**Romanesque Historical Monuments Reconstruction by Using Original Materials and Recycling of Those that Have Lost Their Historical Value** †

**Anamaria Boca 1,\*, Tudor Panfil Toader <sup>2</sup> and Călin Mircea <sup>1</sup>**


Published: 10 December 2020

**Abstract:** The aim of this paper is to present the way of reconstruction of historical monuments of Romanesque architecture by reusing and highlighting the original component materials, related to the subassemblies of the construction, respectively the recycling of those components that have lost their historical value. The Romanesque buildings are part of Romanian national cultural heritage and have been through controversial historical periods, and therefore have undergone important modifications or structural losses. The reconstruction or rehabilitation of the Romanesque historical buildings is a way of sustainable development by adapting the buildings to the new conditions of use.

**Keywords:** Romanesque architecture; cultural heritage; reconstruction; rehabilitation

## **1. Introduction**

Romanesque architectural style developed in the first period of Middle Ages and spread over the whole Catholic Europe between 9th and 13th centuries. Romanesque style presents significant regional variations because of the availability of materials, technologies and the aesthetic tastes. It became the first international ecclesiastical architectural style; therefore, the greatest number of surviving Romanesque buildings are churches. Romanesque style was introduced to Transylvania from Hungary in the 12th and 13th century. The influence on architectural style was initially from Hungary and Germany, and later from France and Italy. Romanian Romanesque churches are generally small and modest churches, compared to the cathedrals from Europe or the following Gothic churches. The construction material was brick or stone depending on the local availability. In Italy, Poland, part of Germany and Netherlands, brick was used on a larger scale. In other areas, the churches were made of stone in small, irregular pieces bedded in thick mortar. In Transylvania, the builders of the period used preponderant quarry and river stones because of the local availability and numerous stone quarries and Roman ruins. Brick was used in the northeast of Transylvania where stone was not available. While a small number remain substantially intact, many churches were sympathetically restored, being extended and altered in different styles. We will take into consideration some examples of rehabilitation of these historical monuments.

If today's buildings are built with a clear differentiation between their architecture and structure, when talking about Romanesque historical monuments the relationship between the shape and structure of a building appears in a mutual conditioning. The massive load-bearing structure of the Romanesque building gives the stability of these building over decades. Therefore, this study will

include complex aspects related to the reconstruction and rehabilitation of the load-bearing structures of the Romanesque buildings, at the interface between history, art, architecture and structural engineering. Preliminary study on the structural problems of Romanesque churches will be carried out in order to define adequate techniques of interventions following the preservation and restoration principles. The use of traditional, modern and innovative materials and techniques is also discussed.

## **2. Structural Diagnosis**

The reconstruction and rehabilitation of the load-bearing structures of Romanesque buildings require the knowledge of conception, of technical details or the materials and traditional technologies used. A deep understanding of the construction is mandatory when choosing the method of intervention based on the minimal intervention concept of historical monuments.

This research is carried out in two stages:


The structural diagnosis is based on the knowledge of each component of the load-bearing structure. The architectural components with impact on the structural subassemblies are also important, such as: door or window frames, details of floors or installations, the water-canal networks, the characteristics of the foundation land and its mechanical properties, etc. This phase requires collaboration of specialists in the field of architecture, engineering, topography, archeology or restoration. The common concern is to identify the characteristics of the Romanesque structures and to propose the optimal rehabilitation solutions.

The load bearing structure of a Romanesque church is composed by: thick walls made out of brick or stone, foundations, columns/piers, floors, barrel or groin vaults and roof trusses. These structural subassemblies are connected to each other in subunits with different spatial rigidities, which work together and give the mechanical behavior of the entire structure. The empirical-intuitive conception of the load-bearing structures, the quality of the interventions carried out during the utilization period, the extensions or modifications made during the exploitation, the quality of the used materials, the depth of the foundations and the geological conditions have an important influence on the mechanical behavior of Romanesque structures.

The most common deficiencies of the Romanesque structures are: vulnerability to horizontal loads, low anti-seismic conformation, lack of effective connections among the structural elements, presence of horizontal structures (floors and roofs) with poor in-plane stiffness; lack of longitudinal bracing subunit of the roof structure, lack of rigidity of the infrastructure compared to the need to embed the superstructure; stiffness asymmetries and irregular morphology, due to continuous modifications, stratifications and extensions occurring during the time; and low capacity for stretching and shear efforts (Figure 1).

The presence of curved elements along with the massiveness of the walls and pillars/pilasters generates gravitational actions, thrusts that produce significant bending stresses on the main load-bearing system (walls, pilasters, columns). The main structural degradations of the Romanesque ensembles are mostly due to these thrusts, being followed by the other causes like landslides, earthquakes and fires. A part of the Romanesque churches from Transylvania have one or two towers attached on the west end of the church. They produce distortions of seismic response that can lead to the detachment of the church tower and then to the danger of a collision of the two oscillating subunits: tower-nave. Examples can be found at the Evangelical churches from Roades and Rotbav where the towers collapsed in 2016. At Rotbav, the collapse of the tower led to the collapse of a part of the nave walls. Cracking or separation in the rigid bodies "tower-ship-altar" may also appear due to the differentiated settlements of the foundation land. It was found that the consolidation of the joint areas of these bodies with different rigidities and different pressures under the foundations would lead to those degradations in their vicinity.

**Figure 1.** Failure mechanisms in Romanesque churches: (**a**) overturning of the facade; (**b**) shear mechanisms in the facade; (**c**) overturning of the apse; (**d**) transversal vibration of the nave; (**e**) vaults of the nave; (**f**) vaults in the presbytery or the apse; (**g**) triumphal arch; (**h**) shear failure of the walls; (**i**) bell tower.

## **3. Structural Consolidation**

The structural consolidation on the Romanesque churches must be made with the main purpose of safeguarding the original structure through the use of compatible materials and traditional techniques that can be supplemented with scientifically grounded modern techniques. If the stability of a building is affected or there is the need of a change in destination's building, the structural modifications will be implemented through reversible solutions with the condition that the new elements have the same reliability with the original ones and they must be distinguishable.

### *3.1. Roof Structure*

Preserved in a relatively small number, Romanesque roof structures are characterized by a structural concept limited to the construction of trusses, without any longitudinal bracing systems. On the longitudinal direction, the trusses are stabilized through the roofing support system. The transmission of the loads carried by the trusses to the supporting subunits is made through the simple wall-plates placed over the longitudinal walls [1].

Both Romanesque roof structures are in a good state of stability but there is need of rehabilitation in order to maintain and increase their durability. There are some subsequent interventions on these roof structures. For example, in the case of Vurpar church, temporary consolidations of the marginal north-western area have been made. The biological degradations have made the rafter tie-beam (nodurile caprior-coarda) nonfunctional; therefore, the decision was to place a metal band for carrying the load resulted from the tie-beam between the rafter and the tie-beam. Also, several reinforced concrete rings were placed below the wall plates (centura de beton armat sub cosoroaba). At Toarcla, the observed intervention method on the roof structure is the integrations of bracings made out of a single piece of wood in the rafters (Figure 2).

**Figure 2.** Romanesque roof structures: (**a**) roof structure Lutheran church in Vurpar; (**b**) roof structure Lutheran church in Toarcla.

#### *3.2. Arches and Vaulting System*

The early Romanesque builders developed the science of vaulting when they wanted to replace the wooden ceilings with vaulting structures with better resistance for fire danger. The most common vaults in Romanesque churches are the barrel (semicircular) vaults and the groin vaults-intersection of two barrel vaults (Figure 3). In the later Romanesque period, the ribbed and pointed vaults were also introduced. Vaults constructed of numerous blocks of material pressing against one another exert not only the accumulated downward weight of the material and of any superimposed load, but also a side thrust or tendency to spread. To avoid collapse, adequate resistance against this thrust must thus be concentrated at the haunches (lower portions) of the vault. The resistance may take the form of thickened walls at the haunches; of buttress placed at points of concentrated thrust as in Romanesque and Gothic architecture; or of vaults so placed that their thrusts oppose and counteract. This necessity has controlled the evolution of masonry vaulting and its use in buildings.

**Figure 3.** The thrusts of a barrel vault and a groin vault.

The structural deficiencies of Romanesque vaults occur mostly as a result of: subsequent faulty interventions, lateral buckling/displacements of the vault's supports, the lack of horizontal connecting elements on the slabs level, the subsidence of the foundations and the decays of the masonry caused by moisture [2].

The structural consolidation of the vaults and the supporting elements system is mostly done through interventions that are meant to enhance the load-bearing capacity of the structure. This can be done through the increase of the cross-section of the deteriorated elements (encasement). Additional elements compatible with the original elements can be also introduced with the same purpose. Found in Romanesque churches are metal tension bars meant to take over the abutment loads from arched and vaulted structures. Metal tension bars/tie rods (tiranti) placed on the springing lines of the triumphal arches that separate the altar from the nave can be found in many churches of Transylvania (Herina, Avrig). In other cases, reinforced concrete ring-beams were placed on the slab's level. Indirect consolidations with additional structures may be also carried out with the purpose to discharge the weak original load-bearing structure of a part of the vertical loads.

The cross ribbed vault of brick at the Calvinist church in Sic was in bad condition due to the lack of a roof structure for the choir for a long period, which led to maceration of bricks on a considerable depth and cracks in the walls. Therefore, the intervention taken consisted of the replacement of macerated bricks, bonding-wedging-grouting and protective plaster on the backs of arches reinforced with geogrid [3].

#### *3.3. System Walls-Piers-Columns*

Historical load-bearing support structures such as load-bearing walls, columns and piers have a deficiency in taking over the efforts of stretching and shearing in the console. The walls of Romanesque churches are one of the most important components for the load-bearing structure. The thickness of the walls allows to carry the weight of the vaults. Otherwise, the wall could become unstable if the loads exceed the strength of the masonry, causing structural collapse.

Stone masonry walls have considerable vulnerability to horizontal seismic action, due to their weak mechanical properties and extensive irregularities. In brick masonry, the problem of long-term sustained loads (creep) acting on massive structures (towers, curtain walls, heavy pillars) may induce sudden unexpected collapse [4].

The alternation of columns and piers together with the walls are a very important structural feature of the Romanesque architecture, but sometimes they are used as decoration as well.

At the Evangelical church of Herina, after the 1886 earthquake, the walls were presenting multiple cracks. The adopted solution for consolidations was the insertion of reinforced concrete beams at the upper level of the walls under the roof line and grouting of cracks with lime paste. The same solution was adopted at the church of Strei. In the case of the Calvinist church in Sic, where the degradations in walls occurred due to unprofessional subsequent interventions and improper treatment of fissures (with cement mortar), the cracked walls needed rehabilitation on 80% of their surfaces. The adopted solutions have focused on reassuring continuity by bonding-wedging-grouting plus reinforcing with stainless helical bars [3].

### *3.4. Foundations*

The subassemblies of foundations were made according to an empirical-intuitive conception and have the role of transmitting to the foundation ground the loads of the load-bearing structures. They were made mainly of stone or brick masonry, with lime mortar (up to M10) or clay mortars, with no protection against underground agents. The geometry of the foundations follows the plan design of the building and its construction was influenced by the nature of the foundation land or by the geographical position of the building. In Romanesque buildings, rigid surface foundations and continuous or isolated foundations were made. They were connected to each other by masonry arches through bonding-wedging technique. A major technological aspect that leads to the degradation of a Romanesque edifice is the deficient cooperation of the foundations made in different epochs that lead to unequal settlements. Foundations are exposed to aggressive soil moisture conditions. The problems that arise are related to the depth of foundation, which is often insufficient in relation to the depth of frost of the site and the depth of wetting of the clay with high contractions and swellings and in relation to the foundation ground. Thus, the foundations of the Romanesque structures do not ensure, most of the time, a rigid level of embedding in Romania.

Degradations in foundations of the church in Sic were due to the soil conditions (uneven settling), unprofessional previous interventions or insufficient foundations depth. For the consolidation, the underpinning and micro-piles system was used [3].

Interventions like sub base grouting were carried out at many of the Romanesque churches due to the degradations over time of the material or the insufficient depth. At Strei, the durations of this intervention had caused cracks in the masonry and movement of the vault with dislocation in the ribs (Table 1).

#### **4. Case Studies**

Case Studies—Three Romanesque Churches from Transylvania and Interventions Applied over Time.


**Table 1.** Summary of applied interventions.

#### **5. Conclusions**

The restoration of historical monuments has become a very important issue in the preservation of cities and communities. Well-preserved and maintained historical buildings improve the quality of community life with which they coexist. The Historic Monuments List drafted by the Ministry of Culture and National Heritage of Romania in 2015 lists 110 monuments built in the 12th–13th centuries [7,8]. Most of these monuments were built initially in Romanesque style but they have undergone additions or transformations in the following centuries; therefore, it is challenging to find the monuments that have kept their originality. The reconstruction and rehabilitation of the evangelical church of Herina was a necessity after the 1886 earthquake. The church of Sic was in an advanced state of degradation due to unprofessional subsequent interventions [9]. Even if the solutions adopted are questionable in correlation with today's principles, nowadays these churches stand as some of the most representative monuments for Romanesque architecture. Following these examples, we would like to raise awareness about the need of conservation or reutilization of the abandoned medieval churches. For example, in Cluj County, the actual state of the church of Nima (uncovered) affects the valuable mural paintings that can be seen on the walls [10]. Even if the monument was cleared out of the vegetation from inside and a roof over the altar was realized in 2006, a complex restoration of the monument was not possible yet due to lack of financial resources.

We should also look into the importance of the reconstruction of the several fortresses built between the 13th–15th centuries. A major reconstruction project for the Bologa Fortress was begun in 2016. The restorations of the stone churches of Santamarie Orlea and Strei have had an important impact on increasing tourism in this area. An approach on the research of small village churches in Romania may reveal the importance of including these almost abandoned churches on a so-called Romanesque Route (following the example of Germany, Portugal, Spain, France) and later on, their insertion on the TRANSROMANICA-The Romanesque Route of European Heritage, along with the St. Michael's Cathedral from Alba Iulia.

The reconstruction and the reutilization, along with some modern intervention techniques, are raising divisive opinions but we must take into consideration that reconstruction is motivated by an interest in value preservation and, in some cases, is imposed by functional needs.

**Conflicts of Interest:** The authors declare no conflict of interest.

## **References**


**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## *Proceedings* **Experimental Study on Hollow Blocks with Wastes** †

## **Ligia Hanuseac \*, Marinela Barbuta \*, Liliana Bejan, Raluca Rosu and Alexandru Timu**

Faculty of Civil Engineering and Building Services, Department of Concrete, Materials, Technology and Management, "Gheorghe Asachi" Technical University of Iasi, 700050 Iasi, Romania; lilbejan@yahoo.com (L.B.); ana-raluca.rosu@academic.tuiasi.ro (R.R.); alexandru.timu@gmail.com (A.T.)


Published: 7 February 2021

**Abstract:** The article presents an experimental study on concrete blocks prepared by using waste types such as fly ash as a cement substitution, waste of plastic bottles and wood waste as replacements for sand and polyester fibers waste as a dispersed reinforcement. The mechanical characteristics of concrete with fly ash and polyester fibers were determined. The influence of the type and dosage of waste on the mechanical strength is discussed. The concretes with fly ash and different dosages of waste were used for manufacturing hollow blocks that were tested in compression, and the behavior under load was analyzed. Failure in compression of hollow blocks was gradual and ductile.

**Keywords:** eco concrete; fly ash; waste

## **1. Introduction**

Concrete is one of the most used materials in construction and engineers have worked to ensure concrete responds to new requirements related to environment protection [1,2]. Production of cement, an important component of concrete, is a cause of CO2 emission (7%), and the huge quantities of natural aggregates used in the concrete composition result in important changes in the natural environment. Non-conventional concretes, with different types of materials in the mix, have emerged just for partially eliminating the above ecological problems. The cement is replaced partially or totally by different materials, such as fly ash, silica fume, slag, rice husk and banana leaves ash [3–8]. Aggregates have been replaced by steel slag, chopped plastic bottles, polystyrene granules, recycled aggregates, chopped sunflower, etc. [9–13]. Fibers of diverse types have also been added in the concrete mix: steel, polyester, hemp, etc. [14–18]. The main objective of the article was to analyze the behavior of hollow blocks manufactured with concrete prepared with a cement substitution with fly ash and waste types such as chopped plastic bottles and wood waste as replacements for sort 0–4 mm and polyester fibers as a dispersed reinforcement. The hollow blocks manufactured with non-conventional concretes will be used to make a non-load-bearing masonry wall. In the next stage, this masonry wall will be built and tested.

## **2. Experimental Program**

## *2.1. Materials*

In the research, a control mix of concrete (C0) was used for preparing hollow blocks which had the following components: cement type CEM I 42.5 R [19] in a dosage of 360 kg/m3; and river aggregates in three sorts, namely 0–4 mm, 4–8 mm and 8–16 mm, which were in the following dosages: 803 kg/m<sup>3</sup> of sand, 384 kg/m3 of sort 4–8 mm and 559 kg/m3 of sort 8–16 mm. We used water in a dosage of 172 L/m3, and 10% of the cement dosage was replaced with fly ash, from CET Holboca Iasi. Fly ash was used before in other experimental tests and presented by the authors in [15,20]. Waste types PET bottles and wood waste were chopped into sorts of 0–4 mm and used as replacements for 20% by volume of the dosage of aggregate sort 0–4 mm in the case of PET and 40% by volume of the dosage of the same aggregate sort in the case of wood. The chopped PET and wood waste had sizes between 0 and 4 mm. Waste from polyester fibers was used, which was cut into 30 mm-long filaments and dispersed as a replacement reinforcement in the concrete, in a dosage of 0.25% of the concrete weight. In the mixture, we used a superplasticizer (Master Glenium SKY 617 from BASF) in a dosage of 1% of the cement volume.

## *2.2. Samples*

The control mix of concrete (noted C0) and the mixes with fly ash and chopped PET (noted C1), fly ash and wood waste (noted C2) and fly ash and polyester fibers (noted C3) were prepared by mixing all the components. The wood waste was moistened before being added to the mix. The samples were poured: cubes of 150 mm in size for determining the compressive strength fc, and prisms of 100 × 100 × 500 mm in dimension for determining the flexural strength fti and split tensile strength ftd [21–23]. The hollow blocks, one of which is shown in Figure 1, labeled HBF1–HBF3 were manufactured only for the concretes with waste (concretes C1–C3). After 24 h, the specimens were removed from the formwork and kept in the laboratory at a temperature of 20 ◦C until testing.

(**a**) Digital drawing of the hollow block (**b**) Hollow block

**Figure 1.** The experimental hollow blocks.

## **3. Testing Result and Discussion**

## *3.1. Mechanical Strength of Concrete Mixes*

The control mix and the concretes with waste were tested at 28 days for mechanical strength. The values are given in Table 1.


**Table 1.** Mechanical characteristics of experimental concretes.

## 3.1.1. Compressive Strength

The value of fc for concretes with waste was influenced by the type of waste. All values of fc were lower than that of the control mix. The replacement of sort 0–4 mm with chopped plastic in a dosage of 20% had reduced fc, with 24.5%, in comparison with the control mix. In the case of the replacement of sand with 40% sawdust, fc was reduced, with 52%, and mix C3 (only with fly ash and polyester fibers) presented a decrease in fc of only 11% in comparison with the control mix. For fc, the highest value was obtained for concrete C3.

#### 3.1.2. Flexural Strength

The value of flexural strength was influenced by the type and dosage of waste. When the aggregates of sort 0–4 mm were replaced, a decrease in flexural strength was obtained. The addition of polyester fibers increased the flexural strength by 10% in comparison with the control mix. For fti, the highest value was obtained for concrete C3.

#### 3.1.3. Split Tensile Strength

The value of split tensile strength was influenced by the type and dosage of waste. The waste type chopped PET as a replacement for sort 0–4 mm in a dosage of 20% resulted in an increase in the strength. The waste type wood waste as a replacement for sort 0–4 mm in a dosage of 40% resulted in a decrease in the strength. The dispersed polyester fibers increased the split tensile strength in comparison with the control mix by 12.8%.

The mechanical strengths of concretes with different waste types as replacements for aggregates were lower than those of the control mix. In the case of concrete with fly ash and polyester fibers, the compressive strength was lower than that of the control mix, but the flexural strength and split tensile strength were highest.

## *3.2. Hollow Blocks Experimental Test*

The blocks of concrete were subjected to axial compression. The compression force was applied along with the height of the block. The maximum value of the compression load was divided by the gross contact area of the block, including holes, noted fcb1, and by the net area, noted fcb2.

The indirect tension stress, according to [24], was computed with the following relation (1):

$$\mathbf{f\_{tb}} = 2\mathbf{P}/\pi \mathbf{L} \mathbf{h}\_{\prime} \tag{1}$$

where P is the value of the maximum compression load, h is the height of the block (140 mm) and L is the split length (82 mm) if the holes are neglected, or 240 mm if the total length is considered.

The results of the experimental tests are given in Table 2.


**Table 2.** Experimental results of the compression test on the hollow blocks.

The compressive strengths fcb of the blocks had different values, depending on the type of concrete. According to [24], the minimum compressive strength must be 7 N/mm<sup>2</sup> and the blocks with fly ash and PET waste (HBF1) and those with fly ash and polyester fibers (HBF3) satisfy this condition for their use in masonry also in seismic areas, as a self-weight masonry for realizing partitioning walls [24]. The block HBF2 can be used for self-weight masonry.

The split tensile strength ftb of blocks also had good values which are in concordance with values given by other authors [11].

The mechanical characteristics of hollow blocks recommend them to be used in construction for realizing masonry walls.

## *3.3. Failure Mode*

During the tests in compression, the blocks failed gradually, and vertical cracks developed throughout the entire depth, especially near holes. The blocks had a ductile failure until the complete damage, as shown in Figure 2.

(**a**) HBF1 (**b**) HBF2

(**c**) HBF3 **Figure 2.** Failure of hollow blocks HBF1, HBF2 and HBF3.

## **4. Patents**

For manufacturing the blocks, the following types of waste were used for preparing concrete: fly ash that replaced 10% of cement in all mixes with waste, chopped plastic bottles (PET) that replaced aggregate sort 0–4 mm in a dosage of 20% by weight, waste of wood that replaced aggregate sort 0–4 mm in a dosage of 40% by weight and waste of polyester fibers that was added in the mix with fly ash.

The compressive strength, tensile strength and split tensile of the concretes with waste were determined. The type and dosage of waste influenced the mechanical properties. For all types of concrete, the value of compressive strength was lower than that of the control mix without waste. In the case of concrete with polyester fibers, the flexural strength and split tensile strength were higher than those of all others mixes. For concretes with saw dust, the lowest values of all mechanical strengths were obtained.

When tested in compression, the hollow blocks presented values of compressive strength and tensile strength comparable with other types of blocks, which means we can recommend their use for realizing walls.

**Author Contributions:** Conceptualization, L.H.G. and M.B.; methodology, M.B.; formal analysis and investigations, L.H.G., L.B., R.R., A.T., resources L.H.G., M.B. and A.T.; data curation, L.H.G. and M.B.; writing—original preparation, L.H.G.; writing-review and ending, L.H.G., M.B., L.B., R.R. and A.T.; supervision, M.B.; project administration, M.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research recived no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.

## **References**


**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

© 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## *Proceedings* **Fracture Energy of Engineered Cementitious Composites** †

## **Anamaria Cătălina Mircea 1,\* and Tudor Panfil Toader 1,2**


Published: 10 December 2020

**Abstract:** The aim of this paper is to present preliminary results regarding Engineered Cementitious Composites (ECC) and their behavior when experimentally assessing their fracture energy, by measuring the flexural tensile strength (limit of proportionality, residual). As a characteristic of a ductile material, fracture energy is an important parameter when assessing ECC post-cracking residual stresses. With 2% fibers addition in the mixtures, the crack width can be controlled and the material's ability to bear a tensile strain-hardening capacity has been assessed. Ninety days flexural tensile strength tests were performed in order to obtain preliminary results on ECC prismatic specimens.

**Keywords:** fracture energy; engineered cementitious composites; limit of proportionality; residual strength; fly ash

## **1. Introduction**

Using 2% of fibers well distributed, Engineered Cementitious Composites (ECC), with the ability to undergo the tensile strain-hardening capacity with over 300 times that of normal concrete, allow cracks to form and control the crack width [1]. Those materials with high ductility and damage tolerance under tensile and shear loadings [1,2] have a distinguished design compared to Fiber Reinforced Concrete (FRC) based on micromechanics of first crack initiation, fiber bridging and steady-state flat-crack propagation mode [3].

Kanda and Li, in 1998 [4], established two conditions of Engineered Cementitious Composites in order to achieve the strain-hardening behavior and to induce the multiple cracking behavior. The first condition is defined by the first crack stress as a strength criterion and the second one is the steady-state cracking defined as energy criterion. Responsible for initiating the micro cracks, the strength criterion ensures the tensile load that allows the micro cracks to be less than the maximum capacity of the fiber fringing. The energy criterion that prescribes switching the Griffith-type crack is the second condition. As the crack length extends, the opening of the crack increases as for FRC to steady-state flat-crack propagation mode in the case of tension-softening behavior [3].

The fracture energy is the amount of energy that is necessary to generate a crack of one unit of area. For concrete, the fracture energy can be determined experimentally by applying flexural tensile strength loading on a notched concrete specimen [5] and can be calculated as the area under the load-deflection curve divided with the net cross section of the specimen situated above the notch. It is shown that the method of determining the fracture energy by means of stable three-point bend tests on notched beams seems suitable for concrete and similar materials [6]. In order to generate the area under the load-deflection curve, the first step is to experimentally evaluate the flexural tensile strength of the concrete by means of Limit of Proportionality (LOP) and residual strength.

The importance in studying fracture energy is to determine the formation of multiple cracks in the material in order to achieve high tensile ductility similar to ductile materials.

According to Mehta and Monteiro [5], the fracture energy increases when the aggregate size increases, among other factors. Delivering dimensional stability and wear resistance, aggregates can also affect negatively the tensile performances due to the increasing of the tortuosity of the fracture path, which leads to a tough Engineered Cementitious Composites matrix [3]. The propagation of the cracks after they occur leads dramatically to the damage of the steady-state flat-crack propagation process in Engineered Cementitious Composites and the sacrifice of the multiple-cracking behavior [7]. Therefore, in order to generate this effect, fine aggregates should be used in the mix-design, such as micro-silica sand with maximum grain size of 250 μm and a mean size of 110 μm instead of coarse aggregates.

To reduce the environmental impact [8–11], fly ash considered pozzolans and by-product materials and are added to concrete as a cement replacement for economic reasons and to improve the workability of the material as well as prevent their waste disposal. Studies have shown that industrial by-products can be successfully used as partially replacing OPC in concrete, but also as raw material by their chemical activation in the production of new, innovative materials [12,13].

To obtain the residual strength curve, the relationship between the fracture strength and the crack length are needed. The residual strength depends on the crack size. For the structures where the cracks grow slow like monolithic and single load paths, the residual strength capability is simple. For build-up structures, multiple load paths and fail-safe structures where the crack grow slow, due to the geometric construction of the components, the residual strength analysis is complicated. The residual strength for a given structure is a function of the service time.

The main objective of the current research is to explore the development of ECC materials and analyze their mechanical properties, as well as to explore initial experimental methods and testing procedures in order to assess the fracture energy (by means of flexural tensile strength) and the potential approach regarding concrete delay of cracks growing when the material present defects under certain loads, in accordance with fracture mechanics.

#### **2. Experimental Program**

Engineered Cementitious Composite mixtures presented in this study were developed based on available literature [14] and starting from a rigorous selection of materials that will be used. The initiation of fracture at pre-existing cracks is affected by the residual stresses in structural materials and can modify the intensity of the crack tip stress field [15]. Depending on the crack opening or crack closure, residual stress can promote or inhibit the initiation of fracture. Residual stress affects the fracture initiation behavior of the materials and their resistance to subsequent crack growth [16]. These parameters can be affected by the raw materials and their specific mix-design ratio.

#### *2.1. Raw Materials and Mix-Design Ratios*

In order to produce the ECC samples, Portland cement CEM I 42.5R, Class F fly ash from a local source, silica-sand (maximum size of 0.3 mm) and river sand (maximum size 4 mm), polyvinyl alcohol fibers (PVA), limestone slurry and a water reducing superplasticizer admixture were used. The characteristics of the PVA fibers used have the length of 8 mm, are chemical resistant, have UV-stability, are hydrophilic and previous studies have shown that they meet the requirements of strain-hardening performance of the Engineered Cementitious Composites [14]. The limestone slurry paste was obtained from a local source and, since water is an important parameter that influences the fresh and hardened state of concrete, the water content was measured (19.7%).

Two types of sand were used in the production of the samples in order to study the influence of this material. One mixture was produced using silica-sand (T1E) and one mixture was produced using normal river sand (T2E).

The mix-design ratio was developed based on ECC developed mixtures used in the literature (Table 1) [14] and are presented in Table 2.


**Table 1.** Engineered Cementitious Composites (ECC) base mix-design [14].

T2E Normal Sand 1.00 1.20 0.76 0.56 0.05 0.22 0.02 values reported to cement quantity, <sup>2</sup> CEM—cement; <sup>3</sup> FA—fly-ash, <sup>4</sup> LS—limestone slurry.

#### *2.2. Testing Methods*

The mechanical properties (flexural and compressive strength) of the ECC samples were determined using 40 × 40 × 160 mm prismatic specimens for each of the proposed mixtures; the mean value of the results was considered relevant for the data interpretation. The testing method was in accordance to EN 196-1 [17] and the samples were tested at 28 and 90 days, under laboratory conditions ((20 ± 2) ◦C and (50 ± 3) % RH).

In order to investigate the 90 days flexural tensile strength (LOP and residual strength) of the mixtures, 150 x 150 x 600 mm samples were produced in the same conditions. The tensile behavior of the samples is evaluated in terms of the residual tensile strength values when subjected to bending, determined by the load-displacement curve of the crack edge, obtained by applying a point load centered on a notched prism. The loading scheme for the test is presented in Figure 1.

**Figure 1.** ECC flexural tensile strength loading scheme.

Tests to determine the stress displacement (CMOD) diagram, and to determine the values obtained for the displacements of 0.5, 1.5, 2.5 and 3.5 mm, were carried out according to SR EN 14,651 + A1 [18]. The Limit of Proportionality (LOP) was determined according to Equation (1) and the residual strength was determined according to Equation (2) [17].

$$\text{ft}^{\text{t}}\_{\text{ct,L}} = \text{(3F}\_{\text{L}}\text{l)} / \text{(2b}\text{h}\_{\text{s}P}\text{ }^{\text{2}}) \text{ (N/mm}^{\text{2}}\text{)}\tag{1}$$

where, ff ct,L is the Limit of Proportionality; FL is the corresponding stress; l is the length between the rollers; b is the width of the sample and hsp is the distance between the lower part of the notch and the upper part of the sample.

$$\text{sf}\_{\text{Rj}} = (\text{3F}\_{\text{j}}\text{l}) / (2\text{bh}\_{\text{sp}}\text{ }^2\text{) (N/mm}^2\text{)}\tag{2}$$

where, fRj is the residual resistance, which corresponds to CMODj (j = 1, 2, 3, 4); Fj is the corresponding stress, which corresponds to CMODj (j = 1, 2, 3, 4); l is the length between the rollers; b is the width of the sample and hsp is the distance between the lower part of the notch and the upper part of the sample. In Figure 2, the specimens equipped, ready to be tested, are presented.

**Figure 2.** Flexural tensile strength: (**a**) loading scheme of the samples; (**b**) transducer used for the displacement measurement.

#### **3. Results and Discussions**

#### *3.1. Mechanical Properties*

The flexural and compressive strength test results of Engineered Cementitious Composites mixtures with different types of sand are presented in Figures 3 and 4.

**Figure 3.** Flexural strength of ECC mixtures at 90 days.

**Figure 4.** Compressive strength of ECC mixtures at 28 and 90 days.

## *3.2. Flexural Tensile Strength (LOP and Residual Strength)*

Three point bending tests (3PB) performed at 90 days on the 150 × 150 × 600 mm samples and experimental results and curves concerning the nominal stress and Crack Mouth Opening Displacement (CMOD) are depicted in Figures 5 and 6. Results regarding the LOP are presented in Table 3 and results regarding the residual strength are presented in Table 4. The mean of the individual specimens' curves is also depicted.

Based on the graph curves above, the residual strengths fR,j (evaluated at four different CMOD values), and the flexural tensile strength (LOP) ff ct,L were presented for both of the ECC mixtures proposed. It can be noticed that the post-cracking flexural behavior of samples is generally exhibiting the desirable high residual strength and toughness performance. T2E samples show a much higher post-cracking flexural behavior than T1E samples. A constant CMOD loading rate was maintained until the end of the test. The end point of the test is at 3.5 mm and that is the point where the fracture energy should be calculated.

To find the fracture energy of the specimens, the areas under the curves need to be calculated based on mathematical formulas that are dependent on the geometrical characteristics of the samples [19].

**Figure 5.** Flexural tensile strength (limit of proportionality, residual)—T1E (90 days).

**Figure 6.** Flexural tensile strength (LOP, residual)—T2E (90 days).


**Table 3.** ECC mixtures Limit of Proportionality.


## **Table 4.** ECC residual strength.

#### **4. Conclusions**

In the present paper, an experimental study was presented aiming at evaluating the cracking behavior in terms of flexural tensile strength of ECC samples produces using local raw materials.

Comparing the test results for compressive and flexural strength with the results obtained in the literature, it was noticed that the mixtures from the literature have a higher compressive strength: 53 MPa at 28 days compared to 45 MPa for the mixtures from this paper, and for flexural strength, 24 MPa at 28 days compared to 18 MPa for the mixtures from this paper.

Post-cracking flexural behavior of the tested samples was generally exhibiting the desirable high residual strength and toughness performance expected for the mix-design of ECC mixtures.

As the fracture energy is tightly bound with the obtained results regarding the flexural tensile strength (LOP, residual) and it can be obtained based on mathematical formulas, further studies will be carried on regarding the output of the presented results in terms of fracture energy and evaluating the parameters that affect this characteristic.

**Author Contributions:** Conceptualization, A.C.M. and T.P.T.; methodology, A.C.M. and T.P.T.; validation, A.C.M.; formal analysis, A.C.M. and T.P.T.; investigation, A.C.M..; data curation, A.C.M.; writing—original draft preparation, A.C.M.; writing—review and editing, A.C.M. and T.P.T.; visualization, A.C.M.; supervision, A.C.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.

## **References**


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