Controlling changes caused by holes by adding �bers to composite concrete components

Crack propagation can be affected by the holes of the placement of rebars in the reinforced concrete and the resulting changes in the homogeneous structure of concrete. This experimental study aimed at investigating the �ber self-compacting concrete failure and crack propagation in concrete specimens with circular central holes and �ber with the radius of 2, 4, and 8 percent. The crack propagation mechanisms under the uniaxial compressive strength of the concrete were studied by investigating the holes created by rebars buried in the reinforced concrete. Three concrete mix designs with different compressive strengths of 27, 30, and 33 were used for detailed analysis of the �ber concrete behavior by 224 cubic specimens. The results showed the insigni�cant impact of small holes with an approximate area of 1.4% of the specimen surface area on the failure structure so that crack propagation in this case was similar to that occurred in the hole-free concrete. The compressive strength of concrete decreased, by increasing the crack prorogation around the holes. The compressive strength of the concrete may be reduced signi�cantly by increasing the hole diameter. Taking into account these factors in the design of the connection core can reduce the compressive strength of concrete by 30% depending on the rebar grid.


Introduction
There are several studies on the effect of crack on the mechanical speci cations of the concrete.
Different factors involved in the crack propagation including cyclic loading and the cause of crack development, bers, additives, and concrete type should be investigated.The use of bers affects the crack path and structure (Chananun et al. 2023;Habil et al. 2023;Oshtolagh. 2023).The majority of these cracks appear at certain locations in the concrete depending on environmental conditions.Concrete surface imaging is a common method to measure the extent of crack propagation (Mazzoli et al. 2015; Zhong et al. 2023;Suarez. 2023).In this classi cation, the crack length and width are also used to investigate the effect of crack propagation in the concrete.Furthermore, the use of additives in the concrete directly affects crack propagation.According to the literature, the level and length of bers added to the concrete change the crack propagation structure (Erdem et al. 2017;Hamrat et al. 2016; Kanema et al. 2016).The results showed a decrease in the crack width in the concrete with increasing the ber length.
Various computer codes and software have been used to investigate the crack images at the concrete surface and crack structure, and also propagation mechanism.These images clearly show the length, width, and distribution of cracks, classi ed by these software (Kim et  There are also some studies on the crack site and spread in the RC beams (Dong et al. 2017, Bindurani et al. 2023;Aryan et al. 2023).The overall structural strength is directly affected by the crack site, speci cations, and angle.Further studies on ber-reinforced concrete beams showed three important points with bending cracks and four important points with shear cracks in the beams, on which more detailed studies were, conducted (Berrocal et al. 2016;Fayyad and Lees. 2017;Jiang and Wu. 2023).
Cracks appear with different shapes and angles in different concrete layers under different loading.The durability and strength of concrete elements and their serviceability depend on these cracks.A common method to prevent crack propagation is to use stirrup and rebar in different areas of concrete elements.Therefore, limiting cracks by using stirrups is a common method to improve the concrete behavior.However, in addition to the reduction of concrete cross-section area, the corrosion of rebars buried in the concrete alone may damage areas around the rebar hole.There are many experimental (Zhu et  Different studies have been conducted on the effects of holes creation in the concrete and other brittle objects and associated residual stresses.The majority of these concrete holes are made for piping and other facilities.The creation of holes in the concrete causes the formation of micro cracks and residual stresses in the concrete (Sicot et al. 2004;Nelson et al. 2006).To prevent cracking, prede ned holes can be made before the concrete gets hardened.The effect of a hole and crack propagation mechanism in brittle materials its vicinity have been studied.In uniaxial specimens, 30% of the cracks were tensile cracks, 40% shear cracks, and 30% were a combination of tensile and shear cracks (Jian et al. 2015; Oshtolagh et al. 2023).In these experimental studies, the circular holes were created in the center of the specimens with symmetric cross-sections.Moreover, evaluation of adjacent holes showed a greater stress concentration between the two adjacent holes.This increase in the stress between the two holes is dependent on the diameter and distance of holes (Yavari et al. 2021).The surface area of the hole relative to that of the specimen cross-section and the distance of the holes from each other relative to their distance from the specimen wall can affect the crack length and mechanical properties of the specimen.
The node-based nite element method was used to model the crack and stress concentration around the holes.This method is capable of modeling holes with different shapes.According to the literature, the presence of a crack around the hole causes high-stress concentration which, in turn, accelerates its propagation in the stress point (Kumar et al. 2014;Cai et al. 2013;Kumara et al. 2015).The existence of rebar buried in the concrete makes the concrete structure more homogeneous relative to a concrete structure with an empty hole.Moreover, an increase in the compressive strength of the concrete may cause a greater difference between the changes in the properties of concrete and rebar hole.
In this experimental study, cubic specimens were used to investigate the effect of rebars buried in the ber self-compacting concrete (FSCC) on the crack propagation structure in the concrete subjected to vertical compressive cyclic loading on the axis of the rebars.There are some studies on the effect of holes in the FSCC blocks created for rebar placement.This experimental study investigates the effects of a circular hole for rebar placement in a concrete structure, and also the effect of increased compressive strength.

Experimental study
Cubic concrete specimens were used to investigate the failure structure at the column-beam joint, the rebars passing the FSCC, and/or the rebars buried in the shear walls in compliance with the research objectives.The specimens were designed to investigate the effect of uniaxial cyclic loading perpendicular on the axis of the holes in the concrete.For a more detailed evaluation, specimens with rebar and without rebar (empty cylindrical holes) were also studied.

Materials
The ber was added to improve the compressive strength of the holes and investigate the effect of ber on the failure structure of the specimens (Table 1).In these mix designs, the type and ratio of gravel and sand were constant.The modi ed Type II cement was used in the mix designs.Low-chlorine tap water with a pH of about 7 was used in the experiments.The addition of ber caused the slump loss while increasing the compressive strength of the concrete.Table 1 lists the amount and type of materials used in the concrete.The compressive strength of standard 150-mm 28-day cubic specimens of concrete was measured.

Curing
The aggregates, cement, and polypropylene ber were mixed and then water (with plasticizer) was added to the mixture in two stages.The slump of each mixture was measured and then poured in a mold.
Plastic tubes were placed in the center of all six molds.The tubes could be removed after drying the concrete.Metal rebars were used in the other nine specimens.In these experiments, different rebar layouts were used: (1) a single rebar in the center of the specimen or (2) two or three rebars with a suitable distance from each other.Three simple cubic specimens were prepared to measure the compressive strength.Five series of each mix design were prepared, resulting in a total of 180 cubic specimens of each mix design.

Specimens
The 224 standard cubic concrete specimens (150 mm) were prepared with six groups of different holes and three different ber percentages.The fabricated specimens include two completely cubic specimens, three specimens with circular central holes with a diameter of 2, 3, and 4 cm, and three specimens with steel rebars.The holes and rebars were placed symmetrically to the cube center.The 27 days specimens were placed perpendicular to the circular hole path under uniform uniaxial compressive cyclic loading.

Loading
In these tests, two types of uniaxial compressive cyclic loadings were used in monotonic and cyclic forms.Thirty-six cubic specimens were exposed to the compressive uniaxial monotonic loading and the others are tested under cyclic loading after 28 days (Fig. 1).Loading was increased stepwise and the increase range of each step of the cyclic loading is 30% f'c.The increase continued up to f'c.After breaking concrete, loading decreases with the same range it increased.

Results and discussions
Despite the asymmetric structure of the concrete, it was assumed symmetrical in the calculations.This asymmetric structure affects the concrete failure mechanism causing different failure planes.A general model can be used to express FSCC behavior by ignoring some of these differences in the failure structure.In this section, the changes in the ber percentage are discussed based on changes in the holes size and holes conditions in the concrete.

Compressive strength
The addition of ber increased the compressive strength of the holes in the specimens.Moreover, increasing the hole diameter in the center of the cubic specimens reduced the concrete compressive strength.This strength reduction was proportional to the circular hole diameter (Fig. 2).According to the results, the concrete specimens with higher ber were more sensitive to changes in the circular hole diameter under cyclic loading.In fact, the compressive strength of the samples after failure had a direct connection with the percentage of bers.The behavior of specimens showed that the presence of steel rebars inside the holes caused a slight increase in the compressive strength of the concrete specimens relative to the specimen with an empty hole.

Crack modeling
The failure structure of the concrete specimens with a small circular hole (1.39% of the specimen surface area) investigated.The results show that the failure structure is similar to the conventional concrete specimens (Fig. 3).The presence of the hole in the concrete did not signi cantly affect the crack path so that its behavior was similar to that in the hole-free specimen.Moreover, the hole-induced compressive strength less than 1.5% of the mean compressive strength of the mixture is negligible.To measure the extent and effect of strength reduction of the cubic specimens with circular holes, the effect of increasing the bers tested.The results show that the effect of the hole on the compressive strength reduction is greater than effect of increasing ber in the concrete with the (Fig. 4).
The compressive strength of the concrete reduced with decreasing the crack length.The results show that the strength reduction in the concrete structure with a circular hole is dependent on the ber percentage.
Besides, the effect of the ber on the concrete behavior increased with increasing the hole size.Unlike the hole-free cubic concrete specimens, the crack begins from the edges and ends at the closest point on the hole.Moreover, other cracks reach the hole from the opposite side symmetrically, causing the failure of the concrete specimen.Concrete with higher percentage of bers shows more resistance under cyclic loading.
The compressive strength reduction and concrete failure structure varied with increasing the circular hole in the specimen.The concrete failure structure also changed with increasing the hole surface area by about 17% of the cubic specimen surface area.These changes clearly show the effect of holes on the original crack paths.Two types of cracks are observed in these specimens: (1) cracks that occur in the hole-free cubic concrete specimens and (2) cracks that begin from the edge of the concrete and reach the closest point on the circular hole (Fig. 5).The change in the behavior of the specimens implies that a softer concrete structure containing microcracks.Therefore, the brittle structure of the concrete specimens is more obvious with decreasing the ber percentages.Further, specimens with the same geographical properties and a compressive strength of 33 MPa were studied (Fig. 6).
Investigating the concrete failure structure revealed the effect of hole diameter on the ber percentages of the specimen so that the concrete failed sooner and exhibited a more brittle behavior around the holes.
The specimen brittleness is measured by the strain along the direction of uniaxial compressive cyclic loading.In this regard, the strain decreased in the specimens with the lower percentage of ber.This part of the study showed a behavioral difference between the points farther from the hole and the points around it.It is therefore expected that the behavior of the concrete core in the center of the beam-column joint is different from the outer parts of the joint and the cracks outside the joint are different from those in the connection core.
In this case, the crack propagated towards the center of the cube.Due to the existence of a circular hole at the center of the cube, the crack propagated along the hole and caused the specimen failure.The cracks on the surface of the concrete specimens propagated into the depth of the specimen, tracing the whole length of the hole along the hole direction (Fig. 7).
The analysis showed that the cracks skirt around the aggregates and then continue their direct path.The crack planes are straighter when the materials are more homogeneous and/or the concrete contains ner aggregates.The hole is in the center of the specimens affects the compressive strength and reduces it symmetrically.

Reinforcement SCC
In this part, cubic concrete specimens with rebars at the center were symmetrically subjected to uniaxial loading perpendicular to the cross-section of the rebars.This structure is observed in a concrete structure where a compressive cyclic loading is exerted perpendicular to rebars in the concrete.These properties are very obvious at beam-column joints and also where the beam is buried in the concrete shear wall.In beams attached to the shear wall, the compressive and/or tensile load inside the shear wall is perpendicular to the direction of the beam rebars buried in the wall.Furthermore, the axial force of the beam and/or column has a similar structure in terms of loading for column or beam rebars located in the center of the connection.In this section, the crack propagation in the concrete was studied according to the compressive cyclic loading perpendicular to the direction of the rebars and the failure mechanism of the specimen.
Studies have shown that the existence of rebars in concrete holes causes certain changes in the major cracks, failure path, and the failure mechanism of the specimens.The key changes in microcracks occur around the buried rebar.In holes with a surface area less than 1.3% of the concrete surface area, the failures are similar to those in the conventional concrete specimens.Besides, the existence of rebars does not signi cantly affect the compressive strength and crack path relative to the specimen with an empty hole.A more detailed investigation revealed that in addition to the original cracks in the FSCC formed at the time of failure, other cracks are also formed due to the presence of the rebar around the hole (Fig. 8).
Considering that the direction of loading is perpendicular to that of the rebar inside the cubic specimen, it is expected that new cracks will be formed due to the difference between the mechanical properties of the rebar and concrete.As seen, cracks have been formed around the rebar perpendicular to the loading direction of the specimens.In fact, in addition to the shape and structure of crack planes in a healthy cubic specimen, new crack planes are created in these models in such a way that the normal vector of the plane is along the loading direction.
At the beam-column joint site or where the concrete beams pass through the shear wall, few rebars are placed similar to the experimental structure modeled in the specimens perpendicular to the compressive load.In this study, one, two, and three rebars with a diameter of 22 and 32 cm were placed in the cubic specimens to investigate the failure conditions (Fig. 9).
Given the diameter of the rebars, the holes were selected at a suitable distance relative to the largest aggregates.Considering the number and difference in the tensile behavior between rebar and concrete, it is expected that the cracks perpendicular to the loading direction can be seen in the space between the rebars.In fact, in addition to the common cracks at the beam-column joint site due to the loads, microcracks are formed in the direction perpendicular to the compressive force axis in the space between the rebars.
Investigation of the failure structure of the specimens showed that when more than one rebar is used, the depth and effectiveness of the crack between two rebars almost doubled.Moreover, major cracks develop in the areas between the other two rebars and separated from the concrete and reduced by increasing ber.Although these areas at the connection core may not be broken, they show a much less strength than the concrete.In the specimens under study, the removal of the rebars from the concrete specimen shows that when three rebars are used, the friction of the middle rebars is about 60% of the lateral rebars after the concrete failure.This nding indicates slightly higher separation of the middle rebars than that expected.Finally, the results show that the bers, instead of the edges, have a greater effect on cracking in the middle part of the samples.
Seven models were investigated to simulate the concrete core behavior in the beam-column joint under uniaxial compressive cyclic loading (Fig. 10).These models were developed with three different mix designs and three compressive strengths.Table 2 presents a decrease in the compressive strength because of the existence of a hole and/or rebar in the concrete specimens according to the provided models.
According to the results, due to the presence of cracks and crack length reduction in the reinforced concrete, the concrete between the rebars has a lower strength than the concrete without rebars.The compressive strength reduction is dependent on the diameter, number, and distance of rebars.The compressive strength of the concrete is reduced with decreasing the distance between the rebars.Besides, the compressive strength is reduced with increasing the diameter of rebars and hole in the specimen.

Conclusion
This experimental study investigated the behavior of reinforced FSCC subjected to uniaxial compressive cyclic loading perpendicular to the rebars axis.The results of this study can be used to investigate the behavior of the beam-column joint site, and also the beams passing the shear wall.The passing rebars under these conditions withstand the compressive stress acting perpendicular to the rebar axis and percentages of ber.This stress causes different concrete failure structures based on the Poisson's ratio and the exural behavior of steel and concrete.Standard cubic specimens were used to investigate this behavior and the effect of the hole on the concrete failure structure.Concrete with a compressive strength of 27, 30, and 33 MPa was examined to investigate the effect of ber on the crack behavior.To investigate the effect of rebar diameter, certain holes with a diameter of 2, 3, and 4 cm were created in the specimens.In some experiments, steel rebars were placed in the concrete specimens to determine the behavioral differences more accurately.The rebars were placed adjacent to each other and with a certain distance in the hardened concrete to investigate the effect of rebar accumulation.
The results show the insigni cant impact of the small holes with a surface area about 1.3% of the specimen surface area on the concrete failure structure in lower compressive strength and higher ber.The decreasing effect of these holes increased with increasing the ber to the self-competing concrete.An increase in the hole diameter changed the crack propagation path towards the hole, reducing the compressive strength considerably.Under this scenario, the failed cubic specimen is made by inducing a diagonal crack.
A more accurate model of the given behavior was achieved by lling the holes in the cubic specimen with rebars, which changed the extent of the strength reduction relative to the specimens with empty holes and lower ber.In this spread, the cracks show an almost same behavior of both hole-free cubic specimens and specimens with a hole.The main difference was in an increase in the number of effective cracks perpendicular to the loading direction.These horizontal cracks began from the rebar and reached the edges of the specimen and reduced with increasing the ber.The depth and severity of the given cracks increased signi cantly when two or three rebars were used in the specimen.The horizontal cracks were along the tensile stress direction and occurred in the concrete between two rebars.The occurrence of a tensile crack in the concrete between the rebars suggests that the concrete strength in this area is lower than that expected.Suitable models for the determination of these crack types can be developed to investigate this difference more accurately.

Declarations Author Contribution
Corresponding author: Hamoon Fathi  Compressive strength of cubic specimens considering hole Properties The failure structure of a cubic concrete specimen (27 MPa) with a circular hole and a surface area equivalent to 1.39% of the cubic specimen surface area (2% ber) The Failure structure of a cubic concrete specimen (30 MPa) with a circular hole and a surface area equivalent to 3.14% of the cubic specimen surface area (4% ber) The failure structure of a cubic concrete specimen (30 MPa) with a circular hole and a surface area equivalent to 17% of the cubic specimen surface area The failure structure of a cubic concrete specimen (33 MPa) with a circular hole and a surface area equivalent to 17% of the cubic specimen surface area Page 16/18 The crack structure in the form of planes at the specimen depth (a hole with the surface area of 5.58% of the cross-section area) Rupture of cubic concrete specimens (27 MPa) with rebar with a diameter of 22 mm al. 2017; Valença et al. 2017; Grisaro.2022).These methods are based on images taken from the surface of dry specimens.The clean and dry concrete surface shows the cracks more clearly.Crack imaging is done 15 min after washing the concrete surface.Another imaging technique is the concrete surface laser scanning employed by Valencia et al. to examine the surface cracks of a concrete bridge (Yang et al. 2015; Chen et al. 2023; Tran et al. 2023).The crack site and propagation stages in concrete buildings can be directly studied by this technique.Another study investigated the effect of thermal and creep cracking in concrete bridges (Gulsan et al. 2022; Firoozi et al. 2023).

Figures
Figures

Figure 1 Uniaxial compressive cyclic loading method Figure 2
Figure 1

Table 1
Properties of the concrete mix (Weight Ratio)