Durability and strength are two most important criteria and requirements for the long-term performance of concrete structures against weathering action, chemical attack and abrasion. Any deficiency in any of the two i.e. durability and strength, could make the structure unfit for the intended purpose. If the structure is not durable, but has sufficient strength, then the strength of the structure reduces with age due to deterioration of concrete and reinforcement due to surrounding conditions.
Environment plays an important role while selecting durable materials for concrete structures. For coastal environments where the rate of corrosion is very high, precautions should be taken for the corrosion allowance of reinforcement, epoxy painting of reinforcement, required cover to reinforcement, grade of concrete to be used, minimum cement content, water-cement ratio, quality of water for construction, alkali content of the aggregates, proper placement and compaction of concrete.
In addition to above precautions fine Pozzolanic/Cementitious materials such as Fly ash (FA), Ground Granulated Blast Furnace Slag (GGBS) and Ultrafine Pozzolanic materials such as Silica fume (SF), Rice Husk Ash (RHA), Metakaolin (MK), Ultrafine Slag (UFS), Ultrafine Flyash (UFFA) were used in concrete to improve the particle packing which in turn reduces the porosity and restricts the movement of moisture, air & carbon dioxide into the core of concrete . In the United States, Secondary Cementitious Materials (SCMs) are usually added to concrete rather than blended with clinker, and currently more than 60% of ready-mixed concrete uses SCMs .
Many research scholars across the globe are involved in investigating the potential use of carbon-based nano material Graphene Oxide (GO) and Carbon Nano Tubes (CNT) as nano reinforcement in concrete. Use of Nano materials improves the micro structure of concrete especially nano pores in high strength concrete. The addition of 0.05% graphene oxide (GO) nanosheets (by weight of cement) can effectively increase the compressive strength and flexural strength of ordinary Portland cement paste by 15% to 33% and 41% to 58% respectively . Probably Nano technology will be adopted as one of the solution for durable concrete structures in future.
Background to High Volume GGBS Concrete
Mineral additive like GGBS enriches the performance of concrete against Chloride attack, Sulphate attack, Alkali Silica reaction [4,5] also helps in low heat of hydration, reduced permeability, Higher long-term strength and improved workability. Blast furnace slag has both filling and dispersing effect in mortar and concrete . With all above benefits of GGBS in concrete it was decided to use High volume in sea structures.
Micro level Studies
Scanning Electron Microscope (SEM) and Particle size analysis studies were done on GGBS to understand the morphology at micro level. Particle size and shape influences the strength gain and packing efficiency  and hence reduces permeability and increases strength of concrete. Particle shape and size on 20-Micron scale shows surface area and bond area of GGBS is better compared to fly ash. In Fly ash, spherical shaped particles have less bonding surface.
Figure 1: Micro-Photographs of Cement, GGBS and Fly ash at 20X Magnification
Real Time Study on Marine Structures
One of the greatest discoveries of 20th century was oil and its exploration initially was concentrated on land. As the need for oil expands in an explosive rate, oil industry moves into deeper water in the search for additional supplies of oil and gas with the construction of offshore platforms and Jetty. Depending on the water depth and environmental conditions the type of offshore structure is designed with structural steels and concrete members. In order to resist aggressive environmental conditions precaution is taken in both steel and concrete to achieve durability which will be explained in below case studies.
Background for Case Studies
In the case studies below, Offshore Platform structure and Jetty is exposed to severe coastal conditions near to Mumbai, India. Other than Gravity loads, Environmental loads and Seismic loads, structure will also be subjected to chloride attack, Sulphate attack, Corrosion of Reinforcement, Penetration of sea water and erosion during its construction and service life. Based on literature, material availability, consistent quality, similarity in physical and chemical properties with cement and compatibility with chemical admixtures it was decided to use GGBS in concrete as per IS 16714 – 2018  in conjunction with ordinary Portland cement.
Case Study 1: An Offshore platform structure exposed under Severe Coastal Condition
As shown in Figure 2 and Figure 3, a composite Offshore structure of fixed platform type shows Concrete Gravity Platforms and Steel Template Structures which is under construction. Deep studies were done in material selection & proportion to manufacture high performance concrete against severe aggressive condition. Each and every ingredients of concrete were tested for its physical and Chemical properties as per Indian Standards (IS) codes to check its suitability for trials mixes and construction. Initially, various combinations of Cement - Fly ash, Cement - UFS were tested for its fresh & hardened properties and later OPC 53 is partially replaced by GGBS at various percentages in concrete starting from 0,10,20,30,40,50,60 & 70 and based on the results GGBS is used up to 60% in M-45 grade concrete for Pile & Pile Cap works. Many mix proportions were carried out for various grades of concrete and tests were done to ascertain the selected concrete for its fresh properties, Hardened properties and durability.
Figure 2 & 3: Images of Offshore Platform under Construction
Keeping all the parameters same, a Comparative durability studies were carried on Actual Concrete sample (ACS) (40% OPC 53+ 60% GGBS) and Standard Laboratory sample (SLS) (100% OPC+ 0% GGBS). Three durability tests namely Chloride Migration Coefficient Test (CMCT) as per NT Build 492 , Chloride Ion penetration test (RCPT) as per ASTM C 1202  and Water Permeability Test (WPT) as per DIN:1048 Part 5-1991 were carried on test specimens.
Test 1: Chloride Migration Coefficient: The chloride migration coefficient method is a measure of the resistance of the tested material to chloride penetration. Non-Steady-state migration test is carried on the test specimens and the results achieved is less than 2 x 10-12 m2/s for Actual Concrete sample compared to Standard Laboratory sample of 9 x 10-12 m2/s. As per Table 1, Actual Site sample showed ‘very good’ concrete quality compared to Standard Laboratory sample and it was decided to use it for Substructure Works. For superstructure, concrete which showed Chloride migration coefficient (2 - 8)*10-12 m2/s is used.
|Table 1: Criteria for Chloride Migration Coefficient Test as per project requirements|
|Non-Steady state migration coefficient (x10-12 m2/s)||Concrete Quality|
|Table 2: Chloride Ion Penetrability based on charge passed as per ASTM C 1202|
|Charge passed (Coulombs)||Chloride Ion Penetrability|
Test 3: Water Permeability Test: This test determines the true resistance of concrete against the penetration of water under hydrostatic pressure. WPT were carried on Actual Concrete Sample and Standard Laboratory sample and the mean depth of water penetration is 7mm and 20mm respectively. As per DIN: 1048 Part 5-1991 and MORTH 5th revision, the mean of maximum depth of penetration shall not exceed 25mm.
Case Study 2: An Offshore Structure with Jetties
As shown in Figure 4, a composite Offshore structure and Jetty is in under construction stage. Mix proportions were carried with different percentages of Cement and GGBS (75% OPC 53 + 25% GGBS, 65% OPC 53 + 35% GGBS, 50% OPC 53 + 50% GGBS) and tests were done to ascertain the selected concrete for its fresh properties, hardened properties and durability. Based on preliminary test results, GGBS is used up to 50% in M-40 grade concrete for Pile, Pile Cap and Precast works.
Figure 4: Image of Offshore Structure platform and Jettie
Table 3 shows the mix proportion of M40 Grade concrete with Compressive Strength result and Workability of concrete in terms of Slump and Flow.
|Table 3: Mix proportion for M40 Grade Concrete|
|Grade of Concrete||Binder Content in Kg/m3||Water/Cement||Chemical Admixture Dosage in %||Compressive Strength in Mpa||Workability in Flow/Slump
|OPC 53||GGBS||Initial||60 Min||120 Min|
|580/ Collapse||550 / ≥ 180|
1) Test 1: Chloride Ion Penetration Test: RCPT test is carried as per ASTM C 1202 on cores from the structure which is cured for 12 days and tested after 28 days. Result achieved is less than 1000 Coulombs with GGBS replacement of 50% in M-40 Pile, Pile cap & Precast work at project site. From Table No 2, we can clearly say that 50% of GGBS in total cementitious material in concrete reduced the chloride Ion Penetrability to ‘Very low’ extent.
2) Test 2: Water Permeability Test: The Mean of Maximum depth of penetration of three test specimens is 7 mm. Obtained test result is within the permissible limit as mentioned in DIN: 1048 Part 5-1991 and MORTH 5th Revision.
Thus, it is an established fact that in order to build environmentally sustainable structures, the 21st Century construction practices must be driven by considerations of durability and sustainability. Being the most important infrastructure players and major consumers of natural resources on the earth, the concrete industry will have to be reshaped through the adoption of environment friendly technologies. The owner, developer, designer and contractors can demonstrate that by using concrete wisely, they will be contributing to sustainability and by incorporating some of the benefits of concrete, save both money and resources during the life of the structure.
The Blast furnace slag products have been included as “designated procurement items.” in Japan for public works under the law concerning the promotion of procurement of eco- friendly goods and the services by the state and other entities (Green purchasing Law) which took effect in 2001 in Japan.
We need to further focus specifically on concrete’s role in the provision of sustainable structures, sustainable architecture, sustainable roads and sustainable human settlement
Following are the few conclusions drawn from above two case studies:
- Chloride Migration Coefficient value decreases with increased percentage of GGBS upto 60% in concrete.
- Comparing both the case studies, we can clearly say that higher the content of GGBS in concrete reduces the Chloride Ion Penetration which in turn reduces the corrosion of reinforcement.
- The presence of GGBS in concrete reduces the water permeability in to the core of concrete this is mainly due to pore refinement and grain refinement.
- Thus in order to achieve durability of concrete Secondary Cementitious Materials and Ultrafine materials to be used in addition to primary binders in concrete at appropriate quantity.
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