test according to the procedure described in DIN EN 1015-3, the air void content by means of the pressure method according to DIN EN 1015-7 and the fresh bulk density identified due to volume and weight measurements according to DIN EN 1015-6.
Hardened Mortar Properties. The hardened mortar properties of the material used within this study are represented at first by the determination of bulk density according to DIN EN 1015-10, true density, and total porosity. The total porosity has been computed by applying a ratio of dry bulk density ρs,dry and true density ρ of specimens aged 2, 7, and 28 days. Subsequently, the total porosity P was calculated to:
$$P=1-\frac{{\rho }_{s,dry}}{\rho }$$
(1)
Furthermore, the flexural and compressive strength according to DIN EN 1015-11, the Young’s modulus following DIN EN 12390-13 (Method B), and the dynamic modulus of elasticity in accordance with DIN EN ISO 12680-1 were tested at the age of 2, 7, and 28 days at temperatures of −20 ℃, 20 ℃ and 60 ℃. Therefore, prismatic specimens were produced according to DIN EN 1015-2 with an edge length d and b of 40 mm and a length L of 160 mm. The dynamic modulus of elasticity was determined using the impulse excitation technique.
3 Results
3.1 Fresh Mortar Properties
Table 4 shows the results of the fresh mortar tests. The influence of the polymers on the fresh mortar properties is significant and existing research results could be confirmed. Generally, a liquefaction of the mortar by the polymer modification was observed. Furthermore, the mortars modified with styrene butadiene rubber copolymer (SBR) showed an increased air void content, what is also often described in relating publications especially using polymers without defoamer.
Table 4.
Fresh mortar properties
Sample ID
Slump [mm]
Consistency
Air void content [%]
Fresh bulk density [g/cm3]
PCM_Ref
102
stiff
4,5
2,30
PCM_SA05
122
stiff
4,3
2,27
PCM_SA15
199
plastic
2,8
2,24
PCM_EVA05
108
stiff
4,9
2,27
PCM_EVA15
143
plastic
5,4
2,20
PCM_SBR05
146
plastic
7,2
2,20
PCM_SBR15
217
soft
10,0
2,09
3.2 Hardened Mortar Properties
Figures 2, 3, 4, and 5 show the results of the hardened mortar tests, which are the first small steps to get an impression of the temperature-dependent behavior of the mortars.
The experimental results reveal influential changes in all tested mechanical attributes for the PCMs compared to an unmodified reference, but there is no general correlation. It was surprising that at −20 ℃ the flexural strength of all samples except PCM_SBR05 decreases with increasing hydration time. It is likely that the ice formation process is responsible for this effect because, especially at early ages, there is still a lot of water in the samples that can freeze. The behavior of the specimens tested at 20 ℃ and 60 ℃ is basically as expected. There is an increase in flexural strength with increasing sample age. The polymer modification leads to a slight increase in flexural strength, which is more pronounced for the high polymer contents. The SBR-modified samples behave a little bit different due to the rather high air void content. A decrease in strength with increasing temperature was observed for all mortars tested, where, contrary to expectation, the temperature dependence of the PCMs was not greater than that of the reference.
Fig. 2.
Flexural strength of the mortars at 2, 7 and 28 days and −20, 20 and 60 ℃
Fig. 3.
Compressive strength of the mortars at 2, 7 and 28 days and −20, 20 and 60 ℃
Fig. 4.
Young’s modulus of the mortars at 2, 7 and 28 days and −20, 20 and 60 ℃
Fig. 5.
Dynamic modulus of elasticity of the mortars at 2, 7 and 28 days and −20, 20 and 60 ℃
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×
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Under compressive stress, the mortars behave largely as expected. The polymer modifications cause a strength decrease of the PCMs compared to the reference. As the hydration time progresses, the strengths and deformation resistance of all mortars increase, with this behavior being most pronounced at 20 ℃. Furthermore, it can be observed that both compressive strength and elastic modulus are lowest at 60 ℃, although the differences between −20 ℃ and 20 ℃ are hardly significant and should be considered individually for each formulation. In conclusion, it can be stated that the thermoplastic polymers do not necessarily lead to a greater temperature dependence of the hardened mortar properties. This knowledge is essential to further investigate the temperature impact on the material and develop appropriate prediction models.
4 Conclusions
The application of different service temperatures leads to significant differences in the mechanical parameters of the mortars investigated, so that temperature must be considered as a significant factor in modelling of the material behavior. The results of the presented investigations are the input parameters beside others which are to be bundled in a semi-analytical multiscale model based on the methods of continuum micromechanics. By means of a bottom-up approach, homogenized properties at the macroscale are determined using the specific microstructural behavior. The microstructure changing as a result of the temperature influence can thus be directly correlated with the macroscopic material behavior. After extending an existing multiscale model [15] by considering principles of thermo-poro-elasticity, the temperature dependence of the mechanical properties of polymermodified mortars and concretes will be predictable.
Acknowledgments
This research is supported by the German Research Foundation (DFG) via research grant for the project “Experimental investigations and microstructure-based modeling of the elastic and visco-elastic behavior of PCC depending on temperature”, which is gratefully acknowledged.
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Evaluation of Strength and Modulus of Elasticity of Polymer-Modified Cement Concrete (PCC) Under Thermal Impact Within a Defined Service Temperature Range
Authors
Alexander Flohr Catharina Rohde Savitha Devarajamohalla Narayana Andrea Osburg
