Experimental evaluation of formwork type and release agents in relation to strength,durability,and surface quality of concrete

Abstract

Concrete surface quality plays a critical role in the durability, aesthetics, and long-term performance of exposed concrete elements, particularly in architectural and bridge structures. Numerous studies have investigated the mechanical and durability properties of concrete; however, the combined influence of formwork material, release agent, and curing regime on surface quality and near-surface performance remains insufficiently addressed within an integrated framework. This study experimentally evaluates the coupled effects of concrete type (normal concrete and self-compacting concrete), formwork material (ABS, PVC, wood, MDF, and iron), mold release agent, and curing condition on mechanical performance, durability-related indicators, and surface quality. The study integrates strength-related tests, abrasion resistance, water absorption, Schmidt hammer rebound, water droplet contact-angle measurements, and qualitative surface assessment within a unified experimental program. A comprehensive experimental program was conducted using standardized test methods on laboratory-scale specimens cast under controlled conditions. Compressive strength, rebound index, water absorption, abrasion resistance, and surface wettability were evaluated, and surface appearance was assessed through systematic visual inspection. The results indicate that formwork material and release agent significantly influence near-surface density, abrasion resistance, and aesthetic quality. Polymer molds combined with appropriate release agents generally produced denser surfaces and improved durability-related performance, whereas wooden molds without release agents resulted in inferior surface quality. Although the tested self-compacting concrete mixture exhibited overall favorable trends, certain normal concrete configurations achieved comparable or higher compressive strength depending on mold conditions. Overall, the findings indicate that surface quality and durability-related performance are governed not only by mixture design but also by formwork–concrete interaction and curing practice. This study provides practical guidance for selecting formwork systems and curing strategies in applications involving exposed concrete surfaces.

Keywords

self-compacting concrete (SCC)architectural concrete formwork and release agents surface quality water absorption

Introduction

Concrete is the most widely used construction material worldwide owing to its strength, durability, and versatility.^1–6 As a composite material composed of cement, aggregates, water, and often chemical admixtures, its performance is strongly governed by mixture design, curing conditions, and construction practices.^7,8 Among its various applications, architectural concrete plays a significant role in facade cladding, where both mechanical performance and aesthetic quality are equally important.^9–11 Architectural concrete must meet structural and durability requirements against environmental actions such as frost, rainfall, temperature variations, and humidity.^9,12,13 It must also satisfy aesthetic expectations, including uniform texture, flatness, and color consistency. Similar performance and appearance requirements apply to bridge structures, where large areas of exposed concrete are directly subjected to moisture, temperature variations, de-icing salts, and abrasion. In bridge elements such as piers, abutments, barriers, and precast components, near-surface concrete quality is critical for both aesthetics and durability. Surface defects and increased porosity accelerate water and chloride ingress, thereby compromising long-term serviceability.^9,10,14 Consequently, understanding the influence of formwork systems, release agents, and curing regimes on surface quality and durability indicators is directly applicable to improving the performance of concrete used in bridge infrastructure. SCC is a highly flowable material that can fill formwork without external vibration. It often provides improved surface finish and placement quality compared to conventional concrete, depending on mixture design and casting conditions.^15–17 Nevertheless, despite its mechanical and workability benefits, SCC may exhibit durability-related challenges depending on its mixture composition and near-surface pore structure. Porosity governs water absorption and the ingress of aggressive agents (chlorides, sulfates, CO₂, and microorganisms), thereby influencing strength development, abrasion resistance, and service life.¹⁸ Compressive strength, measured through destructive testing, remains the most widely investigated property in concrete research and is strongly influenced by porosity, curing conditions, and microstructural changes.^19–24 In this context, abrasion resistance is increasingly recognized as a critical durability parameter for exposed concrete surfaces. Abrasion primarily affects the concrete’s outer layer, influencing both aesthetics and long-term performance.¹⁴ Studies have shown that factors such as aggregate type, water-to-cement ratio, and cement composition play key roles in improving abrasion resistance.^25–28 In parallel with strength and durability, surface quality has become a key consideration in architectural concrete. Defects such as bugholes, non-uniform color, and poor flatness negatively impact the visual perception of concrete facades and, consequently, the overall appearance of the building envelope.^24,29,30 The final surface quality is determined not only by mix design but also by the interaction between concrete and formwork systems, including the selection of release agents and formwork materials.³¹ Recent advances have introduced innovative solutions, such as polymeric skins and permeable textile formworks, which aim to improve surface aesthetics and minimize water retention.^32–34 However, most existing studies address either visual quality or isolated durability indicators, without quantitatively linking surface appearance to durability-related performance. Two key gaps can therefore be identified in the current state of the art. First, most studies examine mechanical properties, durability indicators, or surface quality independently. Integrated experimental frameworks that jointly assess these aspects remain limited. Moreover, the combined effects of formwork material, release agent, and curing regime on abrasion resistance and water absorption remain insufficiently quantified under comparable conditions. Addressing these gaps is particularly relevant for exposed concrete elements in bridge structures, where surface-related deterioration mechanisms directly affect service life and maintenance strategies. The present study addresses these gaps through a comprehensive experimental evaluation. It investigates the effects of concrete type (normal concrete and self-compacting concrete), formwork material, release agent, and curing regime on mechanical performance, durability-related indicators, and surface quality. Durability-related properties—including water absorption and abrasion resistance—are assessed alongside surface-related metrics such as visual appearance and water droplet behavior. This approach provides a dual evaluation framework linking near-surface density to both durability and wettability. By integrating mechanical, durability-related, and surface-quality indicators within a single experimental program, the study provides practical guidance for optimizing formwork selection and curing strategies for exposed concrete applications. Some preliminary results were previously reported in a non-English publication³⁵; the present work substantially extends that study by incorporating additional test parameters, comparing normal and self-compacting concretes, and establishing correlations between durability and surface quality.

Research significance and paper organization

The durability and surface quality of concrete are critical parameters in modern construction, particularly for exposed concrete elements where long-term performance and maintenance demands are strongly influenced by near-surface properties. Previous studies have extensively investigated either mechanical performance or surface-related characteristics of concrete. However, the combined influence of formwork type, release agents, and curing regime on both durability and surface quality has received comparatively limited attention. This gap is especially relevant for applications involving exposed concrete surfaces, where abrasion resistance, permeability, and visual integrity are closely interrelated. Most existing research has focused on self-compacting concrete (SCC), emphasizing its workability and surface finish advantages, while comparatively fewer studies have examined the performance of normal concrete under similar formwork and curing conditions. As a result, a clear comparative understanding of how concrete type interacts with mold material and release agents to govern near-surface density, durability indicators, and surface appearance remains lacking. The significance of this study lies in its integrated experimental evaluation of mechanical performance, durability indicators, and surface quality. Both normal concrete (NC) and self-compacting concrete (SCC) are examined under different formwork, release-agent, and curing conditions. By jointly assessing compressive strength, abrasion resistance, water absorption, water droplet behavior, and visual surface quality, the study establishes clear correlations between near-surface density, durability, and wettability. This dual evaluation framework provides practical insight into how construction-related parameters influence both technical performance and surface integrity. From an engineering practice perspective, the findings offer guidance for selecting concrete type, formwork material, release agent, and curing strategy in applications where exposed concrete performance is critical, including infrastructure elements such as bridge decks, piers, barriers, and other surface-sensitive components. By linking durability-related indicators to surface quality outcomes, the study contributes to more informed material and construction decisions aimed at improving service life and reducing maintenance requirements. The remainder of the paper is organized as follows. Section 3 describes the materials, mixture design, and experimental methods. Section 4 presents and discusses the experimental results. Section 5 outlines the limitations of the study and directions for future research, and Section 6 summarizes the main conclusions and practical implications.

Materials and methods

This section describes the materials, mixture proportions, formwork systems, curing regimes, and experimental procedures adopted in this study. The experimental program was designed to systematically evaluate the influence of concrete type, mold material, release agent, and curing condition. Their effects were assessed on mechanical performance, durability indicators, and surface quality.

Mixture design and proportions

Two concrete mixtures were designed and prepared: normal concrete (NC) and self-compacting concrete (SCC). The detailed mixture proportions are presented in Table 1 and Table 2, respectively. The mix designs were selected to represent concretes commonly used in practical construction applications. The workability levels were comparable to pumpable concretes employed on construction sites. For SCC, a w/c ratio of 0.45 was adopted. Limestone powder and a polycarboxylate-based superplasticizer were incorporated to ensure adequate flowability, stability, and resistance to segregation. Fresh-state properties were checked during mixing to ensure that each mixture achieved the intended workability and stability for its placement method. For the SCC mixture, self-compacting behavior was verified based on slump flow observations and visual stability, while the NC mixture exhibited conventional workability suitable for vibration-based placement. These fresh-state checks were conducted as part of quality control during specimen preparation; however, detailed numerical values were not reported, as the primary focus of the study was on hardened-state performance and surface-related behavior. Concrete specimens were cast in different geometries to accommodate the experimental program. Cubic specimens (150 × 150 × 150 mm) were prepared for compressive strength testing, Schmidt hammer rebound measurements (ASTM C805³⁶), and water absorption tests (ASTM C642³⁷). Mosaic-shaped slabs (100 × 100 × 30 mm) were fabricated for abrasion resistance tests, while rectangular slabs (200 × 200 × 30 mm) were produced for surface quality assessment and water droplet contact-angle measurements. All specimens were demolded after 24 h and subsequently subjected to the specified curing regimes prior to testing. The objective of this study is not to isolate the effect of a single mixture parameter, such as the water-to-cement ratio. Instead, the study compares two concrete classes commonly used in practice—normal concrete (NC) and self-compacting concrete (SCC)—under identical formwork, release agent, and curing conditions. The mixtures were proportioned to achieve representative workability for their respective placement methods. Normal concrete was vibrated, whereas SCC was self-compacting. Therefore, comparisons between NC and SCC are interpreted at the system level, while the effects of mold material, release agent, and curing regime are primarily evaluated within each concrete class. The SCC mixture was designed without coarse aggregate. Fine aggregate and limestone powder were used to achieve the required flowability and surface finish. Accordingly, gravel was not included in the SCC mix proportions reported in Table 2. For SCC, the reported w/c ratio refers to the design value, while the effective water-to-binder ratio accounts for the presence of limestone powder as a filler. The dimensions of the specimens and their corresponding applications in each experimental test are summarized in Table 3. This table provides an overview of the specimen geometry adopted for mechanical, durability, and surface-related evaluations. It facilitates interpretation of the experimental matrix. Accordingly, comparisons between NC and SCC in this study are made at the level of the tested mixtures and should not be interpreted as isolating the intrinsic effect of concrete rheological class alone.

Table 1.

Normal concrete mix proportions.

Material	Cement (kg/m³)	Sand (kg/m³)	Gravel (kg/m³)
Quantity	450	492	1329

Note. Water-to-cement ratio (w/c) = 0.40. No chemical admixtures were used in the NC mixture.

Table 2.

Self-compacting concrete mix proportions.

Material	Cement (kg/m³)	Water (kg/m³)	Sand (kg/m³)	Limestone powder (kg/m³)	Superplasticizer (L/m³)
Quantity	450	180	1500	200	5

Note. Water-to-cement ratio (w/c) = 0.45. The reported w/c value corresponds to the design value. The effective water-to-binder ratio reflects the combined contribution of cement and limestone powder.

Note. Coarse aggregate was not included (or not considered) in the reported SCC mixture.

Table 3.

Specimen dimensions and corresponding tests.

Test application	Sample code	Specimen geometry (mm)
Compressive strength (ASTM C39), Schmidt hammer rebound (ASTM C805), water absorption (ASTM C642)	F	150 × 150 × 150
Abrasion resistance (IS:1237-2012)	H	100 × 100 × 30
Surface quality assessment, water droplet contact-angle test	G	200 × 200 × 30

Note. All specimens were demolded after 24 h and subjected to the specified curing regimes prior to testing.

Normal concrete (NC)

To produce normal concrete (NC), sand, gravel, and cement were weighed according to the mix proportions (Table 1) and placed in a mechanical mixer. In the initial stage, the dry constituents were blended for approximately 1–2 min. Subsequently, the prescribed amount of water was gradually added, and mixing continued for about 3 min until a homogeneous mixture was obtained. The fresh concrete was then placed into pre-oiled steel molds using a suitable release agent. Casting was performed in three successive layers. Each layer was compacted using a vibrating table to ensure uniformity and minimize entrapped air voids, in accordance with ASTM C192.³⁸ After 24 h of casting, the specimens—including both cubic and mosaic-shaped samples—were demolded. Cubic specimens were transferred to a water-curing tank maintained at 20 ± 2°C. Additional mosaic samples were stored under laboratory ambient conditions to investigate curing effects.

Self-compacting concrete (SCC)

For the production of SCC, the pre-weighed dry constituents—sand, cement, and stone powder—were first introduced into a mechanical mixer and blended for approximately 1–2 min. Subsequently, a premixed solution of water and superplasticizer was gradually added. Mixing continued for 3–4 min until a homogeneous and highly flowable SCC mixture was obtained. The fresh SCC was then cast into pre-oiled molds treated with a release agent, without the need for vibration, in accordance with the principles of SCC placement (EFNARC guidelines³⁹). After 24 h, the specimens were demolded. Cubic and mosaic-shaped samples designated for curing were immersed in a water tank maintained at 20 ± 2°C. Additional mosaic specimens were stored under laboratory ambient conditions for comparative assessment.

Materials and mold systems

Concrete specimens were cast in five different types of molds: ABS plastic, PVC plastic, untreated wood, MDF wood, and cast iron. To evaluate the effect of mold release agents on the physical and aesthetic properties of concrete, specimens were produced without release agents and with two types of mold oils. One oil was petroleum-based, and the other was water-based. The specifications of these oils are presented in Table 4. Two curing regimes were considered in this study. In the first regime, specimens were water cured (CW) by immersion in a water tank maintained at 20 ± 2°C until the designated test age. In the second regime, specimens were kept under laboratory ambient conditions without water curing (WCW). The temperature was 23 ± 2°C, and the relative humidity was 50 ± 5%. For the water droplet contact-angle test, an additional condition was considered. Specimens were water cured for 28 days and subsequently stored under laboratory ambient conditions for 7 days, resulting in a total age of 35 days at the time of testing. The curing history of each specimen is explicitly reflected in the specimen identification code. Tests performed included compressive strength, Schmidt hammer rebound, water absorption, abrasion resistance, water droplet contact-angle, and surface quality assessments. Compressive strength, Schmidt hammer rebound, and water absorption tests were performed exclusively on water-cured (CW) specimens. The latter three tests—abrasion resistance, water droplet contact-angle, and surface quality—were carried out on both water-cured (CW) specimens and non-water-cured (WCW) specimens stored under laboratory ambient conditions. Photographs of the specimens were taken at 1 day of age to document and evaluate the aesthetic quality of the concrete surfaces. The cement used in this study had a specific gravity of 3.50 t/m³, and a polycarboxylate-based superplasticizer was employed as a chemical admixture. Potable water from Qom province was used for both mixing and curing. Limestone powder obtained from locally quarried limestone in Qom province served as the mineral filler. The specific gravity and water absorption capacity of the aggregates were determined in accordance with ISIRI 13759 (equivalent to ASTM C127⁴⁰–C128⁴¹). The results are summarized in Table 5.

Table 4.

Specifications of mold oils.

Mold oils	State/condition	Color	Specific gravity (−)	Solubility medium
Petroleum solvent	Liquid	Yellow	0.85	In petroleum solvent
Water solvent	Liquid	White	0.85	In water

Table 5.

Specific gravity and water absorption of aggregates.

Aggregates	Specific gravity (−)	Water absorption (%)
Gravel	2.705	0.93
Sand	2.523	3

Experimental methods

Compressive strength test

The compressive strength of concrete was evaluated in accordance with ASTM C39⁴² using 150 × 150 × 150 mm cubic specimens. After 28 days of water curing at 20 ± 2°C, the specimens were surface-dried and centrally aligned in a calibrated hydraulic compression testing machine with a maximum capacity of 2000 kN. Axial load was applied continuously at a controlled rate of 0.25 ± 0.05 MPa/s until failure. The compressive strength f_c was calculated using equation (1):

f_{c} = P / A

(1)

where f_c is the compressive strength (MPa), P is the maximum load at failure (N), and A is the loaded cross-sectional area of the specimen (mm²). Unless otherwise stated, all experimental tests were conducted under laboratory ambient conditions at a temperature of 23 ± 2°C and a relative humidity of 50 ± 5%. For each mix and mold type, at least three specimens were tested. Results are reported as mean values with corresponding standard deviations.

Schmidt hammer rebound test

The surface hardness of the concrete was assessed using the Schmidt rebound hammer in accordance with ASTM C805.³⁶ Prior to testing, specimen surfaces were cleaned and smoothed to remove dust and laitance. The test was performed on 150 mm cubic specimens at the age of 28 days, under laboratory ambient conditions. To minimize edge effects, impacts were applied at least 25 mm away from specimen edges and corners. For each specimen, 10–15 rebound readings were recorded at different points on the surface, and the median of the valid readings was calculated. Outliers exceeding ±20% of the mean were discarded. The rebound number was used primarily as a comparative rebound index reflecting relative surface hardness and near-surface uniformity among specimens. Manufacturer-provided correlation charts were consulted to express rebound values in terms of an “equivalent compressive strength”; however, these values are not intended to represent absolute strength predictions. Rebound–strength correlations are known to depend on mixture composition and surface condition. Therefore, Schmidt hammer results are interpreted mainly in a relative sense and are used only to support trends observed from destructive compressive strength testing. For each test condition, measurements were repeated on a minimum of three specimens, and the reported results represent mean values. Rebound test points were selected in a quasi-random manner across the central region of each specimen surface. Visible defects, edges, and corners were avoided to account for surface heterogeneity while ensuring repeatability.

Water absorption test

The water absorption of concrete was determined in accordance with ASTM C642.²⁸ After 28 days of water curing, specimens were first weighed in a saturated-surface-dry (SSD) condition to obtain the saturated mass (W_s). Subsequently, the specimens were oven-dried at 105 ± 5°C for 24 h until a constant mass was achieved, which was recorded as the dry mass (W_d). The water absorption percentage was calculated using equation (2):

WA % = \frac{W_{s} - W_{d}}{W_{d}} \times 100

(2)

where WA% is the water absorption percentage, W_s is the saturated mass (g), and W_d is the oven-dry mass (g). This procedure provided a quantitative measure of the pore structure and potential permeability of the concrete mixtures, serving as an indirect indicator of durability.

Abrasion resistance test

The abrasion resistance of concrete was determined in accordance with IS:1237-2012,⁴³ which specifies the method for evaluating the wear resistance of cementitious materials. Mosaic-shaped specimens (100 × 100 × 30 mm) were cut from the larger slabs using a precision concrete cutting machine to obtain uniform dimensions and smooth test surfaces. Abrasion testing was performed using a rotating disc abrasion-testing machine equipped with an abrasive sheet, with corundum powder uniformly distributed on the disc prior to each test. The machine was operated at a speed of 75 revolutions per minute (rpm), and each test cycle was carried out for 1 min under constant load. Each specimen surface was subjected to three successive abrasion cycles under identical conditions. The initial mass of each specimen (W₁) was recorded prior to testing, and the mass after each cycle (W₂) was measured with an electronic balance (accuracy ±0.01 g). The depth of wear (t) was calculated using the equation (3):

t = \frac{(W_{1} - W_{2}) \times V}{(W_{1} \times A)}

(3)

where t = depth of wear (mm), W₁ = initial mass of the specimen (g), W₂ = mass after abrasion (g), V = specimen volume (mm³), and A = abraded surface area (mm²). All tests were conducted under laboratory ambient conditions (23 ± 2°C, 50 ± 5% RH). For each mixture and mold type, a minimum of three replicate specimens were tested, and results are reported as mean values together with the corresponding standard deviations. A fixed amount of corundum powder was applied uniformly on the abrasive disc before each test cycle in accordance with IS:1237-2012.

Water droplet contact-angle test

The wettability of the concrete surface was evaluated using a water droplet contact-angle method adapted from Savukaitis et al.²⁹ The test quantifies absorption kinetics and surface permeability through droplet behavior over time. Droplet placement locations were selected on visually representative areas of the specimen surface, away from edges and visible surface defects. For each specimen, measurements were repeated at multiple locations, and the reported values represent averaged results to reduce the influence of local surface heterogeneity. Three droplet configurations were examined: (i) a small droplet formed by merging three successive drops (≈0.05 mL), (ii) a large droplet formed by merging 20 successive drops (≈0.30 mL), and (iii) a spilled droplet of 0.5 mL applied at two points on the surface using a syringe positioned at 110°. Droplets were carefully placed on the specimen surface, and digital images were captured at predetermined intervals (0, 1, 5, 10, 15, 30, 40, 50, and 60 min) with a high-resolution camera. Image-analysis software was employed to determine the contact angle and total absorption time. Contact angle values were extracted from the captured images by fitting a baseline at the solid–liquid interface and measuring both left and right contact angles of the droplet profile. The reported contact angle corresponds to the average of the left and right angles for each droplet. To account for surface roughness and heterogeneity inherent to concrete, measurements were repeated at multiple locations and on replicate specimens, and the results are reported as mean values. Local surface irregularities may influence instantaneous droplet shape. However, the adopted averaging procedure allows reliable comparative assessment of near-surface density and wettability trends. All measurements were conducted at 22–23°C. Two curing conditions were considered: (i) water-cured specimens tested at 28 days and (ii) air-dried specimens cured in water for 28 days and subsequently stored under laboratory ambient conditions for 7 additional days (tested at 35 days).

For each curing condition and droplet type, a minimum of three replicate measurements were performed, and results are reported as mean values together with standard deviations. This method provided a quantitative measure of the surface density and permeability of the concrete specimens. The water droplet contact-angle method was selected as a surface-sensitive technique capable of capturing near-surface densification and absorption kinetics, which are directly influenced by formwork interaction and curing conditions. Bulk sorptivity tests (e.g., ASTM C1585) and electrical resistivity measurements reflect averaged transport properties over a larger depth. In contrast, the droplet-based approach emphasizes the behavior of the outermost concrete layer. Therefore, this method is used in the present study as a complementary, comparative indicator of surface permeability and wettability rather than as a substitute for standardized permeability tests.

Surface quality assessment

The qualitative characteristics of concrete surfaces were assessed following the methodology adapted from Khayyat et al.¹⁹ Photographs of the specimen surfaces were taken at two ages: 1 day (early-age evaluation) and 28 days (mature condition). Two curing regimes were considered: water-cured specimens, photographed after immersion curing, and specimens exposed to laboratory ambient conditions. Images were captured under both controlled laboratory lighting and natural outdoor light to minimize illumination-related bias. Surface quality evaluation focused on three aesthetic indicators relevant to architectural concrete: (i) visible pores (bugholes), (ii) color uniformity, and (iii) surface gloss or brightness. To improve reproducibility and reduce subjectivity, a semi-quantitative visual rating rubric was adopted. Each surface was classified according to predefined qualitative categories for each indicator, as follows: bughole density (low: almost pore-free; moderate: isolated visible pores; high: frequent visible pores), color uniformity (uniform, slightly non-uniform, non-uniform), and surface gloss (glossy, semi-matte, matte). All specimens were evaluated using the same classification criteria and imaging conditions, allowing consistent comparative assessment of the influence of mold type, release agent, and curing regime on surface appearance. No universally standardized method exists for architectural concrete surface evaluation. Therefore, this semi-quantitative approach was selected as a practical and reproducible compromise between descriptive visual inspection and fully quantitative image analysis. It should be noted that the surface quality assessment was performed on laboratory-scale specimens with limited exposed surface areas. These specimen dimensions were selected to ensure controlled and repeatable comparisons among different formwork materials, release agents, and curing regimes. Accordingly, the surface quality results provide comparative trends under identical conditions. They should not be interpreted as absolute judgments of full-scale architectural surface performance.

Results and discussion

This section presents and interprets the results obtained from the experimental program. The findings of the mechanical, durability, and surface quality tests are reported and interpreted with respect to mix type, curing regime, mold material, and release agent. For clarity and to avoid repetition, the abbreviations introduced in Table 6 are used consistently throughout this section. Unless otherwise stated, all reported results represent mean values obtained from replicate specimens, and variability is expressed using standard deviation (SD). Table 5 summarizes the specimen identification scheme used throughout the manuscript.

Table 6.

Abbreviations used for specimen identification parameters.

Parameters		Abbreviations	Parameters		Abbreviations
Curing conditions	In water	CW	Type of mold	Unprocessed wooden	W
Curing conditions	Out of water	WCW		Processed wooden	MDF
Age of the samples	1 day old	1		Iron	I
	28 days old	28		ABS plastic	ABS
	35 days old	35		PVC plastic	PVC
Dimensions of the samples	150 × 150 × 150	F	Type of mold oil	Water-based separator	WB
	200 × 200 × 30	G		Oil-based separator	SB
	100 × 100 × 30	H		No mold oil used	WMO
Type of concrete	Normal concrete	NC	-	-	-
Type of concrete	Self-compacting concrete	SCC	-	-	-

Note. For consistency, specimen identifiers are kept unchanged throughout the manuscript. The notation used in Table 6 applies to all tables and figures, and the parameters defining each specimen (concrete type, mold material, release agent, curing regime, age, and geometry) can be interpreted accordingly.

Compressive strength

The compressive strength results for conventional concrete (NC) and self-compacting concrete (SCC) are summarized in Table 7 and illustrated in Figure 1. The data highlight the influence of mold type, release agent, and curing condition on the mechanical performance of the mixtures. In both NC and SCC series, specimens cast in unprocessed wooden molds (W) exhibited the lowest compressive strength values. This behavior can be attributed to the high water absorption capacity and surface irregularities of wooden formwork, which promote non-uniform material distribution and increased surface porosity. The values reported in Table 7 represent mean ± standard deviation (SD) obtained from replicate specimens, and the observed scatter did not affect the comparative trends discussed in this section. Among the NC series, the highest strength was observed in NC-WB-ABS-28-F. The combination of a water-based release agent and a non-absorbent ABS mold likely improved surface hydration and reduced pore formation. In the SCC series, the best performance corresponded to SCC-W-I-28-F, where the dense microstructure of SCC and the smooth surface of the iron mold synergistically enhanced compressive strength. Interestingly, despite the technological advantages of SCC, the maximum compressive strength overall was recorded in the NC-WB-ABS-28-F specimen. This behavior may be linked to the specific mix proportions and reduced surface adhesion in ABS molds, which produced higher density and lower porosity compared to SCC counterparts under the same curing regime. Overall, the results demonstrate that both mixture type and formwork conditions significantly influence compressive strength. The findings confirm that appropriate selection of mold material and release agent can lead to measurable improvements in mechanical performance, beyond the inherent differences between NC and SCC. These results indicate that, while SCC generally provides improved homogeneity, appropriate selection of formwork system and release agent can enable NC to achieve comparable or even superior compressive strength under certain conditions. It should be noted that the compressive strength results reported in this study are based on standard cube specimens, which have a relatively high surface-area-to-volume ratio. Consequently, surface-related factors such as mold type, release agent, and near-surface densification may have a more pronounced influence on the measured compressive strength of these small specimens. Accordingly, differences observed between NC and SCC in compressive strength under certain mold and release-agent configurations should be interpreted as near-surface and specimen-scale effects rather than as intrinsic material superiority. In actual structural elements, such as bridge piers, decks, or beams, the cross-sectional area-to-perimeter ratio is significantly larger, and therefore the direct influence of surface condition on global compressive strength may be marginal or negligible. Accordingly, the compressive strength results should be interpreted as indicators of near-surface quality and material uniformity. They should not be regarded as direct predictors of full-scale structural strength.

Table 7.

Compressive strength, rebound-based equivalent strength, and water absorption of NC and SCC specimens at 28 days (water curing). Values are reported as mean ± SD (n = 3).

Tests	Compressive strength (MPa)		Schmidt rebound index (equivalent strength*) (MPa)		Water absorption (%)
Sample names	Type of concrete
Sample names	SCC	NC	SCC	NC	SCC	NC
I-WB-CW-28-F	48.20 ±1.21	39.40±1.28	36.33±1.17	36.43±1.29	1.45±0.01	1.96±0.02
MDF-WB-CW-28-F	43.06±1.35	40.11±1.38	36.50±1.15	40.46±1.23	1.60±0.02	1.93±0.03
W-WB-CW-28-F	24.44±1.25	26.90±1.27	28.86±1.11	33.25±1.08	1.69±0.04	1.97±0.03
P-WB-CW-28-F	41.53±1.15	40.19±1.35	42.53±1.14	40.60±1.09	1.69±0.07	1.78±0.06
ABS-WB-CW-28-F	47.59±1.38	50.70±1.43	42.34±1.31	42.63±1.12	1.46±0.05	1.72±0.04
I-SB-CW-28-F	33.38±1.59	38.62±1.18	31.56±1.21	38.73±1.27	1.36±0.07	1.58±0.08
MDF-SB-CW-28-F	36.66±1.34	38.60±1.39	32.23±1.23	38.70±1.38	1.32±0.03	1.71±0.04
W-SB-CW-28-F	30.62±1.17	26.87±1.18	31.67±1.41	29.10±1.33	1.60±0.06	1.77±0.07
P-SB-CW-28-F	38.42±1.28	40.15±1.41	38.43±1.39	35.43±1.09	1.22±0.03	1.56±0.04
ABS-SB-CW-28-F	45.66±1.38	42.84±1.43	38.80±1.11	38.96±1.16	1.07±0.01	1.40±0.05
I-WMO-CW-28-F	30.72±1.62	36.28±1.33	27.33±1.31	32.13±1.27	1.87±0.08	1.41±0.06
MDF-WMO-CW-28-F	30.48±1.39	36.50±1.24	27.33±1.18	33.33±1.36	1.89±0.01	1.36±0.03
W-WMO-CW-28-F	23.39±1.32	21.54±1.26	24.40±1.26	25.63±1.13	1.92±0.02	1.78±0.04
P-WMO-CW-28-F	37.04±1.29	36.65±1.30	37.83±1.23	35.30±1.14	1.83±0.04	1.30±0.02
ABS-WMO-CW-28-F	44.87±1.21	36.93±1.21	38.30±1.27	36.30±1.31	1.80±0.01	1.26±0.01

*Equivalent strength values were obtained using manufacturer correlation charts and are used only for comparative purposes; they do not represent mixture-specific calibrated compressive strength.

Figure 1.

Compressive strength results of NC and SCC specimens at 28 days under water curing (CW). Values represent mean ± SD (n = 3).

Schmidt hammer rebound

The Schmidt hammer rebound values for NC and SCC specimens are summarized in Table 7 and illustrated in Figure 2. The results reveal trends consistent with the compressive strength data. This confirms the suitability of the rebound hammer as a nondestructive strength index. In both mixtures, the lowest rebound values were consistently recorded in specimens cast in wooden molds (W), attributable to the water absorption and irregular surface texture of wood, which promote non-uniform curing and localized surface defects. Conversely, the highest rebound values among NC specimens were observed in NC-WB-ABS-28-F, while the best performance among SCC specimens was recorded for SCC-WB-PVC-28-F. The superior results of ABS and PVC molds can be attributed to their non-absorbent and smooth surfaces, which limit moisture loss during casting and produce denser near-surface zones. Interestingly, despite the enhanced workability and compaction of SCC, the maximum rebound index overall was measured in the NC-WB-ABS-28-F specimen, reflecting the influence of mold–concrete interaction beyond mixture type. This observation aligns with previous findings by Megid and Khayat,¹⁹ who also reported significant surface-quality effects associated with polymer molds. Overall, the rebound hammer results confirm that mold material and release conditions exert a measurable influence on surface hardness and, by extension, the apparent compressive strength of concrete. The strong agreement with destructive strength results reinforces the validity of the Schmidt hammer as a complementary nondestructive tool for evaluating both NC and SCC. Accordingly, Schmidt hammer results are discussed herein as comparative rebound indices reflecting relative surface hardness and near-surface uniformity, rather than as independent measures of compressive strength.

Figure 2.

Schmidt hammer rebound indices of NC and SCC specimens at 28 days under water curing. Results are presented as mean ± SD.

Water absorption percentage

The water absorption percentages of NC and SCC specimens are reported in Table 7 and Figure 3. The results demonstrate clear differences between mixtures, mold materials, and release agents. This reduction can be attributed to the improved particle packing and reduced bleeding typically associated with SCC mixtures, which contribute to a denser near-surface transition zone and lower capillary porosity. Across all samples, SCC absorption values ranged from 1.07% to 1.92%, while NC values were higher, ranging from 1.26% to 1.97%. On average, SCC absorbed ≈12–15% less water than NC under identical curing and mold conditions. The lowest absorption was recorded for ABS-SB-CW-28-F (SCC: 1.07%, NC: 1.40%), confirming the beneficial effect of polymer molds combined with oil-based release agents. The smooth, non-absorbent ABS mold surface limited moisture exchange during casting, thereby enhancing surface density. The highest absorption was observed in W-WMO-CW-28-F (SCC: 1.92%, NC: 1.78%) and MDF-WMO-CW-28-F (SCC: 1.89%, NC: 1.36%), highlighting the negative effect of wooden molds without release agents. The absorbent nature of wood likely increased water loss at the mold–concrete interface, producing higher porosity and weaker surfaces. Comparisons between SCC and NC under identical conditions (e.g., ABS-WB-CW-28-F) showed that SCC consistently performed better, with reductions in absorption of ≈15–20%. These results align with previous findings by Golewski¹² and Zhang et al.,¹³ who reported that higher porosity levels facilitate greater ingress of water, thereby accelerating durability-related deterioration. In the present study, the reduced absorption of SCC specimens confirms its superior resistance to permeability-driven degradation mechanisms. The results demonstrate that both mixture type and mold system substantially influence water absorption. While SCC generally outperformed NC, the use of appropriate mold materials (ABS, PVC, Iron) and compatible release agents (WB or SB) was equally critical in minimizing absorption and enhancing durability. Specimens with lower water absorption generally exhibited higher compressive strength. However, this relationship should be interpreted as a qualitative trend rather than a statistically calibrated correlation. Water absorption in the present study is used as a durability-related proxy reflecting pore structure and near-surface density, whereas compressive strength represents a bulk mechanical response influenced by specimen size and surface-area-to-volume ratio. Accordingly, the observed inverse trend reflects underlying porosity and hydration quality. It does not imply a direct predictive relationship between water absorption and compressive strength.

Figure 3.

The results of the water absorption test of the samples.

Abrasion resistance

The abrasion resistance results, expressed as both mass reduction (M_r) and depth of wear (t), are presented in Table 8 and illustrated in Figures 4 and 5. The results reveal consistent trends regarding the influence of mixture type, curing condition, mold material, and release agent. For each abrasion test condition, three replicate specimens were tested. The values reported in Table 8 represent mean ± standard deviation (SD), where the SD reflects specimen-to-specimen variability rather than differences between successive abrasion cycles. The observed scatter was limited and did not affect the comparative trends discussed below. Therefore, the results can be considered statistically consistent for evaluating the effects of concrete type, curing regime, mold material, and release agent on abrasion resistance.

Table 8.

Depth of wear (t) and mass loss (Mr) results from the abrasion test.

Test	Abrasion
Test	t(mm)		M_r (gr)
Sample names	Type of concrete
Sample names	SCC	NC	SCC	NC
I-WB-CW-35-H	0.095 ± 0.010	0.099 ± 0.012	2.1 ± 0.10	2.3 ± 0.12
MDF-WB-CW-35-H	0.098 ± 0.010	0.107 ± 0.010	2.4 ± 0.11	2.7 ± 0.12
W-WB-CW-35-H	0.114 ± 0.011	0.107 ± 0.012	2.5 ± 0.11	2.7 ± 0.12
P-WB-CW-35-H	0.098 ± 0.012	0.107 ± 0.012	2.4 ± 0.12	2.7 ± 0.11
ABS-WB-CW-35-H	0.098 ± 0.012	0.087 ± 0.013	2.4 ± 0.12	1.7 ± 0.12
I-SB-CW-35-H	0.139 ± 0.011	0.118 ± 0.013	3.3 ± 0.13	2.6 ± 0.13
MDF-SB-CW-35-H	0.139 ± 0.012	0.118 ± 0.012	3.3 ± 0.12	2.6 ± 0.12
W-SB-CW-35-H	0.153 ± 0.011	0.122 ± 0.011	3.5 ± 0.11	2.8 ± 0.11
P-SB-CW-35-H	0.131 ± 0.013	0.091 ± 0.012	3.2 ± 0.11	2.3 ± 0.11
ABS-SB-CW-35-H	0.123 ± 0.015	0.083 ± 0.013	3.1 ± 0.12	2.2 ± 0.13
I-WMO-CW-35-H	0.119 ± 0.011	0.127 ± 0.012	2.7 ± 0.11	2.9 ± 0.12
MDF-WMO-CW-35-H	0.125 ± 0.012	0.124 ± 0.012	2.9 ± 0.12	2.8 ± 0.12
W-WMO-CW-35-H	0.128 ± 0.011	0.134 ± 0.015	2.9 ± 0.11	3.0 ± 0.12
P-WMO-CW-35-H	0.096 ± 0.012	0.114 ± 0.011	2.3 ± 0.11	2.6 ± 0.11
ABS-WMO-CW-35-H	0.073 ± 0.011	0.113 ± 0.012	1.6 ± 0.11	2.5 ± 0.11
I-WB-WCW-28-H	0.086 ± 0.012	0.151 ± 0.012	2.0 ± 0.12	3.3 ± 0.11
MDF-WB-WCW-28-H	0.091 ± 0.011	0.136 ± 0.012	2.1 ± 0.12	3.0 ± 0.12
W-WB-WCW-28-H	0.091 ± 0.013	0.151 ± 0.011	2.1 ± 0.11	3.3 ± 0.13
P-WB-WCW-28-H	0.109 ± 0.011	0.136 ± 0.011	2.3 ± 0.11	3.0 ± 0.12
ABS-WB-WCW-28-H	0.127 ± 0.012	0.127 ± 0.012	2.3 ± 0.13	2.7 ± 0.12
I-SB-WCW-28-H	0.164 ± 0.012	0.124 ± 0.012	3.4 ± 0.12	3.1 ± 0.12
MDF-SB-WCW-28-H	0.157 ± 0.013	0.152 ± 0.013	3.3 ± 0.10	3.5 ± 0.12
W-SB-WCW-28-H	0.175 ± 0.014	0.221 ± 0.013	3.7 ± 0.13	4.4 ± 0.12
P-SB-WCW-28-H	0.125 ± 0.013	0.122 ± 0.012	3.2 ± 0.13	3.4 ± 0.12
ABS-SB-WCW-28-H	0.125 ± 0.014	0.124 ± 0.013	3.2 ± 0.12	3.1 ± 0.13
I-WMO-WCW-28-H	0.124 ± 0.011	0.136 ± 0.012	2.8 ± 0.11	2.9 ± 0.11
MDF-WMO-WCW-28-H	0.136 ± 0.012	0.136 ± 0.012	2.8 ± 0.11	2.9 ± 0.11
W-WMO-WCW-28-H	0.133 ± 0.013	0.152 ± 0.012	2.8 ± 0.12	3.9 ± 0.14
P-WMO-WCW-28-H	0.098 ± 0.011	0.116 ± 0.011	2.7 ± 0.11	2.9 ± 0.12
ABS-WMO-WCW-28-H	0.118 ± 0.011	0.115 ± 0.010	2.7 ± 0.11	2.8 ± 0.12

Figure 4.

Mass reduction (Mr) of NC and SCC specimens under (a) CW-35 and (b) WCW-28 curing conditions. The y-axis represents mass loss in grams, and the x-axis denotes specimen identification based on mold type and release agent.

Figure 5.

Depth of wear (t) of NC and SCC specimens under (a) CW-35 and (b) WCW-28 curing conditions. The y-axis shows the wear depth in millimeters, and the x-axis corresponds to specimen identification.

Mass reduction due to abrasion

According to Table 8 and Figure 4, mass loss values for SCC ranged between 1.6 and 3.0 g, while NC specimens ranged between 1.4 and 3.9 g. On average, SCC exhibited ≈15–20% lower mass loss compared to NC under identical conditions. This improvement can be attributed to the denser particle packing and reduced bleeding in SCC, which enhance the cohesion of the surface layer and reduce material detachment during abrasion. Under water curing (CW), the lowest mass loss was recorded for SCC-WMO-CW-ABS-35-H (1.6 g) and NC-WB-CW-ABS-35-H (1.7 g). Under without water curing (WCW), the best performance was found in SCC-WB-WCW-I-28-H (2.0 g), while NC samples in the same condition showed higher losses, with NC-WB-WCW-ABS-28-H (2.7 g). The highest mass loss was consistently observed in specimens cast in wooden molds without release agents. This confirms the detrimental effect of absorbent formwork on surface durability.

Depth of wear after abrasion cycles

The depth-of-wear results (Table 8, Figure 5) confirm the superior abrasion resistance of SCC. Wear depths ranged from 0.073 to 0.175 mm for SCC and 0.083–0.221 mm for NC. The best performance was achieved by SCC-WMO-CW-ABS-35-H (0.073 mm), which was ∼12% lower than the best NC specimen (NC-SB-CW-ABS-35-H, 0.083 mm). Among WCW specimens, SCC again performed better, with SCC-WB-WCW-I-28-H (0.086 mm) compared to NC-WMO-WCW-ABS-28-H (0.115 mm). The worst abrasion resistance was observed in NC specimens cast in wooden molds without release agents, where the wear depth exceeded 0.20 mm. Interpretation: These findings demonstrate that SCC consistently achieves lower wear depths and reduced mass loss. This behavior reflects its denser and more homogeneous microstructure. Furthermore, the use of polymer molds (ABS, PVC) and release agents (WB, SB) significantly enhanced abrasion resistance, whereas wooden molds without release agents resulted in poor surface performance. These observations are consistent with the porosity–durability relationship reported by Liu et al.¹⁴ and Horszczaruk.²⁶

Percentage of abrasion

The progression of abrasion at three successive stages (a₁, a₂, a₃) is summarized in Table 9 and Figure 6. The results indicate clear differences between NC and SCC, as well as the influence of mold type and release agent. The abrasion percentages a1, a2, and a3 correspond to successive abrasion stages performed on the same specimen. They reflect stage-wise material loss rather than independent measurements. This indicates that abrasion resistance is primarily governed by the quality of the outermost concrete layer, which is strongly influenced by formwork interaction and curing conditions. On average, SCC specimens lost approximately 20–25% less material in the first stage than NC, indicating a denser and more compact near-surface layer. Specimens cast in ABS molds with oil-based release agents (e.g., ABS-SB-CW-35-H) exhibited the lowest initial wear (a₁ = 0.172% for SCC, 0.137% for NC). By contrast, wooden molds without oil (e.g., W-WMO-WCW-28-H) showed the highest early-stage wear (a₁ = 0.164% for SCC, 0.189% for NC), reflecting weak surface quality. In Stage 2 (a₂), SCC maintained lower wear percentages compared to NC, but the differences became less pronounced. For example, SCC-P-WB-CW-35-H lost only 0.072%, whereas the corresponding NC specimen lost 0.111%. By Stage 3 (a3), abrasion percentages converged. This suggests that the internal matrix of NC and SCC exhibits more comparable density once the weaker surface layer has been removed. However, SCC specimens still outperformed NC, particularly when cast in polymer molds. Overall, SCC exhibited a more gradual progression of wear (a₁ > a₂ > a₃), while NC specimens displayed higher losses in the initial stage, pointing to weaker near-surface compaction. A consistent qualitative, trend-based relationship was observed between the percentage of abrasion in the first stage (a₁) and the water droplet contact-angle behavior. Specimens exhibiting lower early-stage abrasion loss generally showed higher initial contact angles and longer droplet absorption times. This trend-based association indicates that both abrasion resistance and droplet behavior are governed by near-surface density and porosity. It does not represent a statistically calibrated correlation.

Table 9.

Percentage of surface wear at successive abrasion stages (a1–a3) for NC and SCC specimens. Values represent stage-wise material loss measured on the same specimen.

Specimen ID	Surface abrasion percentage (%)
	SCC			NC
	a₁ (%)	a₂ (%)	a₃ (%)	a₁ (%)	a₂ (%)	a₃ (%)
I-WB-CW-35-H	0.106	0.121	0.122	0.136	0.106	0.106
MDF-WB-CW-35-H	0.178	0.104	0.074	0.139	0.139	0.097
W-WB-CW-35-H	0.151	0.110	0.083	0.083	0.119	0.119
P-WB-CW-35-H	0.124	0.072	0.051	0.161	0.111	0.062
ABS-WB-CW-35-H	0.138	0.125	0.069	0.103	0.073	0.073
I-SB-CW-35-H	0.195	0.150	0.150	0.151	0.136	0.106
MDF-SB-CW-35-H	0.192	0.137	0.124	0.147	0.132	0.103
W-SB-CW-35-H	0.196	0.181	0.151	0.155	0.155	0.124
P-SB-CW-35-H	0.149	0.096	0.096	0.108	0.084	0.084
ABS-SB-CW-35-H	0.172	0.086	0.075	0.137	0.074	0.062
I-WMO-CW-35-H	0.113	0.162	0.162	0.123	0.137	0.137
MDF-WMO-CW-35-H	0.183	0.137	0.122	0.101	0.126	0.126
W-WMO-CW-35-H	0.122	0.169	0.154	0.137	0.137	0.137
P-WMO-CW-35-H	0.132	0.102	0.102	0.128	0.102	0.102
ABS-WMO-CW-35-H	0.090	0.077	0.038	0.132	0.118	0.118
I-WB-WCW-28-H	0.071	0.113	0.099	0.187	0.144	0.144
MDF-WB-WCW-28-H	0.133	0.060	0.060	0.153	0.153	0.153
W-WB-WCW-28-H	0.183	0.091	0.045	0.238	0.205	0.119
P-WB-WCW-28-H	0.092	0.079	0.079	0.210	0.162	0.113
ABS-WB-WCW-28-H	0.134	0.151	0.100	0.169	0.135	0.152
I-SB-WCW-28-H	0.190	0.161	0.147	0.175	0.093	0.093
MDF-SB-WCW-28-H	0.162	0.148	0.135	0.162	0.136	0.136
W-SB-WCW-28-H	0.200	0.185	0.185	0.256	0.185	0.185
P-SB-WCW-28-H	0.148	0.124	0.124	0.149	0.149	0.092
ABS-SB-WCW-28-H	0.138	0.115	0.115	0.170	0.109	0.097
I-WMO-WCW-28-H	0.180	0.139	0.111	0.187	0.188	0.169
MDF-WMO-WCW-28-H	0.151	0.136	0.137	0.140	0.140	0.126
W-WMO-WCW-28-H	0.164	0.136	0.109	0.189	0.161	0.117
P-WMO-WCW-28-H	0.104	0.094	0.084	0.122	0.102	0.071
ABS-WMO-WCW-28-H	0.157	0.126	0.141	0.128	0.128	0.103

Figure 6.

The results of the wear percentage of the samples at each stage of the wear test: (A) NC-CW-35-H, (B) NC-WCW-28-H, (C) SCC-CW-35-H, (D) SCC-WCW-28-H.

Qualitative assessment of sample surfaces

Representative images of NC and SCC specimens at 1 and 28 days, under both water-cured and non-water-cured conditions, are shown in Figure 7. The visual assessment revealed clear differences in surface luster, pore distribution, and color uniformity, influenced by curing regime and mold type. Curing regime: Water-cured specimens generally exhibited reduced surface luster and non-uniform coloration, which can be attributed to moisture retention on the surface. It also indicates that curing-related moisture migration affects surface reflectivity and pigment distribution in the early stages of hydration. By contrast, non-water-cured specimens displayed more uniform coloration, although their luster diminished with age and was nearly absent at 28 days. Mold influence: Wooden molds (W) left visible grain marks on the concrete surface, adversely affecting aesthetic quality. In contrast, polymer molds (ABS) provided smooth surfaces with minimal defects, consistent with earlier reports.^12,14,20 The best-performing samples were NC-WMO-WCW-ABS-G among NC specimens and SCC-WMO-WCW-ABS-G among SCC specimens. These samples exhibited surface luster, absence of pores, and uniform color. The SCC sample in particular exhibited superior overall appearance compared to its NC counterpart. Age effect: Across all specimens, luster decreased significantly from 1 to 28 days, suggesting that early-age visual properties may not be reliable indicators of long-term surface aesthetics. Overall, the results highlight that both mixture type and formwork system strongly influence surface quality. SCC generally outperformed NC, providing smoother and more uniform surfaces. These findings align with previous studies emphasizing the role of mold–concrete interaction and release agent selection in governing architectural appearance.^29,30

Figure 7.

High-resolution representative surface images of NC and SCC specimens cast in different mold systems under water-cured (CW) and non-water-cured (WCW) conditions at 1 and 28 days. All images were captured under consistent lighting and camera settings to allow qualitative comparison of surface pores, color uniformity, and surface gloss.

Water droplet contact-angle test

The water droplet test was conducted in three configurations: ABSS (small droplet), ABSC (large droplet), and ABSF (flowing droplet). Representative results are shown in Figure 8. The measurements showed that specimens with higher density exhibited larger initial contact angles (t = 0) and longer droplet absorption times. This reflects lower surface porosity. Ordinary concrete (NC): The specimen NC-WMO-CW-ABSS recorded an initial contact angle of 31° with a complete absorption time of 15 min. In contrast, NC-WMO-CW-ABSC exhibited a lower contact angle (23°) but required 60 min for absorption, suggesting that, although the immediate surface layer was more absorptive, the underlying matrix exhibited relatively higher density. Self-compacting concrete (SCC): The specimen SCC-WMO-CW-ABSS displayed a markedly higher initial contact angle of 53° and a complete absorption time of 60 min, demonstrating superior near-surface density. This behavior is consistent with the reduced bleeding and improved particle packing typically associated with SCC mixtures. Similarly, SCC-WMO-CW-ABSC showed a contact angle of 22° and absorption time of 60 min, confirming enhanced overall compactness. Comparison NC versus SCC: SCC specimens consistently outperformed NC, with higher contact angles and longer absorption times, particularly in water-cured ABS molds without release agents. These findings highlight the advantage of SCC in achieving both surface density and durability. Droplet flow (ABSF): In denser specimens, droplets spread uniformly without rapid penetration or irregular dispersion. This behavior further indicates reduced surface porosity and improved surface cohesion. A consistent trend was observed between the initial abrasion percentage (a1) reported in the Percentage of abrasion section and the droplet absorption behavior. Specimens with lower a1 values generally exhibited larger contact angles and slower absorption rates. This trend suggests that abrasion resistance and wettability are governed by a common parameter, namely, near-surface density and porosity, rather than representing a statistically calibrated correlation.

Figure 8.

Images of the results of water droplet tests on the surface of some samples: (A)NC-WMO-CW-ABS_S-G-35, (B)NC-WMO-CW-ABS_C-G-35, (C)SCC-WMO-CW-ABS_S-G-35, (D)SCC-WMO-CW-ABS_C-G-35, (E)NC-WMO-CW-ABS_F-G-35, (F)SCC-WMO-CW-ABS_F-G-35.

Overall discussion

Across the investigated test program, the tested SCC mixture generally exhibited improved performance trends compared to the tested NC mixture, particularly with respect to durability-related and surface-sensitive indicators such as water absorption, abrasion resistance, and droplet behavior. However, differences in compressive strength were configuration-dependent rather than uniform across all formwork and release-agent combinations. Several NC specimens, especially those cast in smooth, non-absorbent polymer molds with appropriate release agents (e.g., NC-WB-ABS-28-F), achieved compressive strength values comparable to or exceeding those of the tested SCC mixture. These outcomes do not contradict the general advantages of SCC. Rather, they reflect the combined effects of mixture composition, specimen scale, and mold–concrete interaction. In small cube specimens with a high surface-area-to-volume ratio, surface-related effects are amplified. Under certain polymer mold and release-agent configurations, the high flowability of SCC may promote locally paste-rich near-surface layers. This may slightly moderate the apparent compressive strength measured on small specimens. In contrast, vibrated NC can achieve more effective particle packing adjacent to the mold wall under these conditions, resulting in a denser near-surface zone and higher measured compressive strength. Within the scope of the tested mixtures and curing regimes, polymer molds (ABS and PVC), particularly when combined with water- or solvent-based release agents, generally provided more favorable performance trends across mechanical, durability-related, and surface-quality indicators. Beyond surface smoothness and non-absorbency, polymer molds and compatible release agents may influence hydration conditions at the mold–concrete interface. This may improve the quality of the near-surface interfacial transition zone (ITZ). Conversely, wooden molds—especially when used without release agents—were associated with increased surface porosity, higher abrasion loss, and less uniform surface appearance due to moisture extraction and surface heterogeneity. Iron molds exhibited intermediate behavior between polymer and wooden molds. Water curing generally enhanced compressive strength indicators and abrasion resistance but was sometimes associated with reduced surface luster and color uniformity, indicating a trade-off between durability-related performance and surface aesthetics. From a practical perspective, this trade-off suggests that polymer molds combined with compatible release agents and adequate curing are preferable when durability and permeability resistance are prioritized, whereas architectural or exposed concrete applications require careful balancing of curing practice, mold selection, and release-agent compatibility to achieve visually uniform surfaces. From an engineering practice perspective, balancing durability and surface aesthetics requires project-specific prioritization rather than a single universal solution. For infrastructure elements primarily governed by durability demands (e.g., bridge piers, barriers, and exposed structural components), the results suggest prioritizing polymer molds combined with compatible release agents and continuous curing to minimize near-surface porosity, abrasion loss, and water ingress, even if minor reductions in surface luster occur. Conversely, for architectural or visually exposed concrete surfaces where uniform appearance and surface finish are critical, controlled demolding, compatible release agents, and post-curing surface protection measures (e.g., hydrophobic treatments) may be employed to preserve visual quality while maintaining acceptable durability performance. In applications where both durability and aesthetics are critical, an integrated strategy is recommended. This strategy should combine polymer formwork, optimized release-agent selection, and tailored curing and surface-treatment practices to achieve balanced performance.

Limitations and future work

The results of the present study should be interpreted within the scope of the experimental program. All tests were conducted on laboratory-scale specimens under controlled conditions; therefore, surface-related effects may be more pronounced than in full-scale structural elements. Accordingly, the findings should be regarded as comparative indicators of near-surface performance. They should not be interpreted as direct predictors of in-situ structural behavior. The normal concrete (NC) and self-compacting concrete (SCC) mixtures investigated differed in mixture composition and workability concept. As a result, comparisons between NC and SCC reflect the performance of the tested mixture systems rather than an isolated effect of concrete type. In addition, compressive strength was measured on small cube specimens, for which mold–concrete interaction and surface effects may have a greater influence than in real structural members. Durability-related assessment focused on surface-sensitive indicators, including water absorption, abrasion resistance, and water droplet behavior, which primarily reflect near-surface density and permeability. Standardized durability tests addressing key long-term mechanisms for bridge concrete were not directly evaluated. These include chloride ion penetration, freeze–thaw resistance, and long-term performance beyond 28 days. Moreover, interpretations related to porosity, near-surface density, and interfacial transition zone (ITZ) quality are inferred from macroscopic indicators, as no microstructural analyses were performed. Future research should incorporate larger-scale specimens, controlled mixture designs, extended exposure durations, standardized durability tests, and microstructural characterization. These efforts will help validate and generalize the observed trends.

Conclusions

This study experimentally investigated the combined influence of concrete type, formwork material, release agent, and curing regime on the mechanical performance, durability-related indicators, and surface quality of concrete. Based on the results obtained under the investigated laboratory conditions, the following conclusions can be drawn.

It should be emphasized that all conclusions presented in this study are derived from macroscopic mechanical, durability-related, and surface-performance tests conducted under laboratory conditions. Accordingly, discussions related to porosity, near-surface density, and interfacial transition zone (ITZ) quality are intended as indirect interpretations inferred from these macroscopic indicators rather than direct microstructural characterization.

(1) The near-surface performance of concrete is governed not only by mixture composition but also by construction-related parameters, including mold material, release agent, and curing practice. These factors significantly influence surface density, permeability-related indicators, and aesthetic quality.

(2) Under comparable formwork and curing conditions, the tested self-compacting concrete (SCC) mixture generally exhibited more favorable trends in rebound index, water absorption, abrasion resistance, and water droplet contact-angle behavior compared to the tested normal concrete (NC) mixture. However, selected NC specimens achieved compressive strength indicators comparable to or exceeding those of SCC, demonstrating that appropriate mold selection and surface conditions can partially offset mixture-related differences.

(3) Polymer molds (ABS and PVC), particularly when combined with compatible water- or solvent-based release agents, were associated with denser near-surface zones, reduced abrasion, lower water absorption, and improved surface appearance. In contrast, wooden molds—particularly without release agents—produced higher surface porosity, lower abrasion resistance, and less uniform visual quality.

(4) Water curing generally enhanced compressive strength indicators and abrasion resistance compared to specimens exposed to laboratory ambient conditions. However, water-cured specimens often exhibited reduced surface luster and color uniformity, indicating a potential trade-off between durability-related performance and architectural surface aesthetics.

(5) Surface-sensitive tests, including abrasion percentage and water droplet contact-angle measurements, showed consistent trends and provided complementary comparative indicators of near-surface density and permeability. These methods are effective for evaluating relative surface performance but should not be interpreted as substitutes for standardized bulk transport or durability tests.

Overall, the findings highlight that achieving high-performance architectural and exposed concrete requires an integrated approach that considers mixture design alongside formwork system selection, release agent compatibility, and curing strategy. Future studies should extend this work to larger-scale specimens, controlled mixture designs, and long-term environmental exposures to validate and generalize the observed surface-performance relationships.

Footnotes

ORCID iD

Mohamad Reza Shokrzadeh

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

References

Awasthy

Schlangen

Hordijk

, et al. The role of eigen-stresses on apparent strength and stiffness of normal, high strength, and ultra-high performance fibre reinforced concrete. Dev Built Environ 2023; 16: 100277. https://doi.org/10.1016/j.dibe.2023.100277

Shi

Qian

Hao

. Effects of equipment parameters, ambient parameters and concrete surface moisture on the acquisition of fair-faced concrete appearance quality information. J Build Eng 2023; 72: 106476. https://doi.org/10.1016/j.jobe.2023.106476

Qian

, et al. Effects of ambient temperature, formwork type, and demolding time on the thermal deformation of sidewall concrete in underground engineering: experiment, simulation, and engineering practice. Constr Build Mater 2022; 353: 129083. https://doi.org/10.1016/j.conbuildmat.2022.129083

Qian

Xie

. Demonstration application of microbial self-healing concrete in sidewall of underground engineering: a case study. J Build Eng 2023; 63: 105512. https://doi.org/10.1016/j.jobe.2022.105512

Qayyum Khan

Ahmad Awan

Rasul

, et al. Optimized artificial neural network model for accurate prediction of compressive strength of normal and high strength concrete. Clean Mater 2023; 10: 100211. https://doi.org/10.1016/j.clema.2023.100211

da Silva

Coelho

de Brito

, et al. Statistical survey on inspection, diagnosis, and repair of architectural concrete surfaces. J Perform Constr Facil 2017; 31: 04017097. https://doi.org/10.1061/(asce)cf.1943-5509.0001092

Revilla-Cuesta

Faleschini

Pellegrino

, et al. Water transport and porosity trends of concrete containing integral additions of raw-crushed wind-turbine blade. Dev Built Environ 2024; 17: 100374. https://doi.org/10.1016/j.dibe.2024.100374

Schmitz

. Fabric forms for architectural concrete: a state-of-the-art report. In AEI 2015: birth and life of the integrated building - proceedings of the AEI conference 2015, 2015. https://doi.org/10.1061/9780784479070.024

Pereira

Hamadyk

Silva

. Probabilistic analysis of the durability of architectural concrete surfaces. Appl Math Model 2020; 77: 199–215. https://doi.org/10.1016/j.apm.2019.07.031

10.

Popovic

Arnold

. Preventing failures of precast concrete facade panels and their connections. In: Forensic Engineering, Proceedings of the Congress, 2000. https://doi.org/10.1061/40482(280)56

11.

Baird

Diaferia

Palermo

, et al. Parametric investigation of seismic interaction between precast concrete cladding systems and moment resisting frames. Proc. ASCE Structures Congress 2011 2011; 401: 114. https://doi.org/10.1061/41171

12.

Golewski

. Assessing of water absorption on concrete composites containing fly ash up to 30 % in regards to structures completely immersed in water. Case Stud Constr Mater 2023; 19: e02337. https://doi.org/10.1016/j.cscm.2023.e02337

13.

Zhang

Song

, et al. Theoretical model of water vapor absorption–desorption equilibrium of concrete considering the effect of temperature. Constr Build Mater 2023; 375: 130968. https://doi.org/10.1016/j.conbuildmat.2023.130968

14.

Liu

Andersen

Zhang

, et al. Abrasion damage of concrete for hydraulic structures and mitigation measures: a comprehensive review. Constr Build Mater 2024; 422: 135754. https://doi.org/10.1016/J.CONBUILDMAT.2024.135754

15.

Sharifi

Shokrzadeh

Ahmadi

, et al. Experimental investigation of the cyclic behavior of full-scale beam-column connections using high-strength self-consolidating concrete with and without fiber -. Bridge Struct 2025; 21(2–3): 1–15. https://doi.org/10.1177/157324872513434

16.

Aadi

Hilal

Hama

, et al. Mechanical performance of eco-friendly self-compacting concrete (SCC) mixtures and two-way slabs partially containing cement kiln dust as cement replacement and internally reinforced with waste plastic mesh. Structures 2024; 66: 106864. https://doi.org/10.1016/J.ISTRUC.2024.106864

17.

Hussein

Ghalehnovi

. The impact of colloidal nano silica (CNS) and polypropylene fibers (PP) on the properties of recycled concrete aggregate self-compacting concrete (SCC). Case Stud Constr Mater 2025; 22: e04424. https://doi.org/10.1016/J.CSCM.2025.E04424

18.

Hui

Wang

, et al. Investigation on axial compression performance of self-compacting fly ash concrete filled square steel tube column: tests and numerical simulation. Case Stud Constr Mater 2024; 20: e03281. https://doi.org/10.1016/J.CSCM.2024.E03281

19.

Megid

Khayat

. Variations in surface quality of self-consolidation and highly workable concretes with formwork material. Constr Build Mater 2020; 238: 117638. https://doi.org/10.1016/j.conbuildmat.2019.117638

20.

Shokrzadeh

Nateghi-Alahi

Mansoori

, et al. Cold rolling versus rotating friction welding techniques in mechanical splices: study of the inelastic cyclic behavior of reinforced concrete columns. Struct Concr 2025. https://doi.org/10.1002/SUCO.70065;PAGEGROUP:STRING:PUBLICATION

21.

Shokrzadeh

Nateghi-Alahi

Mansoori

, et al. The improvement of the threaded-based mechanical splice by modifying the threaded system: study of techniques cold rolling and rotating friction welding. J Build Eng 2023; 80: 107964. https://doi.org/10.1016/j.jobe.2023.107964

22.

Pourmirza

Sharifi

Amirabadi

, et al. Innovative and conventional pile head connections under cyclic loading: experimental and analytical investigation. Ocean Eng 2025; 341: 122685. https://doi.org/10.1016/J.OCEANENG.2025.122685

23.

Shokrzadeh

Nateghi-Alahi

. Temporary seismic strengthening techniques for damaged RC columns: experimental and analytical investigation. Structures 2025; 82: 110496. https://doi.org/10.1016/J.ISTRUC.2025.110496

24.

Pan

Zhang

, et al. Effect of polymer and conventional molds on the aesthetical surface quality of concretes. Constr Build Mater 2021; 302: 124375. https://doi.org/10.1016/j.conbuildmat.2021.124375

25.

Abid

Shamkhi

Mahdi

, et al. Hydro-abrasive resistance of engineered cementitious composites with PP and PVA fibers. Constr Build Mater 2018; 187: 168–177. https://doi.org/10.1016/j.conbuildmat.2018.07.194

26.

Horszczaruk

. Hydro-abrasive erosion of high performance fiber-reinforced concrete. Wear 2009; 267: 110–115. https://doi.org/10.1016/j.wear.2008.11.010

27.

Sun

. Microscopic, physical and mechanical analysis of polypropylene fiber reinforced concrete. Mater Sci Eng 2009; 527: 198–204. https://doi.org/10.1016/j.msea.2009.07.056

28.

Alaskar

Alabduljabbar

Mohamed

, et al. Retraction notice to “Abrasion and skid resistance of concrete containing waste polypropylene fibers and palm oil fuel ash as pavement material”. Constr Build Mater 2021; 282: 122681. Construction and Building Materials (2021) 282, (S0950061821004414). Construction and Building Materials 2023. https://doi.org/10.1016/j.conbuildmat.2023.131327

29.

Savukaitis

Daukšys

Juočiūnas

, et al. The influence of new and used formwork coated with different release agents on the appearance of the formed concrete surface. J Build Eng 2021; 42: 102807. https://doi.org/10.1016/j.jobe.2021.102807

30.

Garg

Nim

Bajpai

, et al. Enhancement in the quality of near surface concrete using some formwork liners. Constr Build Mater 2019; 207: 722–733. https://doi.org/10.1016/j.conbuildmat.2019.02.124

31.

Lemaire

Escadeillas

Ringot

. Evaluating concrete surfaces using an image analysis process. Constr Build Mater 2005; 19: 604–611. https://doi.org/10.1016/j.conbuildmat.2005.01.025

32.

Djelal

Vanhove

Chambellan

, et al. Influence of the application method of release agents on thickness of mould oils. Mater Struct 2010; 43: 687–698. https://doi.org/10.1617/s11527-009-9521-z

33.

Libessart

De Caro

Djelal

, et al. Correlation between adhesion energy of release agents on the formwork and demoulding performances. Constr Build Mater 2015; 76: 130–139. https://doi.org/10.1016/j.conbuildmat.2014.11.061

34.

González-Coneo

Zarzuela

Elhaddad

, et al. Alkylsiloxane/alkoxysilane sols as hydrophobic treatments for concrete: a comparative study of bulk vs surface application. J Build Eng 2022; 46: 103729. https://doi.org/10.1016/j.jobe.2021.103729

35.

Azizi

Sharifi

Shokrzadeh

. Experimental investigation of the effect of mold type and mold oil on the physical characteristics of self-compacting concrete. Civil Infrastructure Researches 2024. https://doi.org/10.22091/CER.2024.10881.1559

36.

ASTMC805-97 . Standard test method for rebound number of hardened concrete, 2020.

37.

ASTM C642 . ASTM C642 :standard test method for density, absorption, and voids in hardened concrete, ASTM international, United States. Annu Book ASTM (Am Soc Test Mater) Stand 2013.

38.

ASTM

International

. ASTM C192/C192M-16. Standard practice for making and curing concrete test specimens in the laboratory. West Conshohocken, PA, USA: ASTM International, 2016.

39.

EFNARC . Specification and guidelines for self-compacting concrete. Report from EFNARC 2002.

40.

ASTM . ASTM C127 - standard test method for specific gravity and water absorption of coarse aggregate. ASTM International, 2001.

41.

ASTM. ASTM C128-04 . Standard test method for density, relative density (specific gravity), and absorption. Annu Book ASTM (Am Soc Test Mater) Stand 2004.

42.

ASTMC39/C39M . ASTM C39: Standard test method for compressive strength of cylindrical concrete specimens, West Conshohocken, PA, USA. Annu Book ASTM (Am Soc Test Mater) Stand 2015.

43.

IS 1237: cement concrete flooring tiles - specification: bureau of Indian standards: free download, borrow, and streaming: internet archive n.d. https://archive.org/details/gov.in.is.1237.2012/is.1237.2012/mode/2up (accessed September 10, 2025).