Evaluation of bond strength between sand concrete as new repair material and ordinary concrete substrate (The surface roughness effect)

Construction and Building Materials 157 (2017) 1133–1144

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Evaluation of bond strength between sand concrete as new repair material and ordinary concrete substrate (The surface roughness effect) Karima Gadri ⇑, Abdelhamid Guettala Civil Engineering Research Laboratory, University of Biskra, 07000 Biskra, Algeria

h i g h l i g h t s
All composite specimens subjected to flexural loadings display a monolithic failure at the centre, without debonding at the interface.
Sand concrete gives remarkable bond strength and good to excellent bond quality with ordinary concrete substrate.
The sand concrete appears to be a very promising material for repair and rehabilitation of concrete structures.

a r t i c l e

i n f o

Article history: Received 2 May 2017 Received in revised form 23 September 2017 Accepted 26 September 2017

Keywords: Sand concrete Flexural tensile Splitting tensile Bond strength Repair concrete Failure Roughness

a b s t r a c t In the present work, a great deal of importance is attached to sand concrete as a new repair material. An investigation was conducted to evaluate the bond strength and the type of failure in composite concrete bi-layers. The test specimens under study were made of an ordinary concrete substrate and sand concrete (repair material). The substrate was prepared from ordinary concrete of two different classes of strength, i.e. the first one contains a superplasticizer (OCS) and the other has no superplasticizer (OC). To this purpose, four different surfaces used as substrates, namely low roughness (LRG), high roughness (HRG), drilled holes (DH), and combination of high roughness with drilled holes (HRGDH) surfaces were prepared. For the repair materials, two mixes of sand concrete were selected, namely sand concrete with 100% of limestone filler (SCL), on one hand, and sand concrete with 50% of limestone filler and 50% of glass powder (SCG), on the other. Two adhesion tests (flexure test, and splitting test) were performed in order to evaluate the response of the composite test specimens under loading, by determining the mode of failure produced after the test. The results of splitting strength obtained showed that the sand concrete gives remarkable bond strength of 2.54 MPa for cylindrical specimens and 317 MPa for cubic specimens. These values present a good to excellent bond quality, depending on the strength level of ordinary concrete, reflecting a good adhesion to the substrate, which manifested a positive monolithic response. These performances allowed us to consider the sand concrete as a good cementitious repair material. Ó 2017 Published by Elsevier Ltd.

1. Introduction All reinforced concrete structures have a limited service life; many of them require maintenance, repair interventions or rehabilitation. Among the different repairs that can be carried out on damaged reinforced concrete structures, one can cite the nonstructural repair where the thickness of the repair material is less than 100 mm [1–3]. Consequently, the association of a young concrete with an older concrete support produces a composite with two materials having different properties. The composition of the new concrete layer is different from that of the concrete substrate ⇑ Corresponding author. E-mail address: [email protected] (K. Gadri). https://doi.org/10.1016/j.conbuildmat.2017.09.183 0950-0618/Ó 2017 Published by Elsevier Ltd.

(mechanical or physical characteristics). In addition, the dimensional changes cause internal stresses within the repair material and inside the substrate. Consequently, there is a problem of compatibility that is never completely guaranteed. Compatibility is defined as an association between the properties of the two materials in contact [4]. According to Emmons et al. [5], compatibility is seen as the equilibrium existing between the physical, chemical and electrochemical properties of a repair material and the existing substrate. The adaptability of the material is an important parameter for the durability of the repair, although the balance between all the above mentioned properties is never fully guaranteed. It is required to make the suitable choice of the repair material in order to have a durable repair.

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Currently, a great deal of research is devoted to the production of good quality repair materials [6,7], and to develop new assessment methods [8,9]. There are many materials for repair and rehabilitation of concrete structures; these are classified in three categories [4,8,10], namely Polymer or resin materials, Polymermodified cementitious materials (PMC) and Cementitious concretes. The latter, which are the subject of our research, are widely used as repair materials, due to their mechanical properties, which are close to those of most substrate concretes, to their availability on the construction materials market, and also to their low prices [8,10]. The experimental results indicated that cement-based materials exhibit higher bond strength with concrete substrate as compared to resin materials [8]. In other words, cement concretes show significant shrinkage, but that does not prevent them from being used. Besides ordinary concrete [11], other concretes, such as Ultra High Performance Concrete (UHPC) [12], Ultra High Performance Fiber Concrete (UHPFC) [6,13,14], and self-compacting concrete (SCC) [15], have been developed and used as repair materials. It was found that ordinary concrete adheres poorly to the substrate [13,15,16], as compared to the two repair concretes previously mentioned. To estimate the bond strength, many researchers investigated cementitious mortars without additions, like Ordinary Portland Cement (OPC), sulphoaluminate cement (SAC) or magnesia phosphate cement (MPC) [8,17,18], in order to enhance the stability of repair mortars. Others, considered the addition of ultra fines as a supplement, and as a partial replacement of cement, for improving the durability of mortars and concretes; such fines may be blast furnace slag [19], combinations of silica fume and metakaolin [7,20], fly ash and silica fume [21], pure fly ash [22– 24], or pure silica fume [9,24]. Momayez et al. found that the positive effects of silica fume on the bond strength are negligible beyond the content of 7% [9]. Generally, cementitious materials are available and easy to use; they show good performance with a conventional concrete substrate. Several studies have addressed the problem of interfacial bond [9,14,25], from deteriorated concrete substrate, with different cementitious repair materials. They employed a diversity of test methods, in order to estimate the bond properties. The bonding tests are classified in three categories. The first category includes the direct tension pull-off test [9,26,27], the direct tension test, the flexural test (indirect tension) [28,29] and the splitting tension test [1,9,14]. In the splitting tension test, the test specimens used may be of cylindrical or cubic shape. In the splitting test, the specimens are subject to two opposing compressive line loads acting perpendicularly to the axis in the diametric plan, inducing a uniform tensile stress over that plan [30]. Regarding the second category, shear stresses, like the direct shear test (Bi-surface shear), are applied [25]. In the third category, the bond strength is determined from the combination of compressive stress and shear stress. The slant shear test is applied to cylindrical specimens or prismatic specimens having a square cross-section [6,9,31]. The slant shear test has been adopted by several international codes as a test for evaluating the bond of resinous repair materials to concrete substrates. Recently, it was also adopted for assessing cementitious repair materials [7,20]. According to Momayez et al. [9], the measured adhesion strength was found to decrease with the testing process in the order of slant shear, bi-surface shear, splitting, and pull-off. Moreover, the preparation of the substrate is an important step that has an influence on the monolithic behavior of a composite system. Consequently, overlay debonding occurs when the surface is not adequately prepared. It is therefore necessary to take into account the level of surface roughness [32]. Several studies in this field have focused on the phenomenon of surface bonding. According to Julio et al. and Tayeh et al. [14,26], the best bond strength can be achieved with sand-blasting, i.e. when the aggregates are well exposed.

The high costs of repairs are the result of the high expenses relative to the surface preparation of the substrate, and the high prices of other expensive repair materials composed of fines additions as a supplement. Consequently, it is necessary to think of effective and more economical materials to be used in concrete repairs. Good adhesion bond to the substrate is required. The present paper aims to formulate a new repair material. It is a cementitious type of concrete called ‘‘Sand Concrete”, which has not been addressed in previous research works. Sand concrete (SC) has been defined in the Standard NF P 18-500 [33–35]. It consists of sand, but does not contain any gravel. This last component may be present but in a very small proportion, i.e. with a ratio G/ S < 0.7 [33–36]. It may also contain cement, water and filler, and even sometimes different admixtures such as superplasticizers. Sand concretes are comparable to conventional concretes; they have the same cement dosage (250–400 kg/m3), and are able to replace ordinary concrete in certain structures [36–41]. The use of fillers is essential in the composition of sand concrete, in order to have the needed compactness and consequently the adequate strength [42]. Moreover, the use of fillers minimizes the amount of cement used, and this distinguishes mortar from sand concrete. Sand concre[42]te has interesting properties; it has a good workability, which means that less energy is required for its implementation (casting of concrete), thanks to the fine size of its particles (particles of sand and the filler) which allow a better coating of concrete, even in complicated cases (in the case of dense and complicated reinforcement). However, sand concrete presents some disadvantages like the high water demand due to the presence of fine particles and also the large amount of cement used in order to achieve a high workability. Increasing the water content results in bleeding and segregation, and gives weak interfaces within granular materials [40]. Furthermore, high cement amounts may lead to drying shrinkage and considerable creep [43]. Hence, the presence of superplasticizers and fine additions into the composition of sand concrete is amply justified in order to achieve the needed workability and to have the appropriate viscosity. Based on this, sand concrete can be regarded as a special type of concrete; this finding is supported by several studies. Sand concrete is advocated for different applications, such as compacted sand concrete used in pavement [44]; self compacting sand concrete [40,45,46]; sprayed sand concrete [41,47] and lightweight sand concrete [36,39,48]. All the above-mentioned applications have not given much consideration, particularly to the use of sand concrete designed in repairing concrete structures. However, sand concrete has been used for the renovation of some structures in Russia, such as the reinforcement of an old brick bridge (1865), and also for the rehabilitation of Lyubertsy’s thermal power plant in Moscow [49]. The Sablocrete project allowed creating a standard for sand concrete; it reported many applications of flowing sand concrete which was intended to be used in concrete repairs [40]. Sand concrete possesses interesting qualities to be used as a repair material. The non-cracked character of sand concrete is justified by the absence of large aggregates [35,50], as well as by its low modulus of elasticity, uniformity of drying, fine porosimetry and homogeneity [50–53]. These properties allow stating that sand concrete is an interesting repair material. All this shows the interest of investigating the use of sand concrete as a repair material. The main objective of the present study is to valorize sand concrete as a new repair material and evaluate its bond strength to ordinary concrete substrate. In order to determine its bond strength and the type of failure that may occur after loading, an investigation was carried through two indirect tension tests, namely the flexural test and the splitting test.

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K. Gadri, A. Guettala / Construction and Building Materials 157 (2017) 1133–1144 2. Experimental program 2.1. Materials The following materials were utilized throughout this experimental study. 2.1.1. Aggregates For the ordinary concrete substrate, two classes of limestone gravel (3/8 and 8/15 mm) were used, with river sand (0/5 mm) of siliceous nature. In addition, crushed sand with particle size (0/5 mm) brought from the quarries in the region of Biskra (Algeria) was the principal component in sand concrete which was developed for the repair operation. It is a limestone-type of sand, which gives a continuous particle size distribution with lower percentage of 1.5% of fine elements less than 80 l. The granular distribution of all used aggregates shown in Fig. 1. 2.1.2. Fillers Two types of fillers were used for two different sorts of sand concrete. The first one is a limestone type of filler that was brought from the quarry of Oum Settas (El Khroub, Algeria); it has an average diameter of 23 lm. The second one is a white glass powder with a grain size smaller than 80 lm. It is usually obtained from waste of window panels of buildings; it is used after crushing, grinding and sieving. The physical properties of the aggregates with all fillers used are presented in Table 1. 2.1.3. Cement The cement employed in the two types of concrete, namely ordinary concrete substrate (OCS) and sand concrete (SC) as a repair material, is ordinary Portland cement (type II) of class 42.5, also known as ‘‘CPJ-CEM II/A”. It is produced in the cement plant of Ain Touta (Batna) with 370 m2/kg fineness. It is composed of 75% clinker, 5% gypsum and 20% pozzolan. The chemical compositions of the cement used along with the two types of fillers are given in Table 2.

ing components was carried out as recommended by the standard NF EN 12390-2 [55]. In order to evaluate the mechanicals properties of the concrete substrate, as shown in Table 4, the concrete was cast, in its plastic state, in molds of different shapes, i.e. a cubic form with dimensions (100  100  100) mm3, prismatic form with dimensions (100  100  400) mm3, and in the form of a cylinder 160 mm in diameter and 320 mm in height. The substrates had two different shapes. The first one was prismatic with dimensions (50  100  400) mm3; it was obtained by casting concrete in prismatic molds at half height. The second one had a semicylindrical shape; it was obtained after a splitting test on the cylinders of dimensions (160  320) mm2 at age of 28 days. All specimens were removed from their molds after 24 h and then they underwent curing in water at the temperature of (20 ± 2) °C. After completion of a 28 days maturation period, they were taken out, for surface preparation. Next, they were immersed again in water for another period of 90 days. All the test specimens underwent continuous curing, outside water and in the open air, for 540 days. This curing period was decided in order to obtain a relatively old concrete substrate to the repair concrete. The surface of the concrete substrates was prepared using hand and mechanical tools such as steel wire brush and hammer drill with point tools. Four types of surface roughening were adopted, as shown in Fig. 2. Three adopted for prismatic specimens and two for the cylindrical specimens. Table 5 indicated the surface type and roughness level of different substrates. The estimated amplitude of roughness surface of substrates is the similar approach of Concrete Surface Profile (CSP) indicated by International Concrete Repair Institute [56]. The (HRG) surface which correspond to the roughness grades CPS 7-9. It is considered as high roughness compared to the roughness level of as cast surface treated by wire brush considered as low roughness (LRG), which correspond to the roughness grades CPS 5-6. After more than two years of curing, all the OC and OCS substrate specimens were cleaned with a steel wire brush, and dusted with an aspirator, in order to eliminate slurry cement from the external surface of fine and coarse aggregates. Before Table 2 Chemical compositions of cement, glass and limestone fillers.

2.1.4. Superplasticizer To obtain a plastic concrete, with slump values greater than 90 mm, it was decided to use the Algerian superplasticizer ‘‘MEDAFLOW 145”, which is a high range water reducer; it is a modified polycarboxylate ether of brown color. 2.2. Specimens preparation 2.2.1. Substrate concrete Ordinary concrete, with two ranges of strength, namely ordinary concrete with a superplasticizer (OCS) and ordinary concrete without superplasticizer (OC), was used to prepare the substrates. The composition was determined by the Dreux Gorisse method [54]. The mixed proportions are given in Table 3. The procedure of mix-

100

(%)

Cement

Limestone filler

Glass filler

SiO2 CaO Al2O3 Fe2O3 MgO Na2O K2O SO3 ClL.O.I

20,52 63,86 5,13 3,36 1,27 0,14 0,76 2,26 0,027 1,09

0,01 55,88 0,01 0,01 0,14 0,01 0,01 0,11 0,005 43,90

71,43 9,93 1,51 0,31 2,61 12,60 0,35 0,16 0,02 0,27

10

100

crushed sand

Cumulave passing (%)

Gravel(8/15)

80

Gravel(3/8) River sand

60 40 20 0 0.01

0.1

1

Grain size (mm) Fig. 1. Particle size distribution of aggregates.

Table 1 Physical property of the aggregates and fillers.

Apparent density (kg/m3) Specific density (kg/m3) Specific surface (m2/kg) Compactness (%) Fineness Modulus Sand equivalent (Piston test)

River sand (0–5 mm)

Crushed sand (0–5 mm)

Gravel (3–8/8–15 mm)

Limestone filler < 80 lm

Glass filler < 80 lm

1400 2700 – 52 2,3 72

1460 2620 – 58 3,0 65

1450 2600 – 56 – –

1090 2700 530 – – –

1420 2450 420 – – –

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Table 3 Mix proportions with slump values for substrate concrete and repair sand concrete. Substrate (kg/m3)

Concrete type

Ordinary Portland Cement OPC Gravel (3/8) Gravel (8/15) River sand Crushed sand Limestone filler Glass filler Superplasticizer W/C W/C + F Slump value (mm)

SC repair (kg/m3)

OC

OCS

SCL

SCG

400 399 741 630 – – – – 0,5 – 50

400 399 741 630 – – – 1% 0,4 – 90

350 – – – 1480 180 – 2% 0,6 0,39 90–110

350 – – – 1480 90 90 2,2% 0,62 0,41 80–90

Table 4 Mechanical properties of concretes OC, OCS, SCL, SCG. Concretes

OC OCS SCL SCG

Compressive strength (MPa)

Flexural tensile strength (MPa)

28 days

120 days

28 days

120 days

120 days

28 days

28 40 36 42

35 55 46 50

4,5 8,2 9 6,5

5,7 9 9,9 7,2

2.1 3.1 2.4 2

25 33 22 23

RGDH

Splitting tensile strength (MPa)

LRG

HRG

Compressive modulus of elasticity (GPa)

DH

Fig. 2. Surfaces textures of substrates.

Table 5 Surface type and roughness level of substrates. Surface roughness

LRG

Surface treatment description Manuel wire brushing Roughness grades CSP* 5–6 [56] Substrate condition (Prismatic shape) x Substrate condition (Cylindrical shape) *

HRG

DH

HRGDH

Surface chipping Hammer drilling Hammer drilling with chipping CSP* 7–9 [56] Holes (D = 10 mm, Dep = 5 mm) [14] CSP* 7–9 + Holes (D = 10 mm, dep = 5 mm) x x x x

Concrete Surface Profile; D: diameter, Dep: depth.

casting the sand concrete (SC) repair material, the substrate specimens were immersed in tap water for 24 h, to get a saturated surface dry (SSD) condition (the commonly recommended site practice) [57], a few hours before placing the repair material. They were then dried with a clean cloth (removed water from surface) [4,26], and placed into their respective molds, as shown in Fig. 3.

ing the compactness of the granular skeleton. The chosen cement mix(C) was fixed at 350 kg/m3 [42,58,59], assuming a compactness coefficient of the mixture (c) equal to 0.775 [54], which corresponds to a mixture where Dmax = 5 mm. The quantity of sand [S] being determined according to the following equation: [59]

½S ¼ 1000c  C: 2.2.2. Sand concrete repair material The sand concrete used in the present work was composed of cement, crushed sand, fillers and water. In addition to these basic components, it typically included a superplasticizer. Local constituents were utilized in sand concrete (SC) which was intended to be applied as a repair material. It was decided to use crushed limestone sand, in this experimental study, due to its better mechanical strength, as compared to siliceous sand (river or dune), and also to its angular shape, which makes the propagation of cracks more difficult. Moreover, limestone makes the concrete stronger and more elastic [35], which are desirable properties for a repair material. The mix proportions adopted were prepared from a formulation based on an experimental approach derived from previous works on sand concrete [37,42,47]. The mixing proportion approach used here for sand concrete was aimed at optimiz-

The limestone filler is used in sand concrete to fill the voids between sand grains, to increase the compactness by creating a continuous graded mix, and also for their great reactivity with the hydrates [42,53]. Using the filler certainly contributes to reducing the consumption of cement paste [33]. The introduction of additives in a cementitious material improves its granular skeleton, as it has been demonstrated by some authors [34,60]. Therefore, part of sand in mass of 11% is replaced by the limestone filler in order to correct the granular distribution. It has been established that the limestone filler has a physico-chemical activity which promotes the hydration acceleration of cement clinker [34,61]. Two mixes of sand concrete were selected to be applied as a repair material, i.e. a) sand concrete in which the limestone filler is used to 100% (SCL), b) sand concrete in which 50% of limestone is replaced by glass powder (SCG). An economic procedure is proposed

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Fig. 3. Substrate specimens in moulds.

here to use waste glass powder as reactive filler due to its remarkable pozzolanic properties, instead of other fine additions like silica fume, metakaolin, fly ash, etc. These last fine minerals are available at high prices and are not found in each region. However, waste glass powder gives high durability to concrete and reduces its maintenance costs [62]. The use of glass in the form of fine particles results in a positive behavior which is associated with the pozzolanic reaction [62,63]. Sand concrete mixing proportions are given in Table 3. The sand concrete repair material was cast in its plastic state, with slump values between 80 and 110 mm. The mechanical properties were measured at the age of 28 and 120 days. The sand concrete repair material was cast, in cubic molds with dimensions of (100  100  100) mm3 to measure the compressive strength, in prismatic molds with dimensions (100  100  400) mm3 for the flexural strength, and finally in cylindrical molds with dimensions (160  320) mm (diameter  height) for the tensile splitting. The results are reported in Table 4. The sand concrete repair material was then cast in its plastic state, on substrates, inside the molds in order to have full specimens. These composite specimens were then removed from their molds after 24 h, and were subsequently kept covered for 27 days, first with a wet cloth and then with a plastic film. Afterwards, they were uncovered and stored in the laboratory at ambient temperature, for twelve to thirteen weeks. The above-mentioned composite specimens (sand concrete repair material over substrate) were subjected to mechanical testing after a total duration of four months. In this work, two strengths classes of ordinary concrete were used for the substrate, i.e. OC and OCS. Also, two repair sand concretes were used as overlay, i.e. SCL and SCG, and the resulting materials showed different individual properties, as previously shown in Table 4.

Substrat

Repair

2.3. Test methods Two mechanical tests were carried out on the composite specimens (Repair/ Substrate), in order to evaluate the specimens’ response under loading and to investigate the type of failure that may occur. The specimens were subjected to an indirect tensile stress (rf). Since the bond strength depends considerably on the test method employed, it was important to select the most appropriate type of bond test, which represents the actual nature of stress exerted on the concrete structure [25]. Debonding of the repaired layer may result from one or both of the two following causes, namely the mechanical loading and/or the dimensional change. The interface within the composite system was subjected to tensile stresses; the flexural tensile test was applied to evaluate the tensile strength, which in turn allowed estimating the adequate surface preparation as well as the durability of repair material. This test was conducted according to AFNOR Standard EN 12390-5 [64]; it was carried out in accordance with the three-point bending principle. The composite prismatic specimens were simply supported on two rollers. The force was applied perpendicularly to the interface line of the composite specimens. The loading is applied steadily and without shock at rate of 0.05 MPa/s. Fig. 4 shows the flexural test set-up. The flexural tensile strength is calculated by means of this Eq. (1):

rf ¼ 3FL=2bd2 ðMPaÞ

ð1Þ

where F is the applied force (N), L is the spacing between the support rollers (mm), b and d are the lateral dimensions of the specimens (mm). Furthermore, a tensile splitting test was carried out according to Standard ASTM C496 [65] in order to determine the bond strength between the OC substrate and SC repair. It is an indirect tensile test (Fig. 5). It was developed by the authors to evaluate the bond strength of strengthened columns [30]. The applied force was parallel to the bond line interface of the composite specimens which have two different shapes (cylindrical and cubic). The loading is applied steadily and without shock at rate of 0.05 MPa/s. The cubic specimens (100  100  100) mm3, shown in Fig. 5(a), were obtained after cutting the composite half-prisms resulting from the

Fig. 4. Flexural test set-up.

bending test. Moreover, the composite cylinder, shown in Fig. 5(b), was made of two types of concrete (SC repair/OC substrate). The splitting tensile strength is calculated using the Eq. (2):

rT ¼

2F

pA

ðMPaÞ

ð2Þ

where F is the applied force (N), A is the area of the bonding surface (mm2).

3. Experimental results and discussion The objective of the present work is to evaluate the mechanical strength of composite concretes subjected to tensile stress and to examine the interfacial bond failure. The mechanical strength considered is the average of the values obtained for three test specimens. Regarding the three-point flexural test, the external load is applied perpendicularly to the interfacial bonding line, and as a result a strain is produced in the composite specimen. If the layers do not have a monolithic response, then they shift laterally with respect to each other, and consequently, the debonding occurs at the interface of the specimens. The repair concrete was placed in bottom face (tensile side). This orientation indicates the most crit-

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K. Gadri, A. Guettala / Construction and Building Materials 157 (2017) 1133–1144 Table 6 Flexural tensile strength of different composite specimens.

Repair

Surface preparation

Composite

Force (Kgf)

Bond strength (MPa)

STD (MPa)

COV (%)

LRG

SCL1/OCS SCL2/OCS SCL3/OCS SCG1/OCS SCG2/OCS SCG3/OCS SCL1/OC SCL2/OC SCL3/OC SCG1/OC SCG2/OC SCG3/OC

1250 1100 1400 950 1250 1000 900 1100 1000 980 900 931

5,62

0,42

7,47

4,8

0,59

12,3

4,5

0,36

8

4,22

0,15

3,55

SCL1/OCS SCL2/OCS SCL3/OCS SCG1/OCS SCG2/OCS SCG3/OCS SCL1/OC SCL2/OC SCL3/OC SCG1/OC SCG2/OC SCG3/OC

2050 2050 1800 1600 1550 1650 1350 1300 1400 1100 1350 1200

8,84

0,52

5,88

7,2

0,18

2,5

6,07

0,18

2,96

5,47

0,46

8,40

SCL1/OCS SCL2/OCS SCL3/OCS SCG1/OCS SCG2/OCS SCG3/OCS SCL1/OC SCL2/OC SCL3/OC SCG1/OC SCG2/OC SCG3/OC

1800 1600 1800 1350 1050 1300 1200 1200 1100 1100 1150 –

7,8

0,42

5,38

5,54

0,59

10,65

5,25

0,21

4

5,06

0,11

2,17

Substrate

(a) Splitting test for composite cube specimens HRG

DH

(b) Splitting test for composite cylinder specimens Fig. 5. Splitting tensile test.

ical case for the appearance of the debonding [32]. According to Perez, debonding results from the bending imposed on the structure and overlay loading. The splitting tension test is a widely used testing method for evaluating the bond strength. The bond line is placed under two opposite longitudinal compressive loads which produces a biaxial stress state with compressive vertical and lateral tensile stresses; this causes the specimens to split into two halves. 3.1. Flexural tensile strength Table 6 presents the average values obtained from the flexural test for the composite specimens. These results are for different types of composites, and for various kinds of surfaces. The standard deviation (STD) and coefficient of variation (COV) were determined in each test. Standard deviation (STD) is a measure of strength values dispersion out around the average; it is the square root of its variance. The coefficient of variation (COV) expressed as a percentage is the ratio of the standard deviation (STD) to the mean. A small standard deviation, limited between 0.11 and 0.59, was observed; a moderate coefficient of variation (COV) was found between 2.17% and 12.3%, and in most cases it was less than 15%. Hence, a low dispersion was noticed, which indicates that the results obtained are reliable. The three-point flexural test proved a monolithic failure mode for all the specimens under consideration. A deflection was observed at the center; cracking occurs in the tension face and propagates vertically until it reaches the interface towards the sub-

strate, without debonding of the overlay layer as shown in Fig. 6(a). The failure mode occurs at the centre, the composites specimens were separated into two equal prismatic parts as shown in Fig. 6 (b and c). They display a monolithic failure, which is very analogous to that of monolithic specimens. It is worth noting that debonding did not occur, because the tensile stress did not exceed the bond’s tensile strength. This is similar to the case studied and reported by Perez et al. [32] for beam specimens tested in pure bending under static loading. Also, these results agreed with those of Tayeh et al. [29], who have evaluated the bonding between normal concrete and ultra high performance fiber concrete (UHPFC) using the third-point load bending tests on composite prismatic specimens. Hence, a failure occurred through the substrate at the middle of the composite prism as in monolithic prism. Moreover, this results agreed with these of Pattnaik et al. [66], who have studied the bonding between normal concrete and different types of epoxy. They considered the failure mode as compatible when the failure occurs at the center of the beams. The highest flexural tensile strength was obtained for sand concrete repair with limestone filler (SCL) associated to OCS substrate, for the three cases of texture surfaces. It was shown in Table 6 that the best values are obtained for the composites with high roughness surface (HRG), followed by the composites on substrates with drilled holes (DH); the lowest values were recorded for composites with low roughness surface (LRG). In the case of (LRG) surface, the best strength values were 5.62 and 4.8 MPa, obtained with SCL and SCG repair materials, respectively, in the case of OCS substrate. Furthermore, for a high roughness (HRG) surface, SCL repair mate-

K. Gadri, A. Guettala / Construction and Building Materials 157 (2017) 1133–1144

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a Failure mode at the centre of the prism

b Interface with HRG surface.

c Interface with DH surface

Fig. 6. Flexural tensile failure.

Relave increase in flexural tensile strength (%)

57.3 50 60 38.8

50

34.8 29.6

40 30

15.4

16.8

20

20 10 0

SCL/OCS

LRG

DH

SCG/OCS

HRG

SCL/OC

SCG/OC

Composite specimens

Fig. 7. Relative increase in flexural tensile strength for the different types of substrate surface.

rial gave the highest flexural strength, with a value of 8.84 MPa, followed by the SCG repair product, with 7.2 MPa; both of them were associated with the OCS substrate. Similarly, for composites applied on substrates with drilled holes, the SCL repair material gave the best strength value, followed by the SCG repair material. Therefore, as shown in Fig. 7, the same tendency was observed for the four composites with regard to the degree of roughness. Also, a decrease was noted in the flexural tensile strengths of the composites with an OC substrate, as compared to those with an OCS substrate, for any type of surface roughness. This difference depends on the individual tensile strength of each sort of concrete. These results are dependent on the strength of the concrete substrate. It was therefore found that the best tensile strength was obtained with the composite repair material SCL, whose individual flexural stress recorded at the age of 120 days was high; it exceeded that of ordinary concrete (OCS) by 10% at that age.

It can clearly be seen, from Fig. 7, that each composite shows an improved evolution in its flexural strength with respect to the surface roughness. Compared to the as-cast surface, which is considered as a low roughness (LRG) surface, one can say that the relative increase in the flexural strength is more important in the case of high roughness (HRG) surfaces. This increase is considerable, for SCL and SCG repair materials, with 57.3% and 50%, respectively, in the case of OCS substrates. This increase is less significant for surfaces with drilled holes (DH), for the same overlay materials, with 38.8% and 20%, over the same OCS substrate. Moreover, it was found that the surface roughness effect contributed to the improvement of the flexural strength and minimized the risk of debonding, which is in agreement with Perez. The degree of surface roughness of the supporting concrete is an important parameter which can reduce the risk of debonding and thus achieve a monolithic behavior [32]. Contrariwise, the results of the flexural strength obtained by Tayeh et al. [29] confirmed that adhesion bond strength between substrate and repair material was very strong. In case of as-cast surface, the adhesion bonding was stronger than the substrate. 3.2. Splitting tensile strength This phase aims at evaluating the bond strength and the failure form as well as its quality. This test was used to evaluate a representative parameter in order to estimate the repair failure [1]. The splitting test was applied to cubic composites with substrates having two surface textures, i.e. high roughness (HRG) surfaces and surfaces with drilled holes (DH). The same test was also applied to cylindrical composites with surface texture substrates (HRG) and a second high roughness texture with drilled holes (HRGDH). The splitting tensile strength results of composite cube specimens are shown in Table 7. Higher values recorded with composites on high roughness (HRG) surfaces were observed, either with limestone sand (SCL) or with glass powder (SCG), where the aver-

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Table 7 Splitting tensile strength and failure mode of different composite cube specimens.

HRG

DH

Composites

Force (KN)

Bond strength (MPa)

STD (MPa) 0,36

COV (%)

SCL1/OCS SCL2/OCS SCL3/OCS SCL4/OCS SCG1/OCS SCG2/OCS SCG3/OCS SCG4/OCS SCL1/OC SCL2/OC SCL3/OC SCL4/OC SCG1/OC SCG2/OC SCG3/OC SCG4/OC

54,2 41,5 54,1 55,1 51,1 46,2 47,4 51,3 34,2 37,2 40,1 – 38,0 46,5 44,6 37.7

3,17

2,65

0,24

9,05

SCL1/OCS SCL2/OCS SCL3/OCS SCL4/OCS SCG1/OCS SCG2/OCS SCG3/OCS SCG4/OCS SCL1/OC SCL2/OC SCL3/OC SCL4/OC SCG1/OC SCG2/OC SCG3/OC SCG4/OC

33,1 25,2 24,6 – 31,8 25 29,2 – 25,3 26,9 24,6 – 21,3 24,2 28,1 –

1,75

0,24

13,7

1,82

0,17

9,34

1,62

0,06

3,70

1,56

0,18

11,5

3,12

2,36

0,14

0,15

11,35

4,48

6,35

Failure mode D C C D D D D D B B C – D C C B B A A – B B B – A A A A A A –

age strength was found equal to 3.17 and 3.12 MPa, respectively. The standard deviations were recorded between 0.14 and 0.36, with a coefficient of variation (COV) between 4.48% and 11.35%. In the case of surfaces with drilled holes (DH), the standard deviation values were lower than those of HRG surfaces. In addition, values between 0.06 and 0.24 were found, with a coefficient of variation ranging from 3.70% to 13.7%. Note that a coefficient of variation (COV) lower than 15%, the results are considered as homogeneous and significant, which is in most cases, for both types of surfaces (HRG and DH). This is in agreement with the results reported by Tayeh et al. [6] for DH surfaces with COV of 13,55%. Also, in agreement with those of Momayez et al. [9], where it record a COV of results with 6.4% for Low roughness surface and 10.9% for High roughness surfaces. Fig. 8 shows the development of the splitting tensile strength of the composite cube specimens. High strength values were observed for composite cube specimens with a high roughness surface area (HRG) for both OC and OCS substrates, for the two sand concrete fillings (SCL and SCG). High bond strengths were recorded for composites with OCS substrates; these values were found greater than 2.1 MPa, which represents an excellent bond quality, in terms of bond strength, according to the classification of the bond quality proposed by Sprinkel and Ozyildirin [67]. Indeed, in this case, composites with SCL overlays gave the best strength (3.17 MPa), followed by that given by SCG overlay composites (3.12 MPa). There was a slight decrease of 1.6%. The best strength values with DH surface for the same range of substrate were equal to 1.82 MPa and 1.75 MPa, for SCG and SCL, respectively. A decrease of 3.8% was recorded. Moreover, for composites involving an OC substrate, strength values of 1.62 MPa and 1.56 MPa were obtained for SCL and SCG repair concretes, respectively, with a

Split tensile strength (MPa)

Surface preparation

DH 3.17

4

3.12 2.36

3 2

1.75

HRG

1.62

2.65 1.82 1.56

1 0 SCL/OCS

SCL/OC

SCG/OCS

Composite specimens

SCG/OC

Fig. 8. Splitting tensile strength of the composite cube specimens for HRG and DH surfaces preparation.

decrease of 3.7%. It is clear that the substrate strength has an impact on the bond strength of the composite specimens. But, the case of the drilled holes surface led to values of bond strength considerably lower than these of high roughness. This result can be explained since only the holes of a 10 mm lattice were drilled, and most of the substrate surface was not treated. This is in agreement with the partially chipped surface situation studied by Julio et al. [26]. Moreover, the results related to the influence of the surface roughness were found to be in good agreement with those of Tayeh et al. [14,68]. Also, the bond strength depends on the cement concrete strength level. In addition, the composite specimens exhibited different modes of failure, which is due to the bond strength, as illustrated in Fig. 9. There are four types of failures, namely: – Type A: Pure interface failure. – Type B: Mixed-mode failure, with a thin layer of repair material bonded to different wedges of the substrate. – Type C: Mixed-mode failure with an important layer of repair material bonded to different wedges of the substrate. – Type D: Mixed failure with an important layer of repair material bonded to different wedges of the substrate with several aggregate tensile failures. It can clearly be noted from Table 7 that type D is the most common failure mode for high roughness (HRG) surfaces. Type D is also found in the case of OCS substrates with composite specimens of SCG repair. In addition, for the same range of substrates, the SCL repair presents two failure types, namely D and C. Type B failure mode is observed in the case of OC substrates in most composites with SCL repair. In the case of surfaces with drilled holes (DH), the bulk of composites present a pure interface failure (type A), except for composite specimens with SCG repair associated to an OCS substrate, which shows a mixed failure of type B. Finally, the bonding of sand concrete repair is significant in the case of high roughness (HRG) substrate. The failure modes manifested in the different composite specimens were similar to that observed by Espech et al. [1]; which indicate that the bonding is generally strong, as the material presents a tensile failure in the overlay and in the substrate; aggregates also tend to have a tensile failure. This may be attributed to the surface roughness effect. On the other hand, Grigoriadis et al. [69] obtained one only failure mode for all the studied specimens for both normally and microwave cured. This failure obtained for smooth substrate surfaces is similar to the failure type (A), which occurred at the bonded interface between the substrate and the repair material. However, the SCG contributes more to bending, due to the improvement of the hydration of

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Repair

Substrate

Repair

Type A failure

Repair

Substrate

Type B failure

Repair

Substrate

Type C failure

Substrate

Type D failure

Fig. 9. Interface failure modes for split composite cube specimens.

Table 8 Splitting tensile strength and failure mode of different composite cylindrical specimens. Surface preparation

Composites

Force (KN)

Bond strength (MPa)

Failure mode

HRG

SCL/OCS SCG/OCS SCL/OC SCG/OC

154,0 172,1 136,9 161,6

2,54 2,14 2,21 2,01

C B C C

RGDH

SCL/OCS SCG/OCS SCL/OC SCG/OC

204,2 125,5 178,2 122,4

1,91 1,56 1,71 1,52

B and C A B A

cement mixed with glass powder [62,63]. This last mentioned material is supposed to reduce the porosity and to increase the micro-hardness in the interface zone like silica fume and fly ash, as reported by Lukovic et al. [10]. Although, the bonding is less significant in the case of a drilled holes (DH) surface. This is due to insufficient anchoring of the sand concrete repair material into the holes, because of weak bonding. To confirm the results obtained and to examine the effect of surfaces with drilled holes on the bond strength, it was decided to reapply the splitting tensile test on cylindrical specimens for two types of surface roughness. The first one was a high roughness (HRG) surface and the second was a high roughness (HRG) surface with drilled holes (HRGDH), the case where the most of the substrate surface was treated. The results obtained from the splitting tensile test when applied to composite cylindrical specimens are summarized in Table 8. The bond strength is the average of three values obtained from different test specimens. It was found that the best splitting tensile strength results were obtained with the SCL repair material, in the case of composite specimens with high roughness surface (HRG). An excellent quality of bond strength was observed for two ranges of substrate, namely OC and OCS with 2.21 and 2.54 MPa, respectively. These were followed by a very good quality of bond strength in the case of composite specimens used on high roughness with drilled holes (HRGDH) surfaces. However, it was noted that in the case of the SCG repair applied on a high roughness (HRG) surface, there was a very good quality of bond strength

with the OC substrate of 2.01 MPa, and an excellent quality with the OCS substrate of 2.14 MPa. Nevertheless, good bond strength was observed in the case of composites with SCG repair for two ranges of substrate, i.e. OC and OCS, in the case of high roughness surfaces with drilled holes (HRGDH). These results agreed with these of Tayeh et al. [14] obtained with ultra high performance fiber concrete (UHPFC) repair in the case of wire brushed and drilled holes substrate surfaces. The European Standards EN 1504–3 indicates that the tensile strength (bond strength) must be at least 2 MPa in the case of structural repairs, and 1 MPa in non structural repairs. The results obtained in this study are satisfactory as they are all greater than 1.5 MPa. One may clearly see from Fig. 10 that the splitting tensile strength increased more for composite specimens with the both repair materials SCL and Fig. 10SCG applied on high roughness surfaces (HRG), as compared to those applied on (HRGDH) surfaces. For OC and OCS substrates, the values went from 22.6% to 24.8% respectively in the case of SCL repair, and from 24.1% to 27.3% in the case of SCG repair. The findings indicate that the bonding is less significant with the SCG repair in the case of (HRGDH) substrate. This is caused by the insufficient anchoring of the repair material into the holes, due to the weak bond as a result of the lower slump of SCG repair as compared to SCL. Furthermore, the cylindrical composite specimens exhibited different modes of failure due to different bond strength values, as it is shown in Fig. 11. In this case, there are three types of failure modes:

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Spling tensile (MPa)

RGDH

4

2.54

2.21

3 1.91

2

1.71

2.14

agreement with previous related works [6,24]. It is worth noting that using a repair material with a lower elastic modulus than that of the concrete substrate is preferable to employed it. That material is desirable in the case of non-structural repairs in order to achieve the deformational compatibility between the two types of concrete. Otherwise, it is advisable to use it in structural repair. On the other hand, if they are identical, then the repair material should share the loads with the existing structure [10].

HRG

2.01

1.56

1.52

1 0

SCL/OCS

SCL/OC

SCG/OCS

4. Conclusion

SCG/OC

Composites specimens Fig. 10. Splitting tensile strength of composite cylindrical specimens for RGDH and HRG surfaces preparation.

– Type A: Interface failure with a thin layer of repair material bonded in different wedges and holes of the substrate. – Type B: Mixed failure with a thick layer of repair material bonded in different wedges of the substrate. – Type C: Mixed failure with a thick layer of repair material bonded to substrate with minor cracking or damage of the substrate and repair material. It was clearly seen that poor adherence was a result of type A failure. The splitting tensile test applied to the composite made with SCG repair material showed the presence of type A failure in the case of an HRGDH substrate. However, that composite presented a significant failure of type C in the case of a high roughness (HRG) surface. Furthermore, a composite made with SCL repair material presented important failures of types B and C, for both substrate surfaces (HRGDH and HRG). A pure failure was noted at the interface between the repair concrete and substrate, which indicates an unsuccessful repair operation. A mixed failure, due to the damaging of repair concrete or substrate or both, indicates a successful repair. Generally, this finding is in agreement with the explanation given by Soliman et al. [70] for different modes of failure obtained with composites samples exposed to slant shear test. With respect to what had been discussed earlier, these failure modes, caused by the composites, indicate that the bonding of sand concrete is significant in the case of a high roughness (HRG) substrate. There is an adequate compatibility between the two concretes (Repair/Substrate). However, the bond is less important in the case of composites with a substrate with drilled holes (DH), particularly for sand concrete with glass powder (SCG). Furthermore, despite the difference in elastic modulus that exists between sand concrete repair materials (SCL, SCG) and the concrete substrate, no interfacial debonding was observed after the loading of the composites; the results obtained are in good

Substrat

Repair

Type A

The present work was an attempt to valorize sand concrete as a repair material. To do this, two composite concretes, with different substrate surfaces subjected to flexural and splitting loadings, were considered in order to evaluate the bond strength and its quality. Furthermore, different failure types were investigated in both cases. The results obtained allowed us to draw the following conclusions: – All combined specimens, subjected to flexural loadings display a monolithic failure at the centre, without debonding or cracks at the interface, which considered as compatible failure. The sand concrete repair with limestone filler (SCL) showed the best flexural tensile strength of 8.84 MPa, for the high roughness OCS substrate. The surface roughness effect contributed to the improvement of the flexural strength and minimized the risk of debonding. – The bonding is generally strong, as the material presents a tensile failure in the overlay and in the substrate. An excellent quality of bond strength was obtained for two ranges of substrate with the high roughness surface, namely OC and OCS with 2.21 and 2.54 MPa, respectively. The bonding of sand concrete is more significant in the case of a high roughness (HRG) substrate. There is an adequate compatibility between the two concretes (Repair/Substrate). – Sand concrete with limestone filler showed better bond strength than when limestone was mixed with glass powder. The composites made with SCG repair material applied on drilled substrates had less significant bond strength. The less slump caused insufficient anchorage of the repair into the holes, hence weak bonding with the majority situation of pure interfacial failure. – Overall, it can be said that sand concrete has a good adhesion to ordinary concrete substrate, reflecting a good to excellent bond quality. For this study, the results of bond strength obtained with all composites specimens are satisfactory as they are all greater than 1.5 MPa; thus allowing sand concrete using it as structural repair.

Repair

Substrat

Type B

Substrat

Repair

Type C

Fig. 11. Interface failure modes for the bond strength of composite cylindrical specimens.

K. Gadri, A. Guettala / Construction and Building Materials 157 (2017) 1133–1144

Finally, the results obtained showed that Sand concrete appear to be a very promising material for repair and rehabilitation of concrete structures. Therefore, it may be classified as a good repair material.

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