EFFECT OF DIFFERENT MIX RATIOS AND WATER CEMENT RATIOS ON SULPHATE ATTACK ON CONCRETE Saeed Ahmad*, University of Engineering & Technology Taxila, Pakistan Faisal Shabbir, University of Engineering & Technology Taxila, Pakistan
30th Conference on OUR WORLD IN CONCRETE & STRUCTURES: 23 - 24 August 2005, Singapore
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30th Conference on OUR WORLD IN CONCRETE & STRUCTURES: 23 – 24 August 2005, Singapore
EFFECT OF DIFFERENT MIX RATIOS AND WATER CEMENT RATIOS ON SULPHATE ATTACK ON CONCRETE Saeed Ahmad*, University of Engineering & Technology Taxila, Pakistan Faisal Shabbir, University of Engineering & Technology Taxila, Pakistan
ABSTRACT This paper gives useful insight regarding the various factors that needs to be considered in designing a sulphate resistant mix. Sulphate attack is possibly the most common and widespread form of chemical attack on concrete. The durability of concretes exposed to alkaline sulphate soils or water’ containing alkaline sulphates has been a problem of long standing. The research involved an experimental evaluation of the resistance of various cement concrete mixes and various water cement ratios using a Class-1 cement against sulphate type chemical attack. The first portion involved the evaluation of different concrete mixes for evaluating its resistance to sulphate salts. Four different concrete mix ratios were investigated ranging from lean mixes to richer ones using Class-I cement. On the other hand, four different water/cement ratios were also investigated ranging from 0.45 to 0.75 for their exposure to sulphate ions. All the concretes specimens were exposed to sodium sulphates enriched environment at a concentration of 350 gm/litre. The concretes were alternatively dried and wetted to accelerate the damage on the concrete. The limestone content in the tests specimens was kept at 25% replacement level of the cement by weight. In the drying phase, the specimens were also oven heated so that all the salts in the pores of concrete may be crystallized. The destructive and non-destructive tests were carried out on the specimens. The results show that the lean mixes were more affected than the richer ones. The concretes with the same concrete mix ratio and having more than 0.55 water/cement ratio were more affected in case of exposure to soluble sulphate salts. The results also show that the compression members were likely to be more affected than the flexural members on their exposure to sulphates. Keywords: sulphates, durability, degradation, water cement ratio, concrete mix ratio, Limestone.
1-
INTRODUCTION
Concrete durability is one of the most important considerations in the design of new structures and when assessing the condition of existing structures. Concrete construction is becoming increasingly complex and the importance of producing structures that are both cost effective and durable has never been higher. An understanding of concrete durability is fundamental to establishing the service life of new or existing structures. [6,8] Sulphate attack has long been recognized as responsible for concrete deterioration in a wide variety of structures.[3] Sulphate attack in concrete has been known to occur when sulphate solutions, derived either from a constituent in the concrete such as aggregate or from external sources such as groundwater, react with the cement hydrates present in the hardened cement paste to form the products which can occupy greater volume than the reactants. The major sources of these salts are soils (particularly Arid Region), groundwater, seawater, industrial chemicals & wastes, and fertilizers [1,2]. T he t hr ee p r o d u c t s o b t a i n e d as a r es u lt of t h e r ea c t i o n i . e . g y p s um , et t ri g n i t e a nd t h a u m a s i t e h as g re at o r v o l um e s t h a n t h e r ea c t an t s a n d t h u s r es u l t s i n t he f o rm at i on of c rac k s and ev e nt ua l di s r upt i o n i n t h e ha r d en ed c onc r et e. [ 4, 5] T h e m ai n ai m o f t h e r e s ea rc h w a s t o s t u d y :
T h e d e l e t e r i o u s e f f ec t s o f s u l p ha t es e x p os u r e t o P o r t l an d c em en t c o n c r et e.
T h e i nf l u e n c e o f t he d i f f e r e n t c o n c r et e m i x r a t i os t ha t e ns u r es d u r a b i l i t y o f c o n c r e t e, ag a i n s t t h e e x p o s ur e o f s ul ph at e s a l t s
T h e i nf l u en c e o f t he d i f f e r e nt w a t e r c e m e n t r at i os t h a t e n s u r e s d ur ab i l i t y o f c o n c r e t e, ag a i n s t t h e e x p o s ur e o f s ul ph at e s a l t s
D e f i n i n g t he c on t r o l li n g p a r am e t e r s i n t h e m i x o f t he c o n c r et e e x p o s e d t o s u l ph at es
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EXPERIMENTAL PROGRAM Locally available materials were used in this research work as under: 1. 2. 3. 4.
Cement Fine Aggregate Coarse Aggregates Chemicals
Fauji Cement (Class-I) Lawrencepur Sand Margalla Crush Anhydrous SODIUM SULPHATE and Limestone. Testing of constituent materials were carried out as per ASTM specified procedures [7]. The class-I cement is used in the study, having a high C3A content. Four types of different concrete mix ratios and four types of different water cement ratios were taken. The different concrete ratios and the different water cement ratios in this study are shown in Table-1. A fixed amount of 25 % limestone as cement replacement material has also been added in all the concrete pastes. These concretes were cured in fresh water for 28 days and then placed in a specially designed steel tank containing a sulphate rich environment i.e. 350 gm/liter sodium sulphate. These concrete beams were subjected to alternate drying and wetting on daily basis to accelerate the effect of damage. In the drying phase, the beams were also heated in the oven so that all the salt, which may be present in the pores of the concrete, may be crystallized.
S.No. 1. 2. 3.
Concrete ratio 1:2:4 1:2:4 1:2:4
Water/cement ratio 0.45 0.55 0.65
4. 5. 6. 7. 8.
1:2:4 1:1.5:3 1:2:4 1:3:6 1:4:8
0.75 0.55 0.55 0.55 0.55
Table-1: Different concrete ratios and w/c ratios used in the study In the first phase, tests were applied on the concrete after 28 days curing in the fresh water. The 28th day concrete strength has been taken as the controlling strength value. After that all the beams were placed in the sulphate enriched environment and tested after the intervals of 30 days i.e. at 58, 88, 118 and 148th day after casting of beams. The concrete beams of size 4” x 4” x 20” as prescribed in ASTM C 78-84 [7] having four different concrete ratios and four different water cement ratios were made. A minimum of three beams were tested at each testing stage and the ultra sonic pulse velocity, modulus of rupture and equivalent cube strength tests were carried out to check the effects of exposure to sulphates. 3-
RESULTS AND DISCUSSIONS
3.1
ULTRASONIC PULSE VELOCITY
The UPV values are noted on the concrete beams at the age of 28 days after soaking in fresh water as controlling value. After that, these concrete beams were exposed to soluble sulphates and the UPV values were noted on 30, 60, 90 and 120 th days for their exposure to salts. The average ultra sonic pulse velocity values for different concrete mix ratios have been shown in Table-2 and Figure-1. On analyzing, the results taken at 28 th day(controlling values) shows that the values of UPV decreases with the leaner mixes i.e. the values decreases in a steady manner from the mix ratio of 1:1.5:3 to 1:4:8 . The results taken after the exposure of sulphates shows that during the initial period of exposure to sulphates, the UPV values were reduced to a lesser extent as compared to controlling values. However, the last results taken at 148 th day shows that the rate of degradation of the Portland cement concrete has been increased. The decline in UPV values for the mix ratio of 1:1.5:3 and 1:2:4 at the end of test was less than 2% as compared to its controlling values (28 th day strength in fresh water) whereas the decline effect is more evident in the leaner mixes i.e. for the mix ratio of 1:3:6 and 1:4:8, the decline is noted up to 7-9%. The results of UPV values obtained for different water/cement ratios are shown in Table-3 and Figure-2. Analyzing the results for various water cement ratios, it is seen that the water/cement ratios has also influenced the UPV values. Generally, it has been noted that the %age UPV values are decreasing with the increase in the water cement ratio.
3.2-
MODULUS OF RUPTURE
The flexural strength of concrete was determined using 4"x4"x20" prisms with two-point loading on Universal Testing machine in accordance with the ASTM C 78-94. The modulus of rupture values for different mix ratios has also been shown in Table-4 and Figure-3. It has been noted that the values of modulus of rupture are not much affected on their exposure to sulphate salts, even in very lean mix of concrete mix ratio of 1:4:8. The effect on the values of modulus of rupture is even lesser in the mixes of 1:1.5:3, 1:2:4 and 1:3:6. The modulus of rupture values for different water cement ratios are shown in Table-5 and Figure-4. It has been noted that the values of modulus of rupture also are not much affected on their exposure to sulphate salts.
3.3-
EQUIVALENT CUBE STRENGTH
Equivalent cube strength was found, as prescribed by ASTM C 116-90, to confirm the behavior of compressive strength at different stages of exposure of concrete to sulphates. The results and behavior of the concretes for different concrete mix ratios is noted in Table 6 and Figure-5. Now on analyzing the results show that the values of equivalent cube strength decreases with the leaner mixes. The %age decrease in the values for the mix ratios of 1:1.5:3 and 1:2:4 are lesser as compared to concrete mix ratio of 1:3:6 and 1:4:8. The results of equivalent cube strength values obtained for different water/cement ratios are shown in Table 7 and Figure-6. It has been noted that the values of equivalent cube strength decreases with the increase in water cement ratio. The %age decrease in strength for the w/c ratio of 0.65 and 0.75 are more than the equivalent cube strength values for w/c ratio of 0.45 and 0.55. 3.4-
VISUAL INSPECTION OF SAMPLES
During experimentation visual inspection of the cracked samples was carried out. It was found that the sample beams have surface stains and have a characteristic whitish appearance. The crystallization of salts in to the pores of concrete is seen in most of the samples on their exposure to soluble sulphate salts. It was also found that the edges of the concrete has also spalled and broken in some cases.
Concrete mix ratios S.NO
Days
1:1.5:3
1:2:4
1:3:6
1:4:8
1
28
4634.81
4491.38
4160.32
4060.55
2
58
4628.48
4471.61
4116.49
4047.61
3
88
4617.96
4463.75
4044.39
3974.77
4
118
4601.22
4448.12
3990.38
3910.51
5
148
4572.23
4409.51
3901.5
3721.43
TABLE-2: Values of Ultrasonic pulse velocity for different concrete mix ratios and constant water/cement ratio=0.55
Water/cement ratios S.NO
Days
0.45
0.55
0.65
0.75
1
28
4543.61
4491.38
4409.51
4293.96
2
58
4543.61
4471.61
4379.1
4250.84
3
88
4519.35
4463.75
4345.38
4194.68
4
118
4475.55
4448.12
4308.53
4133.24
5
148
4452.02
4409.51
4247.28
4047.61
TABLE-3 : Values of Ultrasonic pulse velocity for different water/cement ratios and constant concrete mix ratio = 1:2:4
Concrete mix ratios S.NO
Days
1:1.5:3
1:2:4
1:3:6
1:4:8
1
28
632.27
570.29
489.7
421.52
2
88
619.88
570.29
495.9
415.32
3
148
619.88
564.09
477.3
402.92
TABLE-4: Values of modulus of rupture for different concrete mix ratios and constant water/cement ratio=0.55
Water/cement ratios S.NO
Days
0.45
0.55
0.65
0.75
1
28
595.08
570.29
564.09
520.7
2
88
595.08
570.29
557.89
520.7
3
148
588.88
564.09
551.69
502.1
TABLE-5: Values of modulus of rupture for different water/cement ratios and constant concrete mix ratio = 1:2:4
Concrete mix ratios S.NO
Days
1:1.5:3
1:2:4
1:3:6
1:4:8
1
28
3931.7
3257.7
1965.85
1572.68
2
88
3987.87
3229.61
1909.68
1488.43
3
148
3847.45
3173.45
1741.18
1291.85
TABLE-6: Values of equivalent cube strength for different concrete mix ratios and constant water/cement ratio=0.55
Water/cement ratios S.NO
Days
0.45
0.55
0.65
0.75
1
28
3538.53
3257.7
3173.45
2780.28
2
88
3510.45
3229.61
3089.19
2667.94
3
148
3426.2
3173.45
2976.86
2583.69
TABLE 7: Values of equivalent cube strength for different water/cement ratios and constant concrete mix ratio = 1:2:4
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CONCLUSIONS AND RECOMMENDATIONS The exposure to soluble sulphate salts on concrete has influenced the strengths of concrete. After the exposure of the concrete beams to sulphates, it has been noticed that the values decreases steadily. It has been noted that the rate of declination of the values obtained for all these tests was more violent during the last 60 days of sulphate solution immersion. Therefore sulphates attack progresses after 60 cycles of alternate drying and wetting. The values of modulus of rupture obtained for different concrete mix ratios and different water cement ratios were not decreased significantly. It can be concluded that the concrete members, which may be subjected to bending are less affected on exposure to sulphate salts. Based on the results it can be said that leaner mixes having the concrete mix ratios of 1:3:6 and 1:4:8 should be avoided in areas where there is possibility of the concrete structures to be exposed to sulphates. Also the higher water cement ratios should also be avoided in the structures having probability of exposure to sulphates such as seawater structures and bridge structures. Furthermore more experimental research is required to study the effects of other sulphates on the properties both in short term and long term.
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REFERENCES:-
[1]
Prof. K. A. Soudki, Umar Rizwan “Causes Of Concrete Deterioration design And Construction Defects Rehabilitation Of Structural Concrete” Department of Civil Engineering, University of Waterloo, Ontario, Canada 97803196.
[2]
Prof. K. A. Soudki, Polaris “Sulphate Attack On Concrete”, 06-01-2000, Department of Civil Engineering, University of Waterloo, Ontario, Canada. (accessed October 2004)
[3]
E.G. Swenson “CBD-136.Concrete in Sulphate Environments”, Canadian Building Digest, National Research Council, Institute for Research in Construction. Canada. 1971.
[4]
M. Collepardi, S. Collepardi, J.J. Ogoumah Olagot, R.Troli “Delayed Ettringite Formation Due To Sulphate Cement Content And Curing Temperatures.” Leonardo da Vinci. Polytechnic of Milan, Italy
[5]
M. Collepardi “A State-Of-The-Art Review On Delayed Ettringite Attack On Concrete” Civil Engineering Faculty, Leonardo Da Vinci, Politechnic Milan, Italy, Cement and Concrete Composites, 25, 401-407, 2003.
[6]
A.M. Neville “Properties of Concrete” 3RD EDITION Longman Group Limited.
[7]
Annual Book of ASTM Standards. ASTM C-78 Concrete and Aggregates.
[8]
Http://Www.Sandberg.Co.Uk/.Htm “Concrete Durability” SANDBERG online (accessed December 2004)
4700
4700 4500
1:1.5:3 1:2:4
4300
4500 w/ c =0 .4 5
4300
1:3:6
4100
4100
3900
3900
3700
3700 0
30
60
90
w/ c =0 .5 5 w/ c =0 .6 5
1:4:8
'w/ c =0.75
0
120 150 180
Fig 1: Ultrasonic pulse velocity for different concrete mix ratios
30
60
90
120 150 180
Fig 2: Values of Ultrasonic pulse velocity for different water/cement ratios
700
700
600
600
w/c=0.45
1:1.5:3 1:2:4
500
1:3:6
w/c=0.55
500
w/c=0.65
1:4:8
'w/c=0.75
400
400
300
300 0
30
60
90
120
150
0
180
Fig-3: Values of modulus of rupture for different concrete mix ratios
30
60
90
120 150 180
Fig-4: Values of modulus of rupture for different water/cement ratios
3900
3900 3600 3300 3000 2700 2400 2100 1800 1500 1200
3600 3300 1:1.5:3 1:2:4
0
30
60
90
120
150
3000
1:3:6
2700
1:4:8
2400
w/ c =0.45
2100
w/ c =0.55
1800
w/ c =0.65
1500
'w/ c =0 .75
180
1200 0
Fig-5: Values of equivalent cube strength for different concrete mix ratios
30
60
90
120
150
180
Fig-6: Values of equivalent cube strength for different water/cement ratios
