Adhesion of Masonry Coating: Effect of Mortar Consistency and Type of Substrate

Tests were carried out on the different mortars and their components (sands and cement) as well as their substrates (concrete and breeze blocks). The number of samples used in each test are three and the result of each test is the average obtained for the three samples.

2.1 Sands

Two types of sand were used in this study. A crushed limestone sand "SC" coming from the quarry in region of Guelma, and a dune sand "SD" coming from the region of Tebessa. These two regions are located at east of Algeria. Table1 summarizes the main characteristics of the two types of sands used in this study. These sands have properties similar to those used in other countries.

Table 1. Physical characteristics of sands (SC, SD)

2.2 Cement

The cement used for the mortars and the concrete support is a CPJ CEM II/A 42.5, from the HdjarSoud -SKIKDA cement. It has an absolute density of 3200 Kg/m³ and a specific surface of 3300-4000 cm²/g. The chemical and mineralogical compositions, of cement and clinker, respectively, are given in Table 2.

Table 2. Chemical and mineralogical composition of cement

2.3 Mixing water

The mixing water used in this research is tap water, available at the Civil Engineering and HydraulicLaboratory (LGCH) at the university 8 May 1945 (Guelma). Its temperature is between 18 and 22℃.The chemical analysis of the water is shown in Table 3 and meets all the requirements of standard [9] for mortar.

Table 3. Chemical characteristics of water (mg/l)

2.4 Properties of the substrates

Two different supports were used in this study. These were masonry substrate (breeze block) and concrete substrate. The composition of 1m³ of the concrete base consists of 582 kg of sand, 911 kg of 15/25 gravel and 153 kg of 8/15 gravel, 315 kg of cement, and 172 liters of water. The material strengths of these two substrates are respectively 5.24 and 26.82 MPa for compression and 0.23 and 3.87 MPa for flexural tension.

2.4.1 Capillary absorption coefficient

The capillary water absorption test was carried out on circular samples of 15 cm diameter and 4 cm thickness for concrete blocks and breeze blocks of 40×20×15cm³. The test procedures applied are those laid down in the standard [10]. Firstly, the specimens were left for a week in laboratory conditions to stabilize the humidity. Then, they are placed in an oven at 105℃ until they are of a constant mass. The weight of the prepared specimen m_o is measured immediately before testing. The bottom side of the specimen is immersed in water (tap water) up to a depth of 5 millimeters. The water level is kept constant during the duration of the test (Figure1). To obtain a unidirectional flow, the sidewalls of the samples were covered with paraffin in several layers. The uptake of water by capillary absorption is measured through the weight of the specimens m(t) at time intervals of 0.5, 10, 20, 30, 60, 180, 360, and 1440 minutes. The water absorption of each test specimen w(t)=m(t)-m_o is recorded and presented for each of the specified time intervals. Together with the absorption data, the area of the tested surface is given for each specimen. The absorption of water per unit area of the test surface [kg/m²] is calculated for the specified suction periods.

The following formula obtains the capillary absorption coefficient: Ca= $\frac{m_0-\mathrm{m}(\mathrm{t})}{\mathrm{S} \cdot \sqrt{\mathrm{t}}}$, it is expressed as $\left(\frac{\mathrm{kg}}{\mathrm{m}^2 \sqrt{\min. }}\right)$.

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Figure 1. Water capillary absorption test

The results of the capillary absorption of the two substrates are shown in the following Figure 2.

Figure 2 presents the results of the capillary absorption of the two substrates. It can be noted that the variation of wet mass/surface area is a function of the square root of time.

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Figure 2. Water absorption by capillary of substrates

The following formula obtains the capillary absorption coefficient: Ca= $\frac{m_0-\mathrm{m}(\mathrm{t})}{\mathrm{S} \cdot \sqrt{\mathrm{t}}} \ldots$ Eq. (1), it is expressed as $\left(\frac{\mathrm{kg}}{\mathrm{m}^2 \sqrt{\min } .}\right)$.

It can be noted that the breezeblock substrate has higher absorption than the concrete substrate. This difference in absorption is mainly due to the porous structure of the breeze block. The breezeblock has a capillary absorption coefficient of Ca=0,037 $\frac{\mathrm{kg}}{\mathrm{m}^2 \sqrt{\min } .}$, which is 75% higher than concrete (Ca=0.025 $\frac{\mathrm{kg}}{\mathrm{m}^2 \sqrt{\min }.}$).

Lap-shear tests evaluated the adhesion between the different mortars and the different substrates on samples with a cross-section of 50×50 mm² (Figure 3). The principle of the test is to apply a compressive load on either side of the interface, parallel to the interface, to stress it in shear. The disadvantage of this test is that it can lead to mixed loading of the interface (tensile/shear), as indicated by Mauroux [11].

The results present the average of tests of three samples for each mortar consistency and each type of substrate.

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Figure 3. Lap-shear adherence test of mortars

4.1 Effect of the nature of the mortar and the substrates

The failure was adhesive between the substrate and the lining mortar in the tests. The best adhesion was obtained with the plastic mortar on the breezeblock and concrete substrates (Figure 4). These 28 days shear strengths are 3.86MPa and 2.01MPa for breeze block and concrete substrates. The composition of the mortars alone cannot explain the differences between the bond strengths of the different mortars. The fluid, plastic, and firm mortars have 28 days compressive strengths of 26.18MPa, 29.92MPa, and 30.64MPa, respectively.

In contrast, their flexural tensile strengths are 6.37MPa, 7.08MPa, and 7.50MPa, respectively. Thus, the strongest mortar did not give the best bond. As stated by study [10], the adhesion between the two materials develops before the cohesion of the mortar is effective. It should be kept in mind that other parameters influence the adhesion of mortars, in particular the nature and surface of the substrate and the thickness of the mortar.

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Figure 4. Shear strength of mortars at 28 days (thick=2cm)

The rough and porous surface condition of the breeze block contributed positively to the adherence of the different mortars. The breeze block substrate has better mortar adhesion than the concrete substrate. The breeze block substrate has higher mortar adhesion than the concrete substrate. The adhesion increases as the roughness of the substrate increases. The mortar, when fresh, penetrates easily into the interstices of the substrate. As Mauroux [11] indicates, increasing the complexity of the surface texture of the substrate promotes the extent of the specific surface area of the interface and thus increases adhesion. There is also another phenomenon of water exchange that occurs when a mortar is applied to a substrate. This water exchange is more or less important depending on the nature of the substrate [12]. The breeze block has a capillary water absorption coefficient 75% higher than that of substrate. Therefore, the suction by the porous substrate of the breezeblock is more important, which decreases the amount of water in the mortar.

4.2 Effect of age

The evolution of the adhesion of mortars over time is affected by their composition and the nature of the substrate to which they are applied (Figures 5 and 6). Fluid mortar increases adhesion between 7 and 28 days by 3% and 147%, respectively, on breeze block and concrete substrates. These increases are 104% and 279% for plastic mortar and 41% and 25% for firm mortar. These changes are greater than the changes in the intrinsic strengths of the mortars. The increase in compressive strength of the other mortars between 7 and 28 days varies from 15 to 29%. The increase in flexural tensile strength of the mortars varies from 23 to 50%. Sadowski and Stefaniuk [13] argue that the evolution of mortar adhesion on concrete substrate tends toward an asymptotic value from the seventh (7) days after the material has been spilled. This finding was not verified by our tests.

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Figure 5. Evolution of shear strength of mortars on breezeblock substrate with age (thick=2cm)

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Figure 6. Evolution of shear strength of mortars on concrete substrate with age (thick=2cm)

It shows that the effect of the nature of the support is more visible at a young age. The differences vary from 35% to 257% at 7 days and only from 8% to 92% at the age of 28 days

4.3 Effect of thickness

The 28 days bond strengths were determined for two mortar thicknesses (1 and 2cm). This was done for the three types of materials (fluid, plastic, and firm) and the two substrates (breeze block and concrete. For the different mortars and their substrates, an increase in adhesion with increasing thickness can be observed (Figures 7 and 8). The plastic mortar shows the highest increase, 118% for the breeze block substrate and 114% for the concrete substrate. The other two mortars have smaller increases ranging from 8% to 67%. These results contradict the results obtained by Khaldi et al. [14]. The authors indicated a decrease in the adhesion of mortars with an increasing thickness. Other research is in line with our results. The thin thickness of the mortar favors the rapid departure of water by evaporation leading to poor hydration. Travailleur [12] indicates highlighted that for a thickness of less than 20 mm, there is not enough water after 24 h of drying to ensure the hydration of the cement. In addition, we can notice the cracks resulting from drying shrinkage that can initiate delamination by edge effect at the ends, as shown by Certain et al. [15]. At the same time, a greater thickness leads to different hydration in the depth of the mortar. In addition, lower hydration of the material in the vicinity of the drying surface can be observed. Therefore, differences in the properties of the material once cured depending on the depth, as indicated by Amba et al. [16], can be noticed. Thus, the adhesion layer will have better properties.

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Figure 7. Effect of thickness on the shear strength of mortars on breeze block substrate, at 28 days

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Figure 8. Effect of thickness on the shear strength of mortars on concrete substrate, at 28 days

The authors thank the LGCH research laboratory at the University 8 May 1945 (Guelma) and the pedagogical laboratory of the civil and hydraulic engineering department (Guelma) for their technical support during the experimental work.