Fracture strength of geogrid-reinforced concrete pavements with carbon-fiber composites insertion

ABSTRACT

Repeated and multiple traffic loads, along with climatic conditions, influence the mechanical behavior of linear pavements, leading to the formation of cracks that propagate significantly across the wearing surface and result in a loss of load-bearing capacity of the pavement body. To remedy this problem, various reinforcement and repair methods (traditional and modern) are applied to address this issue. The use of geogrids, involving the insertion of sheets at the interfaces of the sub-base layers, has proven effective as an alternative solution due to their mechanical and aesthetic performance. However, these geogrids, primarily serving as a separation layer, are sometimes limited in the gains they make in reducing stresses and strains, since these gains do not exceed 5 to 10%. Consequently, researchers have sought other techniques that provide both separation (to prevent crack propagation) and strengthening (to increase the bearing capacity of the pavement). In this article, we propose to study the reinforcement of rigid cement concrete pavements through an experimental approach, using two laboratory batches, each comprising a number of twenty-two (22) small-scale slabs, with dimensions of 400 × 400 × 50 (mm). The first batch was produced at an ambient temperature of 20°C while the second batch was produced at an elevated temperature of 50°C (arid climate). These slabs will be tested in 4-point bending, after reinforcement with different combinations of geogrids and carbon fibers composites. To compare the experimental results obtained, a numerical simulation based on the finite element method, using appropriate software, was conducted. The results regarding stresses and strains, as well as dissipation energy, showed that the combination adopted is very effective, yielding gains of up to 20 to 35%, additionally the integration of geogrids, with the addition of the composite, enhances the reinforced pavement’s longevity, ensuring long-term savings on its upkeep and maintenance.

Keywords:
Concrete Pavement; Geogrid; Composite; Experimental Test; Numerical Analysis

1. INTRODUCTION

Rigid pavements made of cement concrete are utilized for roads, motorways and airport runways enduring excessive, repetitive loads, alongside ambient climatic conditions such as cold, frost, high temperature, and thermal gradient. This type of pavement is designed to possess a high structural capacity to withstand heavy loads, including instances of intense heat generation, as observed in the case of military aircraft [¹]. In addition, rigid pavements are susceptible to surface and joint cracks induced by tensile stresses from recurrent moving loads or even thermal gradient effects [², ³]. These cracks can propagate downward and affect the lower layers and wearing courses (Figure 1), considerably reducing the structural capacity of the pavement and escalating the risk of damage [⁴].

Figure 1
Damage and visible cracks in rigid pavements.

In high-temperature zones, crack propagation and the risk of damage are exacerbated, due to the elevated thermal gradient [⁵]. This phenomenon has been substantiated in a numerous cases involving airfield runways and road pavements, leading to deleterious effects and the onset of fatigue and obsolescence [⁶, ⁷]. Even in regions subject to seasonal frost, there are many diseases typical of cement concrete pavements, including broken slabs, displaced platforms, scattered cracks, exposed aggregates, broken corners and potholes [⁸]. Moreover, the performance of constructions under actual operating conditions is influenced by environmental exposure. It is important that concrete roads and airport pavements withstand not only mechanical damage, but also the effects of alternating freezing and thawing. Indeed, freeze-thaw damage poses a significant threat to concrete pavements in areas with seasonal frost [⁹]. This presents a serious challenge for infrastructure in cold regions [¹⁰].

Accurate detection of these diverse forms of damage and the underlying causes of cracking, at an early stage enables the selection of optimal preservation and rehabilitation strategies. This can only be achieved by analyzing and diagnosing the entire structure of the defective pavement or runway, particularly focusing on joints [¹¹].

A robust diagnosis facilitates the identification of appropriate solutions, thus ensure the extending of service life of the rigid pavement [¹²]. Thorough analysis and diagnoses, empower experts to comprehend the types of deterioration that have occurred and find the most suitable solutions to remedy the problem [¹³,¹⁴,¹⁵]. It is imperative to implement periodic maintenance and upkeep policies to safeguard the integrity of the pavement surface over time [¹⁶] or to give priority to the rehabilitation of the deteriorated pavement.

Several techniques for reinforcing and repairing rigid pavements have been applied over the last 35 years in the context of road and runway rehabilitation contexts [¹⁷,¹⁸,¹⁹,²⁰]. In the literature, there are numerous repair techniques, in addition to traditional techniques involving the application of an improved concrete layer or crack sealing with additive mortar or innovative products that significantly enhance the mechanical properties of the pavements. However, these methods prove ineffective when surface cracks appear and propagate to the depth of the pavement body [²¹].

Various techniques have been developed in the past to address these reflection cracks, with the most significant being the application of geosynthetic materials [²², ²³]. Numerous studies are conducted in this field have demonstrated that optimal utilization of geogrids (a family of geosynthetics) can delay crack initiation and thus reduce propagation rate, given their primary function of separation. The application of these geogrids appears to emerge as an alternative and effective repair solution, given their mechanical performance, provided that careful considerations are made regarding grid material, mesh shape, dimensions, rigidity and positioning within the section to be reinforced [²⁴].

However, many researchers are critical of the choice of geogrid alone in the reinforcement of linear pavements, as the desired objective encompasses both separation (crack attenuation) and increased traction (stress) through deformation reduction as desired [²⁵, ²⁶]. Hence, It is imperative to conduct further research into the combination of two or three products (materials) at the same time, commonly referred to as “hybrid” reinforcement, for the repair or rehabilitation of rigid pavement structures [²⁷, ²⁸].

This paper presents an experimental laboratory study of the behavior of rigid cement concrete pavements (simulated as small-scale slabs), with dimensions of 400 × 400 × 50 (mm). A total of 44 (forty-four) samples (test pieces), were divided into two categories. The first category, comprised 22 slabs, either unreinforced or reinforced by a hybrid combination (geogrid-composite), under ambient temperature conditions of 20°C. The second category, also consisting of 22 slabs, under the same reinforcement conditions, but built up and left at elevated temperature of 50°C (within a temperature-controlled oven). This selection was based on simulations of rigid pavements in Algeria, reflecting two different temperature conditions, 20°c (representative of the northern climate) and 45°c to 50°c (characteristic of the arid in the southern region). These slabs, manufactured with identical concrete composition, are cured at 28 days, and then tested in 4-point bending, after reinforcement by hybrid combinations of geogrids and CFRP composites.

Subsequently, this experimental work was complemented by a numerical analysis, based on the finite element method, and appropriate commercial software, aiming to align the laboratory findings with numerically derived results regarding stresses, strains and dissipation energy. Interpretation of the results found in this study revealed a certain tendency between experimentation and numerical modeling.

The various reinforcements adopted were demonstrated efficacy in terms of stiffness and energy absorption, despite the geogrid reinforcement alone increasing the ductility of the tested pavement slabs. The incorporation of the carbon fibers composite extended the service life of the reinforced pavement, ensuring long-term maintenance cost savings.

2. EXPERIMENTAL PROGRAM

2.1. Principle and materials used

The main objective of this laboratory study is to conduct tests on rigid concrete specimens (slabs), focusing on the 4-point bending behavior of Portland cement concrete slabs, reinforced with various combinations, subjected to monotonic loading until failure under environmental conditions at two distinct temperatures of 20°C and 50°C (Figure 2). To this end, the experimental program included the extraction of the required mechanical parameters including strength, modulus of elasticity, stress, strain and absorption. Technical comparisons among different types of reinforcements considered (a single layer of geogrid, a composite sheet, two layers of geogrid, combination of a layer of geogrid with a composite sheet), will be thoroughly discussed.

Figure 2
Pouring concrete test slabs.

The cement utilized in this study comprises the investigated composition incorporating two (02) different types of geogrid and two types of carbon fiber composite were used to prepare the samples. All these materials were chosen based on test conditions, equipment and instrumentation availability, and compliance with the standards [²⁹, ³⁰]. The materials used were CEM IIA 42.5 aggregates, crushed to a maximum size of 15 mm, along with fine limestone aggregates to form the specimens [³¹] the mix maintains a water/cement ratio (w/c) of 0.51.

A plasticizing admixture and an entraining admixture were added to achieve a high slump and ensure adequate contact and interaction with the geogrid and composite. The composition (per 1 m³ of concrete) of the mix used is detailed in Table 1. The compressive strength of the concrete, measured on day 28, is 39.5 MPa.

Table 1
Test concrete components.

CONSTITUENT	(Kg/m3)
Cement	350
Sand 0/1	252
Sand 0/3	612
Gravel 3/8	416
Gravel 8/15	658
Water	1801
Plasticizer	38
Air entertainer	0.12

The additive materials employed for the reinforcement, i.e. the two types of geogrid and the two sheets of CFRP composite underwent quality control tests to ascertain their mechanical characteristics. These tests included the tensile test on wide strips, conducted using the “Instron 5900” universal tensile testing machine [²⁴], situated at the public works control body (CTTP) in Algiers, these tests were conducted in accordance with standard ISO-10319 [³²], which determines tensile strength in both directions, and standard ISO-524-4, for composites [³³].

The geogrid (type 1) utilized in this study is the “Notex AC 400/30–25” reinforcement grid, featuring acrylic impregnation and high-tenacity polystyrene reinforcement cables. Its technology is recognized as woven and knitted (Figure 3a). The reinforcement geogrid (type 2) denoted as “Notex Glass C 50/50 −40”, incorporates high modulus glass fiber reinforcement cables combined with a 17g/m² polyester fleece. It also uses a “woven-knitted-twisted” geogrid, a technology that ensures immediate tensioning of the technical yarns (Figure 3b).

Figure 3
Types of geogrids used.

As for carbon fiber sheets (CFRP composites), the type 1 selected, based on carbon fibers, is a unidirectional fabric designed to reinforce concrete structures and known as “SikaWrap®-230 C/45” [³⁴]. This fabric can be applied dry, without prior impregnation (Figure 4a). Finally, the last reinforcement material, black in color, is the bidirectional carbon fiber fabric composite (TFC), it comprises carbon fibers oriented at 90°, in both the warp and west, so as to obtain a flexible and deformable reinforcement capable of adapting to the support’s shape (Figure 4b).

Figure 4
Types of composites used.

The physical and mechanical properties of the two geogrids and the CFRP composites are presented in Table 2.

Table 2
Physical and mechanical properties of the geogrids and composites used.

PROPERTY	UNIT	GEOGRID 1	GEOGRID 2	COMPOSITE 1	COMPOSITE 2
Mass per unit area	g/m2	711	270	/	/
Geometry	mm	35 × 35	35 × 35	30 × 30	30 × 30
Elastic modulus	GPa	/	/	234	240
Tensile strength	MPa	421.5	432.6	4 300	4900
Elongation at failure	%	2.9	2.5	1.8	2.1

2.2. Test description

The laboratory simulation, involving the manufacturing of specimens (slabs), encompasses the main elements representing a rigid pavement structure [³⁵]. These specimens underwent a four-point bending test adhering to European standards: BS EN 1339 [³⁶] and EN 12390-5 [³⁷].

Each slab measured 400 mm, with a span of 360 mm. The distance between loads was 120 mm, and load “P” was applied by two similar rollers (P/2) at two points along the span length (Figure 5). The test was conducted using the servo-hydraulic loop testing machine, employing displacement control at a constant rate of 0.04 MP/second and the loads acting on the slabs were measured a 600 kN load cell, with each slab loaded to failure.

Figure 5
4-point test configuration.

The static diagram was selected to induce failure through bending while adhering to the a/h ratio, where a: represents distance between the support and the point of application of the load and h: denotes the height of the slab [³⁸]. A displacements measuring device (sensor) was mounted on the central tension bar in the bending span.

For each test category of 22 specimens (Figure 6a), two different temperatures, 20°C and 50°C, were used, for the different series of slabs tested. The first series (control test) consists of two (2) specimens manufactured without reinforcement, using the concrete composition mentioned above. For the second series (2 × 4), eight slabs reinforced with a layer of G1 or G2 (geogrid), or a C1 or C2 (composite layer). In this second series, the specimens are built: initially, a 20 mm layer of concrete is poured into the mold and compacted using a table-top vibrator.

Figure 6
Preparation and finishing of test slabs.

The geogrid (or composite) was meticulously placed atop the concrete layer, followed by the pouring of a second layer of concrete, which is carefully compacted and finished with a trowel (Figure 6a). A third series of (2 × 2) test slabs, reinforced with two layers of geogrid, totaling 4 slabs, was prepared similarly, i.e. a 10 mm layer of concrete was poured and compacted, followed by a layer of geogrid, then a second 10 mm layer of concrete, another layer of geogrid, and finally a third layer of concrete, well compacted and finished, for the remaining height of the concrete. Similarly, a fourth series, consisting of (2 × 4), i.e. eight (08) test pieces, was fabricated. This series involved combining a layer of composite (C1 or C2) (Figure 6b) and a layer of geogrid (G1 or G2), to obtain the required slabs, which will be stored in a cooling chamber (Figures: 6c, 6d).

Table 3 summarizes the series of specimens (slabs) produced, with the name and identification of each one.

Table 3
Nomination and identification of tests.

NUMBER	TYPE OF REINFORCEMENT	APPOINTMENT	LOCATION
01	Without	R	–
02	Geogrid 1	G1	1/3 of the base
03	Geogrid 2	G2	1/3 of the base
04	Composite C1	C1	2/3 of the base
05	Composite C2	C2	2/3 of the base
06	Two geogrid G1	G11	1/3 and 2/3 of the base
07	Two geogrid G1	G11	1/3 and 2/3 of the base
08	Composite C1 + Geogrid G1	C1G1	1/3 and 2/3 of the base
09	Composite C1 + Geogrid G2	C1G2	1/3 and 2/3 of the base
10	Composite C2 + Geogrid G1	C2G1	1/3 and 2/3 of the base
11	Composite C2 + Geogrid G2	C2G2	1/3 and 2/3 of the base

3. NUMERICAL ANALYSIS

Numerical modeling involves the selection of a three-dimensional (3D) model, using the finite element method (FEM) and developed using the commercial software “Ansys Student/Workbench’’. In the present work, we have taken the dimensions of the test slab 400 × 400 × 50 (mm), thus simulating the rigid pavement using M39 grade concrete. For the boundary conditions and as the rigid pavement rests on the ground, which means that it is an elastic foundation, the stiffness taken is 24 KN/mm3. For the horizontal plane, we took the transverse direction (y–y) at both ends and the longitudinal direction (x–x') [³⁹]. The pavement design is based on a monotonic load, taken from the experiment, with its limit value P/2, equivalent to an axle wheel, placed symmetrically to the track.

The three-dimensional finite element models (3DFE) presented in this study, produced using the Ansys/Workbench commercial software, are based on the geometric and mechanical properties of the materials used (concrete, composite, geogrid), since 3DFE analysis has recently developed into a powerful tool capable of capturing the response of rigid pavements, which cannot be provided by conventional two-dimensional (2DFE) programs [⁴⁰].

The 3DFE mesh of concrete slabs, geogrid sheets and composite sheets [⁴¹] was produced to generate a finite element model (FEM). The material properties for the concrete, geogrid and composite elements were modeled by defining the elastic properties of the materials. The concrete material model was a continuous, isotropic model with uniaxial tensile and compressive response. The geogrid was modeled as a biaxial laminar membrane material with equivalent stresses in both principal directions (transverse and longitudinal) and also with the same failure stress. The composite used, on the other hand, was modeled as ductile and anisotropic, since its mechanical properties are not the same in the main directions (x, y and z). The geogrid had a thickness of 0.9 mm and an average modulus of elasticity of 800 MPa, whereas the composite had a very low thickness of 0.42 to 0.45 mm and a high modulus of elasticity of 234 GPa or 240GPa, depending on the composite used.

This process involves inputting geometric characteristics and mechanical properties of the experimental slab, the incorporated geogrid and the introduced carbon fiber composite. To model the tested beam, the same static scheme depicted in the experimental part (Figure 5), was adopted. Four types of elements were selected to model the cross-section of the test specimen (slab). All of which elements were implemented in the appropriate software, following the Mohr-Coulomb criteria [¹⁴]. For the concrete, isotropic block elements were employed for modeling concrete, while one-dimensional linear elastic block elements were chosen for geogrid (and composite).

The numerical model chosen conforms to the experimental configuration (static diagram), in “4-point” bending, with the dimensioning mentioned in Figure 5. Figure 7 shows a view of the numerical model with the two applied loads of value (P/2). Given the lack of possible analytical equations, especially for the insertion of the geogrid sheet or the composite sheet in rigid slabs, we were obliged to follow the recommendations of previous research converging in this direction, such as the work carried out by SAPOUNTZAKIS and KATSIKADELIS [⁴²] in 2003, showing that the numerical solution is that of the plate bending problem describing the plane stresses and the resulting plane strains, or that of SILVA et al. [⁴³], in the field of flexible pavements, using mechanico-empirical methods of pavement engineering design and calculation for the study of fatigue, or directly by following the implementation recommendations in the technical data sheets for geosynthetic products and composites [⁴⁴, ⁴⁵].

Figure 7
Numerical model with the two applied loads.

The finite element model underwent validation through comparison with the results of the experimental study on a series of laboratory tests. The geometry and simulation of the support layer mirrored those of the experimental work developed. The explanation for the choice of the geometry of the experimental slab is based on the availability of “4 point” bending material. Starting from simulating a rigid roadway (in practice), we took the support layer, in the numerical analysis, as being boundary conditions and as the rigid roadway rests on the ground, we took the equivalent of an elastic foundation, with a stiffness of 24 KN/mm³. This detail completes the explanation given (answer relating to the explanation of the numerical model). Due to the involvement of slab dimensions and optimize computing time, mesh and convergence analyses were conducted by evaluating different models [⁴⁶]. In order to determine the stresses and strains at the various nodes of the slab sample (both before and after strengthening with various combinations), the slab was simulated at the pavement body with rigid layers (Figure 8).

Figure 8
Finite element model of a slab reinforced with geogrid layers.

A comparison for each case of reinforcement will be thoroughly discussed. The characteristics of the various layers analyzed in the numerical model and the types of reinforcement employed are illustrated in Table 4. The parameters of each layer, along with the mechanical properties of the fibers and the reinforcement (geogrid1, geogrid2, composite1 and composite2) are also provided. Modeling was conducted to facilitate a comparison between the obtained results and those derived from the experimental campaign. We used the same configurations and notations for the test slabs as those used in the experimental part, with monotonic loading in 4-point bending, up to failure [⁴⁷].

Table 4
Materials properties for model layer and the type of reinforcement.

LAYER/ REINFORCEMENT	ELASTIC MODULUS E (MPa)	POISSON’S RATIO	THICKNESS (mm)
Cement concrete	30 000	0.2	50.0
Geogrid 1	750	0.3	0.9
Geogrid 2	850	0.3	0.9
Composite 1	234 000	0.3	0.42
Composite 2	240 000	0.3	0.45

4. RESULTS AND DISCUSSION

In this section, the results of the experimental laboratory tests and of the numerical analysis of the selected model are presented for each category of specimens tested, corresponding to temperatures of 20°C and 50°C, respectively. The results encompass stress states, strain states, dissipation energy values and evaluation of Young’s modulus. The objective is to observe and discuss the effects of the hybrid: geogrid – CFRP composite combination, in the various cases examined during the 4-point bending tests.

4.1. Test results and analysis

The results of the experimental analysis are presented below. In all the slabs tested, it was found that when the monotonic load was applied by the testing machine (Figure 9a), signs of damage first appeared in the form of fine vertical cracks on the outer face of the lower fiber of the concrete sample and then propagated upwards until failure (Figure 9b).

Figure 9
Overview of bending test machine and fracture crack.

4.1.1. Flexural strength and fracture of slabs

The initial results obtained during this experimental study are presented in Figure 10, illustrating the evolution of flexural strength. These curves represent the “force-deflection” behavior for the various tested slabs. The maximum load is determined during the complete propagation of the crack, before the ultimate failure of the slab sample. In all conducted tests, apart from the control slabs (R), the specimens failed ductility, without tearing or disintegration of the applied geogrid sheet or composite sheet. This observation was consistent across tests conducted at both ambient temperatures (20°C and 50°C).

Figure 10
Evolution of force as a function of slab deflection (at 20).

Each curve consists of three distinct parts: the elastic part (linearity according to Hook’s law), the elasto-plastic part, up to the limit value (peak of the curve) and the final phase culminating in failure. At a temperature of 20°C, the breaking forces for the reference slab (R) were measured at 15,435 kN, compared with 14,081 kN and 12,641 kN for the geogrid reinforcements G1 and G2, indicating gains of 8.78% (G1) and 18.11% (G2). The composites demonstrated gains of 4.77% for C1 and 3.91% for C2, resulting in forces of the order of 16,170 kN (C1) and 16,038 kN (C2). Double reinforcement with two layers of geogrid resulted in gains of 7.15% (for G11) and 1.236% (for G22).

The combination of geogrid and composite yielded rupture force values around 10,131 kN (for C1G1), 9,572 kN (for C1G2), 9,942 kN (for C2G1) and 8,274 kN (for C2G2), i.e. percentage differences of: 3.423%, 3.798%, 3.559% and 4.639% respectively.

In terms of deflection, it was observed that the geogrids (whether one or two reinforcement layers) did not reduce deflection, unlike the composites (C1 or C2). Specifically, the mid-span deflection for the C1 composite was measured at 0.174 mm (compared to 0.247 mm for the reference R), i.e. a gain of 29.56%, while the deflection for the C2 composite was 0.201 mm, i.e. a gain of 18.63%. Conversely the geogrids, led to increased deflection at break with negative percentages, of the order of: −129.55% (for G1), −106.07 (for G2), −168.82% (for G11) and −116.20%.

The same applies to the combination of composite and geogrid, exhibited percentages ranging from −47.77% to −80.97%. The evolution of cracks and failure modes were also observed. It was found that the reinforcement additives had a significantly beneficial effect on the ductility of the tested slabs. Notably, all failures were concentrated in the middle and at the base of the lower fiber, confirming a flexural failure mode. Figure 11, illustrates the evolution of the crack during monotonic loading. The geogrid, like the composite, did not tear and prevented the disintegration of the reinforced slab tested.

Figure 11
Crack evolution during reinforced slab failure.

At the temperature of 50°C, similar trends in evolution were observed with reduced displacements values (arrows) and increased values for ultimate and breaking strength. All the specimens underwent testing after being placed in an oven, at an internal temperature of 50˚C, with a low relative humidity of 10%, for a total of 50 days following complete curing.

The strain rates were determined on the day of testing, after the 88 days had elapsed (from the day the concrete was poured). The results indicate that the geogrid mat can mitigate deformations in concrete pavements.

Rigid concrete slabs exhibiting less rapid deformation when exposed to high temperatures. The results also revealed that cement concrete slabs reinforced with carbon composite sheets displayed higher resistance at a temperature of 50°C than slabs reinforced with geogrid sheets. In addition, in general, the ultimate force applied by 4-point bending at high temperature for slabs reinforced with hybrid combinations (geogrid-composite) surpassed that of unreinforced concrete slabs (Figure 12).

Figure 12
Evolution of force as a function of slab deflection (at 50°C).

Figures 13a and 13b illustrate the comparisons, in terms of ultimate strength value (Fu) and mid-span deflection value (fu), respectively, obtained for all the slabs tested, at the two temperatures of 20°C and 50°C.

Figure 13
Histograms comparing the ultimate force and deflection at temperatures of 20°C and 50°C.

4.1.2. Dissipation energy and type of reinforcement

Table 5 below shows the respective values of ultimate load (Pu), ultimate deflection (in mm) and dissipation energy, for all types of slabs, with the reinforcements applied. The force-deflection curves show that, for each slab tested, the ultimate load is maximum (Pmax), followed by that at failure (Pr), with a clear advantage for slabs reinforced with composites and/or geogrid sheets. The area under each curve is made up of three zones (elastic zone, intermediate zone and failure zone). The two zones (I and II), areas under the curve up to flexural strength (Pmax), represent the crack initiation energy (Ei), while the area under the curve between Pmax and failure (zone III), is the pre-crack propagation energy (called Eup) (Figure 14). As all our samples (slabs) are made up of two layers (during installation), the dissipation energy (Ep) is the sum of the two energies (Ei + Eup). These values are calculated from the surfaces (areas) under each curve by discriminating these curves [⁴⁸, ⁴⁹]. This confirms the advantage of reinforced slabs over non-reinforced slabs.

Table 5
Summary of flexural test results.

PROPERTY	ULTIMATE LOAD Pu (kN)	ULTIMATE DEFLEXION AT PU (mm)	DESSIPATED ENERGY (kN-mm)
Reference (R)	16.5	0.218	1.79
Geogrid2 (G1)	16	0.40	3.21
Geogrid2 (G2)	13.7	0.36	2.47
Composite (C1)	16.8	0.145	1.22
Composite (C2)	17.1	0.13	1.11
Two geogrids1 (G11)	17.9	0.45	4.03
Two geogrids2 (G22)	14.5	0.44	3.19
Double reinfor. (C1G1)	13	0.35	2.34
Double reinfor. (C1G2)	11.3	0.345	1.94
Double reinfor. (C2G1)	11.65	0.36	2.09
Double reinforc. (C2G2)	11.42	0.372	2.12

Figure 14
Schematic representation of energy dissipation zones.

For single reinforcement, it appears that composites do not exhibit high dissipation energy, compared to reinforcement with a G1 geogrid sheet, which shows a value of 3.21 Kn-mm (gain of 79.3%) or a value of 2.47 kN-mm, for the G2 sheet (a gain of 38%). Double reinforcement proves highly advantageous in terms of energy dissipation, particularly for the two geogrid layers, with values of 4.03 kN-mm for GG1 (125.2% gain) and 3.19 kN-mm for G22 (78.2% gain) respectively. The hybrid combination (composite-geogrid) exhibited lower dissipation energies compared to the previous double reinforcement, with values of: 2.34 kN-mm (30.7% gain for C1G1), 1.82 kN-mm (8.38% gain), 2.09 kN-mm (16.76% gain for C2G1) and 2.12 kN-mm (18.44% gain for C2G2). This proves that reinforcement using geosynthetics improves load capacity, displacement, flexural strength and delays the collapse of concrete slabs, consequently increasing the dissipation energy by almost 126%. It was also noted that the c1 and C2 composites, either alone or in hybrid with the geogrid, slightly increased ultimate load capacity and dissipation energy by approximately 31%, while imparting greater stiffness for the reinforced slabs compared to the unreinforced slabs, this suggests that the use of FRP systems significantly improves the structural capacity of the slabs as built [⁵⁰].

4.2. Numerical analysis results

In this section, the results of the numerical modeling are presented in the figures below, showing the strains and stresses, both before and after reinforcement along the main axis of the monotonic static load. The values obtained indicate that the reinforcements, whether geogrid or composite, or even a combination, are effective and give significant values for strains and stresses, particularly when placed at pavement depth (ready-to-use rigid slab simulation).

The numerical analysis gives the stress and strain values at the various nodes of the modelled slab (for all combinations). On the basis of the states of stress and strain and the reading of the values found, we were able to plot the stress-strain curves for the eleven (11) types of slabs tested, at temperatures of 20°C and 50°C (Figures 15 and 16) .

Figure 15
Numerical results of the stress-strain evolution of the slabs tested (at 20°C).

Figure 16
Numerical results of the stress-strain evolution of the slabs tested (at 50°C).

Interpretation of the results found, by reading from the previous curves and the states of strains (Figure 17) and stress (Figure 18), gives us the following comments:

Figure 17
Strains evolution for different reinforcement combinations.

Figure 18
Stresses evolution for different reinforcement combinations.

Concerning the deformations, at a temperature of 20°C and with a composite layer (C1 or C2), the deformation for the reference beam measures approximately 2.441 µm (Figure 17a), whereas it increases to an average value of 8.66 µm (Figures 17d, 17e), i.e. a difference of 255%, which proves that the composite gives maximum elongation to the slab and thus increases its elastic phase and consequently its ductility.

For reinforcements with one layer of geogrid (G1 or G2) (Figures: 17b, 17c), or with two layers of geogrid (G11 or G22) (Figures: 17f, 17g), the deformation seems to be little influenced, with values averaging 2.35 µm, i.e. a reduction of around 3.8%. For hybrid reinforcements (combination of geogrid and composite), the deformations have average values of 4.36 µm, with gains of the order of : 79.5%, which may be a good alternative for hybrid reinforcement, also with an increase in the ductility of the reinforced slab (Figures: 17h, 17j, 17i, 17k). These results show that composites, as reinforcement materials, have a clear advantage in terms of increasing deformation and are effective, whether used alone for reinforcement or combined with geogrids.

This trend is further corroborated by the stresses observed for the different combinations analyzed, at a temperature of 20°C, as shown in Figures: 18a to 18k. The maximum stress value of 2.685 MPa for the reference beam (R), prior to strengthened, In contrast values of : 2.588 MPa (for G1), 2.533 MPa (beam G2), 11.602 MPa (beam C1), 11.31 MPa (beam C2), 2.77 MPa (beam G11), 2.471 MPa (beam G22), 6. 019 MPa (beam C1G1), 5.779 MPa (beam C1G2), 5.9.04 MPa (beam C2G1) and finally the value of 4.859 MPa for beam C2G2).

These values demonstrate that composites significantly enhance stiffness of the reinforced slab and are highly recommended for reinforcing this type of rigid pavement. The gains range from approximately: 320% (C2) to 300% (C1), which confirms the rigidity of the reinforced slab and thus the increase in its ductility. The hybrid combination (composite-geogrid), while alleviating deformation, yields appreciable stress values, with gains ranging from 124% (C1G1), 115% (C1G2), 120% (C2G1) and 81% (C2G2). Reinforcement with one or two layers of geogrids does not bring any gain in stress, given the separation role they play, but stops the propagation of the crack that appeared in the slabs tested. This can be particularly advantageous for roadways located in arid environments [⁵¹, ⁵²].

At a temperature of 50°C (simulation in an arid zone) and given the expansion phenomenon, we observe stress values of the order of: 2,525 MPa (slab R), 2,415 MPa (for G1), 2,358 MPa (for G2), 2.851 MPa (for G11), 2.784 MPa (for G22). Thus, for G1, G2, G11 and G22, the differences vary between 6.65% and 11.60%. It is obvious that the temperature parameter influences the geogrid reinforcement (whether one or two layers). Unlike geogrid reinforcements, the insertion of composites (alone or combined with geogrids) leads to reductions in stresses at 50°C [⁵³].

The analysis revealed values of 11.56 MPa for C1 (30.28% gain), 12.54 MPa for C2 (42.19% gain), 6.956 MPa for C1G1 (28.22% gain), 6.945 MPa for C1G2 (gain of 22.5%), 6.814 MPa for C2G1 (gain of 25.6%) and finally 6.812 MPa for C2G2 (gain of 25.5%). It clearly appears that the increase in temperature reduced the gain provided by the advantage of the insertion of the composite and the “composite-geogrid” hybrid combination.

4.3. Experimental − numerical comparison

This comparison involves a comparative analysis between the experimental and numerical values obtained for the different tested slabs (with reinforcement by geogrid and/or composite) tested in three-point bending and simulated as rigid pavement (RC). Whether at 20°C or 50°C, the comparison showed that the difference between the predicted numerical values and the measured experimental results did not exceed, on the whole, 6% (Table 6), which could be further improved (smaller difference) in the case of nonlinear behavior of the materials used. Indeed, for stresses, the numerical analysis using the finite element method provided values that were not far from the experimental values found in the laboratory, i.e., the stresses are calculated based on the values of ultimate loads [⁵⁴]. The differences (deviations) ranged from 3.34% to 5.93%, confirming that these values correspond to the experimental results and that the chosen model for the simulation is acceptable. The same applies to the deformations, calculated from the deflections found experimentally. The differences found varied between the values of 2.40% and 5.90%, confirming the correlation between the two 4-point bending tests and the numerical analysis.

Table 6
Comparative values experimental–numerical.

IDENTIFICATION	STRESS (MPa)			STRAIN (µm)
	EXPERIMENTAL	FEM	GAP (%)	EXPERIMENTAL	FEM	GAP (%)
Reference (R)	2.814	2.685	4.58	2.525	2.441	3.32
Geogrid2 (G1)	2.725	2.588	5.03	2.448	2.345	4.21
Geogrid2 (G2)	2.715	2.554	5.93	2.411	2.322	3.69
Composite (C1)	12.07	11.60	3.89	8.988	8.771	2.41
Composite (C2)	11.87	11.31	4.71	8.981	8.550	4.79
Two geogrids1 (G11)	2.912	2.770	4.87	2.651	2.518	5.01
Two geogrids1 (G22)	2.371	2.471	4.21	2.354	2.246	4.58
Double reinfor. (C1G1)	7.214	6.819	5.47	5.371	5.155	4.02
Double reinfor. (C1G2)	6.067	5.779	4.74	4.524	4.369	3.42
Double reinfor. (C2G1)	6.245	5.904	5.46	4.701	4.464	5.04
Double reinfor. (C2G2)	5.024	4.859	3.34	3.902	3.672	5.89

Similarly, the comparison holds for the values obtained at a temperature of 50°C, for both stresses and strains, with differences ranging from 3.50% to 6.80%, favoring the values obtained in the laboratory. It also emerges from the two analyses (experimental and numerical) that geogrids with good adhesion to composites, regardless their type, improve stresses and deformations, which can promote the longevity of rigid pavements and therefore increase their load-bearing capacity, particularly in hot climates [⁵⁵, ⁵⁶].

5. CONCLUSIONS

The experimental analysis and numerical modeling in our study lead to the following conclusions:

1. An experimental study was conducted on a series of concrete slabs, reinforced with various types of geogrids and carbon composites, with the manufacture of 22 types of specimens and tested in 4-point bending, which can simulate a rigid cement concrete pavement, the specimens were statically loaded to failure, under two different temperatures of 20°C and 50°C.

2. The application of innovative products, such as geogrids, combined with carbon fiber composites, shows promising outcomes for reinforcing rigid runways, with improved stresses and reduced deformations, with percentages of up to 20 to 35% and consequently there is an improvement in the modulus of elasticity, resulting in increased ductility and stiffness of concrete slabs

3. Numerical findings, with a carefully selected calculation model, closely align with experimental results by 3 to 7%, particularly evident for hybrid reinforcement (geogrid-composite) and It would seem desirable to opt for an optimized model, taking into account the non-linearity and isotropy of the materials used (geogrid, composite and concrete).

4. Reinforcement with such layers presents a viable alternative for deteriorated and damaged rigid pavements (road pavements and airport runways). This approach can promote the longevity of these pavements and therefore increase their load-bearing capacity, particularly in hot climates.

5. It should be noted that the extension of this type of work to other full-scale linear structures, whether on flexible or rigid pavements, may be of great economic interest for improvement projects and will provide useful data on the long-term performance of reinforced pavements in the future. This is the main objective of this paper, showing a more methodical orientation towards subsequent wide practical use in the safeguarding of existing basic infrastructure assets and developing concrete compositions, with new insertions of innovative plastic products with specified physical and mechanical properties and service life.

6. ACKNOWLEDGMENTS

The authors would like to thank the managers and staff of the public works laboratory of the LCTP in Algiers, for facilitating the task in the experimental part and the managers and directors of the company “Afitex-Algérie”, for supplying the geosynthetic products. The master’s students: Abdessemed Anfel.N.H, for his contribution to the formatting and checking of the translation of the document and the pair “Raissi H and Bendjema S., for their collaboration in the laboratory trials and tests.

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Publication Dates

• Publication in this collection
  02 Sept 2024
• Date of issue
  2024

History

• Received
  11 Apr 2024
• Accepted
  01 July 2024