# Investigation into the mechanical performance of concrete foundations affected by Alkali-Silica Reaction

Sylvain Langlois, Amélie Fau, Maroua Maaroufi, Fikri Hafid, Farid Benboudjema

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Sylvain Langlois, Amélie Fau, Maroua Maaroufi, Fikri Hafid, Farid Benboudjema. Investigation into the mechanical performance of concrete foundations affected by Alkali-Silica Reaction. Proceedings of the 15th fib International PhD Symposium in Civil Engineering, Fédération Internationale du Béton, pp.1127-1133, 2024, 978-2-940643-24-0. hal-05231437

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# Investigation into the mechanical performance of concrete foundations affected by Alkali-Silica Reaction

Sylvain Langlois, Amélie Fau, Maroua Maaroufi, Fikri Hafid, Farid Benboudjema

CentraleSupélec, ENS Paris-Saclay, CNRS, Laboratoire de Mécanique Paris-Saclay,
Université Paris-Saclay,
4 Av. des Sciences, 91190 Gif-sur-Yvette, France

## Summary

Concrete foundations for electrical transmission line face pull-out loads, with aging and pathologies like alkali-silica reaction (ASR) leading to potential stability risks. Understanding the mechanical impact of this pathology is still complex due to its multi-physics nature. The aim of this work is to investigate the mechanical performance of concrete foundations affected by ASR. A pylon pull-out test from a small-scale mortar foundation is carried out in-situ in an X-ray tomograph, along with standard testing to evaluate the material's mechanical properties. Despite a degraded Young's modulus and an unchanged flexural strength by ASR, pull-out resistance increases, and failure modes are impacted. Digital Volume Correlation can be used to analyze swelling heterogeneity, cracking pattern and its influence on the foundation's mechanical strength.

## 1 INTRODUCTION

Concrete foundations and pylon installation began in the early 20th century. When tetrapod towers experience overturning moments, two foundations among the four are under tension, which is the most challenging stress for the foundation. Research efforts have focused on developing analytical models for soil behavior during pull-out conditions, considering geotechnical parameters as outlined by [11]. Additionally, studies have examined the concrete foundation [14] to ensure intrinsic stability. [9] highlights infrastructure aging as a significant contributor to mechanical stability issues in the tower/foundation system. However, studies have not investigated concrete foundation aging resulting from environmental interactions. Alkali-silica reaction (ASR), an internal swelling reaction, may occur due to inadequate concrete manufacturing practices during electrical tower foundation construction.

The degradation of concrete resulting from ASR is ongoing and slow. Over time, alkalis naturally present in cement dissolve amorphous silica from aggregates in an aqueous media. This reaction generates an alkali-silica gel, resulting in gradual swelling of concrete caused by internal pressure. This process may lead to macroscopic cracks which decrease the concrete's performance and lifespan. The multi-scale and multi-physics (non linear) processes make understanding the mechanical impact of this reaction challenging [6]. Recently, numerical simulation has gained significance in the field of ASR. Its utility lies in providing deeper insights into expansion mechanisms under multiaxial loading [13]. However, the models require further investigation to quantify the ASR-induced damage. Additional data are needed to establish the link between macroscopic swelling and material cracking. One way to overcome this issue is in-situ mechanical testing, conducted directly within the X-ray tomograph, allowing degradation to be observed in real-time [5].

This study aims to investigate the effect of ASR on the pull-out bearing capacity and failure mode of a concrete foundation used in electrical transmission lines. A reactive mortar is developed and mechanical properties are characterized. The expansion at a macroscopic level is measured to assess the apparition of ASR. Compressive and tensile properties are measured through compressive and bending tests. Additionally, in-situ pull-out tests are conducted on a 1/50 scale model of the mortar foundation. The force and displacement data from the tensile testing machine are analyzed along with the failure mode of the specimen at various stages of ASR. The findings reveal that CT accurately identifies changes in the foundation's failure mode with the development of ASR, which can be linked to the mechanical properties of the mortar.
# 2 MATERIALS AND METHODS

## 2.1 Materials

A reactive mortar is formulated to induce an alkali-silica reaction, leading to significant swelling within a short timeframe. The aggregates have a 0.25 to 4 mm diameter and are composed of 50% of siliceous limestone reactive aggregates and 50% of limestone non-reactive aggregates to avoid pessimum effect. According to [7], the quantity of reactive silica in siliceous limestone sand is around 6.9%. The cement is CEM II/A-LL 42.5R, with an alkali content estimated at 0.96%. The mortar is manufactured following a ratio W/C=0.5 and a ratio S/C=3. A 1M sodium hydroxide solution enhances the alkali content in the interstitial solution. This substitution results in an alkali content of 8.1 kg. m⁻³mortar, ensuring a sufficient supply of alkaline material for subsequent reactions. The mortar is then poured into moulds to conduct compressive tests, bending tests, swelling measurements and in-situ pull-out tests. Following demolding, the specimens are kept in autogenous curing (two layers of plastic film and two layers of aluminium foil) at 20°C for 28 days to ensure the development of mechanical properties in a non-aggressive environment. Subsequently, they are submerged in a 1M NaOH solution to maintain fully saturated conditions and a constant alkaline concentration in interstitial water (avoiding alkaline leaching). The bath temperature is maintained at 60°C to accelerate the reaction kinetics.

Experiments are conducted at various stages of swelling. Indeed, samples are tested at 0% (no swelling), 35%, 65%, and 100% of the final swelling. These tested samples are denoted as ASR-M-0, ASR-M-35, ASR-M-65, and ASR-M-100, respectively.

## 2.2 Macroscopic mechanical testing

Swelling measurements are carried out in accordance with the standard detailed in [3]. The test measures the variation in length between two opposite faces of prismatic mortar specimens, 4x4x16 cm in size, fitted with brass studs. The specimen is placed in a rigid frame, with the lower face fixed and the upper face fitted with an LVDT sensor. The elongation measurement then corresponds to a longitudinal strain as

$$ \epsilon = \frac{L - L_0}{L_0} \qquad (1) $$

Compressive strength is measured in accordance with standard [1]. A cylindrical specimen measuring 11x22 cm, rectified on its loaded faces with surface area S, is brought to failure with a force F at a loading speed of 10 kN.s⁻¹. Its compressive strength f_c is measured as

$$ f_c = \frac{F}{S} \qquad (2) $$

The specimen is fitted with an extensometer, equipped with 3 LVDT sensors, to measure longitudinal strain ε_l on a 100 mm measurement base, centered on the height of the specimen to avoid fretting phenomena. The sample is subjected to 3 loading cycles of lower threshold σ_a = f_c/10 and upper threshold σ_b = f_c/3. The Young's modulus is calculated according to standard [2] and corresponds to the initial secant modulus calculated during the first loading cycle, with ε_a the strain at the upper stress limit and ε_b the strain at the lower stress limit according to

$$ E = \frac{\sigma_a - \sigma_b}{\varepsilon_a - \varepsilon_b} \qquad (3) $$

The Poisson ratio is measured using 3 LVDT sensors placed in the extensometer to measure transverse strain ε_t during the test, according to

$$ \nu = - \frac{\varepsilon_t}{\varepsilon_l} \qquad (4) $$

The material's flexural strength R_f is determined by a 3-point bending test. A prismatic mortar sample, measuring 7x7x28 cm, is brought to failure. The test is also used to determine the material's tensile strength and fracture energy by the means of Finite Element Method Updated (FEMU). Fracture is reached between 30 and 60 seconds after the start of the test, as recommended by RILEM [12], and the machine is controlled by Crack Mouth Opening Displacement (CMOD) to ensure stable specimen fracture at a rate of 0.15 µm/s.
## 2.3 In-situ mechanical test

The scaled-down structure consists of a Glass-Fiber-Reinforced-Polymer (GFRP) reinforcement, acting as the pylon foot, anchored in a 1/50 scaled mortar foundation (whose dimensions are written in Fig. 1 (left) in mm). The mortar is manufactured and poured directly into an 3D printed mold (using ABS material). A fixing template allows the reinforcement to be sealed centred and vertical to the foundation. Before the test, the base of the foundation is affixed directly to an ABS collar, which is then fastened to the testing machine using screws. A clamp tightens the pylon and is pinned to the testing machine.

<figure><img src="image_2.png" /><figcaption>Fig. 1 Dimensions (in mm) of the specimen composed of the reinforcement and mortar foundation (left), and experimental set-up drawing with boundary conditions (right)</figcaption></figure>

Mechanical pull-out tests were performed in-situ using a tension-compression-torsion machine custom-designed to accommodate the tomograph (North Star Imaging X50+), allowing precise imaging between the X-ray tube, the specimen, and the detector without any obstruction. The foundation is loaded in steps, and Computed Tomography (CT) scans are performed along with the loading history. The pull-out prescribes a vertical (z-axis) displacement with a stroke speed of 3 µm/s on the foundation while the pylon remains fixed ($\delta_v = 0$), as shown in Fig. 1 (right). Each CT scan consists of 1200 projections and takes approximately 30 minutes per scan. The scans are carried out using the X-Ray worX XWT-240-CT scanner, and detailed parameters can be found in Table 1.

<table><caption>Table 1 CT hardware parameters</caption><tr><td>X-ray source</td><td>X-Ray worX XWT-240-CT</td></tr><tr><td>Target</td><td>32.5 W (reflection mode)</td></tr><tr><td>Filter</td><td>None</td></tr><tr><td>Voltage</td><td>210 kV</td></tr><tr><td>Current</td><td>150 µA</td></tr><tr><td>Tube to detector</td><td>615 mm</td></tr><tr><td>Tube to object</td><td>105 mm</td></tr><tr><td>Detector</td><td>Dexela 2923</td></tr><tr><td>Definition</td><td>3888 x 3072 pixels</td></tr><tr><td>Binning</td><td>2 x 2</td></tr><tr><td>Number of projections</td><td>1200</td></tr><tr><td>Angular amplitude</td><td>360°</td></tr><tr><td>Frame average</td><td>15 per projection</td></tr><tr><td>Frame rate</td><td>15 fps</td></tr><tr><td>Acquisition duration</td><td>Around 30 min</td></tr><tr><td>Reconstruction algorithm</td><td>Filtered back-projection</td></tr><tr><td>Gray levels amplitude</td><td>16 bits</td></tr><tr><td>Resolution</td><td>45 µm/voxel</td></tr></table>
# 3 RESULTS

## 3.1 Macroscopic mechanical test

Figure 2 illustrates the longitudinal swelling of the reactive mortar formulation and a standardized (NF EN 196-1) non-reactive mortar. During the 28-day autogenous curing period, the reactive mortar experiences autogenous shrinkage up to -300 µm/m. Swelling becomes evident during curing under 1M NaOH at 60°C, exhibiting rapid kinetics in the initial weeks. After 119 curing days, swelling peaks at approximately 5500 µm/m, surpassing the threshold of 500 µm/m used to assess aggregate reactivity.

<figure><img src="image_3.png" /><figcaption>Fig. 2 Swelling of 4x4x16 cm reactive and non-reactive specimens according to the age (error bars represent the ± standard deviation of 3 specimens)</figcaption></figure>

The mechanical tests were conducted at various stages of swelling due to ASR: 0% (aged 28 days), 35% (aged 42 days), 65% (aged 63 days), and 100% (aged 119 days) of the strain measured at 119 days. As depicted in Fig. 3 (left), the compressive strength starts at 28.9 MPa at 28 days and exhibits a quasi-linear increase with swelling. Meanwhile, Young's modulus, presented in Fig. 3 (right), is measured at 26.2 GPa at 28 days. It experiences a 31% reduction at 14 days of accelerated aging, followed by a slight recovery and further increase as the reaction progresses. Few data are available regarding the evolution of Poisson's ratio. As illustrated in Fig. 4 (left), its initial value is about 0.27 at 28 days, showing a 9% decrease in the third stage of swelling and a 22% decrease at 65% swelling.

<figure><img src="image_4.png" /><figcaption>Fig. 3 Compressive strength (left) and Young Modulus (right) of 11x22 cm mortar cylinder specimen according to different stages of swelling</figcaption></figure>

In the 3-point bending test, the flexural strength (see Fig. 4 (right)) shows a slight decrease from 5450 N up to 4800 at 14 days of accelerated aging. Subsequently, this resistance increases to 5800 N and nearly returns to its initial value at 100% swelling. Notably, these tests exhibit high standard deviations. Overall, the results indicate a significant increase in compressive strength as the reaction progresses, a decrease in sample stiffness, and relatively constant flexural strength reflecting the material's tensile properties.
<figure><img src="image_5.png" /><figcaption>Fig. 4 Poisson ratio (left) of 11x22 cm mortar cylinder specimen and Flexural strength (right) of 7x7x28 cm (span of 24 cm) mortar prism specimen according to different stages of swelling</figcaption></figure>

## 3.2 In-situ mechanical test

Three in-situ tomograph pull-out tests are conducted at each ageing stage. The force and the stroke are recorded during these tests. A typical test curve is depicted in Fig. 5 (left). To ensure full contact within the setup, a preload of 50 N is applied. Before each scan, a pause is marked to allow the specimen to relax before performing a 3D CT scan. Scan 0 is the reference scan, representing the state without mechanical load. Subsequent scans are taken at increasing pull-out forces: scan 1 at 200 N, scan 2 at 550 N, and scan 3 at 920 N. Scan 4 is performed on the fly, ideally near the (unknown) peak force. Following fracture, a post-mortem scan is conducted to get the 3D cracking pattern. Due to limitations in the experimental setup and sample geometry, precise control over crack initiation and propagation is not possible. Failure is characterized by a significant force reduction (quasi-brittle process). This effect is less pronounced in samples with no ASR or few swelling, where the force decreases incrementally, corresponding to a progression of the front process zone. The peak force determines the pull-out resistance. Fig. 5 (right) illustrates the pull-out resistance of foundation samples at various ageing stages. The measurements exhibit a high standard deviation, ranging between 10% and 20% of the mean resistance value, typical of direct tensile tests on concrete. Initially, the resistance remains constant during the first ageing stage. Subsequently, it increases to 20% at 65% of swelling and further to 40% at the final stage.

<figure><img src="image_6.png" /><figcaption>Fig. 5 Pull-out force vs machine displacement during the in-situ test, where different CT scans are represented (left), pull-out strength according to different stage of swelling (right)</figcaption></figure>

Fig. 6 depicts a post-mortem scan comparing a sample unaffected by ASR (ASR-M-0 at left) with one in the advanced stage of swelling (ASR-M-100 at right). On the ASR-affected sample, cracks due to aggregate swelling are observed. These cracks are located within the aggregates, with some extension into the cementitious matrix, which undergoes tension. Pull-out loading causes mode I failure of the foundation. As the reaction progresses, a shift in failure mode is noted. In unaffected mortar, initial cracking occurs at the junction between the chimney and the first slab. Subsequently, as reinforcement anchors in the second slab, the load is transferred to the foundation, culminating in peak failure characterized by a crack between the first and second slabs, leading to sample failure, as illustrated in Fig. 6 (left). However, with the onset of the reaction, cracks emerge solely between the two slabs, as depicted in Fig. 6 (right). Among ASR-M-65 samples, 1/3 of the specimen exhibited the latter failure mode, compared to 2/3 of ASR-M-100 samples.
<figure><img src="image_7.png" /><figcaption>Fig. 6 CT post-mortem scan of an ASR-M-0 foundation (left) and ASR-M-100 foundation (right). Arrows depict the crack due to mechanical loading and ASR swelling.</figcaption></figure>

## 4 DISCUSSION AND CONCLUSION

The mechanical test indicated an increase in compressive strength, a decrease in Young's modulus, a constant flexural strength and Poisson's ratio, and an increase in pull-out strength during the development of ASR. Several hypotheses can be proposed to explain these findings. Cement hydration persists for several months following its production, contributing to the enhancement of the mortar's mechanical properties over time. The silica gel produced during the reaction migrates into the interconnected porosity between the aggregate and the cement paste. The porosity, which is then filled with gel, helps increase the strength of the material. As gel has a higher modulus of elasticity than shear modulus [4], it is possible that it contributes more to mortar resistance in compression than in tension. CT scans reveal the emergence of cracks in the ASR-M-35 samples, occurring two weeks after immersion in a NaOH bath at 60°C. These cracks are mainly located in the aggregates, with some extending into the cement paste. Cracking is the main source of the decrease in Young's modulus [10], which may explain the significant drop in Young's modulus in the ASR-M-35 samples. The increased resistance of the reduced-scale foundation to pull-out forces cannot be explained directly by the material parameters presented above, as the increase in compressive strength alone does not increase pull-out resistance. The swelling induced by ASR in the mortar is constrained by the GFRP reinforcement, creating a swelling gradient that needs further investigation to elucidate its role in the mechanical strength of the foundation.

This study investigates the influence of ASR on the mechanical performance of the concrete foundation/pylon system under pull-out conditions. For this purpose, a protocol was led to create ASR in a 0.25-4 mm mortar. A novel experimental setup was designed to assess a scaled-down model of the pylon/mortar foundation system. CT scanning is employed to monitor the microstructure of the model during loading history. It has been shown that, despite the appearance of intergranular cracks, pull-out resistance increases with the development of ASR. Initially, the foundation cracks in mode I at the junction between the chimney and slab 1, then proceeds to failure when cracking occurs between slab 1 and slab 2. The onset of alkali-silica reaction impacts this failure mode, causing the foundation to fracture solely between the two slabs. Changes in the mortar's mechanical properties cannot explain this impact. Digital Volume Correlation, based on scans taken during mechanical loading, makes it possible to measure the displacements of the reinforcement and the foundation [8] to determine whether the heterogeneous swelling of the mortar induces a prestressing effect.
## Acknowledgements

This research was partly funded by the French National Research Agency (ANR) under the "HEAT COFFEE" project (grant "ANR-22-CHIN-0007-01") and the Electricity Transport Network company (RTE).

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