Repairing of RC Beams using Geopolymer Mortar
5.1 Introduction
5.2.1 Materials
The materials that were used in the process of casting of the RC beams and thereafter their repairing are described elaborately in this section under the following heads:-
5.2.1.1 Geopolymer paste
The fine cracks in the repaired beams were filled using GPP. The paste was prepared using ultra-fine ground granulated blast furnace slag (UGGBS). The properties of UGGBS have been discussed in sub-section, 2.2.2 Ultra-fine ground granulated blast furnace slag of Chapter 2. Table 5.1 presents the mix proportion of the GPP used as repairing agent. Total three numbers of mixes were prepared and tested to evaluate the efficiency in terms of fresh and hardened state properties. The mix consisted of 70 % UGGBS and 30 % flyash (FA).
This percentage was selected as this gives optimum result in terms of workability and strength as found in Chapter 3. Moreover, from the results presented in Chapter 3, 10 M sodium hydroxide (NaOH) was decided to be used as alkali activator. Here, total binder refers to UGGBS and FA together. Alkali in alkali/ total binder (a/b) ratio refers to NaOH solution.
The solid in the water/solid (w/s) ratio refers to binder and the NaOH solids in the solution.
The tests were performed as per guidelines of IS 1727 1967 [81]. Workability test was performed using flow table. The procedure is described in sub-section, 3.2.4 Experiments of Chapter 3. The result is reported in terms of flow index (FI) as defined by equation 5.1.
100
ID ID -
=FD
FI (5.1)
Here, FI is the flow index in percent. FD is the average final base diameter of mortar mass measured on four diameters after jolting as per the codal provisions [81], ID is the original base diameter, which is 100 mm. While performing the experiment for workability, it was observed that the addition of high quantity of alkali activator led to highly flowable
Chapter 5 Repairing of RC Beams using Geopolymer Mortar
GPP. However, when the such GPP was placed in the cube mould, the solid particles settled at the bottom. Compressive strength test was performed at 3 and 28 days on 50 mm cube of GPP to observe early and later age strength development behaviour.
Table 5.1: Mix proportion for GPP.
Mix Binder Alkali/binder
(a/b)
Water/solid (w/s)
UGGBS FA
GP1
70 30
0.55 0.34
GP2 0.60 0.37
GP3 0.65 0.39
From the results in Table 5.2, it was found that the GPP having a/b ratio of 0.60 possessed satisfactory workability and compressive strength. The paste having a/b ratio of 0.55 resulted in lowest workability. Lowest compressive strength was found in the GPP having a/b ratio of 0.65. The low strength attainment is presumed due to the segregation of GPP constituents. Therefore, GPP GP2 was decided to be used as repairing paste.
Table 5.2: Workability and compressive strength tests result of GPP.
Mix Alkali/binder (a/b)
Compressive strength (N/mm2) Workability (FI)
3 days 28 days
GP1 0.55 42.40 51.07 96.67
GP2 0.60 37.33 48.00 158
GP3 0.65 24.27 29.87 200
5.2.1.2 Portland cement paste
The fine cracks in the beams which were not possible to be filled up by the repairing PCM were filled using Portland cement paste (PCP). Ordinary Portland cement (OPC) 43 grade was used for preparing the paste. The properties of OPC are discussed in sub-section, 2.2.1 Portland cement of Chapter 2. Table 5.3 shows the water/cement (w/c) of the PCP tested for using as repairing agent. The mix proportion was selected such that the w/c ratio of PCP almost matches with total water/solid (w/s) ratio of GPP. Laboratory tests were performed to evaluate the fresh and hardened state properties of the PCP. The tests were performed as per guidelines of IS 1727 1967 [81]. Similar to the tests conducted for evaluation of fresh and hardened state properties of GPP, flow table and compressive strength tests were performed
PCP to observe early and later age strength development behaviour.
The test results are presented in in Table 5.3. It was found that the PCP exhibited lower compressive strength at early age than GPP. The strength at 28 days was high and was almost equivalent to that of GPP. However, the PCP possessed very low workability. The FI was 36.67.
Table 5.3: Workability and compressive strength tests result of PCP.
Mix Water/cement (w/c)
Compressive strength (N/mm2)
Workability (FI) 3 days 28 days
CP1 0.40 27.07 46.00 36.67
5.2.1.3 Geopolymer mortar
Geopolymer mortar (GPM) was prepared primarily using UGGBS. FA and superplasticizer (SP) were added as admixtures. The details of UGGBS, FA and SP are presented in sub- section 2.2.2 Ultra-fine ground granulated blast furnace slag, 2.2.3 Flyash and 2.2.6 Admixture of Chapter 2. The fine aggregate used in the preparation are described in sub- section, 2.2.5 Aggregates of Chapter 2. NaOH solution was used as alkali activator. Mix GM20 was selected for repairing from among all the GPM mixes presented in Table 3.1 in Chapter 3. Table 5.4 presents the mix proportion of GM20. Here, UGGBS and FA are together termed as binder. Total solid in the mix considers all the solid particles contributed by UGGBS, FA and NaOH. This was determined to find the value of water/solid ratio (w/s) in GPM which was considered equivalent to w/c of PCM mix used. The SP was added to the fresh GPM after the addition of the NaOH solution.
Table 5.4: Mix proportion for GM20 (kg).
UGGBS FA NaOH concentration (Molarity, M)
SP (% of binding agent)
SP
type w/s
70 30 10 1.5 PE 0.4
Note: Fine aggregate = 300 kg, NaOH = 60 kg.
The selection of GM20 as repairing agent was on the basis of fresh and hardened state properties of the GPM mixes. Compressive strength test was conducted on 50 mm cubes of
Chapter 5 Repairing of RC Beams using Geopolymer Mortar
GPM in Compression Testing Machine and results of 3 and 28 days are presented in the table.
Workability test was performed using flow table when the mortar was in fresh state. The result is reported in terms of flow index (FI) as defined by equation 5.1 in sub-section 5.2.1.1 Geopolymer paste. Vicat apparatus was used for conducting the setting time test. The tests were conducted as per relevant IS Code of Practice [81].
GM20 offered appreciable workability. The setting time was also found to be sufficient for placing the mortar into the cracks. The strengths attained at both early and later ages were also comparatively high. The fresh and hardened state properties of the GPM mix GM20 are presented in Table 5.5.
Table 5.5: Fresh and hardened state properties of GM20.
Test Result
Initial setting time 58 minutes Final setting time 88 minutes
Workability 83.67 (FI)
Compressive strength (3 days) 36.60 N/mm2 Compressive strength (28 days) 45.96 N/mm2 5.2.1.4 Portland cement mortar (PCM)
Portland cement mortar (PCM) was used for repairing the cracks in the RC beams. The mortar was prepared using OPC as described in sub-section, 2.2.1 Portland cement of Chapter 2.
Fine aggregate that was used for preparation of GPM was also used for preparation of PCM.
Table 5.6 presents the mix proportion of CM1 which was used for preparing the PCM. The fresh and hardened state properties of CM1 are shown in Table 5.7. The tests performed were similar to the tests performed in case of GPM.
Table 5.6: Mix proportion for PCM (kg).
Mix Cement Fine Aggregate Water/cement (w/c)
CM1 100 300 0.40
Test Result Initial setting time 95 minutes
Final setting time 265 minutes
Workability 19.67 (FI)
Compressive strength (3 days) 19.58 N/mm2 Compressive strength (28 days) 35.02 N/mm2 5.2.1.5 Portland cement concrete
Portland cement concrete (PCC) was prepared using ordinary Portland cement. The details of OPC as described in sub-section, 2.2.1 Portland cement of Chapter 2. The fine and coarse aggregates used in the preparation have been described in sub-section, 2.2.5 Aggregates of Chapter 2. The mix proportion for PCC has been presented in Table 5.8.
Table 5.8: Mix proportion for PCC (kg/m3).
Mix Cement Fine Aggregate Coarse aggregate Water/cement (w/c)
CC1 360 680 1128 0.55
Mix CC1 was used for preparation of PCC for casting of RC beams. Table 5.9 presents the fresh and hardened state properties of the mix. Compressive strength test was performed on 150 mm cubes of PCC in Compression Testing Machine and results of 3 and 28 days have been presented in the table. Prism specimens of size 150 mm × 150 mm × 700 mm were cast and tested applying four point loading at 3 and 28 days to determine the flexural strength of PCC. For determining the tensile strength, PCC cylinders of 150 mm diameter and 300 mm height were cast and tested in the Universal Testing Machine (UTM). Workability was evaluated from slump test performed using slump cone when the concrete was in fresh state.
The tests were performed as per relevant IS Code of Practice [122, 125, 127 ].
Chapter 5 Repairing of RC Beams using Geopolymer Mortar
Table 5.9: Fresh and hardened state properties of CC1.
Test Result
Workability (Slump) 90 mm Compressive strength (3 days) 15.14 N/mm2 Compressive strength (28 days) 28.04 N/mm2
Tensile strength (3 days) 0.90 N/mm2 Tensile strength (28 days) 2.17 N/mm2 Flexural strength (3 days) 1.12 N/mm2 Flexural strength (28 days) 2.56 N/mm2 5.2.1.6 Reinforcement
RC beams were cast with 2 numbers of 12 mm diameter tor steel bars IS 432 (Part 1) 1982 [95] at top and bottom as main reinforcement. Stirrups were provided using 2 legged 8 mm diameter tor steel bars. Sub-section, 2.2.7 Steel reinforcement bars of Chapter 2 presents the details of the rebars used.