Carbonation Resistance of SCC

Carbonation Resistance of SCC

Gundupalli Bhanu Prakasp, V Prem Kumar2

Dept. of Civil Engineering

Sree Vidyanikethan Engineering College Tirupathi, India

Abstract This study was conducted to investigate the effects of partial replacement of Portland cement (PC) by fly ash (FA) and silica fume (SF) as binary and ternary blends, on the carbonation resistance of Self Compacting Concrete (SCC) and its compressive strength development. Carbonation properties under consideration were evaluated by Accelerated Carbonation Test as per RILEM CPC-18 respectively. An experimental program was planned in which different concrete mixes were prepared using PC replacement level of 35% by FA in binary mixes. The ternary mixes were also prepared by replacing FA by 5% and 10% with SF. The water/binder (w/b) ratio was kept constant at 0.4 and the super plasticizer was used at 1.55% by weight of cement. The compressive strength was determined for each SCC mix, for testing at 7 and 28 days of curing. Also, for each mix testing is done at 28 and 56 days of curing at 2 and 4 weeks of carbon dioxide (CO2) exposure for accelerated carbonation test.

It was observed that the compressive strength, concrete mix containing 65%PC+25%FA+10%SF has given the highest compressive strength at all curing ages amongst all the mixes, whereas in Accelerated Carbonation Test, the best performance was given by same concrete mix containing 65%PC+25%FA+10%SF. Concrete mix of 65%PC+25%FA+10%SF is recommended to be the most appropriate mix based on results of compressive strength and Carbonation Resistance results taken together.

Keywords Self Compacting Concrete; Fly Ash; Silica Fume; Compressive Strengt;, Carbonation; Gelenium-51.

1. INTRODUCTION

   1. Self Compacting Concrete

      Self Compacting Concrete is highly flowable, non segregating concrete that can spread into place, fill the formwork, and encapsulate the reinforcement without any mechanical compaction. The hardened concrete is dense, homogeneous and has the same engineering properties and durability as traditional vibrated concrete.

   2. Carbonation

      Carbonation is a process where carbon dioxide (CO2) presents in the atmosphere reacts in the presence of moisture with the hydrated cement minerals. Carbon dioxide mainly reacts with the calcium hydroxide to form calcium carbonate. Carbonation has two effects, it increases mechanical strength of concrete, but it also decreases alkalinity, which is essential for corrosion prevention of the reinforcement steel. Carbonation is an unwanted process in concrete chemistry.

   3. Fly Ash

      Fly ash is one of the most extensively used by-product materials in the construction field resembling Portland cement). It is an inorganic, non combustible, finely divided residue collected or precipitated from the exhaust gases of any industrial furnace.

   4. Silica Fume

   Silica fume is a byproduct of producing silicon metal or ferrosilicon alloys. One of the most beneficial uses for silica fume is in concrete. Because of its chemical and physical properties, it is a very reactive pozzolan. Concrete containing silica fume can have very high strength and can be very durable.

2. MATERIALS

   River sand was used as fine aggregate. Locally available crushed stone aggregates of 12.5mm nominal maximum size were used as coarse aggregate. Fly ash being used in our Project was brought from Ambuja Cement Plant located in Roopnagar, Punjab. Gelenium-51 is used as Super-plasticizer which is based on a unique carboxylic etherpolymer with long lateral chains which was being brought from BASF, Chandigarh.

3. TESTS CONDUCTED

   1. Cement

      Ordinary Portland Cement (OPC) from a single lot was used throughout the course of the investigation. The physical properties of the cement as determined from various tests conforming to Indian Standard IS: 1489-1991 are listed in Table1. All the tests were carried out as per recommendations of IS: 4031-1988.

   2. Fine Aggregate

      River sand was used as fine aggregate. The particle size distribution and other physical properties of the fine aggregates are listed in Table2. Clumps of clay and other foreign matter were separated out from before using it in concrete.

   3. Coarse Aggregate

      Locally available crushed stone aggregates of 12.5mm nominal maximum size were used as coarse aggregate. Sieve analysis and other physical properties of aggregates are listed in Table3.

   4. Mix Proportion

      Several trial mixes were prepared keeping in view the workability of the mix and the quantities of the materials were then finalized for the durable mix are listed in Table4.

      Table1. Physical properties of cement

      S. No	Properties	Observation
      1	Fineness (90 micron IS Sieve)	4 percent
      2	Initial setting time	58 minutes
      3	Final setting time	280 minutes
      4	Standard consistency	31 percent
      5	28-days compressive strength	42.65 MPa

      Table2. Fineness modulus of fine aggregates

      S.NO	Sieve size	Mass retained (gm)	Percentage retained (%)	Percentage passing (%)	Cumulative %retained
      1.	4.75mm	4	0.4	99.6	0.4
      2.	2.36mm	28	2.8	96.8	3.2
      3.	1.18mm	237	23.7	73.1	26.9
      4.	600µ	230	23.0	50.1	49.9
      5.	300µ	318	31.8	18.3	81.7
      6.	150µ	141	14.1	4.2	95.8
      7.	Pan	39	3.9	Zone II	257.9

      Fineness modulus of fine aggregates = 257.9/100 = 2.57

      Table3. Fineness modulus of coarse aggregates

      S. No.	Sieve size	Mass retained (gm)	Percentage retained (%)	Percentage passing (%)	Cumulative %age retained
      1.	20mm	–	–	100	–
      2.	16mm	65	2.167	97.833	2.167
      3.	10mm	1610	53.67	44.163	55.837
      4.	4.75mm	1238	41.27	2.893	97.107
      5.	Pan	p>87	2.9	EC	155.111

      Finness modulus of coarse aggregates is (155.111+500)/100 =6.55

      Table4. Finalized Mix Proportion

      S. No	Descriptions	Cement (kg)	Fly ash (kg)	Fine Agg (kg)	Coarse Agg (kg)	Silica fumes (kg)	Water (l)	Super- Plasticizer (kg)
      Mix 1	65%PC+35%F A	400	215	846	602	0	246	6.2
      Mix 2	65%PC+30%F A+5%SF	400	185	846	602	31	246	6.2
      Mix 3	65%PC+25%F A+10%SF	400	154	846	602	62	246	6.2

      Concrete mixes were prepared using PC replacement level of 35% by FA in binary mixes. The ternary mixes were also prepared by replacing FA by 5% and10% with SF. For each SCC mix, 6 cubes of size 100 x 100 x 100 mm were cast for testing at 7, 28 of curing. Also, for each mix, 4 cubes of size 100 x 100 x 100 mm were cast for testing at 28 and 56 days of curing at 2 and 4 weeks of carbon dioxide (CO2) exposure for accelerated carbonation test.

   5. Test on hardened SCC

      To check the hardened properties or strength of SCC the tests conducted are:

      • Compressive Strength test

      • Accelerated carbonation test

   1. Compressive Strength test

      Compressive strength tests were conducted on concrete cubes of size 100 x 100 x 100 mm cast from concrete of each series, to check quality by obtaining the 28-days compressive strength. These tests were carried out in accordance with IS: 516-1959 on Compression Testing Machine. The bearing surfaces of the machine were cleaned and the test specimen was placed in the machine such that the load was applied to the faces other than the cast faces of the specimen. The maximum compressive load on the specimen was recorded as the load at which the specimen failed to take any further increase in the load. The average of three samples was taken as the representative value of compressive strength. The compressive strength was calculated by dividing the maximum compressive load by the cross-sectional area of the cube specimen.

   2. Accelerated carbonation test

   For carbonation studies, cube specimens for each mix were prepared. After respective days of curing, cube specimens (100 x 100 x 100 mm size) were cut into 4 prisms of size (50 x 50 x 100 mm) each with the help of cutter Prior to testing, conditioning of specimens was done. Prism samples were kept in oven for drying at (60 ± 5) °C until constant mass was achieved, i.e. not more than 0.1 % weight changed over any 24 h drying period. After that prism samples were placed in the dessicator to cool down. The temperature in the cabinet was allowed to fall to within 2 °C of that of the room. Silica gel was kept in the dessicator in powdered form to absorb any moisture. Specimens were kept in the cabinet until required for testing. After conditioning of the prisms, two coats of epoxy paint were applied on all faces except two opposite parallel faces of prisms (50 x 50 mm). After coating, drying and marking the samples, they were kept in carbonation chamber ensuring that the uncoated faces do not touch the uncoated faces of the other specimens or any other surface. Where controlled relative humidity (50-65%), Carbon dioxide concentration 4±0.2% and temperature 27±2°C was kept controlled. The specimens are being taken out of the chamber after 2, 4 and 8 weeks of CO2 exposure.

   The specimens were split into two halves along the length and split surfaces were sprayed with pH indicator. A standard solution of 1% phenolphthalein in 70% ethyl alcohol was used. In the noncarbonated region with pH values above 9.0, the phenolphthalein indicator turns purple-red and in the carbonated portion with pH less than 9.0, the solution remains colorless.

   Carbonation depth D of freshly broken concrete surface was measured at four places and taken as average value. The measured depth of carbonation was influenced by the time of measuring after (30 min.) application of the indicator solution.

4. RESULTS

   1. Compressive strength results

      40

      Compressive strength

      Mpa

      Table 5. Compressive strength results in MPa

      S. No	Description	7 days	28 days
      M 1	65%PC+35%FA	23.15	30.88
      M 2	65%PC+30%FA+5%SF	28.95	37.8
      M 3	65%PC+255FA+10%SF	35.8	43.95

      7 Days Curing

      M 1

      M 2

      MIxes

      M3

      40

      20

      0

      60

      30

      20

      10

      0

      Compressive Strength

      Mpa

      Fig 1. Compressive strength of mixes at 7days curing

      28 Days Curing

      M 1

      Mixes

      M3

      M 2

      Fig 2. Compressive strength of mixes at 28days curing

      50

      40

      30

      20

      10

      0

      7 Days

      28 Days

      M 1 M 2 M3

      Mixes

      Compressive Strength

      Mpa

      Fig 3. Comparision of Compressive strength of mixes at 7and 28days curing

      Table 7. Carbonation depths at 56 days curing age in cm at respective exposure

      S.No	2 weeks	4 weeks
      M 1	1.01	1.12
      M2	0.69	0.84
      M 3	0.59	0.75

      1.5

      2 Week

   2. Carbonation test results

   Table 6. Carbonation depths at 28 days curing age in cm at respective exposure

   S.No	2 weeks	4 weeks
   M 1	1.15	1.3
   M2	0.78	0.84
   M 3	0.71	0.76

   Fig 4. Depth of carbonation in cm at 2 week exposure

   Fig 5. Depth of carbonation in cm at 4 week exposure

   M1

   MMix2es

   M3

   1.5

   1

   0.5

   0

   1

   0.5

   0

   Carbonation depth in

   cm

   Carbonation depth in

   cm

   Fig 6. Depth of carbonation in cm at 2 week exposure

   4 Week

   M1

   MMix2es

   M3

   Fig 7. Depth of carbonation in cm at 4 week exposure

5. CONCLUSIONS

Based upon the scope of the work carried out in this investigation, following conclusions are drawn:

1. Addition of SF within ternary blends increased the compressive strength with significant increase in 7-day strength, 10% SF addition showed better compressive strength.

2. Mix with 65%PC+25%FA+10%SF gave the maximum compressive strength than all concrete mixes at all ages of curing.

3. Mix with 65%PC+35%FA gave the maximum carbonation depth than all concrete mixes at 2 weeks and 4 weeks of CO2 exposure at both 28 and 56 days of curing period.

4. Mix with 65%PC+25%FA+10%SF gave the maximum

   carbonation resistance than allconcrete mixes at 2 weeks and 4 weeks of CO2 exposure at both 28 and 56 days of curing period.

5. Form the results, it can be concluded that concrete mix containing 65%PC+25%FA+10%SF can be adjudged as the most appropriate mix for compressive strength and carbonation resistance taken together.

REFERENCES

1. ACI Committee 237-R(2007), Self Consolidating Concrete, ACI Manual of Concrete Practice, Emerging Technology Series, Part-I.

2. ASTM: C 1018-94b (1994), Standard Test Method for Flexural Toughness and First Crack Strength of Fibre-Reinforced Concrete (Using Beams with Third Point Loading), Annual Book of ASTM Standards, Vol.04.02, Concrete and Aggregates, pp. 509-516.

3. Atis C.D. (2003), Accelerated carbonation and testing of concrete made with fly ash, Constr Build Mater, 17, pp.14752.

4. Batis G., Pantazopoulou P., Tsivilis S. and Badogiannis E. (2005), The effect of metakaoilin on the corrosion behavior of cement mortars, Cement and Concrete Composites, 27, pp. 125 130.

5. Byfors K. (1985), Carbonation of concrete with silica fume and fly ash, Nordic Concrete Research, Publication No. 4, Oslo, pp. 2635.

6. Ceukelaire l. D. and Nieuwenburg D. V. (1993), Accelerated carbonation of blast-furnace cement concrete, Cement and Concrete Research, by, 23, pp. 1760-1767.

7. Campbell D. H., Sturm R. D. and Kosmatka S. H. (1991), Detecting carbonation. Concrete Technology Today, 12, pp. 1.

8. Castel A., Francois R. and Arliguie G. (1999), Effect of loading on carbonation penetration in reinforced concrete elements,Cement and Concrete Research 29, pp. 561565.

9. Chang C-F, and Chen J-W. (2006), The experimental investigation of carbonation depth, Cement and Concrete Research, 36, pp. 1760-1767.

10. Domone P, Chai H and Jin J (1999) Optimum mix proportioning of self-compacting concreteProceedings of International Conference on Innovation in Concrete Structures: Design and Construction, Dundee. Thomas Telford; London. pp. 277-285

11. Gajda j. (2001), Absorption of Atmospheric Carbon Dioxide by Portland Cement, R&D Serial no. 2255a; Portland cement Association: Skokie, IL.

12. IS: 516-1959, Methods of Test for Strength of Concrete, Bureau of Indian Standards, New Delhi.