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PARTIALLY REINFORCED CONCRETE MASONRY

Ahmad A. Hamid

Professor

Drexel University

Philadelphia

PA,USA

ABSTRACT

Gouda M. Ghanem

Research Fellow,Drexel

University, on Leave fr om

Helwan University,

Cairo,Egypt

Partially reinforced concrete masonry is cheeper than reinforced

concrete masonry and can be used in many applications in areas of low or no

seismic action, where benficial properties of reinforced concrete masonry

such as higher strength, improved out-of-plane flexural, in-plane flexural

and shear capacities and lesser variability can be utilized at a reasonable

cost . This paper presents information on an ongoing research program at

Drexel University aimed at evaluating the material properties of partially

reinforced concrete masonry as well as studying the structural behavior o f

wall elements made of this material using small-scale modeling techniques.

Information on the methodology of small-scale modeling techniques, material

properties, and wall behavior of partially reinforced concrete masonry i s

presented. This research will provide desgin recommendations for wall

elements and analytical or empirical formulae for prediction of material

properties and wall capacity.

INTRODUCTION

Masonry is one of the first durable building materiaIs to be

developed. Most of the early applications utilized its compressive strength

and capacity to arching. Resulting masonry structures were invariably

massive as gravity loads stabilized the structures against any lateral

forces due to wind or siesmic action . Although structural use of masonry

was hampered by its long history as a craft based material, greater

understanding of its structural capacity in compression and shear led to

thinner walls and consequent intensive usage in commercial and residential

buildings. Its continued use today i8 primarily due to is economy,

possibility of construction without large capital expenditure by the

builder, fire resistance, aesthetic appearance, durability ease of

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construction, good thermal and sound insulation,weather protection, and

ability to provide structure as well as sub-division of space. Now it is

considered to be an engineering material, one where the properties of that

material are known and its performance can be predicted.

One major shortcoming of plain (unreinforced) masonry is the

inadequate resistance to out-of -plane bending. Further, masonry walls are

often subjected to out-of-plane and in-plane flexure and shear stresses

from wind or seismic forces and also moments due to eccentricity of gravity

loads and continuity of the floor slabs. Reinforced masonry was developed

to overcome this major shortcoming, it is proved to be very effective in

improving structural behavior as well as reducing the inherent variability

of masonry. It is widely used now and most national codes give design

guidan6e.

Partially reinforced masonry can be considered as a by-product of

reinfoced masonry. It has a greater appeal when used with concrete blocks

and the finished product is partially reinforced concrete masonry. It is

defined as masonry where not alI the cavities are grouted and

reinforcement provided is less than 0.002 times the gross cross-sectional

area of the wall or is spaced greater than 4 feet on- centers l !2). Partially

reinforced concrete masonry, which is stronger than hollow concrete masonry

but weaker than reinforced concrete masonry, has a growing demand lll ) in

residential and commercial construction in situation where tension is small

enough to require steel spaced more than 4 feet. It is criticaI, however,

to check the adequacy of the hollow masonry between the reinforcing

elements to resist lateral loads.

The bulck of research information available so far IS ,6,7) on partially

grouted concrete masonry has been produced by investigations on fully

grouted concrete masonry where extent of grouting is varied from fully

grouting to partial grouting. These research studies have helped to

identify partially grouted concrete masonry in its right and ACI-530/ASCEs

cOde l !), is now specifying allowable flexural tension stresses for design

of partially grouted masonry. Yet, partially reinforced concrete masonry

remains to be researched and a better understanding will help to attain the

true potential of this material.

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In-depth studies on this material has only just begun (3,8,9,10,13) and

more research work needs to be done to achieve the full benefits of

partially reinforced concrete masonry.

Researchers have not yet addressed the proplem of developing

analytical methods for prediction of strength of partially reinforced

concrete masonry. Present state of knowledge indicates the need for more

research to enable partially reinforced concrete masonry to achieve its

full potential. Areas needing further study are material properties such as

axial compressive strength, modulus of elasticity, shear strength and

splitting tensile strength, and behavior of elements such as walls under

in-plane compression, in-plane shear and out-of-plane flexure, and walls

with returns. In the following two sections, a brief description of the

ongoing research program at Drexel University in the area of partially

reinforced masonry is presented .

EXPERIMENTAL PROGRAM

Methodo1ogy of Sma11 Sca1e Mode1ing

The main advantages of a physical model over an analytical model is

that it portrays a complate structure loaded to the collapse stage. The

prime motivation to conduct experiments on structures at reduced scales is

to reduce the cost, this reduction comes from: reduction of loading

equipement and associated restraint frames and reduction in cost of test­

structure fabrication, preparation and disposal after loading. The major

limitations of using structural models in a design environment are those of

time and expense.

Any structural model must be designed, loaded and interpreted

according to a set of similitude requirements that relate the model to the

prototype structure. These similitude requirements are based upon the

theory of modeling, which can be derived from a dimensional analysis of

physical phenomena involved in the behavior of the structure .

A true or replica modeling (i.e. to produce models which obey all

similitude requirements) has to be used. These modeling techniques are

based on the choice of model materials and their methods of fabrication.

Therefore, the small-scale direct model must be satisfy the similitude

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requirements of not only the masonry units but also the mortar joints,

grout, and reinforcement. For replica modeling, the similitude requirements

can be classified in five distinct categories : loading, geometry, material

properties, prediction equation, and designo Based on the assumption of no

significant time dependent effects which influence the structural behavior,

the pertinent parameters that enter the modeling process in case of static

loading have been developed and are documented in Reference(4) .

The choice of a geometric scale factor for a specific type of model is

governed by the physical capability of the test facility. In addition,

physica l geometry limitations and factors such as scale effects dictate the

limit of the models length factor. The introduction of the larger scale is

principally motivated by a need to increase the facility's capabilities, to

acquire wider knowledge in modeling masonry structures and finally to

minimize the scale effects and increase the accuracy of the detailling of

the modelo In this research program, a scale factor of three was is used to

model the concrete block masonry components and elements.

Scope

The overall research program at Drexel University plan is presented in

the flow chart shown in Figure 1. The program is divided into the following

three phases:

Phase 1- This phase is aimed at modeling the components materiaIs of

concrete masonry (units, mortar, grout, and reinforcement) and conducting

structural performance tests to evaluate the material properties of

partially reinforced concrete masonry. The results of this phase will

enable specification of important material properties needed for structural

use such as flexural tensile strength, compressive strength, modulus of

elasticity, shear strength, and splitting tensile strength. Further,

development of analytical formulae will also be pursued to predict the

above properties in terms of prism strengths of grouted and hollow block

masonry.

The study on modeling material components, flexural tensile strength,

in-plane shear , and splitting tensile strength were completed and the

studies on compressive strength and modulus of elasticity are currently in

progresso

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Phase 2- This phase is aimed at conducting structural performance tests

to study the behavior of wall elements. The results will enable development '

of design recommendations for walls under axial compression, in-plane '

loading and out-of-plane flexure. The following are the test results that

will be measured, observed, and documented during the course of the

proposed testing program:

- Cracking load and crack pattern,

- Yielding and ultimate load,

- Failure Mode,

- Load deformation curve (hyteretic loop),

- Ductility and energy dissipation capability of these walls,

- Maximum masonry strain and maximum strain in reinforcement, and

- Control test data such as masonry prism strength, mortar cube

strength, block strength, grout strength, and steel properties.

Currently, the experimental program of wall behavior for in-plane

loading is ongoing(4l at Drexel University. A total of thirteen 1/3-scale

models of partially reinforced concrete block masonry shear walls, were

constructed and will tested under in-plane lateral loads, with and without

axial precompression. Eleven shear walls will be tested under monotonically

increasing loads, and two shear walls will be tested under fully reversed

cyclically loading.

Five types of parameters are investigated: axial precompression, block

strength, lateral load, and amount and distribution of vertical and

horizontal reinforcement. Table 1 presents and summarized the details of

the model shear wall specimens included in this programo

A typical shear wall panel is 3.1 feet in length (7 blocks) by 3.1

feet in height (13 courses) , representing a li 3-scale of 9.3 x 9.3 feet

full-scale walls. The panels were fabricated with a single wyth of 2 inches

wide blocks representing a 1/3-scale model units of the 6 in .

hollow concrete full-scale b1ocks, as shown in Figure 2.

ANALYTICAL STUDY

nominal

This phase is aimed at conducting analytical analysis using finite

element method to study and analyze the behavior characteristics of

partially reinforced masonry walls. Analytica1 formulas to predict the

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flexural and shear capacity of partially reinforced walls will be

developed. This study is essential for the development of design

methodology and building code requirements.

Availabe formulas for predicting flexural strength of reinforced

masonry assuming plain section remains plane and an equivalent rectangular

stress block(12) will be checked and modified if necessary, to reflect the

difference between reinforced and partially reinforced masonry. Two main

differences between fully reinforced walls and partially reinforced walls

are noted. First, not ali the cells are grouted in case of partially

reinforced walls which requires consideration of an effective width for the

compression block. Secondly, the vertical steel usually is uniformly

distributed in reinforced walls whereas it is not in partially reinforced

walls. This requires defferent treatment of the contribution of tension

steel in flexural strength calculations.

Shear strength of reinforced masonry walls depends on amount and

distribution of vertical and horizontal steel. Available formulas for

predicting nominal shear strength of reinforced walls will be checked and

modified, if necessary, to reflect the unique features of partially

reinforced, namely; the larger spacing of horizontal steel and the shear

capacity of hollow masonry between the reinforced parts of the wall.

ANTICIPATED RESULTS

Several significant indicators can be read from the load deformation

curves obtained experimentally such as initial and subsequent slopes,

cracking load, yield point of the wall, ultimate capacity and the ductility

ratio . These indicators will help determine the wall response, wall

parameters can be varied to improve wall response, and relative suitability

for various applications. Similarly, several significant indicators can be

read from the graphs of the hysteretic loops such as ultimate loads,

ductility ratios and stiffness degradation coefficients. These indicators

will help in determining wall response and resistance under low moderate

seismic loads.

Ultimate ductility factors are required to determine ultimate collapse

conditions using inelastic response spectra. Ultimate ductility is also

essential to apply more sophisticated failure analysis methods such as a

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finite element time-history computation. Past-yield envelopes will be

established from the hyteretic curves. The characteristics o f these

envelopes are essential to determine the applicability of standard analysis

methods such as the inelastic yield spectra and ene rgy balance technique to

partially reinforced concrete masonry walls.

The analysis of the above results is expected to produce design

recommendations for walls under in-plane compression, in-plane shear, out­

of-plane flexure, and design information on walls with returns. It is also

expected that limits within which partially reinforced concrete masonry can

be used optimally, between hollow concrete block masonry at one extreme and

reinforced concrete masonry at the other, for various applications can be

identified during this phase of the study.

Finally, the outlined research program is expected to provide

comprehensive design information on partially reinforced concrete ma s onry

which will enable structural designers to utilize the full potential o f

this material. The final goal is envisaged to be the achievement of greater

economy in concrete masonry structures by encouraging the judicious use o f

hollow block, partially reinforced or reinforced concrete masonry to

satisfy optimally the requirements of any particular application.

BIBLIOGRAPHY

1) ACI/ASCE Standareds (ACI 530-88/ASCE 5-88), .. Building Code Requireme nts

for Masonry Structures" ACI and ASCE, 1988 .

2) Amrhein, J.E ... The Future of Masonry" , Proceedings of the 4th North

American Masonry Conference, Los angeles, California, August 1987.

3) Elnawawy, O.A. and Hamid,A . A. , .. Flexural Strength of Partially Grouted

Concrete Block' Masonry Using Small-Scale Model Wall Elements", Report

NO. STL-02/89, Departement of Civil and Architectural Engineering,

Drexel University, Pila, PA 19104.

4) Ghanem,G ... Behavioral Characteristics of partially Reinforced Masonry

Shear Walls Using 1/3-Scale Models" Ph.D. Thesis in-progress o

5) Hamid,A.A . and Drysdale, R.G.," Flexural Tensile Strength of Concrete

Block Masonry", Journal of Structural Engineering, VOL. 114, NO. 1,

January 1988.

6) Hamid,A . A., Abboud, B . E . , Farah, M.W., Hatem, M.K, and Harris, H. G.,

"Response of Reinforced Block Masonry Wal l s to Out-of Plane Static

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Loads", Report NO. 3.2(a), U.S.Japan Coordinated Program for Masonry

Building Research, Departement of Civil and Architectural. Eng., Drexel

University, Piladelphia, PA 19104, 1984.

7) Hamid,A.A., Assis,G.F., and Harris, H.G.,"Materia1 Mode1s for Grouted

Block Masonry", Report NO. 1.2(a)-1, U.S.Japan Coordinated Program for

Masonry Building Research, Departement of Civil and Architectural. Eng.,

Drexel University, Piladelphia, PA 19104, 1988.

8) Hamid,A.A.,Elnawawy,O.A., and Farah,M.,"Strengthening of Existing Hollow

Block Masonry Walls", Proceedings of the 8th International Brick/Block

Masonry Conference, Dublin, Ireland, 1988.

9) Hamid,A.A.and Chandrakeerthy, S .R., "Compressive Strength of Partially

Grouted Concrete Block Masonry Using Small-Scale Model Wall Elements"

10)Hosny,A.H. and Fouad,H.A.,"Behavior of Partially Grouted Concrete Block

Masonry Walls Under Concentrated Loads", M.SC.Thesis, Ain Shams

University, Cairo, Egypt,1988.

11) 'Schneider, R. R. and Dickey, W. L., "Reinforced Masonry Design", Prentice­

Hall,Inc., Eaglewood Cliffs, New Jersey,1987.

12)Uniform Building Code,"Masonry Codes and Specifications", Commentary to

Chapter 24, The Masonry Society, Boulder, Colarado, Ju1y 1989.

13)Vekey,R.C. "The Effectiveness of Concrete Grout for Reinforced

Masonry", Proceedings Of the 8th International Brick/Block Masonry

Conference, Dublin, Ireland, 1988.

Table 1. TEST SPEClMENS OF SHEAR WALLS (PRASE 2)

r------------------r--Vertical Steel

Wall Designation

m

Reinf. P/lo(1)

----------3#5 0.12

3#5 0. 12

3#5 0.12

3#5 0.12

3#5 0.12

3#5 0. 12

3'# 5 + 3 # 4 0.21

3#5+3#4 0.21

3#5+3#4 0.21

3#5+3#4 0.21

----------- -4#4 0.12

4#4 0 .12

4#4+2#5 0.20

(1) Based on gross area

(2) Monotonic loading

(3) Cyclic loading

Horizontal Steel Masonry Strength Axial Stress

-l---_____ ~------------ --... I P1 P2 P3 Ph%(1) Reinf. f m

( psi) (psi)

- - -----t-----r--------3#5 0. 12 I 2350 100

3#3

3#3

3#3

3#3

3#4

3#5+3#4

3#5+3#4

3#4

3#4

0. 05

0. 05

0.05

0.05

0.09

0.21

0 .21

0.09

0.09

2350

2350

2350

2350

2350

2350

2350

2350

o 100

200

1400 I 100

100

100

100

100

100

---- ---t -t---------4#4 0.12

-------t 4#4 0. 12

4#4 0 .12

2350

2350

2350

100

100

100 __________ ---l ________________ -l-____________ _ _

Lateral Load

M (2) c(3)

M

M

M

M

M

M

M

M

M

M

M

c

c

W -...J m

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Experimental program (Phase I)

Materi a I Propert i es

I I I I Axial In-Plane Compression Joint Splitting Flexural and Modu lus of Shear

Tension Tension

Elasticity

Experimenta l program (Phase 2) Wa ll Behavior

Ax i al compression In-P lane Loading Out-of-Plane Flexure

I Analytical Study I I I

I I Analytica l formulae for Design recommendations for predicting the flexural and elements under in-plane, shear strength of part ially out-of-plane loading , and wal reinforced masonry walls w ith returns

Figure 1 - Flow Chert Df the Reseerch PleR

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4

+ 40 In.

~I ~ 1 I - - - I

I

I I I I I I I I I I I I I

I I I I I I I I J I I I I

I I I I I I I 1 .88

I I I I I I

I I I I I I I c

I I I I I I I I I I I I I

I I I I I I I I I I I I I

I -- -- - I lf)

-1 3 1 .... ~ ____ 37_ln_. ---~.l 3 r-H T

Figure 2 - Typicol Mosonry WolI Ponel