and cracking in high-performance and normal-strength cement grouts is ... ever, a random choice of grout ingredients may endanger its long-term durability.
Mechanics of Composite Materials, Vol. 41, No. 1, 2005
ALKALI-SILICA REACTION EXPANSIONS IN HIGH-PERFORMANCE AND NORMAL-STRENGTH CEMENT GROUTS REINFORCED WITH STEEL AND SYNTHETIC FIBERS
R. H. Haddad and A. Qudah
Keywords: fibers, alkali-silica reaction, expansion, contraction The role of brass-coated steel (BCS), hooked steel (HS), and polypropylene fibers in controlling the expansion and cracking in high-performance and normal-strength cement grouts is investigated. The grouts were prepared using the BCS, HS (from 0 to 2.0 vol.%), or polypropylene (from 0 to 0.30 vol.%) fibers. Standard prisms (25 ´ 25 ´ 300 mm3) were cast and cured for 40 days before subjecting to a special treatment to accelerate the alkali-silica reaction (ASR). Expansion measurements were taken for these prisms over an immersion period of up to 98 days, during which the extent of cracking was monitored. The results indicated a significant role of brass-coated and hooked steel fibers at volume fractions of 1.0 and 2.0% and polypropylene fibers at a volume fraction of 0.15% in reducing the expansion due to the ASR. The reduction in expansion of high-performance and normal-strength grouts was also dependent, in addition to the fiber type and content, on the duration of immersion of the grouts in a NaOH solution.
Introduction Cement grouts have been used on a wide range in different structural systems, such as thin plates and shells, as well as facets of wooden buildings. Recently, they are being utilized in the field to rehabilitate deteriorating concrete structural members. Two classes of grouts are usually used: high-performance flowable and normal-strength of moderate consistency. However, a random choice of grout ingredients may endanger its long-term durability. This may occur when an amorphous siliceous aggregate is employed along with a cement of high alkalis content, triggering what is known as an alkali-silica reaction (ASR). This reaction occurs in the existence of ample amount of hydroxide ions and water, producing a complex gel, which expands upon absorption of water and leads to the expansion and later cracking of hardened grout layers [1]. Since the factors that trigger the ASR are inherent in concrete structural members, the traditional rehabilitation and repair methods of these members are limited. Research on concrete structural members have shown that the ASR can create large irreversible strains in concrete and steel, which will adversely affect the serviceability, strength, and stability of reinforced beams. The maximum reported loss in the flexural capacity due to the ASR reached as high as 25% [2, 3]. It was also reported that the expansion in reinforced con-
Department of Civil Engineering, Jordan University of Science and Technology, P.O. 3030, Irbid, Jordan. Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 41, No. 1, pp. 121-130, January-February, 2005. Original article submitted January 30, 2004.
0191-5665/05/4101-0087 © 2005 Springer Science+Business Media, Inc.
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TABLE 1. Chemical Composition of the Modified Type I Cement Oxide
Modified Type I Cement (%)
Si2O
21.21
CaO
63.69
Fe2O3
3.11
Al2O3
5.54
MgO
1.5
SO3
2.63
(Na2O+0.658 K2O)
1.30
Insoluble residue
0.12
Loss through ignition
0.96
crete was greatly reduced as compared to that in plain concrete and that the resulting cracks were mainly parallel to the reinforcement [4]. Field and experimental results have shown that the use of steel fibers in concrete reduces the expansion or contraction of concrete due to its creep or shrinkage, increases the cracking resistance, and improves the ductility [5-13]. Investigations concerned with the effect of fibers in preventing and/or reducing concrete cracking due to the drying shrinkage or creep strains have led to contradictory conclusions [5-7]. While some studies showed that fibers had insignificant effects on unrestrained shrinkage, others indicated a real contribution of fibers in reducing the restrained shrinkage of fiber-reinforced concrete (FRC) [5-7]. As for the creep, most investigations indicated that the creep strain of FRC with steel or polypropylene fibers was higher than that of plain concrete [8, 9]. Mangat and Azari [10] arrived at opposite conclusions. Results from studies on high-strength concrete exposed to heating-cooling cycles and high temperatures pointed to a high potential of fibers in delaying concrete cracking and in limiting its extent [11, 12]. Other studies reported that the concrete cracking caused by steel corrosion could be controlled by using synthetic fibers [13]. One of the very few studies that tackled the role of synthetic and steel fibers in controlling the ASR cracking in Portland cement concrete was that by Haddad and Smadi [14]. The results obtained indicated that the fibers contributed significantly to delaying the initiation of cracks and to controlling their extent, but their contribution to controlling the degree of length expansion was very limited. In this paper, the role of brass-coated steel, hooked steel, and polypropylene fibers in controlling the grout-expansion-produced cracking in high-performance and normal-strength grout matrices is investigated. The high-performance flowable grouts were prepared with different volume fractions of brass-coated or hooked steel fibers. The normal-strength grouts were made using polypropylene or brass-coated steel. Standard prisms (25 ´ 25 ´ 300 mm3) were cast and cured for 40 days before their subjecting to a special treatment to accelerate the ASR. The compressive strength of different grouts was evaluated using cubic specimens (50 ´ 50 ´ 50 mm3), in accordance with ASTM standards C 109. The length expansion measurements were taken over a treatment period of 60-96 days. The extent of cracking was also evaluated by mapping the cracks formed. 2. Experimental Program 2.1. Materials. A mixture of fine particles consisting of 75% silica sand and 25% crushed Pyrex were used in preparing mortar mixtures. The percentages passing for fine particles were chosen according to the ASTM C 1260 requirements for
88
gradation of aggregates tested for the ASR activity. An X-ray diffraction analysis of the Pyrex used indicated a silica content in excess of 80%. The ordinary Portland cement (Type I) was used, including a modified alkaline content equivalent to 1.3% KOH powder. The chemical composition of the modified cement is indicated in Table 1. Three types of fibers, namely BCS, HS, or polypropylene, were incorporated in grout mixtures. The BCS and HS fibers were supplied by Bekaert, Belgium, whereas the polypropylene ones was supplied by the Harex Company, USA. The yield strength, length, diameter, and aspect ratio were 2950 MPa, 6 mm, 0.15 mm, and 40 for the BCS and 1172 MPa, 30 mm, 0.50 mm, and 60 for the HS fibers, respectively. The tensile strength, Young’s modulus, specific gravity, and fiber length of the polypropylene fibers were 650-760 MPa, 3.5 GPa, 0.9 kg/mm3, and 12 mm, respectively. 2.2 Mix proportions and specimen preparation. The high-performance grouts were prepared at a w/c ratio of 0.40 with a fine particles to cement ratio of 0.80 and at different volume fractions of the BCS or HS fibers (from 0 to 2.0%). The normal-strength grouts were prepared at a w/c ratio of 0.50 with a fine particles to cement ratio of 3.0 and BCS (from 0 to 2.0%) or polypropylene (from 0 to 0.3%) fibers. The Cormix water reducer was used at a content of 0.5% (by cement wt) in the high-performance flowable grouts to achieve the consistency required. The mixing was carried out in a mechanical mixer according to the ASTM method C33. The fibers used were previously blended with the fine particles before mixing. Grout prisms (25 ´ 25 ´ 300 mm3) with steel knob-inserts and standard cubic specimens (50 ´ 50 ´ 50 mm3) were cast in order to measure the length expansion (due to the ASR) and to evaluate the compressive strength, respectively. The specimens of normal-strength grouts (having a moderate consistency) were cast by placing the mortar in two layers, each consolidated by a tamper. The high-performance flowable grouts, on the other hand, were poured in molds and consolidated on a vibrating table. The surfaces of the specimens were smoothened by a trowel before their placing in a water bath at 23°C to cure for 40 days. At this age, the compressive strengths for different grouts were obtained. The compressive strengths were 46.3, 48.2, 48.3, and 48.2 MPa for the high-performance grouts with the BCS fibers and 46.3, 46, 45.8, and 59.1 MPa for those with the HS fibers at volume fractions of 0, 0.50, 1, and 2%, respectively. The corresponding values for the normal-strength grouts with the BCS fibers were 24, 24.5, 25.4, and 34.4 MPa. The strengths for the normal-strength grouts with polypropylene fibers at volume fractions of 0, 1, and 2% were 24, 24, and 21.9 MPa. 2.3 Conditioning and expansion measurements. In order to accelerate the ASR, the specimens cured for 40 days were placed in a 0.5 N water solution of NaOH at a temperature of 55°C. The specimens were kept in the solution for about 60-98 days, during which their expansion was measured by using a dial strain gage. Prior to any measurement, all the specimens were left for about half an hour in laboratory air to dry and cool down. 3. Results and Discussion The following discussion covers four points. First, the effect of treatment in the NaOH solution on the expansion behavior and color changes of grout prisms. Second, the role of fibers in controlling the expansion of high-performance and normal-strength grouts. Third, the role of fibers in controlling the cracking. Finally, empirical models for predicting the expansion of high-performance grouts. 3.1 Expansion behavior and color changes of grout specimens. High-performance grout. Figs. 1 and 2 depict expansion histories for the high-performance grout prisms with the BCS and HS fibers, respectively. It is seen that the immersion in the NaOH solution has resulted in triggering the ASR, as indicated by the continuous increase in expansion with time. The expansion rate changed with immersion periods without showing a plateau behavior after 60 days. This is explained by the fact that the expansion measurements were terminated at this age due to the excessive cracking of the prisms. The expansion rate was the lowest during the period of 0-12 days, the highest during the period of 20-50 days, and intermediate during the period of 50-60 days. This behavior was observed for plain grouts as well as for those with the BCS and HS fibers. The expansion of different grouts was accompanied by changes in their color from greenish to whitish. Normal-Strength Grout. The expansion history for the normal-strength grouts with the BCS and polypropylene fibers is depicted in Figs. 3 and 4, respectively. As can be seen, the immersion of the specimens in the NaOH solution has resulted in
89
9000
a
E, me .103
9000
7000
7000
5000
5000
3000
3000
t, days
1000 0
10
30
50
70
b
E, me .103
t, days
1000 0
10
30
50
70
Fig. 1. Expansion history of high-performance grouts with BCS (a) and HS (b) fibers at their volume content of 0 (v), 0.5 (o), 1.0 (F), and 2.0% (m).
4500
a
E, me
4500
3500
3500
2500
2500
1500
1500
t, days
500 0
20
40
60
80
100
120
b
E, me
t, days
500 0
20
40
60
80
100
Fig. 2. Expansion history of normal-strength grouts with BCS (a) and PP (b) fibers at their volume content of 0 (v), 1.0 (F), and 0.3% (m).
triggering the ASR, as indicated by the continuous increase in expansion with time. The expansion rate was the highest during the period of 20-50 days, after which the curves showed a plateau behavior. Compared with the high-performance grouts, the normal-strength grouts attained a much less ultimate expansion, owing to the lower content of cement and hence of alkalis. 3.2 Role of fibers in controlling the expansion and the expansion rate. High-performance grouts. The role of fibers in controlling the expansion of high-performance grouts can be understood with the aid of Figs. 1 and 2, as well as Table 2. As can be seen, the curves pertaining to the grouts with the BCS or HS fibers are located below those corresponding to the plain grouts (without fibers) This points to the significant effect of the fibers on the expansion over the entire immersion period, especially at fiber volume contents of 1 and 2%.
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TABLE 2. Reduction in the Expansion of High-Performance Grouts for Different Immersion Periods Immersion period (days) Fiber type
Content, %
Plain grouts BCS
HS
14
28
63
0
513
1783
8068
0.5
371 (27.7%)
1447 (18.8%)
8187 (0%)
1.0
551(0%)
1385 (22.3%)
6576 (22.7%)
2.0
353 (31.2%)
1122 (37.1%)
6533 (23.5%)
0.5
825 (0%)
1501 (18.7%)
5907 (26.8%)
1.0
614 (0%)
1480 (17.0%)
Not measured
2.0
552 (0%)
1266 (29.1%)
3977 (50.7%)
TABLE 3. Reduction in the Expansion of Normal-Strength Grouts for Different Immersion Periods Immersion period (days) Fiber type Plain grout BCS Polyprophylene
Content, % 14
28
63
96
0
200
2014
3937
4104
1.0
53 (73.5%)
970 (51.8%)
2321 (41.0%)
2351 (42.7%)
2.0
57 (71.5%)
1081 (46.3%)
2302 (41.5%)
2497 (39.2%)
0.15
198 (1.0%)
1527 (24.2%)
2673 (32.1%)
2905 (29.2%)
0.30
182 (9.0%)
1696 (15.8%)
3184 (19.1%)
3349 (18.4%)
The results for 14, 28, and 61 days of immersion are presented in Table 2 along with expansion reduction data upon the use of fibers. It is seen that the incorporation of the BCS fibers at volume fractions of 0.50, 1.0, and 2% has reduced the grout expansion by about 18.8, 22.3, and 37.1% after 28 days, and by about 0, 22.7, and 23.5% after 63 days, respectively. This means that the contribution of the BCS fibers to controlling the expansion depends not only on the fiber content but also on the immersion period (aggressiveness of the ASR). It is clear that the use of the BCS fibers at 1.0 and 2 vol.% is much more effective than at 0.5 vol.% in reducing the expansion due to the ASR. The use of 2.0 vol.% HS fibers reduced the expansion of the grouts by about 51% after 63 days of immersion. The HS fibers at 0.5 and 1.0 vol.% showed almost the same effect. In general, the contribution of the BCS and HS fibers to reducing the expansion depended upon the fiber content and the immersion period: it decreased at lower fiber fractions and increased at later immersion periods. Normal-strength grouts. The role of the polypropylene and BCS fibers in reducing the expansion of normal-strength grouts can be seen from Table 3. The use of 0.15 vol.% polypropylene fibers reduced the expansions of grouts by 24.2 and 29.2% after immersion periods of 28 and 96 days, respectively. The contribution of the polypropylene fibers at a content of 0.3 vol.% was less, reaching only 18.4% after 96 days of immersion. For the grouts with 1.0 and 2.0% of the BCS fibers, the reduction in expansion after 96 days was 42.7 and 39.2%, respectively. The reduced expansion upon the use of fibers may be attributed to their crack arresting capability and to the increased rigidity of the fibrous mixture (with the BCS or HS fibers). This conclusion is supported by the facts that the BSC and HS fibers
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Fig. 3. High-performance (top) and normal-strength (bottom) grout prisms with BCS fibers after immersion in a NaOH solution for 60 days.
TABLE 4. Equation Constants for the High-Performance Grouts with BCS of HS Fibers Constants c
d
Time interval (days)
Fiber type a BCS
HS
b
Coefficient of determination, R 2
0.0158
0.8209
26.69
-0.2299
0-28
0.9883
–0.0604
7.7185
–111.24
–0.1357
28-62
0.9798
–0.0303
1.9831
26.24
–0.1381
0-28
0.9880
–0.0873
9.4187
–128.63
–0.2917
28-62
0.9614
showed a better performance at higher fiber contents and that a rise in the content of the polypropylene fibers from 0.15 to 0.3 vol.% contributed inversely to controlling the expansion. This was because of the insignificant growth in the rigidity of grouts with increasing fiber content and of the more intense fiber-fiber interaction, which made the cement matrix weaker. 3.3. Extent of cracking in the high-performance and normal-strength grouts. The cracking extent was evaluated qualitatively by mapping the cracks on the surfaces of the prisms tested. The cracking patterns were observed over immersion periods from 10 to 60 days. At early ages, the cracking intensity of the high-performance grouts was lower for the specimens with a higher fiber content, indicating an effective role of fibers in controlling the cracking. As the expansion increased, the cracking intensity also increased, and the role of fibers seemed to become insignificant. Contrariwise, the normal-strength grouts received negligible cracking after similar immersion periods, owing to the much lower expansion achieved. Fig. 5 shows typical cracking patterns for the high-performance and normal-strength grouts with the BCS fibers after 60 days of immersion. Similar cracking patterns were observed for the high-performance grouts with the HS fibers. The normal-strength grouts with the polypropylene fibers showed a similar behavior to those with the BCS fibers. 3.4 Predicting the expansion of the high-performance grouts. The prediction models for the high-performance grouts were developed using the statistical package SPSS and the data presented in Figs. 1 and 2. The models can be used to predict the expansion of high- performance grouts, similar to those used in this study, for any immersion period between 0 and
92
62 days and for the BCS or HS fibers of volume fractions between 0 and 2%. They are also useful in determining the content of the BCS or HS fibers so as to ensure allowable expansions. The analytical form of the models is E = ( at 3 + bt 2 + ct )exp ( dF ), where E is the expansion (microstrain me), t is the immersion period (days), F is the volume fraction of fibers (percent), and a, b, c, and d are constants. The constants for the grouts with the BCS and HS fibers are listed in Table 4 along with the multiple coefficient of determination R 2 . This statistical parameter indicates that the model developed fits the experimental data very well. 4. Conclusions Based on the previous discussion, the following conclusions can be drawn: 1. The use of brass-coated and hooked steel fibers in high-performance grouts at volume fractions of 1 and 2% allows one to significantly reduce the expansion of the grouts caused by the alkali-silica reaction. 2. Polypropylene fibers at 0.15 vol.% and brass-coated steel fibers at 1 and 2 vol.% can reduce the ultimate expansions of normal-strength grouts by more than 30%. 3. The alkali-silica reaction brings about much more damage in high-performance than in normal-strength grouts. The use of fibers plays a limited role in controlling the cracking of grouts, especially at late stages of the reaction. 4. The contribution of fibers to reducing the expansion due to alkali-silica reactions depends upon the grout strength, fiber content, and period of immersion in the NaOH solution. 5. The empirical models developed can be useful in predicting the expansion of grouts and in determining the best fiber content based on certain expansion requirements.
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12. K. H. Rebat, Behavior of Fiber Reinforced Concrete as Subjected to Thermal Cycling, M.Sc. Thesis, Civil Engineering Department, Irbid, Jordan (1999). 13. R. H. Haddad and A. M. Ashteyate, “Role of synthetic fibres in delaying steel corrosion cracks and improving bond with concrete,” Canad. J. Civ. Eng., 28, 787-793 (2001). 14. R. S. Haddad and M. Smadi, “Role of fibers in controlling unrestrained expansion and arresting cracking in Portland cement concrete undergoing alkali-silica reaction,” Cement Concr. Res., 34, 103-108 (2004).
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