Patent Application
20200157006
The present invention relates to improved repair mortars and structures implemented by improved repair mortars that can strengthen the beam-column joints in older concrete structures.
Older reinforced concrete buildings may have beam-column joints (BCJs) that lack transverse reinforcements; these beam-column joints have limited shear capacity and limited ductility. In Hong Kong, building codes prior to 2004 did not require transverse reinforcements. In major alteration and addition (A&A) works, the pre-2004 BCJs have to overcome both the deficiency in transverse reinforcements and the additional floor loading. The latter incurs additional force to the BCJs. Demolishing and re-constructing these existing buildings would generate substantial construction waste and pose environmental concerns. Therefore, upgrading the beam-column joints to enhance shear capacity and ductility and to withstand the additional load is required. However, existing repair mortars lack the necessary combined strength and ductility to reinforce these beam-column joints. Therefore, there is a need in the art for improved repair mortars and structures implemented by improved repair mortars that can strengthen the beam-column joints in older concrete structures.
The present invention provides a polymer-modified hybrid fibers cementitious composition having a one-day compressive strength of at least approximately 17 MPa, a 28-day tensile strength of at least approximately 3.8 MPa, an ultimate tensile strain of approximately 3% to approximately 9%, and a 7-day bond strength of at least approximately 2.3 MPa. The composition includes a binder of ordinary Portland cement, fly ash, and silica fume. Other components include limestone powder, sand, superplasticizer and water. The composition further including polymer additives. The additives include one or more of styrene butadiene rubber or ethylene-vinyl acetate copolymer in an amount ranging between approximately 2% and approximately 8% by mass of binder. Fiber additives are also provided. The fiber additives include steel fibers in an amount ranging between approximately 0.3% and approximately 3.0% by volume of the cementitious composition and polymer fibers in an amount ranging between approximately 0.8% and approximately 1.0% by volume of the cementitious composition.
In another aspect, a repaired concrete structure is provided. The repaired concrete structure includes a concrete beam-column joint and a chamfer positioned on the beam-column joint. The chamfer fills the approximately right-angled intersection between a beam and a column, creating an approximately diagonal surface extending between the beam and the column. The chamfer is made from a hardened repair mortar. The repair mortar includes a binder of ordinary Portland cement, fly ash, and silica fume. The repair mortar further includes limestone powder, sand and polymer additives. The polymer additives are one or more of styrene butadiene rubber or ethylene-vinyl acetate copolymer. Fiber additives include steel fibers and polymer fibers.
Repair Mortar
Addition of chamfers to beam-column joints is an effective and less-disruptive technique to strengthen the beam-column joints, thereby enlarging concrete struts in joint zones and relocating plastic hinges away from the joints. However, conventional cement-based mortars are prone to cracking under tension due to low tensile strength. Following cracking, the conventional mortar can no longer resist tensile stress. As a result, the cracked chamfers cannot contribute to part of load transfer between reinforced concrete beams and columns. Therefore, the present invention provides an innovative polymer-modified hybrid micro-fiber repair mortar (named as PHMRM hereafter) to form high-performance chamfers. The novel repair mortar attains superior mechanical properties to enhance joint shear capacity and high deformation capacity to facilitate energy dissipation.
The repair mortar of the present invention includes a binder phase of ordinary Portland cement, fly ash, and silica fume. The binder phase chemically reacts with water to harden the resultant mortar. In general, hydraulic reactions involve calcium, silica, and alumina ingredients. Reaction products, when introduced to water, include calcium silicates and calcium aluminate hydrates. Portland cement is a type of hydraulic cement that typically includes calcium oxides, silica, and alumina in various proportions. Compositions of Portland cement may include CaO in a range of 61-67%, SiO2 in a range of 19-23%, Al2O3 in a range of 2.5-6%, Fe2O3 in a range of 0-6% and sulfate in a range of 1.5-4.5%.
Various compositions of Portland cement are set forth in ASTM C150/C150M-16e1 “Standard Specification for Portland Cement”, available from ASTM International, West Conshohocken, Pa., 2016, the disclosure of which is incorporated by reference herein. Any of these compositions may be used as the hydraulic binder of the present invention.
Fly ash is the leftover product of coal combustion. The range of components depends on the composition of the coal that was burned and tends to be regionally-specific. The main components of fly ash are silicon dioxide (SiO2) in an amount from 20-60 weight percent, aluminium oxide (Al2O3) in an amount from 5 to 35 weight percent and calcium oxide (CaO) in an amount from 1 to 40 weight percent. Iron oxide (Fe2O3) may also be present in an amount from 4 to 40 percent. Note that the materials such as fly ash have variable compositions because they are waste materials and, as such, come from a wide variety of sources. Therefore, the above compositions are merely exemplary of material compositions.
In order to create a high-strength repair mortar, silica fume is included in the composition. Silica fume is typically amorphous silicon dioxide with a fine particle size on the order of 100-300 nm. Silica fume acts as a pozzolan in the repair mortar, that is, a material that, in the presence of water, can react chemically with calcium hydroxide to form a cementitious material. As a result of this chemical reaction, the resultant mortar has an increased strength. The amount of silica fume may be selected depending upon the desired strength of the repair mortar. The amount selected may be approximately 2.5 weight percent or less.
Limestone may be added to the repair mortar composition in an amount equal to or less than approximately 9 weight percent. Limestone may improve the workability of the repair mortar. Further, limestone may prevent sand/binder separation and reduce shrinkage cracking. The limestone may act as a seed crystal for the binder phase, better distributing the reaction products and increasing the reactivity of the cement.
Aggregate, the structural filler of the repair mortar, provides compressive strength and bulk to the repair mortar and may be chosen based on the desired durability, strength, and workability of the repair mortar. The repair mortar of the present disclosure may include sand and/or lightweight aggregate in varying amounts. A total amount of aggregate may range from approximately 25 weight percent to approximately 33 weight percent. In an exemplary embodiment, river sand having a particle size less than approximately 2.36 mm is used as the aggregate/filler in an amount of approximately 25 weight percent.
A superplasticizer in an amount from approximately 0.17 weight percent to approximately 0.24 weight percent may be included in the repair mortar composition. “Superplasticizer,” as used herein, refers to materials used to disperse cement agglomerates. Superplasticizers may be polycarboxylate-based polymers such as polycarboxylate ether-based polymers.
Polymeric materials may be added to the repair mortar in order to increase the bonding strength of the repair mortar. Polymeric materials may also enhance the cohesiveness of the binder, reducing cracking during drying, and increase the ductility and ultimate tensile strain. The polymer additives may include one or more of styrene butadiene rubber or ethylene-vinyl acetate copolymer in an amount ranging between approximately 2 percent and approximately 8% by mass of the binder.
Fiber additives may be used to increase the tensile strength of the repair mortar. Steel fibers may be added in an amount ranging between approximately 0.3 percent and approximately 3.0 percent by volume of the repair mortar. Polymer fibers may help reduce drying cracking of the repair mortar. Polyethylene fibers may be used as the polymer fibers in an amount ranging from approximately 0.8 volume percent and approximately 1.0 volume percent of the repair mortar.
To achieve a one-day compressive strength of 15 MPa, a water-to-binder ratio of 0.35 was selected. Since a larger amount of binder phase is beneficial to disperse fibers in the mortar, the binder-to-aggregate ratio was set at 1.5. Ordinary Portland cement was used as the main binder. According to Concrete Code 2013, fly ash and silica fume can be used to replace Portland cement partially to reduce hydration heat and improve strength. The code recommends a range of substitution of 25% to 35% for fly ash, and less than 6% for silica fume by mass of binder, respectively. In this example, the fly ash ranges from 25% to 35% and the silica is 5% by mass of binder. River sand and limestone powder were used as aggregate. Grace ADVA 109 superplasticizer was employed to adjust workability of mortar. As shown in Table 1, three formulations of repair mortar were tested.
In each formulation, three cubic specimens with dimension of 70×70×70 mm were prepared for one-day compressive strength test. The procedure of preparing the repair mortar specimens is described as follows:
Place ordinary Portland cement, fly ash, silica fume, water and superplasticizer in a mixer and mix them for 30 seconds at low speed. Add sand during mixing. Shift the mixer to medium speed and mix the mortar for 30 seconds. Suspend the mixing and scrape down the mortar on sides of the bowl. Mix the mortar for another 60 seconds at medium speed.
Cast the mortar into molds and compact it using a vibrating table.
The specimens are removed from molds 24 hours after casting, and then subjected to compression test.
Loading rate of the compression test was 0.6 MPa/s. The test was terminated when the compressive load reduced to 85% of its maximum. Measured one-day compressive strength of the repair mortar is given in Table 2.
From Table 2, it is observed that all the mortar formulations achieved the target one-day compressive strength of at least 15 MPa. The mortar formulation F2, which contains limestone powder, demonstrates slightly improved compressive strength. It is likely due to the high fineness of limestone powder that fills the pores in the mortar. Compared to F1, the formulation F3 has a higher cement content, resulting in higher one-day compressive strength. Further increase in the cement proportion may lead to high hydration heat and shrinkage of mortar. Therefore, formulation F3 was selected as the composition for further testing.
As the fiber additive, two types of steel fibers, bare steel fiber (“B”) and copper-coated steel fiber (“C”), and one type of polyethylene fiber (“PE”) are used to reinforce the mortar as shown in
The amount of steel fibers B and C were set at 1%, 2% and 3% by volume of mortar while that of PE fiber is set at 1% by volume of mortar. Fly ash, silica fume, limestone powder, water and superplasticizer were placed in a mixer and mixed for 30 seconds at low speed. The fibers were then added with mixer running for 30 seconds. Lastly river sand was added and the mortar was mixed until uniform. Mortar without fiber was also prepared as reference. In each formulation, cubic specimens with dimension of 70×70×70 mm and dog-bone type specimens were prepared for compressive strength test and direct tensile test respectively. The dog-bone specimens had length of 330 mm with expanded ends, as shown in
The specimens were removed from molds one day after casting. The cubic specimens were used to test one-day compressive strength. The loading rate of the compression test was 0.6 MPa/s. The test was terminated when the compressive load reduced to 85% of its maximum. The dog-bone specimens were cured in air for 28 days and then subjected to a direct tensile test. Axial load was applied by a Mechanical Testing and Simulation (“MTS”) system through the fixtures attached to the two ends of the specimen. The ends of fixtures were connected to ball joints to avoid eccentric loading. The loading rate of the tensile test was 1 mm/min. The axial deformation of the specimen was measured between one pair of aluminum sheets with a gauge length of 80 mm. Relative displacement between the sheets was recorded by a pair of LVDTs connected to the sheets, as shown in
The one-day compressive strength of repair mortar incorporating different contents of fibers is listed in Table 4. The compressive strength is maintained at approximately 25 MPa even when the content of steel fiber B was varied from 1% to 3%. When the steel fiber C was increased from 1% to 3%, the compressive strength increased to 27.2 MPa at 2% and then decreased to 24 MPa at 3%, probably due to the high fiber content which lowered the workability of the repair mortar. As compared to steel fiber B, steel fiber C has a larger length-to-diameter ratio, which contributes to the effect of fibers in restraining repair mortar cracks under compression and results in higher compressive strength. Comparatively, polyethylene fiber slightly lowered the one-day compressive strength, however, it achieved the required strength of 15 MPa.
Axial stress-strain relationships of specimens under tension are depicted in
The stress-strain curve of polyethylene fiber reinforced repair mortar consisted of two stages, pre-crack stage and post-crack stage. The first stage was generally linear. Compared to control specimens, polyethylene fiber reinforced mortar maintains similar tensile stress when the first crack occurred. The tensile stress dropped and the tensile force was redistributed. The tensile stress can be increased back to the proceeding peak value when the displacement was increased. Both tensile strength and ultimate strain of the repair mortar were effectively improved by polyethylene fibers.
Tensile strength and ultimate strain of fibers reinforced repair mortar are given in Table 5. Compared to control specimens, specimens reinforced by 1% of steel fibers B had an improvement of 34% in tensile strength. However, as fiber loading increased, tensile strength descended gradually. It is probably due to hooks at ends of steel fibers B which lead to conglobation of fibers. As a result, higher fiber contents caused uneven distribution and thus lower tensile strength. On the other hand, the tensile strength of specimens containing steel fibers C increased with increasing dosage of fibers. Fiber dosage ranging from 1% to 3% resulted in enhancement from two to three times in tensile strength of mortar. Relatively small diameter of steel fibers C increases interfacial area between fibers and the repair mortar, which lead to a greater improvement in tensile performance. Addition of steel fibers in mortar improved the ultimate strain of the repair mortar up to 0.7%. The best tensile strain performance was observed in the specimens containing polyethylene fibers. With incorporation of 1% polyethylene fibers, the specimens can achieve an ultimate tensile strain of 2.7%.
As seen above, steel fibers C performed better than steel fibers B in improving compressive strength and tensile strength of mortar. Polyethylene fibers achieved significant improvement in tensile strength and ultimate strain of the repair mortar. Both steel fibers C and polyethylene fibers were added into repair mortar to realize the advantages of both fibers. Loading of the two-fiber system was optimized for compressive and tensile performance and cost of the repair mortar. One-day compressive strength, tensile strength and ultimate strain of two-fiber reinforced repair mortar were tested.
Three different formulas with varying fiber loadings were tested, as set forth in Table 6, below. Cement, fly ash, silica fume, limestone powder, water and half of superplasticizer were placed in a mixer and mixed for 30 seconds at a relatively low speed. Polyethylene fibers were then added with mixer running for 30 seconds at high speed. Subsequently river sand, steel fibers and the balance of the superplasticizer were added and the mortar was mixed until uniform. In each formulation, three cubic specimens and three dog-bone type specimens were prepared for compression tests and direct tensile tests, respectively. The specimens were removed from molds 24 hours after casting. The cubic specimens were used to test one-day compressive strength. The dog-bone specimens were under air-curing for 28 days and then subjected to direct tensile test.
The one-day compressive strength of fiber reinforced mortar is tabulated below. Mortar formulation PE1SF1 gave 23.9 MPa. Compared to PE1SF1, PE1SF0.5 had a slight reduction of 0.7 MPa in one-day compressive strength due to decreasing dosage of steel fiber. As dosage of both steel fibers and PE fibers reduced by 0.2%, the one-day compressive strength reduced to 22.4 MPa.
Failure modes of the two-fiber reinforced mortar under direct tension are shown in
The axial stress-strain relationships of hybrid fibers reinforced mortar are given in
Tensile strength and ultimate strain of two-fiber reinforced repair mortar are listed in Table 8. From the ultimate strain results, lowering the steel content can increase both the ultimate strain and tensile strength. The formulation PE0.8SF0.3 exhibited the best performance.
Incorporation of polymers in the repair mortar is desirable as it can enhance the tensile strength and bond strength between repair mortar and concrete substrate. Two types of polymers, including styrene butadiene rubber (SBR) and ethylene vinyl acetate (EVA), were used to enhance the tensile bond strength of the repair mortar. Five dosages for each polymer including 0, 2%, 4%, 6% and 8% by mass of binder were mixed into mortar without fiber (Table 9). Tensile strength, bond strength, one-day compressive strength of the polymer-modified mortar were tested.
The procedure for preparing polymer-modified repair mortar was similar to that used above. When fresh control mortar was ready, polymer was added into the mortar. The mortar was mixed for an additional 30 seconds at high speed. In each formulation, three cubic specimens were prepared for the compression test. The method of the compression test was the same as that set forth above. Six dumb-bell shaped specimens were prepared and cured in water for 7 days, and then subjected to the tensile test and bond test, respectively.
The one-day compressive strength of the polymer-modified mortar is given in Table 10. Both EVA and SBR can weaken the one-day compressive strength of the polymer-modified repair mortar. As the percentage of EVA and SBR increased, the polymer-modified mortar experienced gradual deterioration in compressive strength. When dosage of polymer increased to 8% by mass of binder, the compressive strength of mortar was very close to the required compressive strength of 15 MPa.
Failure modes of the polymer-modified mortar in tensile tests and bond tests are shown in
The optimized PHMRM is a cement-based mortar with water-to-binder ratio of 0.35. The dosage of PE fiber and steel fiber of 0.8% and 0.3% by volume of mortar respectively. The dosage of SBR is set to 2% by mass of binder. Compression test, direct tensile test and bond test were conducted on the optimized mortar to verify the mechanical properties, such as one-day compressive strength, tensile strength and strain, and bond strength. The corresponding specimens, including cubes, dog-bone specimens and dumb-bell specimens, were prepared and tested. For the dumb-bell specimens, a mortar primer was coated on the specimen interface, as shown in
Chamfer
As discussed above, the repair mortars of the present invention may be used to form chamfers at beam column joints to strengthen the beam-column joints. Adding chamfers to beam-column joints enhances the load-carrying capacity, improves energy dissipation, and suppresses brittle joint shear failure (which is an undesirable failure mode for building and particularly during seismic events). Typically, smaller chamfer sizes (desirable from a “usable floor space” point of view) receive larger compressive and tensile stresses, requiring better chamfer material to distribute the stresses, as with the material of the present invention. The dimension of chamfers is dependent on column width/beam depth, reinforcement configuration, mechanical properties (strength and elastic modulus) of concrete and chamfers. Two important parameters are the column width/beam depth and properties of the chamfer material.
Seismic Performance of Interior Beam-Column Joints Strengthened by Chamfers:
A pair of chamfers were installed at the beam soffit of interior beam-column joint specimens, as schematically depicted in
Four interior beam-column joint specimens (“IJ-NC”, “IJ-SP”, “IJ-C150FM” and “IJ-C150FMN”) were prepared. The control specimen IJ-NC represents an interior beam-column joint without chamfers. The specimen IJ-SP represents an interior beam-column joint designed according to seismic requirement of Hong Kong Concrete Code 2013. A pair of stirrups with diameter of 8 mm (“R8”) was adopted as joint shear reinforcement in specimen IJ-SP. The other specimens IJ-C150FM and IJ-C150FMN represent beam-column joint strengthened by chamfers. The dimension of chamfers is based on the least dimension (“LBC”) of beam depth and column width, i.e. the column width of 300 mm in this study. Lengths of chamfers (“LC”) are taken as one half of LBC, i.e. 150 mm for interior beam-column joints (Table 12). Dimension and reinforcement details of specimens are given in
The concrete grade for the test specimen was C30 according to the as-built record. Ready-mixed concrete with a slump of 150 mm was used. All specimens are cured in air after demolding. The compressive strength of concrete “fcu” of each specimen was measured using 100 mm cubes one day before the quasi-static test, as listed in Table 13.
After curing, specimens IJ-C150FM and IJ-C150FMN were strengthened by chamfers using the repair mortar determined through Example 1, above. Two different strengthening methods including fixed formwork method and lifting formwork method were applied in strengthening work of specimens IJ-C150FM and IJ-C150FMN, respectively. The procedure of strengthening works is listed as follows and depicted in
The compressive strength of the repair mortar of each of specimens IJ-C150FM and IJ C150FMN are 32.0 and 43.7 MPa, respectively. The strength was measured using 70 mm cubes one day before the quasi-static test.
First, an axial load of 0.25 fcuAg was applied vertically at the upper column and kept constant throughout the test. Here, fcu is the compressive strength of concrete and Ag is gross cross-sectional area of the column. After imposing the axial load, each specimen was tested under reversed horizontal displacement. Horizontal loading was applied according to the loading scheme shown in
Failure Mode:
Flexural cracks of specimen IJ-NC were first observed in the beams. Several vertical cracks then appeared in the joint. Subsequently, diagonal cracks appeared in the joint at a drift ratio of 0.82%. Drift ratio is defined as the ratio of horizontal displacement at the tip of the upper column to the overall length of column (2600 mm). With progressive increase in the drift ratio, more flexural cracks emerged on the beams and existing cracks propagated. Shear cracks intersected at the center of joint area. However, only three cracks were observed on the lower column throughout the loading history. It is due to the beneficial effect of column axial load that reduced tensile stress and suppressed the crack formation on the columns. When the load reached its peak, diagonal cracks fully developed in joint area, as shown in
Hysteretic Behavior:
Since a constant axial load was applied, a P-Δ effect was induced to the specimens and the effect could become significant under large drift ratio. The strengthening schemes may stiffen the specimens and thus a large horizontal force is responded in the stiffener specimen at the same displacement. For comparison, the P-Δ effect was determined and integrated into the hysteretic loops by the following equation.
where, Ph is the horizontal load considering the P-Δ effect; Ph0 is the horizontal load applied on the upper column; Pv is vertical load; Δ is horizontal displacement; l is overall length of the column.
All the interior beam-column joint specimens failed due to shear failure of the joint. The shear strength of joint is linked to the strength of the concrete. In order to eliminate the effect of different concrete strengths, the horizontal load is normalized by the following equation.
where, Ph,e is equivalent horizontal load; fcu.SP and fcu.X are compressive strength of concrete of specimen IJ-SP and specimen X, respectively.
Plots of the equivalent horizontal load versus displacement at the column tip are shown in
Table 14 shows peak values of equivalent horizontal force. Different from the control specimen, peak loads of specimens IJ-SP, IJ-C150FM and IJ-C150FMN in the pull direction were larger than those in the push direction. This is because cracks propagated in the concrete under a pulling load. When the horizontal load turned to the push direction, cracked specimens cannot reach the peak load in the pull direction. Stirrups effectively improve horizontal loading capacity of the beam-column joint. Compared with control specimen, average peak value of specimen IJ-SP was increased by 21%. Although bonding failure occurred between the column and the chamfers, the chamfers can transfer load under compression and improve shear capacity of the beam-column joint. Peak load of IJ-C150FM was increased by 36% as compared with that of control specimen and was 12% more than that of specimen IJ-SP. Additionally, the average peak value of specimen IJ-C150FMN is slightly higher than that of specimen IJ-C150FM. This is probably attributed to the higher strength of chamfers cast using lifting formwork in specimen IJ-C150FMN.
Stiffness Degradation
Stiffness of a specimen is defined as the slope of a line connecting the maximum equivalent load under reversed horizontal load in each hysteresis loop, as shown in
Strain of Chamfers
Strain gauges were installed at the centroid of chamfers to measure the strain of PHMRM parallel to the hypotenuse of chamfers.
Shear Distortion
The shear distortion of the joint region under different horizontal displacements is shown in
Energy Dissipation
Seismic Performance of Exterior Beam-Column Joints Strengthened by Chamfers
Exterior beam-column joint specimens were prepared and strengthened by chamfers using the repair mortar determined in Example 1. Quasi-static load tests were performed to evaluate the seismic behavior of the specimens with and without chamfers. This part will describe specimens and chamfers for strengthening, followed by test setup and results. The failure modes, hysteretic loops, peak loads of specimens and strain gauge results are presented. Effect of PHMRM chamfers on the seismic performance of BCJs will be examined.
Preparation and Strengthening of Exterior BCJ Specimens
Three exterior beam-column joint specimens (“EJ-NC”, “EJ-SP”, and “EJ-C200FMN”) were prepared. The control specimen EJ-NC represents an exterior beam-column joint without chamfers. The specimen EJ-SP represents an exterior beam-column joint designed according to seismic requirement of Hong Kong Concrete Code 2013. A pair of stirrups R8 was adopted as joint shear reinforcement in specimen EJ-SP. The specimen EJ-C200FMN represents a beam-column joint strengthened by a chamfer with a dimension of 200 mm, i.e., LC to LBC ratio of 0.67 (Table 15). Dimension and reinforcement details of specimens are given in
Similar to the interior beam-column joint specimens, ready-mixed concrete C30 with a slump of 150 mm was used. All specimens are cured in air after demolding. The compressive strength of concrete “fcu” of each specimen was measured using 100 mm cubes one day before the quasi-static test, as listed in Table 16. After curing, specimen EJ-C200FMN was strengthened by a chamfer using the lifting formwork method.
The procedure of the quasi-static test on exterior beam-column joints is similar to that of interior beam-column joints. An axial load of 0.25 fcuAg was kept constant throughout the test. After imposing the axial load, each specimen was tested under reversed horizontal displacement. Horizontal loading was applied according to the loading scheme shown in
Failure Mode
Flexural cracks of specimen EJ-NC were first observed in the bottom of the beam. Several vertical cracks then appeared in the joint and the top of the beam. Subsequently, diagonal cracks appeared in the joint at a drift ratio of 0.78%. With progressive increase in the drift ratio, more flexural cracks emerged on the beams and existing cracks propagated. Shear cracks intersected at the center of joint area and propagated to both the upper column and lower column. When the horizontal force reached its peak, diagonal cracks fully developed in the joint zone, indicating that specimen EJ-NC failed due to shear failure in joint. Under the reversed cyclic load, repeated opening and closing of diagonal cracks on the joint accelerated spalling of joint cover at the post-peak stage, as shown in
The crack pattern of specimen EJ-SP was similar to that of specimen EJ-NC. Flexural cracks appeared first in the bottom of the beam and then in the top of the beam. Subsequently, diagonal cracks appeared in the joint at a drift ratio of 0.81%. With an increase in the drift ratio, the flexural cracks increased and developed towards the neutral axis of the beam. Shear cracks intersected at the center of the joint area and propagated to column. Due to the stirrups in the joint, the number of diagonal cracks in specimen EJ-SP was fewer than that in the control specimen (
Hysteretic Behavior
Horizontal load-displacement relationships of exterior beam-column joints are shown in
Table 17 shows peak values of equivalent horizontal force of exterior beam-column joint specimens. Specimen EJ-SP had close loading capacity as compared with the control specimen in both directions of horizontal loading. The seismic provision in the joint of specimen EJ-SP seems to play little role in improvement of seismic behavior. In contrast, specimen EJ-C200FMN showed almost the same peak load as the control specimen under positive horizontal displacement (i.e., the pull direction), while it improved by 22.3% in peak load than the control specimen under negative horizontal displacement (i.e., the push direction). The difference in seismic performance of specimen EJ-C200FMN is ascribed to effectiveness of the chamfer in transmission of load under opposite horizontal displacement. The chamfer at the beam soffit was subjected to tensile load under positive horizontal displacement. Insufficient bonding between the beam and chamfer failed to carry the tensile load. The chamfer separated from the beam and lost its contribution to loading capacity of specimen EJ-C200FMN. However, when the horizontal displacement was negative, the chamfer succeeded to resist compression and thus improve the horizontal loading capacity of beam-column joint specimen.
Stiffness of specimens is shown in
Strain gauges were installed at the centroid of the chamfer of specimen EJ-C200FMN to measure the strain of the repair mortar parallel to the hypotenuse (diagonal surface) of chamfers.
Thus, as demonstrated by the Examples, a high-performance repair mortar was developed having high compressive and tensile strength and high bonding strength. Chamfers made from the repair mortar are installed at beam-column joints were extensively tested. As shown by the testing, chamfers made from the inventive repair mortars can provide higher horizontal loading capacity, stiffness, and energy dissipation to the beam-column joints. Both fixed formwork and lifting formwork may be used to for strengthening beam-column joints.
Those skilled in the art will appreciate from the foregoing description that the broad techniques of the embodiments can be implemented in a variety of forms. Therefore, while the embodiments have been described in connection with particular examples thereof, the true scope of the embodiments should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the specification, and following claims.
This application claims priority from the U.S. Provisional Patent Application Ser. No. 62/770,173 filed Nov. 20, 2018, and the disclosure of which is incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 62770173 | Nov 2018 | US |
