Evaluation of flexural failure for concrete beams partially reinforced by geogrid sheets

ABSTRACT The behavior of reinforced concrete beams with biaxial and uniaxial geogrids as a partial reinforcement with traditional steel bars under flexural loads has been experimentally studied. The effect of geogrid layers with traditional steel reinforcement in concrete beam specimens in compression and tension zones has also been investigated. Geogrid is one of the geosynthetic materials that exhibits both tensile and flexural behavior similar to traditional steel. To examine the effect of geogrids on traditional steel as a composite reinforcement system, strain gauges were fixed to the concrete beams and bottom reinforcement (steel bars). The studied parameters in this investigation are the number of geogrid layers, types of geogrids, and their position in concrete beams. First cracking load, failure load, initial and post-cracking, deflection, and ductility of beam specimen were investigated. The experimental results indicated that using geogrid sheet layers as partial reinforcement greatly contributed to resisting flexural stresses of concrete beams. An increase in the ductility and the capacity load of the concrete beam specimen was also observed. Based on this experimental investigation, the geogrids can be applied as additional or alternative reinforcement with/without traditional steel bars in concrete beams.


Introduction
Geogrids are geosynthetic materials commonly made from polymer materials, such as polyester, polyvinyl alcohol, polyethylene, or polypropylene (PP).Geosynthetics have long been used as reinforcement and stabilization elements in various heavy civil and infrastructure works.The geogrids can be obtained in various shapes and materials to suit different uses and applications based on their contribution to various fields.
Geogrids are formed into two major types: Uniaxial geogrids exhibit a large tensile strength in one direction only, whereas biaxial geogrids exhibit a high tensile strength in two directions.The high demands and applications of biaxial and uniaxial geogrids in construction are because they are good in tension and have a higher ability to distribute the load over a large area, and also, geogrids are enabled to be used as an alternative to a traditional steel bar which has many concerns related to corrosion, leading to a reduction in the durability of the structures.
One of the major advantages of using geogrids as a partial or full replacement of reinforcement in structural elements, such as beams, slabs, columns, etc., is that they are extremely lightweight when compared to traditional steel bars reinforcement, and their high corrosion resistance must be also considered as its advantages.Ramakrishnan, Arun, Loganayagan, and Mugeshkanna (2018) concluded that the geogrids carried tensile forces at the tension zone as well as their flexural strength increased by increasing the number of geogrid layers.Imam, Tahwia, Elagamy, and Yousef (2021) reported that the externally bonded CFRP strips increased the flexural and stiffness capacities of reinforced concrete beams by 12-40% and 10-79%, respectively, with the observance of decrease in deflection for all strengthened specimens.El Meski and Chehab (2014) tested simply supported plain concrete beams reinforced with various types of geogrids, and the increase in the concrete ductility, high fracture energy, and high flexural strength with large deflection was investigated.Rakendu and Manoharan (2017) concluded that the load-carrying capacity and flexural strength increased by increasing the number of geogrid layers.Ahmed Yousif, Shahada Mahmoud, Abd Hacheem, and Mohammed Rasheed (2021) studied the effect of a geogrid layer on the performance of reinforced concrete beams and concluded that the geogrid layers lead to an increase in post-cracking stiffness of concrete beams.Shobana and Yalamesh (2015) reported that the uniaxial geogrids give better post-peak flexural behavior compared to biaxial geogrid.When tested, fabricated simply supporting beam specimens were fully reinforced by uniaxial and biaxial geogrids with two and three layers for each user type of geogrids in the reinforced concrete beam.Tang, Chehab, and Kim (2008) investigated the flexural behavior of plain concrete reinforced by fully triaxial geogrid and concluded that the geogrid reinforcement specimen exhibits significant deformation after the initial cracking and before the ultimate failure.Tang et al. (2008) investigated the effect of geogrid as a fully flexural reinforced concrete beam, and test results illustrated an increase in concrete ductility, although the flexural strength of the reinforced concrete beam is not necessarily improved due to the inclusion of geogrids.The present investigation studied the contribution of the geogrids against flexural behavior of reinforced concrete beams using two different types of geogrids (uniaxial and biaxial).This research clarified that the capability and the compatibility of geogrid layers can be used as a partial reinforcement with traditional reinforced steel bars for concrete beams.

Research significance
Current research aims to study the increase in reinforced concrete beams capacity regarding flexural failure using embedded geogrid layers as a partial reinforcement of traditional steel bars.The present research leads to understanding the behavior of concrete beams reinforced by both geogrids and steel bars, which aids in assessing the potential of using geogrids as a partial reinforcement in structural concrete elements.

Experimental program
The experimental work consisted of testing a total of nine simply supported beam specimens, as detailed in Table 1; they were tested under flexural loading.All the reinforced concrete beam specimens were designed to undergo flexural failure and had the same overall dimensions and shear reinforcement.The beam specimens had an overall width, depth, and length of 200 mm, 250 mm, and 1100 mm, respectively, as shown in (Figures 1, 2, 3, 4 and 5).The experimental program was divided into three groups based on the parameters study (number of geogrids layers, types of geogrids, and reinforcement position of geogrids) as illustrated in Table 2.

Concrete
Trial mixes were conducted to reach the target concrete compressive strength of 30 MPa after 28 days by CEM I grade 42.5 with 10 mm of maximum nominal size of aggregate of concrete mix.Concrete mix proportion designed by weight of the quantities of materials is listed in Table 3.To control the quality of the concrete design mix, six cubes were tested for 7 and 28 days (BS EN 1992(2004); Egyptian code of practice for design and construction of reinforced concrete structures ECP203 (2018), Design of concrete structures).Three concrete cubes and cylinders randomly poured from the same batches of the beam specimens were tested at the date of tested beam specimens to determine the concrete compressive strength and the concrete tensile strength (BS EN 1992(2004), Part 1-1), respectively, as shown in Figure 6(a) and (b) and as listed in Table 4. Figure 1.Typical reinforcement detailing for control beam specimen.

WATER SCIENCE
High-tensile-strength deformed bars of 10 mm are used to overcome flexural stresses, and 8 mm diameter mild steel is used as both compression steel reinforcement and stirrups.Furthermore, to find a tensile strength, three specimens of each bar diameter were tested and failed to find and determine the mechanical properties of steel bars reinforcement (ACI 318, 2014;ACI Committee 318, 2008.Building code requirements for structural concrete and commentary.American Concrete Institute (ACI), Farmington Hills, MI).The mechanical properties of steel bars reinforcement are summarized in Table 5.

B) Geogrids
A uniaxial geogrid type (RE-560) with 235 mm × 16 mm apertures size and a biaxial geogrid type (LBO-440) with 32 mm × 32 mm apertures size were used as a partial reinforcement of concrete beam as shown in Figures 7 and 8, respectively.The relationship between the geogrid aperture opening size and the maximum nominal aggregate size allows for interlocking between the geogrid and several large aggregates, thus leading to a better interaction between geogrid and concrete.
The geogrid with a high density of polyethylene (HDPE) sheet for uniaxial and with PP sheet for biaxial is illustrated in Figures 7 and 8.The geogrid structural elements are made up of ribs and junctions (NODES).The manufacturer provided physical and mechanical geogrid characteristics, and a series of tensile tests were performed on biaxial and uniaxial PP and HDPE geogrids to determine their actual tensile strength as stated in American Society for Testing and Materials ASTM D 6637 (2011) (Wide Width Tensile Method) using a universal testing machine Testometric CM-500 as illustrated in Figure 9, and the physical-mechanical properties of the tested geogrids are summarized in Table 6 for the two types of geogrids.
The standard test method to determine the tensile strength properties and to plot the stress-strain curve of uniaxial and biaxial geogrids was conducted on five specimens with dimensions of 200 mm width and 470 mm height for each HDPE RE-560 and PP, respectively (American Society for Testing and Materials ASTM D 6637, 2011).The steel clamps were used at the outer ribs geogrids specimens to prevent any slippage failure of specimens during the test, and the tests were performed with extension rate of 30 mm/min.Stress-strain relationships for tensile test of HDPE and PP geogrid are shown in Figure 10) and (Figure 11), respectively.

Specimens fabrication
The concrete mix used to cast the tested reinforced concrete beam specimens consisted of Portland cement, gravel, and natural sand as a coarse and fine aggregate, respectively, and water.Specimens were cured at about 95% relative humidity.Water was added and mixed thoroughly after dried cement and sand were mechanically mixed.The mixing operation was continued after adding water until a uniform color was obtained.The mixing proportion of different materials was by the weight of cement.The concrete was cast in wooden formworks having a smooth surface, and these surfaces were coated with oil before casting.Geogrids sheet layers were placed below and above the bottom and top steel bars reinforcement, respectively, as shown in Figure 12.

Test setup and instrumentation
All beams considered in this experimental study were tested till failure under symmetrically applied and   gradually increased two-point loads.They were also supported over two rigid supports.Specimens were tested under load control with increments of 5 kN up to failure using a hydraulic jack attached to a load cell with a capacity loading frame of 800 kN.The load was applied vertically at the center of 800 kN.The loads were applied vertically at the center of the stiff steel beam, which transmitted and distributed the concentrated load equally onto the bearing resting on the top of the beam and spaced at one-third of the clear span of beam specimens.Deflections of beams were measured using a linear variable differential transducer attached at mid-span.A typical test setup for beam specimens is shown in Figure 13.
A strain of traditional reinforcement steel bars was measured using a strain gauge attached directly to the rebar at the tension zone and a strain gauge directly attached to a top surface of concrete beam specimens at the compression zone, as shown in Figure 14.The measured data were recorded by a data logger connected to the computer system program by "LabVIEW" software.

Results and discussion
From the experimental outcomes of beams tested regarding first cracking load (P cr ), which appeared immediately in between the two-loading point in the bottom zone for the reference beams specimens (without and with geogrids), yield and maximum failure measured load as well as their relative vertical deflections for concrete beam specimens, and types of first crack.Modes of failure are listed in Table 7.

Crack pattern and failure mode
All reinforced concrete beam specimens were subjected to a two-point loading system until they eventually failed.The cracks were observed and marked as each load increased continuously during the loading time.The cracking patterns and deflections were        Based on experimental work results, the use of geogrids sheet layers as partial reinforcement in concrete beam specimens greatly enhanced the behavior \and performance of concrete beams regarding the number of cracks and widths.The number of cracks with decreasing formed cracks width for beam specimens B3, B4, B7, and B8 increased compared to control beam B0.Using geogrids sheets as partial reinforcement in both compression and tension zones leads an increase in the number of cracks in the flexural zone and formation of flexural shear, as shown in the behavior of concrete beam specimens B4 and B8 compared to the behavior of concrete beam specimens B3 and B7.However, there is no effect on the crack propagation, the number of cracks, and the width of the crack when using just one layer of geogrids as partially tensile reinforcement, as shown in beam specimens B1 and B5 compared to control beam specimen B0.
Although using one layer of geogrids as partial reinforcement at both tension and compression zones gives a slight enhancement in crack propagation, it brings about a slight increase in the number of vertical flexural cracks recorded, as shown in beam specimens B2 and B6 when compared to control beam specimen B0.No difference was observed in the crack pattern propagation and the crack width when using uniaxial and biaxial geogrids as partial reinforcement in concrete beam specimens.Finally, it was found that the spread of cracks increases by increasing the number of geogrid layers in concrete beam specimens from 16% up to 40%.
The cracking loads ranged from 16.81% to 20.96% of the failure loads for beam specimens with geogrids increased up to 31.87% compared to beam specimen B0 without geogrids.This is attributed to the presence of geogrids as a partial reinforcement that delayed the formation of cracking load, increasing the failure load capacity of specimens.By comparing beam specimens with geogrids reinforcement in both tension and compression zone and beam specimens with geogrids just reinforced in the tension zone, it was found that the use of geogrids layers in the compression zone leads to an increase in the cracking load and failure load capacity up to 23.47% and 20.45%, respectively.The increasing range between the failure capacity load after yielding load up to 40% for tested beam specimens reinforced as partially reinforced by geogrid indicated that the geogrid layers were contributed with steel bars reinforcement to resist the flexural stresses on the beam specimens.

Load-deflection behavior
The load-deflection relationship curves measured in the mid of the clear span of beam specimens are illustrated in Figure 16, and Table 8 lists the ultimate load capacities ratio of the tested beam specimens compared to the control beam (B0) and their corresponding deflections.Generally, using geogrids layers as partial reinforcement increased the maximum capacity load up to 38.23% when compared to the control beam specimen (B0).At both tension and compression zones reinforcement increasing, the maximum failure loads up to 14.60% when compared with beam specimens reinforced by geogrid layers in the tension zone only.This is attributed to the fact that the total load will be directly transferred to geogrid and traditional steel bars reinforcement after concrete failure.So the beam reinforced with geogrid sheet layers can take a further load regarding the number of layers and their reinforced position.Whereas, as listed and shown in Table 8, increasing geogrids layers in the flexural zone leads to an increase in failure load capacity by 7.619% and 14.76% for beam specimens B3 and B1 when compared to control beam specimen B0, respectively, and by 2.65% and 9.93% for beam specimens B5 and B7 compared to control beam specimen B0, respectively.Above results were matched with the results in previous researches.Ramakrishnan et al. (2018) reported that the ultimate load carrying capacity of geogrid beams were found to increase as the geogrid layer was increased because of the contribution by geogrid layer in load carrying capacity of up to 25% higher strength than conventional beam.
It can be observed that using geogrids in the flexural zone and compression zone leads to an increase in failure load capacity by 12.07% and 20.45% for beam specimens B2 and B4, compared to beam specimens B1 and B3 reinforced by geogrids in the flexural zone respectively.On the other hand, a slight increase in failure loads capacity was discovered when using different types of geogrids (uniaxial geogrid and biaxial geogrids) as partial reinforcement in the beam by 4.83%, 4.41%, 4.39%, and 6.38% for beam specimens B1, B2, B3, and B4 compared to beam specimens B5, B6, B7, and B8, respectively.
There was a decrease in the beam specimens B1, B4, B5, and B8 deflection at yielding load by 32.45%, 49.62%, 29.76%, and 43.36%, respectively, compared to B0.This can be attributed to an increase in geogrids layers, whereas no effect was observed due to a reduction in deflection when using geogrids at both tension and compression zones.On the contrary, deflection at a failure load capacity of beam specimens B1, B2, B3, and B4 increased by 29.98%, 138.11%, 199%, and 241.3%, respectively, compared to B0, and the beam specimens B5, B6, B7, and B8 increased by 206%, 191.22%, 179.90%, and 203.50%, respectively, compared to B0.This is attributed to the significant contribution of geogrid sheet layers to resist flexural failure after concrete failure, as shown in Figure 17.

Steel and concrete strain
All bottom reinforcement partially contributed to carrying the tensile force after applying load to the tested beam specimens, and after the formation of the first crack (cracking load Pcr), the geogrid sheet layers began carrying the tensile force concurrently with the steel bars.Then, the strain values of steel bars were increased gradually until failure concrete beam specimens were observed, decreasing steel strain values and increasing the number of geogrid sheet layers.Figure 18 illustrates the load-strain curve of steel bars at the flexural zone and concrete at the compression zone per each concrete beam specimen compared to the concrete control beam specimen.
All bottom reinforcement partially contributed to carrying the tensile force after applying load to tested beam specimens.After forming the first crack (cracking load Pcr), the geogrid sheet layers began carrying the tensile force concurrently with the steel bars.Then, the strain values of steel bars were increased gradually until failure concrete beam specimens were observed, decreasing steel strain values and increasing the number of geogrid sheet layers.
There was up to 15.16% reduction in the steel bar strain with three geogrid sheet layers on both reinforced positions (Table 9).There was up to 9.28% reduction in the steel bars strain when three layers of geogrids are in the flexural zone.There was a minor effect in removing the steel bars strains up to 2.28% when using uniaxial and biaxial geogrids as partial reinforcement in the concrete beam.The steel strain affected the position of geogrids in the concrete beam.The steel bars strain was recorded as a reduction for concrete beam specimens were reinforced by geogrids in flexural positions and compression up to 7.65% when compared with concrete beam specimens that were just reinforced by geogrid layers in the flexural zone.It should be mentioned that using geogrids as partial reinforcement in the compres-   sion zone of beam specimens significantly contributed to increasing concrete to resist the compression stresses at the top of the flange of the concrete beam up to 29.30%.

Ductility
The ability of the material to withstand permanent deformation under a tensile load without rupture is called ductility (Nasr Abdelmonem Mohamed, El Sebai, & Shaban Abdel-Hay Gabr, 2020;Tharani, Mahendran, & Vijay, 2019).In this study, the ductility was investigated for all the tested specimens by calculating the ratio of total energy (Au; the area under the ultimate load-deflection curve) to yielding energy (Ay; the area under the yielding load-deflection curve) (Abd-Elmohsen, 2017).Biaxial and uniaxial geogrids reinforcement in the concrete beam specimens leads to an increase in fracture energy.The highest for the biaxial type is mainly due to the significantly large ductility.Figure 19 shows the ductility of all tested beam specimens.An increase was observed in the ductility of concrete beam specimens B4 and B8 using geogrid sheet layers as partially reinforced in both bottom and top reinforcements.It was six times the ductility value of control concrete beam specimen B0, increasing the ductility by 130% and 149%, compared with beam specimens B3 and B7, which used geogrids as a partial reinforcement at flexural zone only, respectively.
It can be observed that an increase in geogrid layers in concrete beam specimens leads to an increase in the ductility up to 85% when compared to control beam specimen B0.Using geogrid layers as a partial reinforcement in both position flexural and compression zones significantly enhanced the ductility up to 48% when compared with beam specimen just reinforced by geogrid layers at flexural zone.

Initial and post-cracking stiffness
Initial and post-cracking stiffness of the tested concrete beam specimens was calculated based on the slope of the load-deflection curve before and after crack, respectively (Xiaochao & Jlilati, 2019).Figure 20 shows that the initial and post-crack stiffness of tested beam specimens was reinforced by geogrids as partially reinforced with traditional steel bars.
Based on the experimental work of the tested beam specimens, initial and post-cracking stiffness increases by increasing geogrid layers as partial reinforcement.In concrete beam specimens, there in an increase in initial cracking up to 49.63% and 59.41%, while there in an increase in the postcracking stiffness up to 40.62% and 65.71% when compared with control beam specimen.Beam specimens using one layer and three layers at both reinforced flexural and compression zones B2 and B4, respectively, have given great results due to the stiffness when compared with control beam specimen B0.Also, it can be observed that using geogrids as partial reinforcement in both flexural and compression zones significantly contributed to an increase in the initial and post-cracking up to 17.51% and 11.76%, respectively, for initial crack.In contrast, the beam specimens reinforced at compression and tension zones, B2 and B4, were given increasing in the post-cracking stiffness by 12.66% and 30.51% when compared to the beam specimens using one layer and three layers just at flexural zones B1 and B3, respectively.Biaxial and uniaxial geogrid layers were used as partial reinforcement, and slight differences in pre-and post-cracking stiffness were observed.Above results were matched with the results in previous researches.Many researchers concluded that the use of all types of geogrid reinforcement provides a ductile post-cracking behavior, high fracture energy, high  flexural strength, and large deflection (El Meski et al., 2014).

Conclusions
The following conclusions can be drawn based on the analysis and discussion of the test results obtained during this investigation: The tensile strength of geogrids and the number of geogrid sheet layers positively correlate with vertical flexural cracks, deflection, failure load capacity, flexural strength, and the ductility index.The number of geogrid sheet layers plays the most crucial role in the flexural behavior in concrete beams.The increase in the geogrid layers leads to an increase in the first crack load-carrying capacity of beam specimens due to the contribution of the geogrid layers in capacity-carrying loads.Moreover, the uniaxial and biaxial geogrid layers provide a ductile post-cracking behavior, high flexural strength, high fracture energy, and large deflection.
Using uniaxial geogrid sheet layers as a partial reinforcement in concrete beam provides higher values in terms of the ultimate capacity load, ductility, and crack propagation when compared with biaxial geogrids.In addition, using geogrid sheet layers as partial reinforcement in both position's compression and flexure in the concrete beams gave a good behavior.The reinforced concrete beam regarding initial and post-cracking stiffness, ductility, ultimate load capacity, and reduction in cracks width when compared with concrete beam specimens using geogrid sheet layers as a partial reinforcement in the flexural zone only.So, the cracks appeared in the mid-span of beam specimens where the flexural cracks are formed for all tested beam specimens reinforced with geogrid.
The stresses' transition mainly depends on the bonding between concrete and geogrids.The friction between the geogrids and concrete interface is weak.A debonding failure is achieved.It is induced by a flexural and/or flexural-shear crack (intermediate vertical crack) as the main failure mode of geogrid concrete beam specimens.This debonding failure mode is unique to beam specimens reinforced with geogrids due to the weakness of its bond.

Figure 2 .
Figure 2. Typical reinforcement detailing for one bottom of geogrid beam specimens.

Figure 3 .
Figure 3.Typical reinforcement detailing for one bottom and one top layer of geogrid beam specimens.

Figure 4 .
Figure 4. Typical reinforcement detailing for three bottom layers of geogrid beam specimens.

Figure 5 .
Figure 5.Typical reinforcement detailing for three bottom and top layers of geogrid beam specimens.

Figure 6 .
Figure 6.Quality control of concrete: (a) Standards concrete cubes specimen.(b) Standards of concrete cylinder specimens.

Figure 10 .
Figure 10.Stress-strain for tensile test of HDPE geogrid at tensile rate of 30 mm/min.

Figure 11 .
Figure 11.Stress-strain curve for tested biaxial geogrid sample at tensile rate of 30 mm/min.

Figure 12 .
Figure 12.Casting of concrete and placing the reinforcement for one bottom layer of geogrid beam specimens.

Figure 13 .
Figure 13.Test setup of beam specimens.

Figure 14 .
Figure 14.Strain gauge locations of beam specimens.

Figure 15 .
Figure 15.Crack patterns at failure load for tested beam specimens.(a) Crack patterns at failure load for tested beam specimen B0.(b) Crack patterns at failure load for tested beam specimen B1.(c) Crack patterns at failure load for tested beam specimen B2.(d) Crack patterns at failure load for tested beam specimen B3.(e) Crack patterns at failure load for tested beam specimen (f) Crack patterns failure load for tested beam specimen B5.(g) Crack patterns at failure load for tested beam specimen B6.(h) Crack patterns at failure load for tested beam specimen B7. (i) Crack patterns at failure load for tested beam specimen B8.

Figure 16 .
Figure 16.Load-vertical deflection relationships of all tested beam specimens.

Figure 17 .
Figure 17.Flexural failure bottom concrete beam.Figure18.Midspan longitudinal steel and top surface concrete strain of tested beam specimens.

Figure 18 .
Figure 17.Flexural failure bottom concrete beam.Figure18.Midspan longitudinal steel and top surface concrete strain of tested beam specimens.

Table 4 .
Concrete characteristics at compressive strength and tensile strength.

Table 5 .
Mechanical properties of steel reinforcement bars.

Table 6 .
Physical and mechanical properties of geogrids.
Note: MD is the machine direction; TD is the transverse direction; HDPE is high-density polyethylene.

Table 7 .
Experimental results of tested beam.

Table 8 .
Ultimate load capacity ratio and their corresponding deflection.

Table 9 .
Ultimate strain characteristics in longitudinal reinforcement ratios.