Structural Masonry, Civil, IISc

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Behaviour of brick masonry subjected to out-of-plane static and dynamic loading

It is well known that one/two storeyed masonry buildings suffer a great deal of damage during earthquakes leading to significant loss of life. Mainly the damage to masonry buildings is due to out-of plane collapse of walls. It is evident that to prevent out-of plane collapse of walls, ductility has to be imparted to the brittle masonry. In this investigation a new method of reinforcing masonry called "containment reinforcement" has been developed to impart ductility to masonry. Containment reinforcement consists of small diameter wires/rods provided on both faces of masonry walls, held together with the help of lateral ties provided through bed joints. In this investigation both experimental and numerical studies have been conducted on unreinforced and reinforced brick masonry beams, columns and walls subjected to static and lateral dynamic loading. The ductility measurement of masonry provided with 6mm diameter mild steel rods and 2-3 mm diameter galvanized wires as containment reinforcement has revealed significant ductility values. The comparison of experimentally obtained static load response of contained masonry models with that of analytical response shows good correlation. 

Frequency and mode shape analyses of simple box type masonry buildings have been done. A simplified lumped mass analysis has been made to obtain the sway mode and torsion mode frequencies of masonry buildings. The results obtained by the simplified analysis have been compared with results of finite element analysis. The walls of masonry building analysed were modelled using orthotropic shell elements. This investigation shows that there exists a correlation between fundamental frequency of the building and the frequency of its cross walls.

Shock table studies have been conducted on 1/6th and 1/ 2.5-scaled masonry building models with and without earthquake resistant features. A simple version of the shaking table is a shock table, which is a horizontal table on which building models are mounted and subjected to base shocks. This is used to simulate the effects of ground motion by subjecting the model to a series of base impacts. By using simple pendulum impact device it is possible to control the magnitude of base shock. Also after each shock it is possible to study the progress of failure of walls. In this study an attempt has been made to study the scaling effects. Totally five models were tested in which 3 were of 1/6 th scale and 2 were of 1/ 2.5 scale models. In all the models door and window opening were provided in cross walls and there were no openings in shear walls.  One of the models did not have any earthquake resisting features. In two of the models (one of 1/6th scale and another of 1/ 2.5 scale) had horizontal bands at sill, lintel and roof levels as prescribed by BIS. In one of the 1/6 th scale model both horizontal bands and vertical containment reinforcement was provided. In one of the 1/ 2.5 scale model horizontal bands, vertical corner reinforcement and vertical containment reinforcement was provided. In these tests the amount of energy imparted to all the models either before they collapsed or became non-functional has been measured. The highlight of this investigation was that the model with containment reinforcement and horizontal bands was able to sustain significantly larger amount of energy and walls did not collapse even after 60 base impacts whereas the models with only horizontal bands collapsed after 25 base impacts.

 Further, full scale testing of two walls; unreinforced and reinforced with vertical containment   reinforcement and horizontal bands subjected to lateral soft impact load was done. A simple device was developed to measure the contact period of impact, which is vital experimental information for conducting theoretical analysis. The experimental responses were compared with a non-linear transient dynamic finite element and as found to be in good agreement. 

 

Strength characteristics of unreinforced brick masonry vaults

Experimental and analytical investigations have been conducted on collapse analysis of brick masonry vaults. Brick masonry vault of length 3.0m, rise 0.52m, thickness 0.075m and chord width 1.5m was constructed with lean soil-cement mortar (1:10:8). The vault was supported on all edges and subjected to patch load up to collapse by monitoring the formation of cracks at different stages of loading.  Elastic properties and flexural strength of brick masonry with lean soil-cement (1:10:8) have been determined experimentally. The influence of axial compression on the flexural strength of the masonry has been determined experimentally by testing brick masonry arches of different rises keeping the span and thickness of the arch constant. It has been found that flexural strength increases with increases in axial compression. This information is vital for conducting the numerical analysis.  The vault has been analysed using non-linear finite element method to predict the collapse load and formation yield lines. The collapse load has been determined using a progressive failure technique by an iterative method. In the FEA layered shell element has been adopted. Failure mainly due to cracking has been modelled using smeared crack approach. The results of first crack load, collapse load and pattern obtained by FEA compares reasonable well with the corresponding experimental results. A case study of an ancient vault at Gingee, Tamil Nadu was made to understand the implication of horizontal thrust on the stresses in the walls of the vaulted structure. Our study revealed how our ancient builders cleverly connected the vault to the walls eccentrically to offset the tensile stresses in the vault and the walls.

 

 

Behaviour of masonry using low-strength & low-modulus mortars under compression

Mortar modulus is kept lower than the masonry unit (brick/block) modulus for better compatibility, especially when the masonry is under compression. Burnt clay bricks available in southern Indian states have very low modulus (400 � 1200MPa) as against the modulus of most commonly used cement mortar (1:6) being in the range of 3000 � 6000MPa. Properties of low modulus mortars using soil-cement have been examined. Behaviour of masonry using such mortars and the local low-modulus bricks when subjected to compression is examined through a series of tests on wallettes. The results throw more light on the strength and deformation characteristics of low strength masonry, which ultimately helps in broadening the masonry design code (I.S. 1905).

 

Fracture Mechanics of Brick and Brick Masonry

Fracture toughness of brick masonry was obtained for the first time and it looks encouraging. Brick being relatively more brittle than mortar and brick masonry being more brittle than concrete, fracture mechanics of brick and brick masonry should be more relevant. It is hoped that it would start a new school of thought.

 

 

Utilisation of industrial solid wastes for masonry blocks

This work pertains to the studies on Bharath Gold Mines Ltd. (BGML) tailings (from KGF). BGML has accumulated fine gold mine tailings in 13 heaps amounting to 36 million tons for the past 100 years of operations. Properties of these tailings have been characterized and studies have been conducted to utilize the tailings for the production of stabilized blocks.

The mine tailings mainly consist of crushed rock dust. Hence cement stabilization techniques were explored for the production of building blocks. Also, mine tailings were explored for making burnt bricks. The following parameters were studied with respect to preparation of stabilized blocks:

  • Grain size distribution of tailings at different depths of the dumps

  • Influence of density on strength

  • Influence of cement content on strength

  • Influence of additives like fly ash and sand on the characteristics of blocks

  • Long-term strengths

Grain size distribution analysis reveals that the mine tailings is a well-graded material having more percentage of fines (<75 micron) except at the bottom portion of the dump, where sand size fraction is high. Mine tailings are very fine non-plastic crushed rock dust samples. The increase in the sand size fraction i. e. coarser particles at the bottom of the dumps may be attributed to cementation of particles due to very slow consolidation with time, or it may be due to type of ore crushing methods employed during earlier mining activities.

Influence of density on wet compressive strength of stabilised specimens was studied. Two cement contents (6% and 12%) were tried. These results clearly indicate the that 1) The wet compressive strength of cement stabilized blocks using mine tailings increases with increase in dry density irrespective of cement content for both the samples. 2) The compressive strength is very sensitive to the density of the block.

BGML mine tailings contain very high percentage (> 60%) of fines (< 75 microns size) especially at the surface and depths up to 10 -15m of the dumps. The mine tailings are cohesionless particles. Compacting such fine materials to blocks of higher densities will be a difficult task. Addition of coarse sand and industrial waste materials like fly ash will help in achieving higher density for the block. Experiments were conducted to understand the strength of cement stabilized blocks using mine tailings and other additives like fly ash and sand. The results show that the strength increases with increase in fly ash content up to about 20% fly ash, beyond which there is hardly any increase in strength. Optimum fly ash content for strength is about 20%.

The broad conclusions of these studies are:

  • The experiments conducted have clearly shown that mine tailings can be utilised to produce high quality stabilised blocks as well as burnt bricks.

  • Mine tailings are non-plastic and devoid of clay minerals and hence mine tailings alone will not result in good quality burnt bricks. A mixture of local soil and mine tailings results in a good quality burnt brick.

  • These investigations have proved the possibility of using mine tailings in bulk quantities for the manufacture of blocks/bricks. For instance, a 100m2 residential building needs about 25,000 bricks (= 75 tons). Present stock of 32X106 tons of tailings at KGF can result in 12 X 109 bricks worth Rs. 2400 crores (at the present market rates). This can meet the entire requirement of bricks for Bangalore city for about a decade.

 

Lime stabilised steam-cured blocks for masonry

Sandy soils containing predominantly non-expansive clay minerals are ideally suited for cement stabilised soil blocks. Soils containing expansive clay minerals and also high clay soils are difficult to stabilise using cement alone as a stabiliser. Lime is essential to stabilise such soils. At ambient temperatures (20 - 30OC), lime stabilised soil blocks require longer duration of curing (>1 month) for satisfactory strength development. Curing at elevated temperatures can accelerate lime-clay reactions. Steam curing of lime stabilised blocks for a short period (~10 hours) at 80OC, can lead to satisfactory strength development. A study was undertaken with the scope as follows:

  1. Understanding the properties of lime stabilised steam-cured compacted soil blocks using expansive soils and high clay soils

  2. Monitoring long-term strength of lime stabilised steam cured blocks

  3. Developing small-scale production systems for lime stabilised steam-cured blocks.

Influence of steam-curing period, lime content and fly ash content on compressive strength of soil blocks was studied through laboratory experiments. Duration of steam curing controls the strength development in lime stabilised blocks. There is a sharp increase in wet compressive strength as curing period is increased. Steam curing at 80OC for 10 hours leads to 3 to 6-fold increase in strength when compared to 28 day wet cloth curing at ambient temperature.

The compressive strength of steam cured blocks increases with increase in lime content until an optimum content beyond which the strength falls. Percentage of clay fraction decides the optimum lime content for block production from the considerations of strength. Addition of fly ash greatly improves the strength. Higher quantity fly ash demands higher percentage of lime for obtaining higher strengths. Lime-clay ratio of >0.85 and lime-clay-fly ash ratio of >0.30 show stable long-term strength.

Steam cured blocks can be produced in a small-scale production system. Adjusting the mix ratios can easily control the strength of such blocks. Such blocks can be energy efficient alternatives to burnt clay bricks. Commercial scale plants using this concept are under construction involving private entrepreneurs.