Retrofitting of reinforced concrete columns with honeycombed cover

In this research, a significant attention has been given to the experimental investigation of retrofitting and rehabilitation of honeycombed reinforced concrete (RC) columns. Ten RC columns specimens were tested and discussed in the experimental tests; five of them are for the square section with a dimension of cross section (200 X 200) mm, and five for the circular section with a cross section diameter of (200) mm. The total length for both sections is (1200) mm. The honeycombed zone in the base and middle of specimens was considered as a percentage of the total columns. The honeycombed columns were repaired with cement-based materials prior to the test. All columns were tested axially until failure. For each tested RC columns, the ultimate load, cracking load, mode of failure, and load versus lateral and axial displacement were recorded. Test results showed the strength and load capacity of the treated column exceed the one for the control column with different percentages based on the cross-section of RC column, and location and size of the honeycomb defect. The increase in ultimate load ranges from 3% to 16% for square section, while for circular section the range of increase starts from 3% to 17%. It is worth noting that this percentage of increase is higher as the size of the honeycomb increases and vice versa.


Introduction
The (RC) column is the most important member in the RC structures that resist vertical loads and provide bracing against the lateral loads.In columns, the damage perhaps is caused by cracking, surface deterioration, surface deposits, and construction defects or construction features.One of the most common construction defects in concrete structure is honeycomb.Honeycombing occurs when the concrete against the formwork is not filled resulting in a rough textured surface with voids in the concrete.It's also known as hollow voids genereted in concrete mass where concrete can't be accessed.The honeycomb defect appears to be incredibly rough surface.It results in various problems to the concrete exterior and the concrete's long-term durability in structural member.There are many possible causes, all of which lead to cavities forming in concrete once it has hardened.It is advisable and better to minimize or stay away from honeycombing as far as possible.The rehabilitation of honeycomb should ensure that it's either fully concealed; or removed the concrete damaged and replaced with good quality of materials that is accomplished by efficient and experienced.
Concrete is one of the most common important structural materials in the world.So, the sustainable and retrofitting of concrete structure is very important and necessary.Retrofitting development challenge the construction industry to create building that acknowledge the life-cycle of structure and seeks to addition of new features to older buildings, it is part of circular economy to increase building efficiency and reducing the amount of newly manufactured.
Sasaki and Kobayashi [1] presented a test of cyclic loading using RC columns with repaired honeycombed concrete at column base to confirm the impact of the reparied part on deformation performance and on behavior of the interface.As a result of this, difference in strength and deformation performance were not discovered.While, with using the conventoinal repair method on the columns, interfacial peeling occurred with small comprassive load.From the other side, with using the proposed repair method on the specimen, interfacial peeling didn't happen.With noting that the damage with the conventional repair method specimens was larger than proposed repiar method specimen.Dawari and Vesmawala, [2] performed an experimental study for honeycomb damage identifciation in RC beams.Methods has been proven based on difference between modal curvature and modal flexibility differences.Damage is found by comparing the typical dynamic properties of the damage and undamaged structure.It is considared as a localized reduction in stiffness of structure.This study showed that this method have been effecitively discovered the existence of damage and is able to select the position of damage for single and multiple damage scenarios for beams and the results of numerical demonstrated the effectiveness of these methods in locating single and multiple damage scenarios in beams.Karim et al. [3] showed that the different techniques about the strengthening of the damaged RC columns such as RC jacketing, steel jacketing, and wrapping by glass fiber reinforced plastic.It was determined that, the jacketing of steel raised the stiffness of column between (10% and 20%) due to addition confinement from the jacket.In addition, the ductility and strength of RC columns raised significantly by wrapping the damaged RC columns by glass fiber straps.As well as; strengthening the damaged RC columns by concrete jacketing was stronger.Wang [4] proposed a common quality defects; examined their characteristics, perforemed a cause analysis from; such five aspects as design, construction, environment management and raw materials.The common quality defects, of any projects in construction are large number, and are fundamentally embodied in three aspects such as crack, filtration, leakage.The researcher summarized the important control measures of errors in quality defects of construction projects preceding; during; and after construction.Ismail and Ong [5] presented a technique to find the danger caused by honeycomb and the location in a RC beam, by using the regression focus of the frequency mode shape on a small amount of information.Simply supported beams were produced, using two volumes ratios of the materials used for honeycomb inserted at the middle of the beam.The method compelled the function of linear and analyses Eigen on the control RC beam.The analysis of non-linearity on the test beams, with a damage local-stiffness , obtained indicators from an regressive-state using the algorithm of Levenberg-Marquardt and reside duals from regressionstate of shapes of frequency mode; using the rational of Chebyshev-series, transformation and application into the 4th order centered finite divided difference rule that have been used.Two methods were effective and important in finding the location of honeycomb defects.However; the Chebyshevseries rational method was unable to determine the severity of damage.Volker and Shokouhi [6] presented a systematic approach for fusion of multi-sensory nondstructive testing data.The data set consists of impact echo; ultrasonic pulse echo and ground pentrating radar data collected on a large scale specimens of concrete with built-in honeycombing defects.From each data set; the most significant signatures of honeycomb, were extracted in the form of featura.Two simple data fusion algorithms; on the data were applied: Dumpster's-rule of combination and the Hadamard product.The performance of the fusoin rules versuss the single sensor test was evaluated; the fusion rules exhibit a slight improvement of false alarm rate over, the best single sensor.
Based on past studies; it was discovered there were extremely limited number and a few studies about performance of rehabilitated of honeycombed RC columns.Thus, the purpose of this work is to investigate the impact of honeycomb damage in the cover of RC columns.

Methodology
Ten specimens were tested in the experimental work and separated into two groups.The first group is represented by five square section RC columns with honeycombed zone in cover with fixed volume ratios (0% for control ,0.03, 0.06 in midspan, and 0.06, 0.13 in the base of column).The second group consisted of five circular section RC columns in which honeycombed zone were distributed by the same mechanism of the square section with volumetric ratios (0% for control, 0.12, 0.16 in midspan, and 0.04, 0.08 in the base of column).The specimens' defects were considered as a percentage of the total columns size.The experiment took place at structural lab of the Al-Qadisiyah University's College of Engineering.

Materials
All the RC columns work in this investigation were cast with , ordinary Portland cement from local supplier.Tests results were conformed to the requierments of the limits of the specification of Iraq standard of cement test (IQS No. 5 2019) [7].The Fine aggregate was tested and the {grading, sulfate} content meet Iraqi regulation ( No. 45/2021 ) [8].Coarse aggregate was also used according to Specification of Iraq No.45-2021 [8].

Experimental Program
The RC columns with square section have dimensions of (200×200) and a total length of 1200 mm, while the RC columns with circular section has a dimeter of 200 mm and a total length of 1200 mm.Depending on the design compressive strength of 30 MPa.The reinforcing details of the tested specimens are shown in figure 1.All specimens were cast using the ordinary concrete and, mixed in a fixed propertion (3:2:1).The concrete strength was evaluated after (28 days) of casting by, compression tests based on (150×150×150) mm cubes, where the average compressive strength was determined to be (33.5)MPa.As part of the research program, ten RC column specimens divided into two sections circular and square have been designed, manufactured, and tested.In general, all specimens (circular and square columns) were reinforced with steel bars.As; for the square section, it was reinforced with (8) Of (Ø10) mm steel bars with a spacing of (160 mm) from center to center.While, the column with circular section; it was reinforced with (6) of (Ø10) mm steel bars with a spacing of (50mm) from center to center.The slenderness ratio KL/r is (20) and (24) for square and circular columns respectively.sothe square columns was classified as a short columns and circular columns was classified as a long columns .The slenderness ratio defined as for columns reinforced with steel bars, Where; (k) is the effective length factored, equal to (1) for the unsupported length of column; (L) is the column's effective length and(r); is the radius of gyration.Based on; ACI 318-19 [9].The honeycombed zone was created in the middle and base of RC columns in calculated and accurate proportions for the purposes of sceintific research.The specimens' defects were considers as a percentage of the total columns size, which was (48000 cm 3 and 58875cm 3 ) for square and circular section, respectively as shown in table 1 and table 2, respectively.The first letter in the sample name refers to the type of section, C for circle section and S for square section.The following M and B letters refer to, the location of defects with the length of RC column, mid-length and base respectively.Finally, number 1 and 2 in the sample name represent the size of the defect in each of the samples.
Foam was used to configure honeycomb and make honeycomb in the cover defect.By Using foam pieces that were cut using a CNC-cutting machine in shapes and sizes to meet the requirements of scientific research.And was fixed on the reinforcing steel before placing it in the casting formwork, as shown in figure 2.

Retrofitting honeycombed
Before starting retrofitting of honeycombed the foam pieces were removed and the place was cleaned using an electric brush.Then, the weak area was removed until the strong layer was reached.In general; as a first step; after cleaning them, Cempatch-AB was used on all samples without exceptoin because, it is an acrylic bonding and curing agent for cement mix and concrete repair.The tensile strength was (2.5 MPa) at 7 days according to ASTM C109/109M [10].It is a one componentt modified acrylic liquid polymere specially formulated for use as a bonding ageant and curing aid for cementitoius concrete repair systems.Then; the speicmens was treated with a Cempatch-S, for honeycomb in cover defect.Cempatch-S is a one component polymer modified and, fiber reinforced repair mortar.It's a blend of dry powders; selected aggregates; and fibers.Which when mix with water produces a thixotropic mortar suitable for verticall and overhead application.The compressive strength of Cempatch-S was 60 MPa at 28-days according to ASTM C109/109 M.

Experimental setup
In this experiment, ten specimens were tested by applying a gradually increased axial load using universal testing machine with computerized load cell.To measure the displacement; a dial-gauges was used to measure the axial displacement for each column specimen until failure as shown in figure 3.
Through the test, many data were monitored and recorded such as: the load at first crack, the ultimate failure load, the lateral displacement at the side of column, the axial displacement, and then the loaddeflection relationships of each specimen.

Results and discussions
The results of the experimental investigation are compared and presented in this section.When the columns are axially loaded; the concrete and steel are subjected to stresses; then when the load are high compared to the cross section area of column; concrete and steel reach yield stress.Finally the RC column fails with experiencing small lateral deformation.The RC concrete column is crushed; and the collapse of the RC column is leads to the failure of the materials.The specimens failed at different values of loads, as shown in figure 4 and figure 5, respectively.
When the vertical load started and increased; the crack began and develops at the base, top, and middle height of RC column specimens and propagated in different places.For the column of square section, the control column failed at load (351.64 kN), while the specimens (S-M-1, S-M-2, S-B-1 and S-B-2), have a cover defect and failed at load (388, 408, 362 and 379) kN respectively.And the increase in the ultimate load was (10%, 16%, 3% and 7.7%), respectively.
On the other side; the RC columns speicmens of circular-section failed with the same failure mechanism as the square-section.So, the control column failed at load (397.01)kN .And the other four columns specimens of cover defect ,(C-M-1, C-M-2 , C-B-1 , C-B-2 ) ,failed at load level of (442 , 465 , 408 , 420 ) kN, respectively.And have an increase in ultimate load by (11%, 17%, 3%, 6%) respectively.Eventually, the failure mode of all columns (compression members) was local and pure compression failures.Table 3 and table 4 showed the results of the experimental tests of ultimate load, load of first crack, Maximum lateral and axil deflection ,and finally percentage of increase in ultimate load .On the other hand; in all columns, the failure site indicates that it was not within the treatment area.

Load-displacement relationship
It is important to know from the experimental results, the important of the Load-displacement relationship for each specimen, square and circular section and compare it with control column.The load displacement relationship for the treated honeycombed square and circular columns as compared with control column are shown in figures 6 to 9 for all site of defects.

Conclusions
According to the findings of this research, these conclusions have been drawn.1.All RC columns had pure compression failures.Furthermore, failure site indicates that it was not within the retrofitting area.
2. The investigation of structural behavior of RC columns with different depths and size of honeycomb should be done after treating those columns.
3. The experimental results in square columns, showed that the strength of the treated columns has been fully restored and exceed the strength of the control column with different values ranged from (3% to 16%).For circular section, the values ranged from (3% to 17%).Based on the size, and location of the honeycomb defect.4. Treated materials from solid sources and skilled worker must be used to prevent failure in structural member, and to ensure accurate repairing 5.In general; treated RC columns have failure load and strength higher than the control column, and this is more evident in speicmens with size of honeycomb more than (10%) for the same treated materials and concrete mix and under the same condition.

Figure 1 .
Figure 1.Cross section details for square and circular section.The slenderness ratio KL/r is (20) and (24) for square and circular columns respectively.sothe square columns was classified as a short columns and circular columns was classified as a long columns .The slenderness ratio defined as for columns reinforced with steel bars, Where; (k) is the effective length factored, equal to (1) for the unsupported length of column; (L) is the column's effective length and(r); is the radius of gyration.Based on; ACI 318-19[9].The honeycombed zone was created in the middle and base of RC columns in calculated and accurate proportions for the purposes of sceintific research.The specimens' defects were considers as a percentage of the total columns size, which was (48000 cm 3 and 58875cm 3 ) for square and circular section, respectively as shown in table 1 and table 2, respectively.

Figure 2 .
Figure 2. Foam fixation mechanism for some samples.

Figure 3 .
Figure 3. Test Set up and dial gauges location.

8 Figure 6 .
Figure 6.Load-displacement relationship for square control column with base defect.

Figure 7 .
Figure 7. Load-displacement relationship for square control column with middle defect.

Figure 8 .
Figure 8. Load-displacement relationship for circular control column with base defect.

Figure 9 .
Figure 9. Load-displacement relationship for circular control column with middle defect.

Table 1 .
Size and location for honeycombed zone in RC columns of square section.

Table 2 .
Size and location for honeycombed zone in RC columns of circular section.

Table 3 .
Experimental results of square section.

Table 4 .
Experimental results of circular section.