233
Indonesian Journal of Science & Technology 11.
233-254 Indonesian Journal of Science & Technology Journal homepage: http://ejournal.
edu/index.
php/ijost/ Experimental and Design-Oriented Investigation of Rectangular Concrete Columns: Leveraging Low-Cost Steel Clamps and Waste Aggregates Phromphat Thansirichaisree 1 .
Ali Ejaz2.
Hisham Mohamad3.
Preeda Chaimahawan4,*.
Saharat Buddhawanna5.
Qudeer Hussain6 Faculty of Engineering.
Thammasat School of Engineering.
Thammasat University Rangsit.
Pathumthani.
Thailand National Institute of Transportation.
National University of Sciences and Technology (NUST).
Islamabad.
Pakistan Civil & Environmental Engineering Department, cc.
Seri Iskandar.
Malaysia School of Engineering.
University of Phayao.
Phayao.
Thailand Civil Engineering Department.
Kasem Bundit University.
Thailand Faculty of Engineering.
Thammasat School of Engineering.
Thammasat University Rangsit.
Pathumthani.
Thailand *Correspondence: E-mail: preeda.
ch@up.
ABSTRACT
Steel clamps provide a low-cost method for strengthening concrete, but their role in confining rectangular specimens remains underexplored.
This study performed monotonic compression tests on rectangular specimens confined with steel clamps, examining unconfined compressive strength, number of clamps, and coarse aggregate type, including recycled brick aggregates.
All specimens showed two-phase stressAestrain behavior, with significant strength and ductility gains from passive confinement.
Strength improved up to 19%, while peak strain increased by 150%, especially in specimens using recycled brick aggregates from higherstrength bricks (CB.
CB1-based concrete exhibited the greatest enhancement.
Confinement ratio strongly affected Analytical models predicting peak strength, peak
strain, and post-peak modulus achieved high accuracy (RA > A 2026 Tim Pengembang Jurnal UPI
ARTICLE INFO
Article History:
Submitted/Received 24 May 2025
First Revised 20 Jun 2025
Accepted 24 Aug 2025
First Available Online 29 Aug 2025
Publication Date 01 Sep 2026
____________________
Keyword:
Analytical modeling.
Concrete.
Confinement.
Rectangular columns.
Regression.
Steel clamps.
Thansirichaisree et al.
Experimental and Design-Oriented Investigation of Rectangular ConcreteA | 234
INTRODUCTION
Managing the waste generated from demolished structures remains a significant environmental challenge.
A sustainable and durable solution is essential to minimize the ecological impact of this debris.
In recent years, researchers have investigated the potential of incorporating demolition waste as a partial substitution for natural coarse .
Ae.
or fine aggregates .
Ae.
in concrete production, resulting in what is commonly referred to as Recycled Aggregate Concrete (RAC).
Utilizing such waste helps mitigate its environmental impact .
while also conserving rapidly depleting natural aggregate sources .
Currently, global concrete consumption is estimated at around 30 million tons annually .
Forecasts suggest this demand could rise to approximately 18 billion tons per year by 2050 .
, with an average consumption of three tons per person each year .
RAC offers a promising approach to reduce reliance on finite natural stone resources .
Research has demonstrated that substituting half of the amount of natural aggregates using recycled brick aggregates (RBA) may lower carbon releases by up to 30% and decrease landfill usage by around 40% .
Additionally, concrete made with RBA can attain compressive strengths ranging from 25 to 40 MPa, indicating its viability for structural use .
Extensive research on RAC has been carried out and is largely regarded as comprehensive and mature .
This is evidenced by the development of formal standards such as GB/T25177Ae2011, which pertains to recycled coarse aggregates, and GB/T25176Ae2011, which addresses recycled fine aggregates for use in concrete and mortar.
As a result, current research trends have shifted toward exploring alternative types of construction and demolition waste, including brick .
, ceramic .
, rubber .
, and glass materials .
Recent investigations highlight a consistent and rapid annual increase in brick waste generation .
In the United States alone, environmental reports from 2012 to 2014 revealed that roughly 44 million tons of brick waste were produced from construction and demolition works .
This significant quantity emphasizes the urgent necessity for effective brick waste recycling strategies to decrease dependency on landfilling.
The negative effect of RBA on concrete characteristics is generally more significant than the benefits they offer .
The crushing value of RBA ranges between 20% and 40%, indicating that these aggregates can withstand moderate compressive stress before failure .
RBAs exhibit higher apparent density, water absorption, and crushing index related to natural coarse aggregates .
Furthermore.
RBA typically have a less uniform particle size distribution and often include a wider range of sizes.
This heterogeneity stems from the brick production procedure, which yields particles of varying shapes and dimensions.
On the contrary, natural aggregates tend to have greater uniform size distributions, shaped largely by their geological formation processes .
The density of recycled brick aggregate concrete (RBAC) is generally lesser than that of concrete made with natural aggregates .
This lower density contributes to RBACAos enhanced water absorption capacity .
Additionally, the elastic modulus of RBAC tends to decrease as the replacement level of brick aggregates increases .
The mechanical properties of RBAC are often significantly inferior to those of natural aggregate concrete .
For example, a reduction of approximately 40% in compressive strength and 50% in elastic modulus when comparing RBAC to natural aggregate concrete .
Replacement ratios below 15%, the reduction in compressive strength was minimal.
however, when the replacement surpassed 30%, strength reductions surpassed 20% .
A full replacement of natural aggregates with RBA and observed a complete slump loss after 165 minutes in concrete made with oven-dried brick aggregates, whereas this issue was not present when DOI: https://doi.
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Improvement in porosity of up to 10% in RBAC compared to conventional concrete.
Larger mean particle sizes of brick aggregates negatively impacted the tensile strength of RBAC .
Nonetheless, some studies have indicated that the failure modes of RBAC are comparable to those of natural aggregate concrete .
The primary drawbacks of RBAC are ascribed to the higher water absorption of brick aggregates, which is a result of their inherently porous structure .
On the positive side.
RBAC offers advantages such as reduced weight relative to natural aggregate concrete, due to the higher porosity of the aggregates, and improved fire resistance, owing to the natural refractory properties of brick materials .
Several strategies have been implemented to improve the performance of RBAC.
These comprise physical modification .
or chemical surface treatments .
of brick aggregates, mix design optimization .
incorporation of fibers into the concrete matrix .
, and external confinement using various jacketing techniques .
Ae.
Microbially brought carbonate precipitation (MICP) improved the quality of RBA and improved the mechanical performance of the concrete .
Furthermore, enzyme-induced carbonate precipitation (EICP) provided even better results by ensuring a more uniform distribution of CaCOCE precipitation.
Extending the incubation period in MICP treatment led to a significant decrease in the water absorption capacity of brick aggregates, addressing a major drawback .
a cost-effective and environmentally friendly artificial reef concrete utilizing 100% recycled brick-concrete aggregate.
A comparative analysis showed that using recycled aggregates in combination with industrial by-products like fly ash and silica fume decreased carbon releases and construction expenses by up to 64%, while maintaining equivalent performance to natural aggregate alternatives .
Fiber-Reinforced Polymer (FRP) sheets are commonly employed in strengthening The effectiveness of FRP sheets in enhancing the compressive behavior of RBAC has been investigated by several studies.
Gao et al.
observed significant improvements in both peak compressive strength and corresponding strain of RBAC when confined with Glass FRP (GFRP) or Carbon FRP (CFRP).
Jiang et al.
found that up to a 30% substitution of natural aggregates with brick aggregates did not adversely affect the mechanical performance of RBAC.
However, further increases in the replacement ratio beyond 30% led to reductions in both elastic modulus and peak compressive stress.
Although FRP confinement effectively enhances the structural performance of RBAC, the high cost associated with these materials presents a major limitation in rehabilitation projects .
For instance.
CFRP, while offering excellent performance, can cost around 150 USD per square meter per layer .
Therefore, identifying economical and efficient alternatives to FRP confinement is essential to achieving cost-effective yet structurally sound solutions.
Steel spiral confinement to improve the mechanical properties of recycled concrete.
Their findings indicated that while the elastic modulus decreased with increased confinement, both confined strength and corresponding strain experienced significant enhancement .
A cost-effective external confinement method using standard steel hose clamps to enhance the compressive behavior of RBAC of circular cross-section .
They tested circular concrete cylinders with two target strength levels and applied 3, 5, and 11 steel clamps.
The confinement offered by steel clamps substantially improved the confined strength and altered the failure pattern of RBAC, yielding better response than that of concrete made with natural aggregates.
Recently, steel clamps to strengthen the compressive response of square-shaped concrete made with natural or RBA.
Also, proposed a uniform model by combining the specimens tested by their study .
Previous investigations by .
established the efficacy of steel clamp confinement in enhancing the compressive performance of RBAC.
However, these studies were restricted DOI: https://doi.
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Experimental and Design-Oriented Investigation of Rectangular ConcreteA | 236 to specimens with circular or square cross-sections.
It is well established that confinement effectiveness diminishes when the cross-sectional geometry transitions from circular to square, or from square to rectangular.
Therefore, evaluating the performance of steel clamps on rectangular specimens is essential.
This study includes an experimental program involving monotonic compression tests.
Key variables examined include the unstrengthened compressive strength of concrete, the quantity of steel clamps, and the kind of coarse aggregates used.
METHODS
Summary of Rectangular Specimens Table 1 presents the test samples along with their respective group classifications.
Two primary groups were defined depending on the kind of brick aggregates: G1 and G2 included recycled brick aggregates identified as CB1 and CB2, respectively.
CB1 aggregates originated from cement-clay interlocking bricks, while CB2 came from demolished cement-clay interlocking brick walls.
In both cases, 50% of natural coarse aggregates in the concrete mix were replaced with recycled brick aggregates.
Each group was further split into two subgroups based on target compressive strengthAi15 MPa (Type-A) and 25 MPa (Type-B).
Every subgroup consisted of four specimen types: an unconfined control specimen and 3 specimens confined with 3, 5, or 11 steel clamps.
The specimen ID system included three components:
concrete strength type, aggregate type, and clamp configuration.
For example.
B-CB2-11CL refers to a Type-B specimen using CB2 aggregates and confined with 11 clamps.
Figure 1 graphically illustrates the naming convention.
Two alike samples were tested for each formation, following methodologies from prior studies .
Figure 2.
shows the crosection details, while Figure 2.
shows the clamp layout.
A minimum corner radius of 13 mm was used to reduce stress concentrations, in line with ACI 440 guidelines.
Figure 1.
Implemented terminology for samples in this study.
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sectional details, .
front elevation, .
control, .
3CL, .
5CL and .
11CL (Dimensions are presented in units of m.
Properties of Materials Recycled brick aggregates (Figure .
were sourced from cement-clay interlocking and demolished cement-clay interlocking brick walls, with their properties evaluated following standard procedures and summarized in Table 2.
To determine water absorption, 5 brick specimens were oven-dried at 110 AC for one day, chilled to ambient temperature, and weighed in their dry state.
The bricks were then immersed in water for another day, surfacedried using a damp cloth, and weighed again to determine their saturated surface-dry mass.
Water absorption was calculated based on the difference between dry and saturated weights.
Portland cement Type I served as the binding material, while natural river sand was employed as fine aggregate.
The concrete mixes were designed to achieve target slump values of 75 mm for Type-B and 85 mm for Type-A mixes, respectively.
The mix proportions are provided in Table 3.
Coarse aggregates used in the mixes ranged in size from 9.
5 mm to 19.
5 mm.
The tensile stress vs.
strain behavior of the steel clamps (Figure .
was evaluated following the ASTM E8 standard, as shown in Figure 5.
Each clamp had a width of 9 mm and a thickness of 1 mm.
Figure 3 presents the typical geometry and appearance of the steel clamps used in this study.
These clamps were chosen for their ease of application, as they can be installed without specialized equipment.
The failure pattern observed during testing is depicted in Figure 3.
To ensure accuracy and realistically simulate the confinement process, the fastener was employed inside the gage length of the specimen.
Fracture consistently occurred outside the fastener zone, and the resulting tensile stressAestrain curve (Figure 3.
) demonstrated a ductile behavior of the steel clamps.
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Experimental and Design-Oriented Investigation of Rectangular ConcreteA | 238 Table 1.
Summary of rectangular specimens used in this study.
Group
Subgroup
Specimen ID
Coarse Aggregate Concrete Number of Type
Clamps G1A
A-CB1-CON
NCS CB1
A-CB1-3CL
NCS CB1
A-CB1-5CL
NCS CB1
A-CB1-11CL
NCS CB1
G1B
B-CB1-CON
NCS CB1
B-CB1-3CL
NCS CB1
B-CB1-5CL
NCS CB1
B-CB1-11CL
NCS CB1
G2A
A-CB2-CON
NCS CB2
A-CB2-3CL
NCS CB2
A-CB2-5CL
NCS CB2
A-CB2-11CL
NCS CB2
G2B
B-CB2-CON
NCS CB2
B-CB2-3CL
NCS CB2
B-CB2-5CL
NCS CB2
B-CB2-11CL
NCS CB2
Note: NCS refers to Natural Crushed Stone.
CB1 and CB2 denote coarse aggregates sourced from cement-clay interlocking and clay burnt, respectively.
Table 2.
Mechanical characteristics of bricks.
Aggregate type Density .
g/m.
CB1
CB2
Compressive Strength (MP.
Water Absorption
(%)
Table 3.
Mix design of Type-A and Type-B concrete.
Concrete Type Ingredients .
aya/yayc ) Cement
Sand
NCS
Brick Aggregates Water Specimen Preparation Locally available recycled bricks were first crushed into smaller fragments using a mechanical crusher.
Sieve analysis was then performed on the crushed material to obtain recycled brick aggregates within the required size range, with a maximum size limit of 25 mm for all coarse aggregates.
Concrete specimens were cast using steel molds.
Following the guidelines of ASTM C943, concrete was placed in three equal layers, with each layer compacted using a vibration table to ensure proper consolidation (Figure .
After 24 hours of casting, the specimens were demolded and subjected to a curing period of 28 days.
The strengthening procedure was initiated after the concrete specimens had undergone 28 days of curing.
Steel clamps were installed in their designated positions and secured by tightening A torque wrench was used to ensure consistent tightening across all specimens.
accordance with previous studies .
, each clamp was tightened with a uniform torque of 5 NAmm to ensure standardized confinement conditions.
Typical low-cost steel clamp specimens are shown in Figure 7.
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Waste aggregates used in this study.
Figure 4.
Low-cost steel clamps used in this study.
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Experimental and Design-Oriented Investigation of Rectangular ConcreteA | 240 Figure 5.
tensile stress vs.
strain response of three clamps, and .
typical fracture observed during tensile testing.
Figure 6.
Construction of specimens.
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Typical low-cost steel clamp confined specimens.
Test Setup and Instrumentation Compressive loading was employed utilizing a Universal Testing Machine (UTM).
Prior to testing, steel plates were attached to both ends of each specimen to guarantee uniform load The reduction in specimen height during loading was measured using Linear Variable Differential Transformers, as illustrated in Figure 8.
Axial strain corresponding to each load increment was estimated based on the recorded height reduction of the specimens.
Figure 8.
Test setup illustration, .
lab view, .
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RESULTS AND DISCUSSION
Observed Failure of Specimens Figure 9 illustrates the observed failure mechanisms across all test specimens.
For better visualization, failure modes are compared within each group based on the level of confinement provided.
In the control specimens, failure was primarily governed by splitting and crushing.
Crushing was most evident near the loading zone, while splitting cracks extended along the specimen's length.
Strengthened specimens displayed a uniform failure response under loading.
Initially, no noticeable surface cracks appeared before the specimens reached their peak load.
However, once peak capacity was exceeded, localized bulging developed in concrete areas not directly strengthened by the steel clamps.
This bulging was ascribed to axial compressive stresses transferred through the steel clamps.
During the load application, the core concrete continued efficiently confined by the clamps until clamp fracture occurred.
Similar failure characteristics have also been reported in studies involving steel batten confinement systems .
In specimens with three steel clamps, lateral protruding of the concrete was minimal.
Failure of the steel clamps typically occurred at or near the screw connections, and concrete crushing was primarily concentrated in the upper half of the specimen.
With five clamps, more pronounced and evenly distributed bulging was observed, indicating improved confinement performance compared to the three-clamp Specimens reinforced with eleven clamps showed the most effective confinement, as evidenced by extensive crushing and splitting patterns.
These results imply that the elevenclamp configuration provided superior resistance to both crushing and out-of-plane Notably, many of the upper clamps failed ascribed to the stress concentration.
Similar failure modes in concrete cylinders subjected to steel clamp confinement, further supporting these findings .
Figure 9.
Images taken at failure of specimens.
Compressive Response of Specimens Figure 10 presents the stressAestrain behavior recorded for all tested specimens, categorized by their respective unconfined concrete strengths for clarity.
The stressAestrain curves can be idealized into two distinct segments: .
an initial ascending branch leading up to the maximum stress .
enoted as the peak strengt.
, and .
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A comparable trend was also observed in previous research on circular specimens confined using steel clamps, as documented .
, and square specimens, tested by .
Across all confinement groups, a clear improvement in peak strength was evident.
However, the stiffness of the post-peak segment, designated as Z, was more gradual related to that of the corresponding unconfined specimens.
The elastic modulus, representing the stiffness of the concrete before peak stress, remained unchanged between confined and reference samples within all groups.
This suggests that the steel clamps functioned as passive confinement elements, activating only once the concrete exhibited significant lateral expansion.
Since lateral dilation is closely linked to the level of axial compressive strain in the concrete core .
, the steel clamps had minimal effect at low strain levels.
Furthermore, the strain at which peak strength occurred varied across the different groups, indicating that confinement influenced not only strength but also deformation characteristics.
Detailed quantitative analysis of these parameters is presented in the following sections.
An idealized response is also shown in Figure 10.
It is noted that several key points need to be approximated if the complete compressive behavior of confined concrete is desired.
These include the peak strength yceycayca , the corresponding strain yunycayca , the elastic modulus yayca , and the slope of post-peak strength degradation branch ycs.
In this work, separate equations for these parameters are proposed in Section 4.
Figure 10.
Measured response of specimens made with .
low strength and CB1 bricks, .
high strength and CB1 bricks, .
low strength and CB2 bricks, .
high strength and CB2 bricks, and .
idealization of the typical response.
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Experimental and Design-Oriented Investigation of Rectangular ConcreteA | 244 Summary of The Peak Compressive Strength and The Related Strain Table 4 summarizes the average values of peak strength and the corresponding strain at peak strength for each specimen group.
These values represent the mean response of all samples within a given category.
Significant improvements were recorded across all groups.
The results indicate that enhancement in strength and strain was not uniform across the In particular, subgroup G1A exhibited notable increases, with peak strength and strain at peak strength rising by up to 91.
19% and 150.
0%, respectively.
It is important to note that this improvement varied across different subgroups, highlighting that the unconfined compressive strength and the type of recycled brick aggregates are important in determining the effectiveness of steel clamps confinement.
Considering this, the following sections break down the effects of both these parameters on steel clamps confinement quantitatively.
Table 4.
Summary of key parameters observed.
yeNyeEyeE Increase in yeNyeEyeE yyayeEyeE Increase in yyayeEyeE (MP.
(%) (%) A-CB1-CON
0020 A-CB1-3CL
A-CB1-5CL
A-CB1-11CL
B-CB1-CON
0019 B-CB1-3CL
B-CB1-5CL
B-CB1-11CL
A-CB2-CON
0020 A-CB2-3CL
A-CB2-5CL
A-CB2-11CL
B-CB2-CON
0018 B-CB2-3CL
B-CB2-5CL
B-CB2-11CL
Note: yceycayca and yunycayca represent the peak compressive strength and the corresponding compressive strain.
Name Impact of The Confinement Ratio The confinement ratio, referred to as the ratio of lateral confinement pressure yceyco to the compressive strength of unstrengthened concrete yceycaycu , is a key parameter used to assess the effectiveness of external confinement systems .
This ratio helps establish a relation between the level of applied confinement and the resulting improvements in the axial compressive behavior of concrete.
As reported in the literature, an increase in confinement ratio leads to greater axial strain for a given level of lateral strain .
In this work, the confinement pressure was estimated using a methodology analogous to that given by .
However, since the original formulation by .
is specifically intended for reinforced concrete, it was adapted to be relevant for plain concrete elements.
It is widely acknowledged that confinement in rectangular or square cross-sections is less effective than in circular sections .
To account for this, a horizontal confinement efficiency factor, ycoEa , is presented, as illustrated in Figure 11.
Additionally, vertical discontinuities between successive confinement layers, such as internal hoops .
, external steel confinement .
, or FRP battens .
, lead to further reductions in confinement effectiveness.
This phenomenon is addressed using a vertical efficiency factor, ycoyc , depicted in Figure 12.
At the planar level.
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The area beyond this boundary is considered ineffectively confined and is approximated mathematically as .
cc Oe ycIyca )2 /6 or .
ca Oe ycIyca )2 /6, where b and d are the crosectional dimensions and ycIyca is the radius at corners.
The complete area of ineffective confinement planar to steel clamps, denoted as yaycnycaEa , is calculated using Equation .
ccOeycI )2 .
caOeycI )2 yaycnycaEa = 3 yca 3 yca By the subtraction of yaycnycaEa from the gross area yayci , the planar area under effective confinement yayceEa may be deduced.
Therefore, calculated using Equation .
ccOeycI )2 .
caOeycI )2 yayceEa = ycaycc Oe 3 yca 3 yca The ratio of yayceEa to yayci results in the horizontal confinement efficiency factor ycoEa , stated in Equation .
yayceEa ycoEa = ya = .
ccOeycIyca )2 .
caOeycIyca )2 .
cayccOe .
ycaycc yci Since the steel clamp confinement is discontinuous along the height, it is essential to take into account the reduced effect of confinement between consecutive clamps.
Following the recommendations by .
, the effectively confined area midway between consecutive clamps is defined in Equation .
ycA ycA yayceyc = .
ca Oe 2 ) .
cc Oe 2 ) .
where the clear distance between two clamps is denoted by yc A.
Analogous to ycoEa , the vertical confinement efficiency factor ycoyc is written in Equation .
ycoyc = ya = ycA caOe ).
ccOe ) yci ycaycc The resulting confinement efficiency factor ycoyce is the product of ycoEa and ycoyc , as recommended by Mander et al.
in Equation .
ycA ycA .
caOe ).
ccOe ) .
ccOeycIyc.
caOeycIyc.
cayccOe ycoyce = ycoyc y ycoEa = .
ycaycc ycaycc It is noted that different confinement pressures exist in orthogonal directions of rectangularshaped specimens .
The confinement pressures in two orthogonal directions .
ay x and .
may be written in Equations .
yceycoycu = 2yaycyca yceycycyca 2yaycyca yceycycyca yceycoyc = yccyc where equations .
take into account the effects of clamps cross-sectional area yaycyca and yield strength yceycycyca .
The effective confinement pressures are obtained by multiplying Equations .
with ycoyce .
yceycoyceycu = yceycoycu y ycoyce .
yceycoyceyc = yceycoyc y ycoyce .
An overall lateral equivalent pressure yceycoyce is thus obtained by taking a weighted average .
in Equation .
yceycoyceycu yca yceycoyceyc ycc yceycoyce = .
yca ycc The computed efficiency factor ycoyce for specimens confined with 3, 5, and 11 steel clamps 17, 0.
21, and 0.
26, respectively.
These correspond to effective confinement pressures 33 MPa, 0.
63 MPa, and 1.
57 MPa, respectively.
In equations .
, the yield strength of the steel clamps is utilized under the assumption that the clamps yield prior to the specimen reaching its peak load.
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Experimental and Design-Oriented Investigation of Rectangular ConcreteA | 246 considered constant, in line with recommendations from previous studies .
Accordingly, the yield strength is applied in all confinement-related calculations for both the ascending and descending branches of the compressive stressAestrain response.
The resulting confinement ratios across all specimens range from 0.
013 to 0.
Figure 11.
Visual representation of strengthened and unstrengthened areas at: .
planar level and .
along the height.
Figures 12.
illustrate the variation in peak strength and strain at peak strength, respectively, for confined concrete made with different brick aggregates.
The data reveal that the rate of improvement in both parameters differs depending on the type of aggregate used.
Specifically, for a given confinement ratio, concrete incorporating CB1 aggregates exhibited the greatest enhancement.
It is important to note that CB1 bricks generally possess higher compressive strength than CB2 bricks.
These differences suggest that aggregate type significantly influences the confined behavior of concrete and should be incorporated into analytical models.
Despite these variations, all aggregate types showed consistent gains in both peak strength and strain at peak strength with increasing confinement ratios, as depicted in Figures 12.
Figure 12.
Effect of confinement ratio on: .
yceycayca /yceycaycu and .
yunycayca /yunycaycu .
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September 2026 Hal 233-254 Modelling of Stress vs.
Strain Compressive Response To facilitate the practical application of the experimental findings, an analytical model was formulated to define the axial compressive stress vs.
strain behavior of rectangular concrete specimens confined with steel clamps.
This model is based on the widely recognized Popovics equation .
, presented in Equations .
, which is commonly utilized to characterize the compressive response of both confined and unconfined concrete.
The objective of this modeling approach is to offer a straightforward and reliable method for predicting the stressAestrain performance of steel clamp-confined concrete, using calibration from the present experimental findings.
yceyca = yceycayca [ yco= yayca yun yco ycoOe1 ( yca ) yunycayca yco( yca ) yceycuyc yunyca O yunycayca yce yunycayca yayca Oe ycayca In this method, the rising portion of the stressAestrain curve is captured using a nonlinear expression introduced by Popovics, as shown in Equation .
The shape-controlling parameter .
is determined through Equation .
, which integrates the elastic modulus yayca , peak compressive strength yceycayca , and the corresponding peak strain yunycayca .
Figure 7.
provides a schematic representation of both the modeled and experimental stressAestrain responses, highlighting essential parameters including the post-peak modulus Z.
Regression based expression for yeNyeEyeE An empirical model was formulated through regression analysis of the experimental data to estimate the normalized peak compressive strength of rectangular concrete specimens confined with steel clamps, as a function of the confinement pressure yceyco and a normalized value of brick compressive strength yceycayca .
This is because both these parameters showed significant impact on yceycayca , as noted in Section 3.
The resulting predictive equation is presented in Equation .
= 1 yca y .
ce yco ) yca y ( yceycayca ) ycaycu where Gauss-Newton based nonlinear regression returned the values of 6.
106, 1.
136y1011, and 30.
574 for a, b, c, and d, respectively.
The coefficient of determination, i.
ycI2 , was approximated to be 0.
94, as shown graphically in Figure 13.
Regression based expression for yyayeEyeE Analogous to yceycayca , the strain at yceycayca was also dependent on the confinement pressure yceyco and a normalized value of brick compressive strength yceycayca , as observed in Section 3.
Thus, taking into account these independent variables.
Equation .
is proposed based on nonlinear regression analysis.
= 1 yca y .
ce yco ) yca y ( yceycayca ) ycaycu Using Gauss-Newton-based nonlinear regression, the parameters a, b, c, and d, were determined to be 4.
592, 0.
601, 8.
560y1010, and 30.
413, respectively.
The model demonstrated a strong fit with the experimental data, achieving a coefficient of determination ycI2 of approximately 0.
88, as illustrated in Figure 13.
DOI: https://doi.
org/10.
17509/ijost.
p- ISSN 2528-1410 e- ISSN 2527-8045 Thansirichaisree et al.
Experimental and Design-Oriented Investigation of Rectangular ConcreteA | 248 Regression based expression for yeA For the post-peak modulus Z.
Equation .
is proposed after several trials on different forms of nonlinear regression.
Again, the independent variables were chosen to be the normalized brick compressive strength and the confinement ratio.
ycs = yca y .
ceycaycu yceycaycu
yceyco yca yceycayca yca
) (
The Gauss-Newton-based nonlinear regression yielded values of -222.
723, 95.
408, -42.
004 for the parameters a, b, c, and d, respectively.
The model showed a good agreement with the experimental data, with an ycI2 of approximately 0.
96, as depicted in Figure 10.
Figure 13.
Agreement between the predicted and observed values for: .
peak compressive strength, .
strain at peak compressive strength, and .
the post-peak modulus.
Agreement between predicted and observed response of steel clamp-confined To evaluate the reliability of the presented analytical expressions, the predicted stress vs.
strain curves were evaluated against the experimental results for steel clamp-confined rectangular concrete specimens.
These comparisons, categorized by concrete strength levels.
A and B are shown in Figure 14.
As illustrated in Figure 14, the predicted curves align closely DOI: https://doi.
org/10.
17509/ijost.
p- ISSN 2528-1410 e- ISSN 2527-8045 249 | Indonesian Journal of Science & Technology.
Volume 11 Issue 2.
September 2026 Hal 233-254 with the experimental data across all strength categories and confinement levels.
For all groups, the models effectively reproduce both the ascending segment and the peak region of the stressAestrain response.
The post-peak behavior is also captured with reasonable accuracy, showing only slight discrepancies.
Figure 14.
Agreement between observed and predicted response for specimens in subgroup: .
G1A, .
G1B, .
G2A, and .
G2B.
CONCLUSION
This study includes an experimental program involving monotonic compression tests on rectangular specimens confined with steel clamps.
Key variables examined include the unconfined compressive strength of concrete, the number of steel clamps, and the type of coarse aggregates used.
The following conclusions were derived from this work.
The stressAestrain response of all specimens exhibited a consistent two-phase behavior with clear improvements in peak strength due to confinement.
Steel clamps provided passive confinement, activating only after significant lateral expansion, thus not affecting the initial stiffness.
The post-peak slopes were more gradual in confined specimens, indicating enhanced ductility compared to unconfined counterparts.
Variation in peak strain across groups highlights that confinement influenced both strength and deformation characteristics.
All groups showed substantial improvements, though the degree of enhancement varied, with up to 91.
19% and 150.
0% increases in strength and strain, respectively.
These variations underscore the influence of unconfined compressive strength and the type of recycled brick aggregates on confinement effectiveness.
Particularly, concrete made with brick aggregates of greater compressive strength showed better improvement in the compressive response.
DOI: https://doi.
org/10.
17509/ijost.
p- ISSN 2528-1410 e- ISSN 2527-8045 Thansirichaisree et al.
Experimental and Design-Oriented Investigation of Rectangular ConcreteA | 250 .
Concrete made with CB1 aggregates demonstrated the most significant improvements at a given confinement ratio, likely due to their higher inherent compressive strength of These findings highlight the critical role of aggregate properties in confined concrete behavior and their relevance in model development.
Nonetheless, all aggregate types exhibited consistent performance gains with increasing confinement, confirming the effectiveness of the strengthening technique.
Predictive models were established to estimate the peak strength, peak strain, and postpeak modulus of steel clamp-confined rectangular specimens.
These models demonstrated strong accuracy, achieving RA values above 0.
85, and were validated through comparison with experimental stressAestrain data.
ACKNOWLEDGMENTS
This project is funded by National Research Council of Thailand (NRCT) and Thammasat University (Project Code: N42A670.
We also thanks to Asian Institute of Technology.
Thailand for partially supporting testing facilities.
AUTHORSAo NOTE The authors declare that there is no conflict of interest regarding the publication of this The authors confirmed that the paper was free of plagiarism.
REFERENCES