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Secondary materials in mortars for use in historical buildings

5 July, 2019 8 min reading
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Based on the thesis of: Menard Kilumile, Beijing Jiaotong University | NJTU · Department of Civil Engineering Contact

Secondary materials in mortars for use in historical buildings

Ancient lime mortars are weak and poorly set in high humid areas in which most of the historical structures are found. Most of historical buildings are founded in areas where the environmental dynamics are high. As a result, in some parts mortars in historical buildings have failed to resist the environmental damage.

 

The principal objective of this thesis by Menard Kilumile is to assess the suitability of the secondary fine aggregates for use in historical mortars.

The definition

The terms masonry refers generally to brick, tile, stone, concrete-block or combination thereof bonded with mortar. In some cases, dry joints are applicable. The use and evolution of material in Historical buildings has been a result of human civilization in which the application and discovery of simple working tools has had enabled production of simple portable units.

 

During the ancient period, it was a tradition to use local available material. The function of the building and the richness of the owner are among other factors e effected the choice and quality of the construction materials. Wood, earth, bricks, stones and iron materials have been for a long time used in not only construction but also repair of Historical buildings. Among other material used in historical buildings include the binders which differed depending on the environment and cost of production in terms of cost and energy requirements.

 

The problem

Historical repair mortars should develop high early strength enough to support the elements of the buildings. Natural hydraulic lime which is compatible with historical binders is therefore suitable for repair of ancient buildings.

 

Population pressure and the growth of construction industry with modern designs has resulted to the replacement of old structures with new ones leading to the accumulation of the construction and demolition wastes that requires sites for disposal, the process which litters the nature. The demand and exploitation of the non-renewable construction materials on the other hand has led to the depletion of the same and environmental degradation.

 

Most of such construction and demolition wastes however have undergone heat treatment during their manufacture processes converting their chemical compositions and forming more reactive compounds when activated. Ceramics for example contain clay and some recycled aggregates are calcite. With lime, the materials undergo pozzolanic reactions forming hydration products or phases that are responsible for strength development of the matrix.

 

It is thus possible to recycle and examine carefully the engineering properties of the secondary aggregates mortar matrices which if found suitable will provide double advantages; the improvement of the engineering properties of historical lime mortars and the conservation of the environment for the future generations at minimum possible expenses.

 

Historical masonry materials

  • Adobe and red brick units

Adobe can be referred to either earthen constructions or buildings with sun dried bricks. The focus of the subject is on to the sun dried bricks. The adobe, or sun- dried brick, is one of the oldest and most common building materials known to man.

 

Traditionally, adobe bricks were never kiln red. Unbaked adobe bricks consisted of sand, sometimes gravel, clay, water, and often straw or grass mixed together by hand, formed in wooden molds, and dried by the sun.

 

The strength of the units depends on the amount of the clay in the sand. Higher amount improves the strength. However, the main problem with the material in constructions is the dimensional and strength instability when moisture varies. In some areas the problems were combated by incorporation of stabilizing agents into the sand matrix during brick making. Unburnt bricks can absorb as high as 35% of their weight in water. Fired bricks technology introduced a magnificent development in historical building materials. With the introduction of kilns, it was possible to produce bricks of relatively higher stability and strength even at elevated moisture content. The need for fuel however was a setback in production of large quantity of bricks in most of the regions limiting the use of red bricks to a few areas of importance such as spiritual buildings and to some dwellings of rich people.

 

The engineering properties of historical brick units vary greatly. The variations are reported to be resulted by multidimensional factors. Among other factors include; variability in production, environmental pollution and differences in age of production. Kiln type and the burning temperature which influenced heat distribution during ring are other causes of strength and density disparity of the historical brick units. Bricks in the 18th and 19th century were soft and porous absorbing 20-25% of their weight in water. A study by Mc Burney in 1929 reports the range of some engineering properties of the bricks in the 19th century. Compressive strength ranged from 19,375 kN/m2 to 71,019kN/m2, 1,531 to 4,144kN/m2 for tensile strength, 4,358 to 10,136kN/m2 bending strengths and 7,585 to 24,477kN/m2 for shear strengths.

 

In Historical buildings, masonry brick units could be found in the Great Wall of China, Rome – Coliseum (72 – 80 AD), Sakkara, Egypt – sun dried brick pyramid – 3,900 BC, the ancient city of Ur, Iraq: 2125 – 2025 BC, and the ancient city of Babylon (MSRIT, 2015).

  • Masonry stone units

Granite stones, limestone and sandstones are among other materials used in construction of historical structures. The stones varied in strength not only between buildings but also within the building or the element. The choice of the type of the stone for a particular building was a function of the proximity of the quarry. The Great Wall of China for example consists of several sections of different stones quarried from nearby mountains.

 

The properties of historical stones units are vary depending on the area of formation. Physical properties depend on composition and texture or geological origin of the lock in the given locality. In mechanical point of view, stones are brittle and behave elastically to failure, have high compressive strength and for porous sandstones, the strength is dependent on the porosity and is weakened when subjected to water under pressure. Of the important physical properties which affect the mechanical properties are porosity and permeability which decides the amount of water ingress into the units leading to decay and corrosion of the masonry.

 

  • Ancient Mortars

Mortar is a plastic mixture of sand, water and a binder. In masonry, mortars bind the masonry units together, cancel the effect of the irregularity of the stones or bricks units to facilitate stacking of the units and prevent concentration of stresses by distributing uniformly the compression stresses. Ancient mortars were non- hydraulic. Studied in history of construction and conservation, was the following types of mortars; Mud. This was mostly used in adobe units. Discovery of re bricks necessitated the use of higher strength mortars, for which the use of bitumen (in Mesopotamia), gypsum (in Egypt) and lime in other parts of the world was adopted. The mortars however were poor and the properties were greatly influenced by the environment. Bitumen melts in high temperatures, gypsum, although it is abundant in nature, requires less energy than lime during production and it can easily be extracted, it is problematic as it sets quickly, it is hygroscopic and has high water absorption capacity.

 

Old non-hydraulic lime mortar required low humidity and air for hardening. Unfortunately, most historical constructions were found in high humidity areas. This further led to discovery and use of cementitious materials. In 300 BC as an example, Romans used a mixture of slaked lime and volcanic ash named pozzolana and other admixtures such as animal fat, milk and blood. Pozzolana cement is reported to have been used in 200 AD to build the vault of the Pantheon in Rome. In 18th century, the discovery of Portland cement substituted ancient historical binders in mortars used for both construction and intervention purposes.

 

Conclusions 

An intensive analysis of the laboratory test results shows that recycled concrete and bricks fine aggregates are suitable for use in historical hydraulic lime mortars. This deduction is on the basis of engineering properties required for historical mortars for which mortars containing recycled material have found to surpass mortar with natural river sand which in the study was used as a reference mortar. The following conclusions are based on specific:

 

In terms of mechanical properties, the properties of historical bricks are found to vary greatly. Input parameters during analysis of historical buildings should therefore be carefully determined and preferably using a large representative samples.

 

RBS mortar attains higher early flexural strength and after seven days the rate of hardening increase steadily. This is due to the existence of pozzolanic reactions in the mix. For this reason, RBS mortar is a better repair material in historical buildings where high early strength is paramount. The development of mechanical properties in natural sand and recycled concrete sand mortars are comparable and are therefore compatible repair material. The mechanical properties of the stacked prisms are alike irrespective of the type of mortar used, and therefore it is possible to say the properties independent of the type of the mortar and greatly in influenced by strength of brick units.

 

Use of recycled fine aggregate in mortars improve significantly the physical properties of mortars. However, air content and ow table of the reference mortar and mortars containing the recycled ne sands seem to be similar. This might have been resulted by similarity in particle size distribution.

 

In both fresh and hardened state, mortars containing recycled concrete and bricks are less dense and thus can significantly reduce the dead weight of the building.

 

The characterized bricks units were found to be equally dense.

 

Bricks were established to have similar porosity and water absorption capacity. RCS and RBS mortars have higher porosity and water absorption than the natural river sand mortar.

 

For hygric properties, the bricks have different water absorption velocities. The RCS and RBS mortars have high absorption velocity than the reference mortar. Such mortars can therefore be used as sacrificial finishes or material to drain masonry units in buildings. However, the desorption characteristic of the mortars with the recycled material seem to be poor than that of the reference mortar. At early age, this can have been resulted by hydration reactions the process that consume water.

 

With drying shrinkage, mortars with recycled aggregates holds significant amount of water which is useful for hydration and reduces formation of cracks. During the first 4 days, the contraction of mortars with recycled aggregates is much lower than that of the reference water. After 28 days, shrinkage of all the three mortars was the same.

 

The heat conduction of the RCS and RBS mortars are comparable and lower than both the heat conduction of the reference mortar and the brick units. For this reason, the recycled fine aggregates are suitable for insulation mortars.

 

The coefficients of thermal expansion of recycled concrete aggregates and the recycled brick fine sands are lower than that of the reference mortar and the brick units. Thus, mortars with recycled materials can be used to envelop buildings against environmental deterioration.

 

The thermal response of masonry wall is greatly influenced by the thermal characteristics of the masonry units and not the mortar used.

 

With recycled aggregates, it is possible to blend the materials to form mortars of different natural colors.

 Future studies

The characteristics of recycled bricks and concrete fine sands mortars studied have been found suitable for historical mortar. However, the following, which have not been dealt with in this study and greatly affect the stability of both the repair and the repaired material should be explored;

 

– Bond strength in influences the shear strength of the masonry walls and therefore bonding characteristics between the mortars with recycled aggregates and different substrates should be assessed.

 

– Stiff mortars are not suitable due to the reduced flexibility when the structure is subjected to movements. For this reason, it is necessary to establish the stiffness of the mortars which will ensure compatible use of the mortars and will avoid any possible damage due to concentration of stresses in masonry joints.

 

The main problem with mortars in historical buildings are the salt attack (crystallization) and frost damage. The resistance of the studied mortars to such attacks has not been established. Hence, in the future it is important to examine the durability of mortars with recycled concrete and brick aggregates.

 

– Study on the evolution of the established characteristics in this thesis over a long period of time. The study has been conducted with in a duration of 45 days or less. The characteristics which have been found better within this short period may probably worsen with time. It is thus necessary to have a long term captured trend of such properties which will useful in maintenance and planning for repair works.

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