Load-Bearing Capacity of Mechanical Fastening in Lightweight Concrete

Abstract

In traditional masonry construction, roof trusses are anchored to walls using conventional anchors embedded directly into the reinforced concrete ring beam. However, in lightweight structures, installing a ring beam may impose an additional load on the wall, which may not necessarily improve its bearing capacity. The use of lightweight concrete, due to its specific properties, represents a significant advancement in modern construction, but requires special consideration in the design of anchoring systems. Based on the anchoring solution proposed by the author, steel, galvanized, screw-type anchors installed directly into lightweight perlite concrete blocks were assumed. Experimental tests and analyses of these anchors provide a basis for the development of design approaches for lightweight structures. The results demonstrate the feasibility of using bonded anchors in perlite concrete and indicate their potential applicability in practical engineering design.

1. Introduction

In lightweight masonry structures made of perlite concrete, the use of a traditional reinforced concrete ring beam for anchoring roof trusses may be structurally inefficient and can unnecessarily increase wall loads. This creates a need for alternative anchoring solutions applied directly within lightweight masonry elements.

The present study addresses this engineering challenge by proposing and experimentally validating a novel anchoring concept based on bonded anchors installed directly into perlite concrete blocks, eliminating the need for a reinforced concrete ring beam.

The novelty of this paper lies in the experimental verification of anchorage performance in highly porous perlite concrete and in demonstrating its potential applicability under real structural conditions. To the best of the authors’ knowledge, such solutions have not yet been systematically investigated for this type of material.

1.1. Background on Perlite Concrete

In the context of the rapid development of construction technologies, lightweight building materials are gaining increasing attention due to their low bulk density and favorable thermal properties [1,2]. One such material is perlite concrete, which combines a relatively low density with sufficient mechanical performance for selected structural applications. However, compared to conventional concrete, perlite concrete exhibits lower compressive strength and stiffness. Its properties depend strongly on the proportion of lightweight aggregate, which influences both strength and deformation behavior [3,4,5,6,7]. Although this material offers clear advantages in weight reduction, its high porosity and relatively low tensile strength pose challenges for structural applications. The porous internal structure of perlite concrete results in reduced bond strength and lower resistance to localized stresses. These characteristics must be carefully considered in the design of structural connections, particularly those relying on load transfer through adhesion or mechanical interlock.

Sengul et al. [5] investigated the relationship between the unit weight of perlite concrete and its compressive strength.

A clear positive correlation can be observed: a higher unit weight corresponds to greater compressive strength. Concretes with very low density (approximately 300–800 kg/m³), associated with high perlite content, exhibit a very limited load-bearing capacity. In contrast, an increase in the unit weight improves mechanical performance, with compressive strength exceeding 20–30 MPa at densities approaching those of conventional concrete (1800–2100 kg/m³). Demirboğa et al. [6] examined the influence of perlite aggregate content on compressive strength after 7 and 28 days. The results indicate an increasing trend in strength up to approximately 60% perlite content, at which the highest values are observed. Despite the reduction in density, concretes with a moderate perlite content show stable strength development, and the difference between the 7-day and 28-day results confirms ongoing hydration and curing processes that are typical of lightweight aggregate concrete. These findings indicate that the use of perlite aggregate enables a significant reduction in the concrete weight, which is advantageous because it reduces strength; therefore, the selection of perlite content must account for load-bearing requirements. In practice, perlite allows the production of lightweight concretes with satisfactory mechanical properties, particularly when the aggregate content ranges between 40% and 60%, making it a promising material for optimized structural applications. In addition to strength, the relationship between the unit weight and stiffness has also been investigated. Sengul et al. [5] presented both experimental results and theoretical predictions based on the ACI 213R-03 [7] model, illustrating the dependence of the modulus of elasticity (GPa) on the unit weight (kg/m³) for perlite-based lightweight concretes. The experimental data show that low-density concrete (approximately 1100–1300 kg/m³) exhibits low stiffness, with a modulus of about 3–4 GPa. As the unit weight increases to 1500–1800 kg/m³, the modulus rises to approximately 10–14 GPa, while at densities above 1900 kg/m³, it reaches values of 18–20 GPa. This confirms that the deformation properties of lightweight concrete are strongly influenced by the proportion of lightweight aggregate and the resulting porosity. For example, a mixture containing 40% perlite exhibits a modulus of elasticity of approximately 9.4 GPa. Although the theoretical curve from ACI 213R-03 [7] captures the overall trend, some deviations from experimental results are observed, indicating that the actual behavior of lightweight concrete may be more variable than predicted. In practical terms, a reduction in the concrete density not only lowers strength but also significantly decreases stiffness, which must be accounted for in structural design. Recent studies provide further insight into the performance of perlite concrete in construction applications. Szlachetka and Dzięcioł [8] presented a comprehensive review of expanded perlite in civil engineering, highlighting its potential in low-carbon and circular construction. Their analysis confirms that perlite-based concretes can reduce the environmental impact of construction while maintaining sufficient mechanical performance for selected structural and non-structural elements. Based on this analysis, the review [8] identifies priority application areas with the highest development potential, with particular emphasis on single-layer walls in buildings as one of the most promising directions of use. In this context, the authors underline the importance of optimizing material composition to balance the thermal insulation and mechanical performance, which is crucial for envelope elements that are exposed to combined environmental and load conditions.

Additional experimental work by Szlachetka et al. [4] focused on the mechanical properties of perlite concrete for external wall applications. The results demonstrate that, despite reduced compressive strength compared to conventional concrete, perlite concrete can achieve adequate performance for building envelopes, particularly when optimized mix compositions are employed. The study emphasizes that the balance between the density reduction and mechanical capacity is a key design parameter, especially for elements that are subjected to combined thermal and mechanical loads. These findings are consistent with the trends discussed above, confirming that perlite concrete—despite its lower stiffness and strength—can be effectively used in construction, provided that its material characteristics are properly considered in design.

Moreover, recent research highlights that the benefits of perlite-based concretes extend beyond mechanical performance and low density to include environmental and sustainability aspects. Dzięcioł and Szlachetka [9] demonstrated that perlite concrete can contribute to sustainable materials management, particularly within circular economy frameworks. Their study indicates that perlite-based materials can reduce waste streams and improve resource efficiency when evaluated over the full lifecycle. Additionally, perlite concrete has been shown to support the immobilization of potentially toxic elements (PTEs) in soil–water environments, which is particularly relevant in applications involving interaction with natural ecosystems. These findings suggest that perlite concrete not only enables weight reduction and material efficiency, but may also enhance environmental protection. Consequently, its potential applications extend beyond traditional structural uses to include earthworks, road subbases, and other geotechnical applications where the interaction with soil and groundwater is significant.

1.2. Anchoring Problem in Lightweight Concrete

Anchors are critical components that are responsible for transferring loads between structural elements and ensuring overall structural stability. While anchorage mechanisms in conventional concrete are well understood and widely codified in design standards, their behavior in lightweight, highly porous materials differs significantly due to reduced strength and altered failure mechanisms. Despite numerous studies on anchoring systems in both normal- and lightweight-concrete substrates, the performance of bonded anchors in highly porous perlite concrete remains insufficiently understood. In particular, failure mechanisms governed by bond degradation and material heterogeneity differ substantially from those observed in conventional concrete. Consequently, experimental verification of anchorage performance under realistic conditions is required, especially for structural applications such as roof truss connections subjected to uplift forces.

In lightweight masonry systems, anchoring roof trusses directly to walls without the use of a reinforced concrete ring beam represents a potential alternative solution. However, such an approach requires careful verification of the load-bearing capacity and reliability of anchors embedded directly in perlite concrete.

The load-bearing capacity of anchors depends on several interacting parameters, including the embedment depth, spacing, edge distance, and installation quality [10,11,12,13,14,15]. Among these, embedment depth is particularly important, as increasing depth enlarges the failure cone and enhances pull-out resistance. However, geometric constraints in masonry elements may limit the achievable embedment depth. Anchor spacing and edge distance are also critical: reduced spacing leads to the interaction of failure cones and a reduction in group capacity, while proximity to edges decreases the available failure surface and overall resistance. Additional factors influencing the anchorage performance include the drill hole quality, cleanliness, and environmental conditions such as temperature and humidity, which directly affect resin curing and bond development [10,11,12,13,14,15].

Recent studies have further emphasized the complexity of anchorage behavior in non-standard materials and under varying loading conditions. Both experimental and numerical investigations indicate that the performance of bonded anchors is highly sensitive not only to the embedment depth and installation quality, but also to the mechanical characteristics of the base material, including the porosity and fracture behavior [15,16,17]. The load transfer mechanism in bonded anchors is strongly governed by the interaction between the adhesive layer and the surrounding concrete, particularly in materials with reduced cohesion and a heterogeneous structure [14,15]. These findings suggest that simplified design approaches may not adequately capture the actual behavior of anchors in non-conventional concretes, thereby highlighting the need for dedicated experimental validation.

In bonded anchorage systems, adhesion between the resin and the base material plays a decisive role. In highly porous materials such as perlite concrete, this mechanism may be significantly weakened due to the reduced contact surface and irregular resin penetration. Understanding these effects is essential for assessing whether bonded anchors can provide sufficient performance in lightweight masonry systems.

Anchors used in construction can generally be divided into two main categories: mechanical anchors and adhesive (bonded) anchors. Mechanical anchors are the most commonly applied solution in concrete structures and rely on mechanical interlock or frictional resistance between the anchor and the surrounding materials [11]. Their fundamental mechanism involves expansion during installation, which increases the radial pressure on the concrete. Common types include sleeve anchors, wedge anchors, screw anchors, and drop-in anchors. Their key advantages include immediate load-bearing capacity after installation, independence from ambient temperature, suitability for multi-point fastening, and predefined embedment depth determined by geometry [10].

Sleeve anchors can be installed in through-fastening applications using the same drill bit diameter as the anchor. They offer several embedment-depth options, enabling flexible adjustment to varying base-material thicknesses and fixture properties.

Adhesive anchors are typically used in more demanding engineering applications due to their ability to transfer high loads and perform reliably even in cracked concrete or seismic conditions. Their mechanism is based on adhesion between the resin and both the concrete and the steel element. Bonded anchors include epoxy, hybrid, and polyester systems, each characterized by specific curing times, load capacities, and environmental resistance.

The use of threaded rods in bonded anchorage systems provides significant installation flexibility and allows for precise adjustment of the connection using nuts, washers, or spacers. This makes such systems particularly suitable for structural applications where geometric tolerances and installation constraints must be accommodated.

An analysis of European Technical Assessments (ETA) of commonly used hybrid bonded anchors [12,13] indicates a high level of consistency in fundamental installation parameters, particularly the relationship between the threaded rod diameter and nominal drill hole diameter. However, significant differences exist in minimum anchor spacing (s_min) and minimum edge distance (c_min), reflecting variations in resin composition, mechanical performance, and manufacturer-specific requirements (

Table 1. Comparison of installation parameters of selected hybrid bonded anchors according to European Technical Assessments, based on [12,13].

ETA	Threaded Rod Size:		Ø8	Ø10	Ø12	Ø16	Ø20	Ø24	Ø27	Ø30
ETA-17/0594	Nominal drill hole diameter	d0 (mm)	10	12	14	18	24	28	-	35
	Effective anchorage depth	hef,min (mm)	60	60	60	60	80	96	-	120
		hef,max (mm)	160	200	240	320	400	480	-	600
	Minimum anchor spacing	smin (mm)	40	40	40	40	40	50	-	60
	Minimum edge distance	cmin (mm)	40	40	40	40	40	50	-	60
	Maximum installation torque	Tinst (Nm)	10	20	40	80	120	160	-	200
ETA-11/0493	Nominal drill hole diameter	d0 (mm)	10	12	14	18	22	28	30	35
	Effective anchorage depth	hef,min (mm)	60	60	70	80	90	96	108	120
		hef,max (mm)	160	200	240	320	400	480	540	600
	Minimum anchor spacing	smin (mm)	40	50	60	75	90	115	120	140
	Minimum edge distance	cmin (mm)	40	45	45	50	55	60	75	80
	Maximum installation torque	Tinst (Nm)	10	20	40	80	150	200	270	300

). These differences highlight the importance of selecting anchorage systems based not only on their nominal diameter but also on certified installation and geometric constraints.

The mechanical properties of threaded rods are standardized according to DIN 975 [18], DIN 976-1 [19], and EN ISO 898-1 [20]. They are available in several strength classes, (e.g., 4.6, 5.8, 8.8, 10.9, and 12.9), corresponding to increasing tensile and yield strength. The appropriate selection of the strength class is essential to ensure compatibility with the anchorage system and to achieve the required structural performance.

Given these mechanical characteristics of perlite concrete, anchorage performance becomes a governing factor in connection design. The embedment depth remains one of the most influential parameters: increasing depth enlarges the failure cone and improves the load capacity, although geometric limitations may restrict its implementation. Similarly, reduced spacing between anchors decreases the group capacity due to overlapping failure cones, while proximity to edges reduces resistance by limiting the available failure surface [21,22,23].

Drill hole quality, including diameter, cleanliness, and moisture conditions, plays a crucial role, particularly for bonded anchors. Environmental factors such as temperature and humidity influence resin curing and mechanical properties, making installation conditions a key design consideration. Furthermore, deviations from perpendicular installation may alter load transfer mechanisms and reduce the anchor capacity [21,22,23].

A crucial aspect of anchorage design is ensuring sufficient embedment depth. Inadequate anchorage length may result in stresses exceeding the bond capacity, leading to pull-out failure and local material damage.

According to EN 1992-1-1, the required anchorage length is determined based on both the basic anchorage length $l_{b,rqd}$ and the design anchorage length $l_{bd}$, as defined by the following relationships [24]:

$$l_{bd} = \propto l_{b,rqd}\ge l_{b,min}$$

(1)

where

• $\alpha =1.0$ or $\alpha ={\alpha }_1{\alpha }_2{\alpha }_3{\alpha }_4{\alpha }_5$
• $l_{b,min}$—minimum anchorage length:
• $l_{b,min}=max\left\{\begin{array}{c}\left\{0.3l_{b,rqd}; 10\varphi ; 100 \mathrm{m}\mathrm{m}\right\} \mathrm{f}\mathrm{o}\mathrm{r} \mathrm{t}\mathrm{e}\mathrm{n}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{b}\mathrm{a}\mathrm{r}\mathrm{s}\\ \left\{0.6l_{b,rqd}; 10\varphi ; 100 \mathrm{m}\mathrm{m}\right\} \mathrm{f}\mathrm{o}\mathrm{r} \mathrm{c}\mathrm{o}\mathrm{m}\mathrm{p}\mathrm{r}\mathrm{e}\mathrm{s}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{b}\mathrm{a}\mathrm{r}\mathrm{s}\end{array}\right.$
• $l_{b,rqd}$—basic anchorage length:

$$l_{b,rqd}={\displaystyle \frac{\varphi }{4}}{\displaystyle \frac{{\sigma }_{sd}}{f_{bd}}}$$

(2)

where

• $\varphi$—diameter of the bar;
• ${\sigma }_{sd}$—design stress in the reinforcement at ULS;
• $f_{bd}$—design value of the bond strength:

$$f_{bd}=2.25{\eta }_1{\eta }_2{f}_{ctd}$$

(3)

where

• ${\eta }_1$—coefficient depending on bond conditions and bar position during concreting:
• ${\eta }_1=\left\{\begin{array}{c}1.0 \mathrm{f}\mathrm{o}\mathrm{r} \mathrm{g}\mathrm{o}\mathrm{o}\mathrm{d} \mathrm{b}\mathrm{o}\mathrm{n}\mathrm{d} \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{d}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{s}\\ 0.7 \mathrm{f}\mathrm{o}\mathrm{r} \mathrm{b}\mathrm{a}\mathrm{r}\mathrm{s} \mathrm{i}\mathrm{n} \mathrm{e}\mathrm{l}\mathrm{e}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{s} \mathrm{c}\mathrm{a}\mathrm{s}\mathrm{t} \mathrm{u}\mathrm{s}\mathrm{i}\mathrm{n}\mathrm{g} \mathrm{s}\mathrm{l}\mathrm{i}\mathrm{p} \mathrm{f}\mathrm{o}\mathrm{r}\mathrm{m}\mathrm{s},\\ \mathrm{u}\mathrm{n}\mathrm{l}\mathrm{e}\mathrm{s}\mathrm{s} \mathrm{g}\mathrm{o}\mathrm{o}\mathrm{d} \mathrm{b}\mathrm{o}\mathrm{n}\mathrm{d} \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{d}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{s} \mathrm{c}\mathrm{a}\mathrm{n} \mathrm{b}\mathrm{e} \mathrm{d}\mathrm{e}\mathrm{m}\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{d}\end{array}\right.$
• ${\eta }_2$—coefficient dependent on bar diameter. ${\eta }_2=\left\{\begin{array}{c}1.0 \mathrm{i}\mathrm{f} \varphi \le 32 \mathrm{m}\mathrm{m}\\ {\displaystyle \frac{132-\varphi }{100}} \mathrm{i}\mathrm{f} \varphi > 32 \mathrm{m}\mathrm{m}\end{array}\right.$
• $f_{ctd}$—design tensile strength of concrete (not exceeding that assigned to concrete C60/75):

$$f_{ctd}={\displaystyle \frac{f_{ctk}}{{\gamma }_c}}$$

(4)

• $f_{ctk}$—characteristic tensile strength of concrete;
• ${\gamma }_c$—partial safety factor for concrete.

A crucial aspect of anchoring is ensuring that the minimum anchorage depth is achieved.

The pull-out resistance of an anchor may be expressed as [25]:

$$T=\pi d{\int }_0^{l_{bd}}\tau dx$$

(5)

where

• d—anchor diameter (mm); $\tau$—bond stress (N/mm²).

It should be noted that these formulations are based on provisions developed for conventional concrete. In the present study, they are used as a reference framework, while the actual behavior of bonded anchors in perlite concrete is evaluated experimentally using a component-based approach that accounts for the interaction between the steel anchor, adhesive layer, and porous substrate.

2. Materials and Methods

To determine the load-bearing capacity of adhesive anchors, galvanized steel threaded rods with a diameter of 16 mm, a length of 1000 mm, and a strength class of 5.8 were used. The bonded anchors consisted of polyester-based, styrene-free resins intended for general-purpose applications under medium loads.

Drill holes with a diameter of 18 mm were prepared in lightweight perlite concrete masonry units with a density of 350 kg/m³. The holes were filled with polyester resin, after which the threaded rods were inserted to an embedment depth of 400 mm. It should be explicitly noted that the investigated anchorage configuration corresponds to a top-down vertical installation along the height of the masonry element. Therefore, the embedment depth of 400 mm refers to vertical anchorage within the wall and is not related to the wall thickness. The results presented in this study are applicable only to this specific configuration. Prior to testing, the specimens were placed on the base plate of a universal testing machine and secured using transport straps with a combined capacity of 4 tons. During testing, the specimens were placed on a rigid base plate and restrained in the horizontal direction, using steel straps to prevent lateral displacement and ensure continuity of the masonry system. An oriented strand board (OSB) panel, matching the specimen surface and provided with a drillhole for the threaded rod, was positioned between the straps and the specimen to distribute the applied load and ensure uniform contact with the testing machine.

Load–displacement relationships were recorded using Bluehill Universal materials testing software version 2.36 in conjunction with the INSTRON 8806 testing machine (Manufacturer: Instron, High Wycombe, United Kingdom), including the precise moment of bond failure between the anchor and the perlite concrete. The tests were conducted under displacement-controlled conditions with a constant loading rate of 5 mm/s. All tests were performed under controlled laboratory conditions. The ambient temperature was approximately 21 °C and the relative humidity was about 45%, which is consistent with the standard conditions specified in EN 1504-6 [26].

Figure 1 illustrates the key stages of the experimental procedure. Figure 1a shows the anchors embedded in perlite concrete blocks prior to testing, representing the specimen preparation stage. Figure 1b presents the test setup during the pull-out test conducted using a INSTRON 8806. Figure 1c shows the condition of the specimen after failure, highlighting the characteristic cracking pattern and material detachment. Finally, Figure 1d presents the extracted anchor after pull-out, illustrating the failure surface and the bond zone within the perlite concrete.

3. Results

The results are summarized in

Table 2. The pull-out force values.

No. Sample	Embedment Length (mm)	Maximum Load F (N)
1	400	11,357.96
2	400	13,259.35
3	400	9712.58 1
4	400	10,765.79
5	400	11,671.78
6	400	1412.07
7	400	11,879.21
8	400	11,689.96
9	400	13,886.99
10	400	14,484.52
11	400	7881.52 1
12	400	6997.29 1
13	400	17,327.96
14	400	23,228.53 1
15	400	17,925.80
16	400	14,829.64
Average value (N)		13,457.59
Standard deviation (N)		2379.6
Coefficient of variation (%)		17.7

¹ Excluded sample.

. A total of 16 specimens were tested. Samples no. 11 and 12 were excluded from further analysis due to confirmed technological defects associated with improper resin curing. In addition, samples no. 3 and 14 were treated as extreme boundary observations and were not included in the calculation of the representative mean value. This approach was adopted to reduce the influence of non-representative extreme results on the assessment of typical anchoring performance in a highly heterogeneous material such as perlite concrete. The unusually high value observed for sample no. 14 is likely attributable to local material densification or enhanced resin penetration and is not representative of typical anchoring conditions in perlite concrete.

The statistical values reported in Table 2 (mean, standard deviation, and coefficient of variation) are calculated after the exclusion of specimens no. 3, 11, 12, and 14, as described above. The average pull-out resistance was determined as 13.46 kN, with a standard deviation of approximately 2.38 kN. The coefficient of variation was approximately 17.7%, which is acceptable given the high heterogeneity and porosity of perlite concrete. The observed variability is primarily attributed to differences in local material structure, including pore distribution and resin penetration depth. The failure mode observed in the tests is illustrated in Figure 1d. The extracted anchor exhibits a distinct failure surface within the perlite concrete, indicating that the dominant failure mechanism was governed by bond degradation and local material detachment, rather than the yield of the steel element.

This observation is consistent with the relatively high scatter of results and reflects the heterogeneous nature of the material.

A conservative estimate of the characteristic pull-out resistance (5th percentile) is approximately 10.8 kN, confirming the applicability of the proposed anchoring solution under practical design conditions. The characteristic value (5th percentile) is therefore considered more appropriate for practical design considerations than the mean value.

For comparison, the anchorage capacity was evaluated using the approach provided in EN 1992-1-1 for conventional concrete. The basic anchorage length ($l_{b,rqd}$) was calculated for threaded bars with a diameter of 16 mm and class M5.8. The tensile capacity of the bar was assumed as 100.5 kN, while the characteristic tensile strength of perlite concrete, determined experimentally, was $f_{ctk}$ = 0.5 MPa.

The design tensile strength is given by the following:

$$f_{ctd}={\displaystyle \frac{f_{ctk}}{{\gamma }_c}}={\displaystyle \frac{0.5}{1.4}}=0.357 \mathrm{M}\mathrm{P}\mathrm{a}$$

(6)

The design bond strength is as follows:

$$f_{bd}=2.25{\eta }_1{\eta }_2{f}_{ctd}=2.25·1·1·0.357=0.803 \mathrm{M}\mathrm{P}\mathrm{a}$$

(7)

The basic anchorage length is as follows:

$$l_{b,rqd}={\displaystyle \frac{\varphi }{4}}{\displaystyle \frac{{\sigma }_{sd}}{f_{bd}}}={\displaystyle \frac{16}{4}}{\displaystyle \frac{66.93}{0.803}}=333.41 \mathrm{m}\mathrm{m}$$

(8)

The design anchorage length is therefore as follows:

$$l_{bd}=\propto l_{b,rqd}=1·333.41=333.41 \mathrm{m}\mathrm{m}$$

(9)

The minimum anchorage length is as follows:

$$l_{b,min}=max\left\{0.3l_{b,rqd}; 10\varphi ; 100\right\} \mathrm{m}\mathrm{m}=\left\{93.88; 160; 100\right\} \mathrm{m}\mathrm{m}=160 \mathrm{m}\mathrm{m}$$

(10)

Since $l_{bd}\ge l_{b,min}$, the calculated anchorage length satisfies the requirements for conventional concrete. It should be emphasized, however, that this comparison serves only as a reference framework, as Eurocode provisions were developed for normal-weight concrete. In this study, the design stress is related to experimentally determined pull-out resistance in perlite concrete. This comparison does not constitute a design verification of the bonded anchorage system in perlite concrete.

4. Discussion

The dominant failure mechanism observed in the tests was associated with the loss of adhesion between the resin and the perlite concrete. Due to the highly porous structure and weak interfacial bonding, failure occurred primarily as pull-out without the formation of a typical concrete failure cone.

The experimental program provides valuable insights into the anchorage behavior of bonded anchors embedded in perlite concrete, a material that is significantly different from conventional concrete in terms of density, stiffness, and tensile capacity. As shown in previous studies, the mechanical properties of perlite concrete—particularly its reduced compressive strength and modulus of elasticity—are directly linked to the lightweight aggregate content. These characteristics inevitably affect the anchorage mechanism, which relies on the integrity of the concrete surrounding the resin–steel interface.

The pull-out tests revealed noticeable variability among the specimens, which is consistent with the heterogeneous and highly porous structure of perlite concrete. Even after excluding inconsistent samples, the results exhibited a relatively high scatter. Nevertheless, the average pull-out resistance remained at a level of approximately 13.46 kN. Considering the low tensile strength of the perlite concrete used in this study ($f_{ctk}=0.5 \mathrm{M}\mathrm{P}\mathrm{a}$), the obtained anchorage resistance is relatively high and indicates that bonded anchors are capable of effectively mobilizing the available tensile capacity of the surrounding material. Calculations based on conventional anchorage theory suggest that a 16 mm-diameter threaded bar requires an embedment depth of approximately 334 mm, which is consistent with the embedment depth adopted in the experimental program. This agreement indicates that, despite structural differences between perlite concrete and conventional concrete, simplified theoretical approaches based on tensile capacity may provide a reasonable first approximation of anchorage behavior.

A practical comparison can be made between the experimentally obtained pull-out resistance and typical design loads acting on lightweight roof structures. According to [27], peak wind suction under unfavorable conditions does not exceed approximately 1 kN/m². For a building with a width of 10 m and a roof slope of 30°, this corresponds to an uplift force of approximately 10 kN per meter of roof projection.

The experimentally determined characteristic resistance (approximately 10.8 kN) is therefore of the same order of magnitude as the expected design load, while the mean resistance (13.46 kN) indicates an additional reserve of capacity. This suggests that the proposed anchorage system may be suitable for practical applications, particularly in lightweight roof structures. It should be emphasized, however, that this comparison is indicative only and does not constitute a full structural verification. The reported characteristic resistance does not represent a design value and should be appropriately reduced by applying partial safety factors in accordance with relevant design standards.

The results also highlight the sensitivity of the bonded anchor performance to the installation quality. Samples affected by improper resin curing exhibited a significant reduction in load-bearing capacity, underscoring the importance of correct resin mixing and installation procedures. Furthermore, factors such as embedment depth, edge distance, spacing, and environmental conditions may have an even greater influence in perlite concrete than in conventional concrete, due to its lower stiffness and higher porosity.

Overall, the findings demonstrate that, despite its reduced mechanical parameters, perlite concrete can be considered a viable substrate for bonded anchors when appropriate design procedures and strict installation control are applied. The interplay between the material properties, anchorage depth, and installation quality is critical and must be carefully addressed in engineering practice. This study contributes to a better understanding of anchorage behavior in lightweight concretes and provides a basis for further development of design recommendations tailored to alternative construction materials such as perlite concrete.

5. Conclusions

Based on the conducted experimental investigations, it can be concluded that chemically bonded anchors ($\varphi$16 mm threaded rods) embedded in perlite concrete blocks are capable of transferring significant pull-out loads. The results indicate that such anchorage systems may be considered for structural applications, particularly in lightweight masonry systems.

The analysis showed that an embedment depth of 400 mm provides a sufficient anchorage capacity for the specific top-down vertical anchorage configuration considered in this study. Furthermore, the findings suggest that bonded anchors may serve as a viable alternative to traditional solutions based on reinforced concrete ring beams, provided that appropriate design verification is performed.

However, several limitations of the study should be acknowledged. The experiments were conducted under short-term laboratory conditions and for a single anchor diameter and embedment depth. The influence of long-term effects, such as creep, environmental exposure, and the durability of the resin–concrete interface, was not examined. In addition, the observed variability in results—although acceptable—reflects the heterogeneous structure of perlite concrete and may affect reliability in practical applications.

Further research is therefore required to investigate long-term performance, alternative anchorage configurations, and the influence of material variability on bond behavior in perlite concrete.