ALKALI AGGREGATE REACTIVE MORTAR CYLINDERS PARTLY RESTRAINED BY EXTERNAL CFRP FABRIC EXPANSION AND CONFINEMENT

Transcription

1 ALKALI AGGREGATE REACTIVE MORTAR CYLINDERS PARTLY RESTRAINED BY EXTERNAL CFRP FABRIC EXPANSION AND CONFINEMENT B. J. WIGUM SINTEF Civil and Environmental Engineering, Cement and Concrete N-7465 Trondheim, Norway Corresponding address; ERGO Engineering Geology, Ltd. Grundarstigur 2, 101 Reykjavik, Iceland Mortar specimens with alkali reactive aggregates have been tested in accelerated laboratory conditions. The specimens were partly restrained by external CFRP (Carbon Fibre Reinforced Polymers) fabric or partly restrained by elastic tape. Expansion results for the various types of specimens are presented in this paper. Preliminary assessments of the results indicate that the CFRP fabric is capable of hindering alkali aggregate volume expansion above a certain limit. Final results will be used to make implications of full-scale relevance of CFRP fabric as a repair medium to prevent AAR expansion. INTRODUCTION A wide variety of aggregate types in common use, particularly those with a siliceous composition, are vulnerable to attack by the alkaline pore fluid in concrete. This attack, which in wet conditions produces a hygroscopic and hydraulic gel, can cause cracking and disruption of the concrete. The deterioration mechanism is termed Alkali Aggregate Reaction (AAR). An example where AAR has disrupted a concrete structure is Elgeseter Bridge in Trondheim, Norway. The 50 years old structure is a continuous 220 m long reinforced concrete beam bridge supported by eight rows of columns, each consisting of four columns with diameter of 80 cm each (Figure 1). AAR was diagnosed in 1990 and in-situ measurements of crack development and humidity were established in 1995 (Jensen 2000) 1. The concrete beam bridge has expanded by 200 mm in the free direction and several vertical cracks, up to 2 mm in width, are in the columns and can be followed from ground level to the underside of the supported beams (10 meters). 1

2 2 2 mm 200 mm Figure 1. Overview of the Elgeseter Bridge near the Nidaros Cathedral in Trondheim. Picture to the left shows close-up of vertical cracks in columns due to AAR. The concrete beam bridge has expanded by 200 mm in the free direction and several vertical cracks, up to 2 mm in width, are in the columns. The necessity of repairing the structure is evident, however the main concern has been what kind of repair methods and materials are best fitting, with the purposes to reduce humidity in the columns, stop further crackdevelopment and finally strengthen the columns. Wrapping the columns with CFRP fabric has been proposed as a possible solution. The positive strengthening effect of columns with CFRP fabric has already been demonstrated in full-scale at SINTEF (Thorenfeldt 2000) 2. The main purpose of this post-doctoral study has been to assess CFRP fabric as a repair material in order to strengthen and repair bridge pillars damaged by AAR. Advantages and disadvantages of the material for this particular purpose will be considered in a final report. In this paper, the preliminary results from the laboratory test programme are reported and discussed. The main questions addressed were how expansion and cracking due to AAR would develop for mortar specimens partly restrained by external CFRP fabric. The laboratory programme is scheduled to finish late autumn 2002.

3 EXPERIMENTAL PROCEDURES 3 Preparation of mortar mix, cylinders and initial storage of cylinders A mortar-mix, expected to be highly alkali-reactive, was prepared using a high w/c-ratio (0,65), enhanced alkali content in the cement and external added NaOH (total 9,5 kg Na 2 O eq./m 3 ). The well-documented highly reactive Spratt aggregates from Canada were used (siliceous dolomitic limestone), with a grading curve according to requirements in ASTM 1260 (1994) 3. A total of eight mortar cylinders (10 x 45 cm) were cast, with fixed studs (for expansion measurements) in the ends of four cylinders. To initiate the reaction, all cylinders were pre-stored in accelerated conditions (1 N NaOH at 80 o C) for 14 days. Subsequent to the initial storage, the cylinders were kept at room temperature, mortar surfaces dried and cleaned with sandpaper to get a clean surface. Two of the cylinders, one prepared with elastic tape (sample C2) and one with CFRP fabric (sample A1), were pre-loaded up to about 10 MPa. The purpose of this was to examine any influence on the E-modulus caused by the introductory accelerated exposure. Since the behaviour at the very beginning of the loading was important in these tests, they were carried out deformation-controlled. However, the tests gave no indication of any change in the E-modulus compared to that of the initially cast concrete. Preparation of samples for further testing Four of the cylinders were wrapped and restrained with single bands of CFRP fabric embedded in epoxy resin, while four cylinders were wrapped with bands of an elastic non-permeable asphaltic tape with the purpose to simulate the same surface area exposed to water (as for the cylinders with CFRP bands), without any restraining effect. Each cylinder was covered with seven parallel bands, each of 53-mm width, and in between were six open areas, each of 13-mm width. Two of the CFRP fibre wrapped cylinders (Samples A1 & A2) and two taped cylinders (Samples C1 & C2) were without end-studs. These cylinders were fixed in rigs with four Ø 14 mm steel rods with 30 mm thick steel plates at both ends (see Figure 2). A total number of eight strain gauges (WFLA-6) were fixed in each sample; one at each of the four rods, two parallel in the length direction of the cylinder and two parallel in the circular direction of the cylinder. Strain gauges in relation to the elastic tape were glued on the concrete before

4 4 placing the tape, while strain gauges related to CFRP were glued upon the CFRP. Small loads were initially placed on the specimens by screwing down bolts at the end of the four steel rods. Strain was introduced into the rigs according to table 1. Figure 2: Configuration of mortar cylinder fixed in rig. Table 1. Initial strain in rigs and mortar specimens Specimens No. Initial Microstrain Strain in Mortar (Mpa) A A C C The two CFRP fibre wrapped cylinders with end-studs (Sample B1 & B2), and the two taped cylinders with end-studs (Sample D1 & D2) were covered with epoxy at both ends. A total of four strain gauges (WFLA-6) were fixed in each sample; two parallels in the length direction of the cylinder and two parallels in the circular direction of the cylinder.

5 5 Figure 3. Configuration of the four different specimen set-up (each set-up with a duplicate sample); Samples A1 & A2: CFRP fibre wrapped cylinders, fixed in rigs; Samples B1 & B2: CFRP fibre wrapped cylinders, with end-studs; Samples C1 &C2: Elastically taped cylinders, fixed in rigs; Samples D1 & D2: Elastically taped cylinders, with end-studs. Exposure and measurement during testing period The strength of the epoxy resin decreases with increased temperature, and the influence of 1N NaOH solution on the properties of epoxy was uncertain, hence all samples were stored in water at 38 o C. Manual measurements of expansion between end-studs and weight changes of cylinders B1, B2, D1 & D2 (weighed both in air and water) were carried out after 1, 3, 6, 8, 10, 13, 21 & 28 weeks respectively, accompanied by a visual inspection of crack development. The strain gauges of all the samples were logged automatically every hour, and results stored in a computer.

6 6 PRELIMINARY RESULTS AND DISCUSSION Expansion- and stress measurement in longitudinal direction Expansion results in longitudinal direction for the four specimens not fixed in rigs after 39 weeks exposure in 38 o C water are found in Figure 4. The percentage expansion in longitudinal direction is almost linear, and reaches values in the order of 0,30 0,43% after 39 weeks of exposure. One of the two taped specimens (D2) exhibits slightly higher values than the CFRP wrapped specimens (B1 & B2). A possible explanation for this could be differences in properties of the CFRP fabric versus the elastic tape, with the former preventing water ingression into the mortar. Cylinders with elastic tape could get more water in the weight increase is greater for the taped samples than the CFRP wrapped samples, see Figure 5. It is difficult to compare the expansion with other standard test results due to the non-standard size of the mortar cylinders. However, the Spratt aggregates used in this study have exhibited 0,29 0,50% expansion in the accelerated mortar-bar test after 14 days of exposure and 0,17 0,19% expansion in the Canadian concrete-prism test after one year of exposure. (RILEM 2002) 4. The linear development of the expansion indicates a potential of further expansion Expansion (%) B1 (CFRP) Expansion B2 (CFRP) Expansion D1 (Elastic-tape) Expansion D2 (Elastic-tape) Expansion Weeks of exposure in water (38 o C) Figure 4. Results of longitudinal expansion (%) after 28 weeks.

7 Preliminary results from the strain gauges measurements are not fully interpreted yet. However, it appears that cylinders, which are free to expand in longitudinal direction, i.e. cylinders with end-studs (B & D), reach a higher longitudinal expansion, % after 120 days compared to % expansion in the cylinders fixed in rigs (A & C). It is however difficult to depend upon the strain gauge measurements, as it seems that they have not been durable in the water storage. At least this is the case for strain gauges not embedded in the epoxy resin Change of weight (g) B1 (CFRP) Change of weight B2 (CFRP) Change of weight D1 (Elastic tape) Change of weight D2 (Elastic tape) Change of weight Weeks of exposure in water (38 o C) Figure 5. Results of change of weight (g) after 21 weeks. Volume expansion For volume changes, the various samples were weighed in water and in air. The volume change was then taken as the difference of the weight in air and water. As evident in Figure 6, shrinkage appears after three weeks of exposure. No explanation is found for the enhanced shrinkage for the cylinders with elastic tape. The effect of shrinkage is not found in the measurement of the longitudinal expansion. After 21 weeks of exposure the volume expansion for the taped cylinders (D) is in the order of 1.10%, while volume expansion of the CFRP wrapped cylinders is lower (0.70%). These are approximately three times the values of the longitudinal expansions, as expected.

8 8 It is however very interesting to notice that for the cylinders with CFRP fabric, the volume expansion reaches a certain level and slows down after 21 week the volume expansion development for both B1 and B2 are much lower after 39 weeks. It is apparent that the CFRP fabric is starting to manage the expansion completely, and will continue to keep the volume almost constant to a potential failure. For the cylinders with the elastic tape the volume expansion is continuing to rise. Preliminary results from the strain gauges measurements up to 120 days, show higher circular expansion for CFRP cylinders fixed in a rig (sample A) (3,0 4,0 o/oo), compared to CFRP cylinders (sample B) free to expand in longitudinal direction (1,5 2,0 o/oo). This indicates the expansion favouring the direction of less resistance. However more strain gauge data (beyond 120 days) should be needed to verify this. However, this will not be carried out due to the durability difficulties with the strain gauges. Cylinders not fixed in the longitudinal direction, i.e. sample D with elastic tape, show a little higher circular expansion (2,5 o/oo) than sample B with CFRP fabric. Weeks of exposure in water (38 o C) Volume expansion (%) B1 (CFRP) Volume-expansion B2 (CFRP) Volume-expansion D1 (Elastic-tape) Volume-expansion D2 (Elastic-tape) Volume-expansion Figure 6. Results of volume expansion (%) after 28 weeks. Visual inspection After 21 and 39 weeks of exposure a detailed visual inspection was carried out for all specimens. No visual cracks were observed for cylinders that were fixed in the rigs (A1, A2, C1 & C2). However, cracks ( 0,1 mm) could be observed for all cylinders that were free to expand in longitudinal

9 direction. For cylinders with CFRP fabric the cracks appear approximately 45 o to the longitudinal direction. Cylinders with elastic tape however exhibited cracks both in longitudinal direction and 90 o on that direction (se Figure 7). 9 Figure 7. The picture to the left shows cracks in cylinder with CFRP, while the picture to the right shows cracks in cylinder with elastic tape. CONCLUSIONS FURTHER RESEARCH & IMPLICATIONS (a) Mortar cylinders wrapped with CFRP fabric free to expand in longitudinal direction reach a volume expansion level after 21 week. Prolonged expansion data for all relevant specimens, will in conclusion demonstrate if this is genuine. (b) For further evaluations of the stress-strain relationship, some of the cylinders will be loaded deformation-controlled to failure at the end of this investigation. (c) Crack development will be studied by polished plan-section and petrographic thin-sections prepared from the various mortar specimens. (d) The final conclusions will be used as a basement in assessing and calculating a proposal for a full-scale pilot repair test.

10 10 ACKNOWLEDGEMENT This study is part of the EU TMR network; Development of Guidelines for the Design of Concrete Structures, Reinforced, Prestressed or Strengthened with Advanced Composites. The author wishes to acknowledge the European Commission for funding the EU TMR Network "ConFibreCrete" and Sika in Norway for providing materials. Colleagues at SINTEF and ERGO are acknowledged for technical assistance and discussion during the process of the work. REFERENCES 1. Jensen, V., In-situ measurement of relative humidity and expansion of cracks in structures damaged by AAR. 11 th International Conference on Alkali-Aggregate Reaction, Québec City, QC, Canada, June 2000, pp Thorenfeldt, E., Forsterkning av betongsøyler med karbonfibervev (in Norwegian). SINTEF Civil and Environmental Engineering, Cement and Concrete. Project nr. 22M17600, 18 pp. 3. Annual Book of ASTM Standards, Volume 4.02, Concrete and Aggregates, C , Standard method for potential alkali-silica reactivity of aggregates (mortar bar method), 1994, pp RILEM, Outline Guide to the use of RILEM Methods in Assessments of Aggregates for AAR Potential. RILEM Recommended Test Method AAR-0. Direction of Potential Alkali-Reactivity of Aggregates, DRAFTS 4 (Version 2), May 2002, 8 pages with appendix.