2"x2" cubes of cement mortar with the inclusion of fJyash are .... Let us consider the formula, y = a+bx+cx2 ---(i) and find out the values of a, b, and c which will ...
EFFECT OF ELEVATED TEMPERATURE ON THE STRENGTH OF CEMENT MORTAR WITH THE INCLUSION OF FLY ASH T.U. Ahmed, Rajshahi University of Engineering & Technology, Bangladesh Md. Shafiuddin Miah, Rajshahi University of Engineering & Technology, Bangladesh Md. Akhtar Hossain*, Rajshahi University of Engineering & Technology, Bangladesh Md. Niamul Bari, Rajshahi University of Engineering & Technology, Bangladesh Md. M. Sazzad, Rajshahi University of Engineering & Technology, Bangladesh 29th Conference on OUR WORLD IN CONCRETE & STRUCTURES: 25 - 26 August 2004, Singapore
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29th Conference on OUR WORLD IN CONCRETE & STRUCTURES: 25 - 26 August 2004, Singapore
EFFECT OF ELEVATED TEMPERATURE ON THE STRENGTH OF
CEMENT MORTAR WITH THE INCLUSION OF FLYASH
T.U. Ahmed, Rajshahi University of Engineering & Technology, Bangladesh
Md. Shafiuddin Miah, Rajshahi University of Engineering & Technology, Bangladesh
Md. Akhtar Hossain*, Rajshahi University of Engineering & Technology, Bangladesh
Md. Niamul Bari, Rajshahi University of Engineering & Technology, Bangladesh
Md. M. Sazzad, Rajshahi University of Engineering & Technology, Bangladesh
Abstract The compressive and bond strength are of great practical importance in designing concrete structures. The structures may be subjected to accidental fire breakout and explosion during its service life. The durability as well as serviceability of structures is highly influenced by these parameters. In this paper, an attempt has been made to assess the effect of degree of deterioration of the compressive and bond strength of cement mortar with the inclusion of flyash. Moreover, a relationship is developed between compressive strength and bond strength with temperature. Also, the effect of inclusion of fJyash with cement mortar is studied. 2"x2" cubes of cement mortar with the inclusion of fJyash are prepared, cured for 28 days, heated at different elevated temperature and cooled down to room temperature. The test results indicate that the compressive and bond strength of cement mortar decrease with the increase of temperature and the bond strength of cement mortar completely vanishes at 600 C. At the same temperature the compressive strength of mortar reaches its minimum value. D
Keywords: Flyash, Cement Mortar, Bond, Temperature, Compressive Strength 1. Introduction Concrete structures usually used in various forms. In modern time, such structures encounter various types of loading. In such cases, knowledge of strength of material at most adverse conditions of loading is vital for reducing the potential damage. The blast type of loading is one of the major problems nowaday due to terrorist act. In such a situation, the structures undergo rapid change of temperature for a short period of time, the properties of material degrade and finally the structures fail due to absence of load carrying capacity of the material. Although blast type loading produces tremendous dynamic loading but the structures finally fail due to degradation of the material properties. So the degradation strength or properties of material under such elevated temperature is one of the vital issues to mitigate potential hazard. Shetty M.S. (1982) discussed the effect of temperature on the compressive strength of concrete for different mix proportion . Price, Shalon studied various aspects of temperature influence upon the concrete both in laboratory and at the site of construction. Bai et al. (1998) discussed the influence of temperature of concrete at elevated temperature. Authors used three different grade of concrete in the test program. In those studies, the high strength concrete was prepared by using super-plasticizer. However, there is no information available regarding the bond strength of concrete at high
135
temperature in their test programs. In a seperate study Kumruzzaman (2002) discussed the temperature susceptibility of flyash mixed cement mortar with respect to compressive strength. Sazzad et at. (2003) discussed the effect of elevated temperature on bond and compressive strength of ceme nt mortar. The study showed that the compressive and bond strength of cement mortar decreased with the increase of temperature and the bond strength of cement mortar completely vanished at 800°C. At the same time the compressive strength of mortar reached its minimum value at 1OOO°C. Authors discussed the degradation of properties of normal cement mortar without inclusion of flyash or plasticizer. The existing literature indicates that a little work has been conducted to study the influence of temperature on the properties of concrete . In this paper, an attempt has been made to study the temperature response of cement mortar with the inclusion of flyash in the laboratory to establish a relation between the bond strength of cement mortar with steel bar and temperature.
2. Theoretical Approach
2.1 Bond stress Two adjacent sections at a distance dx is shown in the following fig . 1 i.e. section x-x and section y-y, distance dx apart. The bending moment at the section x-x is M and that at section y-y is (M+dM) . These moments cause tensile force in the steel reinforcement equal to T and (T +dT) respectively .
--..x M
Y
M+dM
~
i
1
- . - . -.- . - .~. - - - . -.- - - . ~ . - . - . - : - .
.....
x
dx
z
·-·-N.A
T .......1 --______---,.; •• T+dT
I----.T
...i...
1----. T+dT
-+
y
Fig. 1: Sectional View of Concrete
T. z = M, also (T+dT).z = (M+dM) dM
=(dT) .z
dT= dMlz Where , z = lever arm
The increase in tensile force, i.e . (T+dT)-dT = dT is resisted by bond stress developed between steel and concrete in length dx. The total resistance due to bond stress may be obtained by multiplying the total surface area of the bars with unit bond stress. From this concept, the average bond stress is obtained as follows :
Sb =
S zLo ···· . .... ....·.. ... ···· ... ·.. ·.. .... ······· (1 )
Where , Sb =Bond stress S = Shear force developed at the section L 0 = The sum of perimeter of all the steel bars resisting tension Sb in equation (1) represents the bond stress induced at a particular section of the beam; hence it is called local bond stress. This bond stress is also known as flexure bond since it is developed due to change in bending moment.
136
Mortar cube
F
0
A Bar A
0
•
Max m bond Stress
Bond Stress d istri but ion
c Fig. 2: Bond Stress Distribution for Bar Embedded in Concrete
Fig . 2 indicates typical bond stress distribution along the length of bar due to application of direct force. In case of plain bar without any surface deformation, initial bond strength was provided only by the relative weak chemical adhesion and mechanical friction between steel and concrete. Once adhesion and static friction were overcome at larger loads, a small amount of slip led to interlocking . However, this natural bond strength is so low that in beams reinforced with plain bars the bond between steel and concrete is frequently broken and finally the beam will collapse as the bar will pull through the concrete .
2.2 Bond length In order to develop the full bond stress in a bar, it should be embedded in the concrete for a sufficient length . This is known as bond length of the bar. Though the variation of bond stress along the length of bar is of nonlinear nature .(vide .fig. 2) , it is reasonable to assume uniform for most of the practical cases . Therefore , the average bond stress (Sa) may be expr~ssed as follows :
Force in the bar Area of the bar in contact with concrete or,
-lr d 2 er ' Sa = 4 lrdl
=
F mil
der ,
41
der, or, 1= ...... .... ... ...... ...... .... .... ... ..... 4So
(2)
Where, d = Diameter of the bar I = Bond length of the bar
er, = Stress developed in the
bar
F = Force in the bar
2.3 Methodology This method says that the best representative curve is that tor which the sum of the squares are of the residuals are positive quantities. the requirement that their sum shall be as small as possible to ensure that the numerical values of the residuals will be small; and this means that in the case of the series of plotted points the best representative curve will pass as closely as possible to all the points
137
[Scarborough, 1966]. Let us consider the formula, y = a+bx+cx2 ---(i) and find out the values of a, b , and c which will made the graph that will pass as near as possible to each of the (n) points ( Xl, yd, (X2' Y2 ), ------- (Xn , y,, ). The equation will not in general, be satisfied exactly by any 0 f the ' n' pairs, substituting in (i) each of the 'n ' pairs of values in turn , the following residual equations are obtained. [Rajaraman . V , 1994J ' VI = a+bx ,+cX/-YI
V2 = a+bX2+cX/ -Y2
Vn = a+bXn+cxn2-Yn The principle of least square says that the best values of the unknown constants a, b" b 2 are those which make the s~uares residuals a minimum, or V = V/ +V/ + ..... .. ..... ... ............... +V/ must be a minimum, hence _,2+ ................ + ( a +bxn + C Xn2-Yn/,2
(a +b X+ cx2 -y)2 -- (a +b Xt + CXt 2 -Y, ) 2+(a +b X 22+ CX22-Y2I is to be a minimum
The representation of experimental results are very important both from the point of view of theoretical and practical aspects . In practice , two forms of curve fittings are usually adopted , one is exponential form and the other is polynomial form. In this study, the experimental results indicate that the polynomial is best suitable . [Rajaraman . V , 1999]
3. Properties of Materials Used in the Test Cement is a cementing material, which acts as a binder. The cement used in ' the laboratory investigation was CEMEX brand OPC. It was tested according to ASTM standard . The initial setting time was 1 hour 22 minute and final setting time was 2 hour 8 minute. The compressive strength of cement was 2937 .5 psi (7 days) and specific gravity is 3.14. Local Sand was used in preparing the model cubes . The FM of the sand was 1.74 . Flyash is a powdery substance obtained from the dust collected in the electrical power plants that use coal as fuel. The major constituent in most of the flyash particles is alumino silica glass. The flyash that can be represented as reactive silica (non-crystalline silica glass) combines with the calcium hydroxide released by the hydration of calcium tri silicate (principal component of clinker) to form calcium silicate hydrate, the principal binder of cement.
4. Preparation and Testing of Model Cubes Fresh Portland cement was mixed with the sand prepared by ASTM standard . The ratio of cement and sand was 1:4 and 20% cement (by weight) was replaced by the inclusion of flyash . Optimum water cement ratio obtained which was 0.46 and used for the preparation of the specimen . The mortar was casted in the mould for preparing 2" cube models. During casting the mortar in the mould , mild steel bar of 1/8/1 diameter were centrally penetrated in the mortar up to a certain depth and mould was kept in wet place for 24 hours , then prepared cubes were removed from the mould and placed in water for 28 days at room temperature. Later those cubes were heated in the oven . At a time eighteen cubes i.e. six sets of cubes were prepared . These sets were heated for one hour at six different temperatures (2S0c, 100°c, 200°c, 3S0°c, 4S0°c and 600°c) . After that these sets were remain in the room temperature for 24 hours for cooling . For testing of Bond Strength, Torsee's Universal Testing Machine was used . The model is AMU-S . Its capacity is SO ,OOO N and range is 10 N. The model cubes were hold properly and load was applied gradually. The values of ultimate or failure load from the dial gauge reading was determined . Versa Tester 30 M machine was used for testing compressive strength. Its capacity is 30 ,000 Ib and range is SO lb. The cubes without rods were tested for Compressive Strength The value of ultimate or failure load from the dial gauge reading was determined .
138
5. Test Result Chart Table 1: Bond and Compressive Strength of Cement Mortar with the Inclusion of Flyash at Different Temperature for W/C = 0.46
(IJ
.... :::J
ro~
0
z
.... 0
(lJo a.~
-
CJ)
E (IJ
Mixing Proportion
Bar Dia (inch)
Average Bond Strength (psi)
Average Compressive Strength (psi)
f-
1
25
14
1/8
56.35
1495.83
2
100
14
1/8
73.61
1667 .85
3
200
1:4
1/8
80.20
1750.25
4
350
1:4
1/8
68 .98
1533.50
5
450
14
1/8
52.79
1291 .67
6
600
1:4
1/8
4.06
791.67
2000-r------------------------------~--------------------_,
1800
y = a+bx+cx2 a = 1479.53794
1600 ~
'Vi Q.
-
b c
1400
.c:.
OJ
c::
~
(/)
1200
= 2.22038 = -0.00568
1000
Cll
> 'Vi
800
~
600
(/)
Q.
E
o
u
400 200
o
100
200
300
400
500
600
700
Tem perature Fig. 3: Variation of compressive strength with respect to temperature
139
800
100.------------------------------------------------------.
y = a+bx+cx2 a 52.44353 b = 0.24227 c = - 5.39041 E-4
80
-c..
=
.iii
'--'
-
..c
60
/
/
OJ C
•
Q)
.....
If)
40
-0 C
0
CO
20
o
100
300
200
400
500
600
700
800
Temperature Fig. 4: Variation of bond strength with respect to temperature
Table 2: Relation Between Compressive Strength (y) & Temperature (x) Mixing proportion ,
1:4
y
=
Equation of parabolic curve y = a+bx+cx2 1479.53794+2.22038x-O.OO568x"
Table 3: Relation Between Bond Strength (y) & Temperature (x) Mixing Proportion
1:4
Equation of parabolic curve y= a+bx+c~ y 52.44353+0.24227x-O.000539x"
=
6. Discussion From fig.-01, it is obvious that ,the compressive strength of the cement mortar has increased with the increase of temperature up to '200°c and then decreased gradually, From flg,-02, it is also prominent that the bond strength has increased up to 200°c and after that it decreased gradually, It is clear from the experimental investigation that inclusion of flyash produces increase in strength at lower temperature, Results indicate that inclusion of flyash changes the pattern of degradation of concrete properties. This is clear bifurcation of results produced by Sazzad et al. (2003). Also such phenomenon indicates the pr'olonggain in strength of cement mortar due to inclusion of flyash. 7. Conclusion A significant volume of research works has been performed on the development of high strength concrete. However, a little study is conducted on the effect of elevated temperature on the strength of concrete with the inclusion of a dmixtures. I n the p resent study, the experimental investigations are carried out of cement mortar cubes with the inclusion of f1yash as a replacement of cement {20% of
140
cement) From the extensive experimenta l and theoretical study , the following conclusion may be draw n . . 1. Compressive strength of cement flyash mortar iRftially increases up to definite temperature and after that it decreases with the increas~ of temperature. 2. Bond strength of cement flyash mortar initially increases up to definite temperature and then decreases with the increase of temperature. 3. The effect of temperature on the strength may be simulated by well equation of the polynomial form. References
[1]
Rajaraman . V, Ph .D "Computer oriented numerical methods" Third edition, New Delhi 1994 .
[2]
Neville, A.M., "Properties of concrete" second edition, Pitman publishing Co. sa (PTY) Limited , Bath , Great Britain, 1975 .
[3]
Shetty, M.S., "Concrete Technology" First edition , S. Chand & Company Ltd ., New Delh i 110055, 1982.
[4)
Kamruzzaman, M., Zahurul Islam , Nizamud-Doulah (2002) "Performance of pulverized fuel ash in cement concrete ."
[5]
Bai , Sharada Bai , Swamy, Jagadish "Effect of ,elevated temperature on high strength concrete" [National seminar on advances in special concretes). pp 294-304, 1998.
[6)
Sazzad, M. M , Ahmed, T. U., Ali M. M . Y, Sultana, K., Shamim. A. (2003) bond and compressive strength of cement mortar at elevated temperature ."
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"Degradation of
