Investigation of rain surface erosion and bonding strength of different wall care putty materials along with different walling materials

Highlights

• Tested durability in terms of rain surface erosion and bonding strength of wall care putties with walling materials.

• Accelerated erosion test and lap shear strength test were conducted and bond breaking patterns were examined.

• X-ray diffraction analysis was conducted to investigate the chemical compositions of putty mixtures.

• Results showed that cement block walling material and Putty C +10% cement have the highest durability and bonding strength.

• Cement added putty mixtures showed high durability and bonding strength than putty in its pure form.

Abstract

In tropical countries such as Sri Lanka, wall care putties are applied as primers in building constructions providing a smooth shield for the wall. This research study was conducted to investigate the durability performance in terms of rain surface erosion and the bonding strength by shear testing of different wall care putty materials along with walling materials. Four types of walling materials and ten types of wall care putty mixtures were selected. Accelerated erosion test and lap shear strength test were conducted and bond breaking patterns were examined. X-ray diffraction analysis was conducted to investigate the chemical compositions of putty mixtures. Results demonstrate that cement block walling material and putty C+10% cement wall care putty mixture have the highest bonding strength. Similarly, cement added putty mixtures showed higher durability and bonding strength than putties in their pure form.

Introduction

In building design, walls are the core structural elements which separate indoor and outdoor environment. Outdoor environmental conditions such as rainfall, solar radiation and wind have more critical effect on the wall of a building. In tropical countries such as Sri Lanka, people have the habit of selecting solid walling materials due to high solar radiation and heavy monsoon rain [1,2]. At present Sri Lanka's most available walling materials are cement blocks (CB), cement stabilized earth blocks (CSEB), fired clay bricks (B) and cabook blocks. Mud concrete blocks (MCB) and geo polymerized earth blocks have been recently developed and possess sustainable characteristics [1,3]. Similarly, coatings are applied on the wall in order to protect it from extreme climatic conditions [4]. Coatings can be categorized as sealers, primers and top coats according to their functionality [5]. In Sri Lanka, masonry coatings such as cement plaster and rough cement plaster are used as a sealer in building constructions. Wall care putty materials are used as the primer and paints are used as the top coat in the building sector.

Wall care putty materials provide a protective base for the sealer and a smooth surface which bring out a finished appearance suitable for paint application. The minerals such as calcium carbonate, quartz and talcum are the main constituents of most wall care putty materials [6]. In the past, lime putty was the traditional material with a wide application in the construction [7]. At present, white cement wall putty, acrylic wall putty and plaster of paris (POP) are some of the well-known commercially available putty types. White cement wall putty which is a mineral, is the trendy putty type. It is constituted by white cement and polymer based putty of solid base formulated to give white coating, bright, smooth and superior finishes on wall surface. Acrylic wall putty is an acrylic and water based ready to use putty type of liquid base. POP is basically gypsum, a white powder of solid type which sets and gets hard when mixed with water and is used as a base.

Wall care putties are bonded to both masonry and paints where, bonding strength is a crucial factor. Bond strength can be explained as the amount of stress required to rupture an adhesive bond between two surfaces [8]. It can be quantified by bond energy and bond dissociation energy where the nature of the coating affects the bonding strength [9]. Bonding strength between a coating and a substrate can be described as adhesion where several mechanical, physical and chemical forces come into play. These forces have effects on each other [10]. Wall care putty materials play an important role in guaranteeing the quality in engineering projects where it should depict an appropriate durability, high bonding strength and good crack resistance [11]. The durability performance of the walls increase with the durability of the wall care putties [6].

Durability is one of the properties that usually provide measures of the performance and functions of the materials under the expected conditions [[13], [12]]. According to the ASTM standard E 241 durability can be defined as the safe performance of a structure or a portion of a structure for its expected design life [14]. Hence it is essential to select durable material components to perform their functions over their expected lives [15,16]. Stress factors such as mechanical, thermal, environmental or biological factors can cause a material to degrade over time and to produce poor durability performance. Solar radiation, wetting due to monsoonal rainfall, wind erosion are the typical stress factors which affect the durability in a tropical climate [17]. Out of the above mentioned stress factors, rainfall has the most impact on durability performance under Sri Lankan condition [18]. Being a tropical country, Sri Lanka has a humid warm climate constant throughout the year. As it is an equatorial country it receives intensive sunlight creating relatively high temperature [19]. The temperature fluctuates between 26.5 °C and 28.5 °C with a mean annual temperature of 27.5 °C [2]. Monsoonal, convectional and depression rains account for a foremost portion of rainfall with annual values varying between 900 mm in driest parts and 5000 mm in wettest parts (Fig. 1) [20]. Surface decaying is the most common phenomenon occurring due to rainfall which affects the durability performance. Bouncing rain and wind driven rain are the most significant rain types that contribute to surface decaying [1,17]. When the durability performance of the wall care putties is high, it ensures the durability performance of the wall as well as the top coat.

In this research study, the first objective is to identify and compare the durability performance of different wall care putty materials along with walling materials in terms of surface decaying caused by spray erosion testing. The second objective is to determine the bonding strength of different wall care putty materials regarding walling materials by shear testing and characterizing bond breaking patterns. According to the literature, durability performance and bonding strength of different walling materials have been investigated [21]. However, durability performance and bonding strength of wall care putties with different walling materials have not been assessed so far.

Spray erosion test or accelerated erosion test is the ideal testing method used for quantifying the surface decaying caused by rainfall. In other terms it measures the resistance to weather under extreme conditions [22]. The surfaces of a wall should represent its first line of defense against deterioration, because once the surface is eroded, it can rapidly lead to the exposure of less dense, less compacted, and more vulnerable inner core. This exposures can lead to irreversible and accelerated damage to the whole block [13]. Density of the surface material and surface roughness are two of the prime factors which affect surface decay. Similarly, water absorption is affected by the density of the surface material. Wall putty is applied to an average thickness of 3–5 mm in Sri Lanka [23]. In exterior environments, wall putty acts as a protective covering to the wall. It reduces the surface roughness and increases the density of the material surface [21,24]. By spray erosion testing we can quantify the depth to which surface decaying is caused in wall putties.

There are different types of bond strength test methods where shear, tensile, peel testing with torsional and pull-off tests are the most commonly performed. These bond tests allow determining the adhesive bond strength in a given direction or under a given type of stress. Lap shear strength test determines the shear strength of adhesives for bonding materials when testing on a single-lap-joint specimen [25]. Wall putties are scaled off through shear forces. Therefore lap shear strength test can be used to identify the bonding strength of wall care putty materials. Adhesive strength, surface preparation parameters and adhesive environmental durability can be determined by this test. An adhesive is applied between the two substrates and its bond strength and the ability to hold the two substrates together while under stress is determined [26]. A force is applied, steadily increased and the bond fails until the materials separate; the highest possible joint strength is identified. By characterizing the bond breaking patterns, strength of the adhesive and substrates can be critically analyzed [27].