BMS College of Engineering, Bangalore Department of Civil Engineering

Determination of Fracture energy and Fracture toughness of

Compressed Stabilized Earth Blocks

Report on Phase I proceedings

Submitted by:Supervised by:

Amit Yalawar Dr. Sakey ShamuAnuradha KamathAyan Anil GargJagannath N

CERTIFICATE

Date: 21/12/2013

This is to certify that the report entitled ‘Determination of Fracture Energy And Fracture Toughness of Compressed Stabilized Earth Blocks’ is an authentic work carried out by

Amit Yalawar 1BM10CV006Anuradha Kamath 1BM10CV008Ayan Anil 1BM10CV012Jagannath N 1BM10CV021of 7th semester and submitted during the academic year 2013-14 as part of Phase I of the Major Project.

Signature of Guide (Dr. Sakey Shamu)

Name of the Examiners Signature1.

2.

Acknowledgement

We have put in some efforts to complete Phase I of this Major project, however it would have been impossible to complete it without the valuable support and guidance of many individuals. We would like to extend our sincere thanks to each one of them.

We wish to thank our project supervisor Dr. Sakey Shamu for making time out of their very busy schedules and being our constant support continuously throughout the Phase I of this project. His valuable guidance and instructive inputs have constantly inspired us to carry out this project work.

We are highly indebted to all the teaching and non-teaching staff of the Department of Civil Engineering for their support, guidance and constant supervision and for providing us with all the necessary information regarding the project and also for their support in completion of the project despite their busy schedule.

Table of contents :

I. Identification of Problem

II. Objectives

III. Introduction

IV. Methodology

V. Progress so far

VI. Work to be carried out during phase II

VII. Proposed Outcome

Identification of the problemWith the growing concerns about environmentally friendly construction in the areas of health and resources as well as the increasing cost of wood and the environmental concerns of fired brick and concrete products, a new source of building material is becoming increasingly important for the industry. CSEBs make for a wonderful alternative that fulfills these requirements. However, awareness has to be spread for mass adoption of this material and technology.

Objectives

• To obtain the optimum mix design for Soil procured from Ghati Subramanya

• To determine the soil characteristics of the procured soil

• To manufacture CSEBs using cement as stabilizer

• To evaluate characteristics of manufactured blocks

• To determine fracture energy and fracture toughness of CSEBs

Introduction

Soil has been used as a construction material since time unknown. Adobe Masonry, Rammed Earth, Wattle and Daub, etc., have historically been used for building mud walls and are still in use even today. Soil represents an eco-friendly, recyclable, economical material and offers some significant advantages over other construction materials. The usage of soil as a structural material has drastically decreased since the advent of other materials like concrete and timber. Concerns about the environmental impact of fired bricks and concrete has rising steadily, while the costs of the conventional materials have inflated. With the latest developments in soil stabilising techniques, soil blocks are stronger and more durable than ever. As a result, soil-structures are undergoing a renaissance of sorts. As of now, there are more than 30,000 buildings that use Compressed Stabilised Earth Blocks (CSEBs) [1]. With this project, we make an attempt to understand the various properties of Compressed Stabilised Earth Blocks by manufacturing them in-house and conducting tests on them. We also attempt to determine the fracture energy and toughness of CSEB, which has not been reported anywhere as of this writing. Comprehensive reading has been done to prepare ourselves to be able to execute the project successfully in a timely manner.

Compressed Stabilized Earth blocks (CSEBs)

Compressed stabilized earth blocks a type of construction material manufactured in a mechanical press that forms a compressed block out of an appropriate mix of fairly dry inorganic soil, non-expansive clay, fine aggregate, and a small amount of cementIt is a known fact that soil when compacted at optimum moisture content reaches maximum dry density which depends on the energy

input during compaction and is used to calculate its compressive strength in dry state. But it loses its strength during saturation. To accommodate this loss, a stabilizer is required.Necessity of Stabilization: Stabilizers namely lime, cement, bitumen can be used for following purpose:

i) Loss of strength during saturationii) Abrasion due to rain impact.

Limited data sources on CSEB around the world, with different types of soils and stabilizers, is present regarding standard performance under service loads and behavior under ultimate loads.Advantages of CSEBs:

Earth is a local material and the soil is available virtually everywhere.

Earth construction is an easily adaptable and transferable technology.

It is a cost and energy effective material. It is much less energy consuming than country fired bricks (about 4

times less). It is much less polluting than country fired bricks (about 4 times

less). CSEBs are bio-degradable It facilitates effective management of resourcesLimitations:

 Proper soil identification is required or unavailability of soil. Wide spans, high & long building are difficult to do. Low technical performances compared to concrete. Low social acceptance due to counter examples.

Fracture Mechanics

Failures in structures/materials over centuries have occurred for many reasons:-

1. Uncertainties in the loading or environment,2. Defects in the materials,3. Inadequacies in design, and 4. Deficiencies in construction/production or maintenance.Cracks/flaws are present invariably in all structures to some degree. These cracks often act as stress concentrators and result in catastrophic crack propagation under increasing load. Majority of large scale failures in metals and structures have been attributed to presence of crack originating from a natural flaw. It has been observed that, even though these structures are designed using traditional design methods i.e. stress developed even at stress concentrators are well below permissible limits of ultimate load, cracks develop and grow to large sizes, resulting in complete brittle fracture at loads much less than the strength of structure. Such failures being of very great concern, have led to development and evolution of Fracture mechanics and devising a technique of design called Damage Tolerant Design Approach.

In early years of testing and study on failure, yielding of worst loaded point was considered the sole cause of failure. The idea then was to combine all the three-dimensional stress acting on the stress concentrated portion of the structure to principal normal stresses which act along principal planes. How the structure fails was then approached using several yield theories like Tresca failure criterion using maximum shear stress, von Mises theory using maximum distortion energy in the portion, etc. However it was found that structures fail even before that point reaches σy. Therefore, researchers started exploring the various causes of failure that can occur during service of the structure.

MethodologyManufacture of CSEBs

Photo credits: AnyWay Solid Environmental Solutions Ltd.

Soil required for manufacturing of CSEBs for this project is obtained from Ghati Subramanya, where BMSTRI is proposed to be built. Soil shall be subjected to various lab tests in order to determine its composition and other properties. The tests conducted on soil are:

• Grain Size Analysis

• Hydrometer Analysis • Determination of Moisture Content

• Determination of Atterberg limits• pH of the soil

After the test data is

Composition:

Code of Practice

Approximate proportions

Cement IS-8112-1989 5-10%Sand IS-2116-1980 65-75%Clay - 15-20%Water Potable in nature Optimum for

mixing

obtained, Soil compositions are adjusted to approximate proportions, if required.The steps involved in manufacturing of CSEBs are as follows[1]:

i. Generally, top 10-15cm of the soil is discarded since it is most likely to contain organic impurities.

ii. Screen the suitable soil and sand using 4.75mm sieve in dry state.

iii. Mix the soil and sand first and then mix with requisite quantity of stabiliser (cement/lime). Soil, sand and stabilisers should be mixed thoroughly to obtain uniform mixture. These mixing operations should be carried out in dry state.

iv. Mix the soil-sand-stabiliser mixture with optimum quantity of water. Optimum quantity of water does not depend upon the Proctor OMC. The compaction process used for compacting the stabilised mud blocks is a static compaction process whereas Proctor test employs dynamic compaction with a fixed amount of compaction energy input. Optimum quantity of water to be used depends upon the workability of the wetted mix. The workability should be such that the wet mix should not stick to the machine mould while ejecting the block and should have enough green strength to handle. It has been found that for any given dry density the blocks compacted with higher moulding water content give better strength. The block characteristics greatly depend upon dry density of the block.

v. Measure a known weight (depending upon dry density and block dimensions) of the wetted mix, feed into the machine mould, compact it through piston operation, eject the block and stack it for curing.

vi. The blocks shall be stacked closely without gap, one above the other up to 5-6 blocks height. The stack of blocks should be covered with a wet hessian cloth or straw or similar material and under shade.

vii. The block curing shall commence a day after the manufacture. Sprinkle water on the stack and keep the entire stack of blocks always under moist condition for at least 28 days. Cured blocks shall be air dried and then used for construction.

Once the blocks are manufactured, they have subjected to all the prescribed tests in order to ascertain the quality of the blocks. The various tests performed are:

Tests Code of Practice1. Wet compressive strength IS-3495-Part I-19922. Water absorption IS-3495-Part II-19923. Selection criteria for no of bricks &

Methods of samplingIS-5454-1978

4. Handbook on earth building and design of earth structures

NZS-4297-1998 &Australian Handbook on earth building-2002

5. Specifications of soil based bricks in building construction

IS-1725

6. Linear Expansion and Weathering Test

Indian standard code of practice for manufacture and use of SMB for masonry by Dr. B.V. Venkatarama Reddy

7. Required strength for masonry IS-1905-1987

After the blocks are cast, we perform three-point flexure test (TPB) and Four-point bending test (FPB) on blocks with single edge notch in depth and in width. The load- deflection curves of the bending tests are plotted.

Progress so farLiterature Review

Key points from various publications and research works have been listed below:Gernot Minke in the paper titled ‘Building with Earth – 30 Years of Research and Development at the University of Kassel’ observed that in Silty loam mortars additions of 2 % to 6 % cement will not increase but decrease the resistance against compressive forces but not so much reduction in compressive strength is noted with the addition of lime.K S Jagdish in paper titled ‘Earth Construction Today: Prospects and Tasks’The various barriers that hinder the dissemination of this technology are:1. Earth Construction is Taboo in the Modern Context

The primary reason for the rejection of traditional earth construction appears to be its proximity to poverty. The ‘urge’ for development frequently leads to a rejection of all practices which symbolize poverty and deprivation. Use of brick and concrete are shining examples of developed societies and there is an in exorable movement towards such construction in the developing world.

2. Inadequate Understanding of the Process of Stabilization - type of soil, amount of stabilizer, degree of compaction.

a) In the experiences at ASTRA a sand content of 65% to 75% is crucial for satisfactory performance.b) 7% cement may have to be used along with 65% sand content to achieve wet compressive strengths of the order of 3.0MPa. c) A fresh density of 2.05 gm/cc was arrived at by ASTRA as a satisfactory measure of compaction.

Tejas Kotak in Constructing Cement Stabilized Rammed Earth Houses in Gujarat after 2001 Bhuj Earthquake stated that Ideal soil for stabilized earth technology should contain 75%-80% sand and 20-25% of clay and silt.

Auroville Earth Instistute has been at the forefront in the research and development of CSEBs. The following information has been obtained from their website.

CSEB made in Auroville with 5% cement, have an average dry compressive crushing strength of 5MPa and a wet compressive crushing strength of 2.5MPa. The water absorption is around 10%. Country fired bricks have around 35 kg/cm² for the dry compressive strength and 12% water absorption. The cost of CSEB walls (at Auroville) is 25.9 % cheaper than country fired bricks.

K R Ganesh in his paper titled ‘Stabilised Mud Blocks in Architectural Design Process’ Stabilised mud Blocks are available in three sizes (mm): (1) 305x143x100, (2) 230x190x100 and(3) 230x95x100The surface smoothness both on the internal and external surfaces of the masonry results in saving the plastering.

B V Venkatarama Reddy published ‘Indian Standard Code of Practice for Manufactureand use of Stabilised Mud Blocks for Masonry’ at International Symposium on Earthen Structures held at IISc, Bangalore, during 22-24 August 2007

C Jayasinghe in his paper titled ’Characteristics of Different Masonry Units Manufactured with Stabilized Earth’Recent advances in manually and hydraulically operated machines have allowed the manufacturing of compressed stabilized earth bricks and blocks (CSEB) which could easily compete with burnt clay bricks and cement sand blocks.Cement as a chemical stabilizer has shown better performance in terms of strength and durability of CSEB (Rigassi, 1985, Bryan, 1988, Rahman, 1987). All the CSE bricks and blocks were tested under dry and wet conditions and found that the wet strength is more than

0.4 x dry strength of the unit as recommended in the relevant standards.

Fracture toughness references : Code of Practice & JournalsDetermination of fracture energy, process zone length and brittleness number from size effect, with application to rock and concrete

Z.P. Bazant and M.T. Kazemi

Introduction to Fracture Mechanics David RoylanceFracture Mechanics Igor KokcharovFracture Mechanics of Brick Masonry Pietro Booca, Alberto

Carpenteri, Silvio ValentiFracture Toughness of Brick and Brick Masonry S. Raghava Rao

Fracture Mechanics in a broad sense is a part of science of strength of materials and structures, which relates to study of load carrying capacity of a body with initial cracks and the laws governing propagation of these cracks. Thus it relates the maximum permissible stress to size and location of crack and predicts rate at which this crack will grow to a critical size. Study of fracture mechanics is necessary for determining rapid propagation and arrest of moving cracks.Study by Irwin on linear elastic fracture mechanics (LEFM), which is applicable to all structures where inherent inelastic deformation surrounding the crack tip is small, has led to an understanding that there exists a plastic zone over which fracture process is concentrated and is not concentrated at a point alone. The size of this plastic zone (also called Fracture Process Zone-FPZ) relates material properties to its tensile strength. He considered that increased displacement and reduced stiffness result due to formation of this plastic zone whose size is greater than the real crack.

Where rp* = the size of process zone

KI = stress intensity factor/fracture toughness = σ √(πa)

σyld = tensile strength ft’ of material/failure yield stress

Why this tensile strength is necessary comes from the fact that, all structures fail due to stresses exceeding tensile strength of the material. Accordingly, the study of fracture toughness is of prime importance. The stress intensity factor also relates two more important parameters, namely Modulus of elasticity E and strain energy release rate G which are defined by Griffith’s work.The energy-balance approach

Griffith-1921 explored disparity in glass rod of different sizes. As such, he had postulated that this can be explained by the presence of internal flaws (idealized as elliptical) and then used Inglis solution to explain this discrepancy. The total energy which must be supplied to separate atom at surface also called surface energy S or Uo is:

Where, ϒ = surface energy density and factor of 2 is due to the fact that upon separation, we have two distinct surfaces.The strain energy released in forming crack is given by: for plain stress conditions

In which E is modulus of elasticity and a is the crack length

Thus, we accept Griffith (energy) theory as the valid criteria to explain crack growth, then the crack will grow in the direction along which the elastic energy release per unit crack extension will be maximum and the crack will start to grow when this energy reaches a critical value. It is readily seen that crack becomes unstable when crack length reaches critical value ac.

Thus,

This equation is significant because it relates the size of the imperfection (2a) to the tensile strength of the material, observing experimentally that small imperfections have a much less damaging effect on the material properties than the large imperfections.For ductile materials released strain energy was absorbed not by creating new surfaces, but by energy dissipation due to plastic flow in the material near the crack tip - Irwin and Orowan

Each time the crack jumps ahead, say by a small increment δa, an additional quantity of strain energy is released fromth e newly-unloaded material near the crack.Thus, small things tend to be stronger: they simply aren’t large enough to contain a critical-length crack.A number of means are available by which the material property Gc can be measured. One of these is known as compliance calibration, which employs the concept of compliance as a ratio of deformation to applied load: C = δ/P

U = ½ CP2

The stress intensity approach – Modes of failure approach: Irwin considered crack tip stresses under conditions of generalized plain stress/strain conditions which can adequately characterize stress intensity factor KI.MODES OF FAILURE:

Mode I is a normal-opening mode and is the one we shall emphasize here, while modes II and III are shear sliding modes.The semi-inverse method developed by Westergaard shows:

For distances close to the crack tip (r ≤ 0.1a), the second and higher order terms are to be neglected. KI = stress intensity factor for Mode I

.Thus,

The toughness, or resistance to crack growth, of a material is governed by the energy absorbed as the crack moves forward. In an extremely brittle material such as window glass, this energy is primarily just that of rupturing the chemical bonds along the crack plane. But as already mentioned, in tougher materials bond rupture plays a relatively small role in resisting crack growth, with by far the largest part of the fracture energy being associated with plastic flow near the crack tip. A “plastic zone” is present near the crack tip within which the stresses as predicted would be above the material’s yield stress σY. To a first approximation, the distance rp this zone extends along the x-axis can be found by:

In order for the measured value of KIc to be valid, the plastic zone size should not be so large as to interact with the specimen’s free boundaries or to destroy the basic nature of the singular stress distribution. Cases in which enough ductility exists to make it impossible to satisfy the above criteria, the stress intensity view must be abandoned and alternative techniques such as the J-integral or the crack tip opening displacement method used instead. The fracture toughness as measured by Kc or Gc is essentially a measure of the extent of plastic deformation associated with crack extension.The critical thickness t* is that which causes the specimen to be dominated by a state of plane strain, as opposed to plane stress.

vTo guarantee that plane strain conditions dominate, the specimen thickness t must be such that t>>2t*. From either a stress or a strain viewpoint, the extent of available plasticity is reduced by making the specimen thick.Experimental Programme:

Standard Size CSEB of 230mmx190mmx100mm is to taken. Notch of 30mm and 35mm is made using stone cutting machine in separate blocks in depth(100mm) and in width(190 mm).

These notched bricks are subjected to three-point flexure and four-point flexure using standard flexure testing set-up. In case of four point bending test, the loading arrangement/or point load rollers are such that they are at a length of span/4 from the reaction rollers.Maximum Load(in N) is to be applied in small increments and value of deflection(in mm) is noted

1. Fracture energy/Critical strain energy release rate GF or Gc (in N/mm)= Area under the load-deflection curve(called Work of fracture N-mm) per area of ligament Alig(= width*depth of notched section in mm2) of the initial uncracked ligament.

GF=W F

Alig

2. Fracture toughness/Critical stress intensity factor KIC (in MPa√mm)= root of product of corresponding fracture energy and modulus of elasticity(in MPa).

K IC=√EGF

The above is calculated and repeated for all bricks and mean is calculated. The results are tabulated for comparison.Also, KIC from Srawely’s equation is found out and compared with experimental values of fracture toughness.

K IC=βσ √πa

where, σ = Mmax/ZZ = BW2/6W = depth of blockB = width of brick Mmax = Mmax due to self wt + Mmax due to load

Without M due to self wt equation is: for TPF test

where, a = flawP = point loadS = span of brick under consideration

Work to be carried out during phase IITests to be performed on the Soil• Grain Size Analysis• Hydrometer Analysis • Determination of Moisture Content• Determination of Atterberg limits• pH of the soil

Tests to be performed on CSEBs• Pocket Penetrometer test• Determination of Dry Density• Determination of Compressive Strength• Determination of Water absorption• Determination of Linear Expansion on saturation• Weathering Test

Fracture testing of CSEBs• 3 point bending test• 4 point bending test

Proposed Outcome• Basic Understanding of Earth, CSEBs in particular, as a structural

material• An understanding of the feasibility of CSEBs in Bangalore

• An investigation of pros and cons of CSEBs• Basic understanding of Fracture mechanics in Quasi-brittle

materials