Effect of acrylic copolymer addition and interface treatment on the bond strength of polymer modified mortar and concrete

. Polymer-modified mortar is made of mortar with the addition of copolymer acrylic. An acrylic copolymer is one of the additives suitable for mortar repair because it can increase the overall durability of cement. This research investigates the bond strength between polymer-modified mortar and concrete substrate with treatment on the concrete surface. This research investigation focused on determining the effect of acrylic copolymers on bond strength. In addition, this study also investigated the effect of surface treatment with the addition of a bonding agent on bond strength. This study uses a test method: the slant shear test. The results showed that adding a bonding agent and acrylic copolymer increased the adhesive strength of the repair mortar on the concrete substrate. Generally, the higher the amount of bonding agent, the greater the expected bond strength. Meanwhile, in a ratio of 1:1.5 with cement, the acrylic copolymer can maximize bond strength.


Introduction
Concrete is a construction material that is widely used today.The advantages of using concrete include high compressive strength, high durability, fire resistance, easy to form, and the constituent materials are locally available [1].But concrete also has weaknesses which result in shorter service life.One type of degradation that is often seen is spalling due to corrosion of reinforcement [2].Repairs are needed to restore damaged concrete to its original state.The repair material must be compatible with the concrete substrate.According to Emmons et al. [3], compatibility is the balance between repair materials and the physical, chemical, and electrochemical properties of concrete.The ability of the repair material to develop a bond to the concrete substrate is important in assessing the success of a patch repair system.The bond strength of the repair material is affected by several factors such as surface roughness, moisture level of the concrete substrate, and the age of the repair mortar.In addition, the surface treatment and composition of the repair material have the most significant influence on the success of interfacial bonding [4] Therefore, the repair material must be carefully selected to achieve good interfacial strength.
Repair materials can be classified into three categories [5]: polymer-or resin-based mortars, cementbased mortars, and polymer-modified cement-based mortars.Pacheco-Torgal et al. [6] showed that epoxy based mortars developed lower bond strength on damp surfaces than cement based mortars.One way to improve the weakness of the mortar is by adding polymer.Recent research has shown that polymer * Corresponding author: adityachesar8@gmail.com modified mortar improves strength and adhesion.In addition, adding polymers to mortar improves moisture and chemical corrosion resistance.This is because the polymer fills the mortar pores, seals them tightly, and increases the durability of the cement mortar [7].Acrylic copolymer was used in this study as a polymeric material.The use of acrylic directly on fresh mortar can increase the bond strength with the matrix [8].In addition, in practice, the use of acrylic can also increase the adhesion between the repair material and the concrete.
Some surface treatment methods can also achieve increased bond strength.The most common is the roughness of the concrete surface.The surface roughness increases the friction and interlocking surface between the repair material and the concrete.The surface roughness of the substrate affects the mechanical adhesion [9].Guo et al. [10] reported that the interfacial bond strength increased continuously with increasing surface roughness within a certain range of values.Another surface treatment method is to apply a binder to the concrete surface.Previous studies have shown that the addition of a 0.5 mm thick bonding agent to a smooth concrete surface produces the same bond strength value as a rough surface without a bonding agent [11].This is because the bonding agent can combine the pores between the mortar material and the concrete thereby reducing porosity [12].
This research is part of the development of repair materials using polymer modified cementitious mortar.In this regard, this study aims to determine the effect of adding acrylic copolymer in the repair mortar mixture on the surface bond strength between the repair mortar and the concrete substrate.In addition, the effect of surface treatment using a bonding agent is also interesting.In this study, a test was carried out in the form of a slant shear test to determine the bond strength between the repair material and the concrete substrate.This research is expected to get the best composition of polymer modified cement-based mortar in terms of interfacial bond strength.

Specimens preparation
Prismatic concrete with 2 distinct sections.The first part is filled with substrate concrete and the other part is filled with repair mortar.The compressive strength of the concrete substrate is 30 MPa, and the ratio of water to cement is 0.48.Slump is targeted at ± 12 cm.The concrete mix design follows Indonesian standards [13].Surface treatment in this study used a bonding agent which was coated on the smooth surface of the concrete substrate.Untreated concrete substrates were also prepared to evaluate the bonding effect of using a bonding agent.The surface treatment procedure is as follows: first, clean the concrete surface to be coated with the bonding agent using a brush.Mix the bonding agent and cement in a certain ratio (Table 1), stir until homogeneous, no clumping occurs, and apply evenly to the concrete surface.After the concrete substrate has been prepared, the repair mortar is then poured according to the calculated proportions.

Mix proportion
The polymer used is made of acrylic copolymer, while the bonding agent is made of styrene acrylic emulsion.
The repair material used in this study is a cement-based mortar with the addition of polymer.The repair materials used in this study are planned into two series.the first repair material is ordinary repair mortar without polymer addition with a water-to-cement ratio of 0.35.This ratio includes the number of accelerators.This type of repair is applied to a concrete substrate that is given a bonding agent on the surface of the concrete substrate in various proportions (Table 2).This series is intended to determine the effect of surface treatment by a bonding agent on the strength of the interfacial bond between ordinary mortar and concrete.Bonding agent for surface treatment of concrete substrates is made by mixing the bonding agent with cement in various ratios, namely 1:1, 1:1.5, and 1:1.75.The second series are polymer modified mortars with various proportions of acrylic copolymer content, as shown in Table 3.The polymer in the mix displaces the water to maintain a liquid-cement ratio of 0.35.In the second series, polymer modified mortars can be categorized into two groups, namely mortars with bonding agent applied as surface treatment and without bonding agent.The proportion of bonding agent and cement for the surface treatment of the concrete substrate used the highest value from the results of the first series.The number of test objects is 30 prisms.Each type of specimen was tested as many as 3 pieces with a slant shear test.

Slant shear test
The slant shear test was carried out following BS-EN 6319/1984 [14].The specimen consists of two parts, prismatic concrete substrates and prismatic repair materials, with an inclination angle of 30° to the vertical axis with a size of 100 mm x 100 mm x 400 mm (Fig. 1).In this study, the surface of the specimen is smooth textured.The production of slant shear test specimens consisted of two steps, following the steps adopted by the previous study.First, the normal concrete substrates were cast with a bond line of 30° to the vertical axis.Subsequently, the concrete was stored in the curing room for 28 days at a temperature of 23° C±2° C [12].After 28 days, the bond surface of the concrete substrate was treated with a coated bonding agent.Then, the repair material was cast on the other half of the specimen.The specimen was cured again until the time of testing.
The slant shear test was then performed by applying compressive stress on the prismatic specimen using Universal Testing Machine (UTM), as shown in Fig. 2. The bond strength from the slant shear test was calculated by dividing the maximum load by the bonded area, which can be represented by Equation 1.Note : P(a)N = polymer-modified mortar at polymer ratio an applied on a concrete surface without bonding agent, P(a) BA (i) = polymer-modified mortar at polymer ratio and applied on a concrete surface with bonding agent at ratio I, Poly = Polymer, BA = bonding agent, Acc = Accelerator.

𝑆 = 𝑃 𝐴𝑟
(1 where S is the bond strength (MPa), P is the maximum force recorded (N), and Ar is the area of the slant surface (mm2).In this case, the slant surface area can be taken as a nominal value of 100 x 100/sin 30 = 20,000 mm 2 .

Effect of bonding agent on the interfacial bond strength
Previous studies have shown that the addition of a bonding agent positively and negatively affects the interfacial bond strength [15].In general, repair mortar with the addition of a bonding agent is able to increase the shear bond strength.composite specimens can show directly a significant change in the effectiveness of the bonding agent in increasing the interfacial bond strength.In the current study, the effect of bonding agent on interfacial bonding strength is shown in Fig. 3. Concrete substrate specimens without additional bonding agent on the surface of the bonding plane (NN specimens) showed the lowest interfacial adhesive strength, namely an average of 1.23 MPa if determined by slant shear test.the application of a binder on a smooth concrete substrate surface can increase the interfacial strength to 41-143%.For example, NBA 1:1 specimens have an average bond strength of 1.75 MPa by slant shear test or 42% higher than NN specimens.The increase in bond strength continues to occur along with the increasing dose of the bonding agent.at the cement to bonding agent ratio of 1:1.75, an increase in the bond strength tested by the slant shear test was recorded by 143%.According to Zhang et al. [16], the increase in interfacial bond strength is due to the bonding agent working to close the pores on the concrete surface to form bonds with the repair material.Apart from acting as an adhesive, the content of acrylic styrene emulsion in the bonding agent can prevent corrosion [17].All specimens indicate an interfacial failure mode where the repair material is separated from the concrete substrate (Fig. 4).This type of failure can be related to the smooth interfacial texture of the specimens, where this texture provides less friction than expected on a rough surface.However, applying a bonding agent on the smooth interfacial texture can significantly increase the bond strength Fig. 4. Effect of bonding agent on surface adhesion on 3 days.

Effect of acrylic copolymer on the interfacial bond strength
The influence of the polymer (acrylic copolymer) on the interfacial bond strength between the polymer modified mortar and the concrete substrate is presented in Fig. 5.In general, the addition of acrylic copolymer can significantly increase the interfacial bond strength as determined by slant shear tests.a 100% increase in bond strength occurred in the repair material with the addition of acrylic copolymer compared to mortar without acrylic copolymer (NN specimen) Previous studies have shown that the addition of 5, 10, 15, and 20% acrylic copolymer can increase the bond strength respectively by about 17, 24 , 28, and 22% [18].Other studies have also stated that the addition of acrylic copolymer to the mortar can increase the bond strength by 2.6 MPa [18].The reaction between the cement and the acrylic copolymer affects the strength of this bond.Based on research from Wang R et al. [19], acrylic copolymers can react with hydrated cement and form interpenetration networks with large molecules composed of ionic bonds, thus compacting the hydrated cement, resulting in less water penetration.The results showed that the addition of acrylic copolymer variedly increased the bond strength although not significantly.addition of acrylic copolymer with a ratio of 1:1 -1:2 to cement resulted in an increase in bond strength of less than 7% when determined by a slant shear test.This insignificant difference occurred both in the polymer modified mortar that was applied with a bonding agent or without a bonding agent.The results also showed that the composition of acrylic copolymer with a ratio of 1:1.5 to cement content provided the greatest adhesive strength.The failure model generated by both tests shows that the interface failure mode occurs in all specimens.

Correlation between compresive strength and interfacial bond strength
The correlation between the compressive strength of the repair material and the interfacial bond strength between the repair material and the concrete substrate is very interesting to study. it can estimate the interfacial bond strength based on the compressive strength which is known to be correlated.but this needs to be considered because the development of bond strength is not only in adhesion but also affected by friction.Table 4 shows the compressive strength of the repair mortars investigated in this study.The compressive strength is determined on a cube test object of 50 mm x 50 mm x 50 mm according to ASTM C109 [20].This data is used to generate the relationship between compressive strength and adhesive strength.From these data and juxtaposed with Jais et al. [21], Aliabdo et al. [22], Hasan et al. [23] obtained a graph of the relationship between the slant shear test and compressive strength and linear regression.Meanwhile, graphs and linear regressions of compressive strength and adhesive strength based on the split tensile test (Fig. 6) were generated from the researcher's data compared to Jais et al. [21], Aliabdo et al. [22], Hasan et al. [23], and Pothinathan et al. [24].

Conclusions
The conclusion of this work can be drawn as follow.The inclusion of acrylic copolymer into repair mortar as polymer-modified mortar improves the bond strength between the concrete substrate and the repair mortar.Once the acrylic copolymer is included in the mixture of repair mortar, no significant increase in bond strength can be expected about the dosage.The ratio of the acrylic copolymer and bonding agents on the bond strength of polymer-modified mortar is 1:1.5, with a bonding agent shown by the specimen P 1:1.5.BA 1:1.75.The failure mode that occurs in this study is an interface failure.The compressive strength is sufficient to withstand the applied force.The effect of compressive strength on the strength of the slant shear test tends to be linear.

Fig. 3 .
Fig. 3. Effect of bonding agent on surface adhesion on 3 days.

Fig. 5 .
Fig. 5. Influence of acrylic copolymer on 3-day slant shear strength of repair mortar

Fig. 6 .
Fig. 6.Slant shear bond strength and compressive strength correlation This work was part of SMARTcrete Research Group "On the development of fibre reinforced for concrete repair," which was financially supported by the Hibah Mandatory Research Scheme, Sebelas Maret University.(Contract No 260/UN27.22/HK.07.00/2021)

Table 1 .
The surface type of concrete substrate
Note: NN = Normal mortar applied on a concrete surface without a bonding agent, NBA (i) = Normal mortar applied on a concrete surface with a bonding agent at ratio i, Acc = accelerator, BA = Bonding agent

Table 4 .
The 3-day mechanical properties of repair mortar with bonding agent and acrylic copolymer