Cohesive mortar

The types of water-soluble polymers used for the thickening cement slurries, mortar and concrete are shown in Table 6.6. Although many polymers shown in Table 6.6 can be used to increase the viscosity of the water in the mix, they are not all pseudoplastic polymers compatible with cement systems. Only a few can be consistently combined with water-reducing admixtures (WRAs) and superplasticizers to produce concretes with cohesive yet highly flowable mixtures [40, 41, 43]. 

The outermost layer of the skin, the cornified layer or stratum corneum, has been identified as the principal diffusion barrier for substances, including water [2,3]. It is approximately 10 to 20 pm thick when dry but swells to several times this thickness when fully hydrated [17], It contains 10 to 25 layers lying parallel to the skin surface of nonviable cells, the corneocytes, which are surrounded by a cell envelope and imbedded in a lipid matrix. This architecture is often modeled as a wall-like structure, with the corneocytes as protein bricks embedded in a lipid mortar [18]. Similarly to the viable epidermis, desmosomes (corneodesmosomes) contribute to the cell cohesion. 

Unlike polymer-modified mortars, unmodified mortars require moist cure for optimal effect. ASTM codes. C = cohesive failure A = adhesive failure. 

Adhesives are nonmetaUic substances used to join two surfaces by means of surface adherence (adhesion) and inherent strength (cohesion), DIN 16920. This definition of adhesives does not cover water glass adhesives, adhesive ceramics, or adhesive mortars. The substances used as adhesives are polymers that go through a liquid phase at least once (reactive adhesives) or more than once (hotmelts, thermally activated adhesives). The liquid phase can also be achieved by dissolution in suitable solvents (nonreactive adhesives). In dispersion adhesives, the polymer molecules are dispersed (finely distributed) in a liquid - usually water - whereby the polymer molecules themselves are not dissolved. Fig. 6. These adhesives are also known as water-based or aqueous adhesives. It must be remembered that solvents are contained in these adhesives in addition to the water. Genuine aqueous adhesives contain less than 5% solvents in the liquid phase. The dispersions crnitain, in contrast to the solute adhesive molecules, additional substances, disposal of which requires specific additional measures. Since the dispersions represent stable systems in water, the water-resistance of such adhesives is reduced. Their thermal and water resistance can be increased by additional crosslinking (usually with isocyanates). 

Good workability is important to ensure that the mortar is easily applied and can be effectively worked into the joints and crevices of the background material. A high degree of cohesiveness ensures that the mortar adheres to the trowel and to the background. It is also essential when using mechanical spraying equipment. 

Lime-based mortars have excellent workability and plasticity, a high degree of cohesiveness and spread easily under the trowel. These properties help to increase productivity and minimise wastage by droppings. They are eminently suitable for use with mechanical spraying equipment. 

High shear mixers are the mechanized equivalents of the mortar and pestle. Most mixers involve some shearing of the powder and some displacement and mixing will result. High shear mixers exert a high and local nip or shear on the powder and are especially useful for breaking down small aggregates in a bulk cohesive powder. 

M Cohesive Failure (Failure in latex-modified mortar)... 

S Cohesive Failure in Substrate (Ordinary cement mortar)... 

Abstract. The presence of water-soluble polymers affects the microstructure of polymer-modified cement mortar. Such effects are studied by means of SEM investigation. Polyvinyl alcohol-acetate (PVAA), Methylcellulose (MC) and Hydroxyethylcellulose (HEC) are applied in a 1 % polymer-cement ratio. The polymers provide an improved dispersion of the cement particles in the mixing water. The tendency of certain water-soluble polymers to retard the flocculation of the cement particles minimizes the formation of a water-rich layer around the aggregate surfaces. They also provide a more uniform distribution of unhydrated cement particles in the matrix, without significant depletion near aggregate surfaces. Both effects enable to reduce the interfacial transition zone (ITZ). The polymers also provide a more cohesive microstructure, with a reduced amount of microcracks. 

The paper describes the backscattered electron image analysis procedure, as well as the effects on thicloiess and composition of the ITZ and the cohesion and cracking of the bulk cement paste of water-soluble polymer-modified cement mortar. 

A difference in adhesion between the aggregates and cement paste of unmodified and modified mortars is difficult to detect. However, the cohesion of the bulk cement paste is improved by polymer modification. Irrespective of the crack initiation mechanism, a much higher amount of microcracks is detected in the bulk paste of unmodified mortar. Fig. 7. 

In the experiments carried out so far, it was shown that the longitudinal change behaviour of mortars is negatively influenced by polymer modification. In order to draw conclusions what kind of effect this has as far as cohesion in the PCCs is concerned, model tests were carried out on stone samples which were coated on both sides with modified cement paste. In preliminary works before, the development of the dynamic E-modulus as well as of the tensile strength of the cement stones had been investigated. 

In the course of hydration - when the increase of the tensile strength of a mortar caused by polymer modification is not so significant and when deformation is hindered - an exceeding of the tensile strength in the mortar matrix can occur, this leads to cohesion failure - cracks are the consequence. 

The extensibility of the swollen objects depends on the conditions of preparation and on the degree of swelling, but lies in the order of magnitude of 100% (for the air-dry filaments, which contain about 15% water, the extensibility is usu Jly smaller and below 100%). Fully dried filaments cannot be extended at all, they are hard and brittle as glass and can be pulveris ed in a mortar. The presence of water is essential in order to make a cellulose gel deformable. Dry cellulose is comparable with rubber cooled to temperatures far below 2 ero, say — 100°. In both cases the cohesion between the chains is so considerable that there is no more question of an internal mobility, which is essential in order to render large deformations possible,... 

Most physical models in tsunami studies consist of scaled offshore bathymetry and a shoreline to simulate a beach, island, or coastal structure such as a breakwater or seawall. The models are usually fixed bed since sediment transport issues are not considered. The fixed bed model is typically constructed using templates and sand with a thin mortar cap or veneer. If a movable bed is used, then scahng issues become a concern since it is difficult to properly scale both the sediment and the model with the same scale factor. Sediment grains can be minimized only so much before changing from noncohesive (i.e., sand) to cohesive behavior (i.e., mud). 

Properties of mortar depend, to a large extent, on the type and quantity of sand used. Good mortar is made from sharp, clean, and well-screened sand. When sand is too fine, the mortar has less give and water works out of it, making it stiff and difficult to trowel. It may also set before the bricks can be placed. Too much sand robs the mortar of its cohesive consistency and makes it difficult to work with. 

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