Durability of concrete is an important factor considered for structures. Effect of GGBFS of durability properties of concrete is discussed in this article.
Durability is an important factor that must be appreciable for any structure so that it is performing for the requirement for which it is built for. It is a time period requirement. Every structure irrespective of what material is employed should gain this property.
Hence, the choice of materials in building construction also must made based on its durability properties. We will discuss the variation in the durability property of concrete structure that has undergone a replacement by GGBS.
Durability Properties of Concrete with GGBFSFollowing are the durability properties of Concrete with GGBFS:
Creep and Shrinkage of Concrete with GGBFSThe measure of deformation of concrete under sustained load with age is named as a creep. Shrinkage can be either drying, chemical or autogenous shrinkage. The prominent one is drying shrinkage that is mainly caused due to the evaporation of water from the voids of the concrete structure. The two parameters creep and drying shrinkage is considered as important time-dependent factors, that mainly affect even the properties of high-performance concrete (HPC). The figures below shows the variation of creep and shrinkage of an HPC specimen with no replacement (Concrete A), GGBS replacement (Concrete B) and silica fume plus GGBS replacement (Concrete C). The creep study was conducted at the age of 180 and the shrinkage at the age 210 days.
Fig.1: Creep Variation in Concrete A, B and C (Jianyong and Yan, 2001)
Fig.2: Variation of Drying Shrinkage in Concrete A, B and C (Jianyong and Yan, 2001)From figures-1, the replaced samples possess a lesser value of creep compared with a sample with ordinary portland cement alone. This can be given as the property of slag in gaining strength with age. As the variation shown in figure-2, the drying shrinkage value too is found lesser compared to the sample with OPC alone. This may be explained by the primary factor concerned, i.e. lack of voids. GGBS are effective in making the concrete denser and free from voids. This would help in entrapping of water within the voids. This would facilitate the reduction of water vaporisation and hence no shrinking effect.
Chloride Resistance of Concrete with GGBFSA study conducted by Dhir et al. (1996) showed that the chloride resistance of ordinary portland cement concrete in increased with the addition of ground granulated blast furnace slag. The study was conducted by testing the chloride binding capacity of OPC concrete and replacement samples with GGBS in 33.3, 50,60 percentages. The test was carried out by not varying the water cement ratio at 0.55. The results were:
- The chloride binding capacity of concrete increase with increase in GGBS content
- A higher replacement above 50 % showed 5 times greater chloride binding capacity
- If the chloride exposure increases, the binding capacity also increase. Letting it know that they are proportional to each other.
Sulphate Resistance of Concrete with GGBFSThe sulphate attack is an undesirable factor to which most of the hardened concrete are subjected to. It results in deterioration of the respective concrete structure. Now how extend the sulphate attack persists depend upon the time of sulphate ion and the cement type. The sulphate attack can be spread through different ion concentration of the clayey soil to which the concrete is in contact with. Clayey soils mostly consist of salts which are either in the form of magnesium or calcium sulphates. These salts have a possibility to react with the calcium hydroxide ions or the calcium aluminate hydrate (C3A) that is present in the concrete. The products formed in the reaction are calcium sulphoaluminate and gypsum which occupies a larger volume than their parent compounds. This inadequacy in space results in expansion and with temperature changes, contraction also takes place. This is the root cause of the disruption. Now all these are only possible if the sulphate ions are allowed to penetrate into the concrete. Here comes the role of GGBS that they give a concrete highly impermeable, thus limiting the impact of sulphate. The figure-3 below shows expansion rate of concrete for 0,25,35 and 50% of cement by GGBS. It is seen that higher the replacement amount of GGBS lesser is the expansion.
Fig.3: Expansion Rate of Concrete with GGBFSIt is found that, even special cement like sulphate resisting Portland cement (SPRC), that itself have a low C3A content performs less compared with GGBS concrete. The resistance against sulphate is done by the GGBS particles in three different ways as mentioned below:
- The GGBS itself have no C3A. More the amount of GGBS replaced with cement, lesser the overall constitution of C3A.
- The GGBS as a part of pozzolanic reaction utilizes a majority of calcium hydroxide. This will leave less CaOH2 FOR C3A to form ettringite.
- The GGBS resist the penetration of sulphate ion by giving higher permeability for the concrete.
Freezing and Thawing of Concrete with GGBFSThe variation of concrete properties with the change in temperatures is an important factor questioning the durability of the structure. Both freezing and thawing are primarily caused due to the entrainment of water with the concrete. This water behaves differently for cold and hot temperatures. This behavior is freezing and thawing. This increase and a decrease of volume with the concrete creates small cracks which later propagates to larger ones. This rapid deterioration of concrete is highly resisted by good quality concrete mix. The GGBS incorporated concrete gives a good quality concrete. The GGBS particles help in reducing the porosity which is core reason of water entrainment. This itself give an entirely different mineralogy for the concrete.
Corrosion of Reinforcement in Concrete with GGBFSStudies have shown that there are a great relation and dependence between the porosity and the corrosion activity of a concrete. The exposure of concrete with sea water have chances of chloride ion penetration that in turn results in the corrosion of reinforcement. The replacement of GGBS with silica fume with the concrete conventional mix have proved better performance that OPC concrete under corrosion resistance.
Fig.4: The variation of surface area corroded with varying GGBS ratio, for two type of cement under ASTM (Yeau and Kim,2005)As per the figure below, the rate of surface area corroded is less for 25,40 and 55 percentage replacement with GGBS compare to zero percentage replacement of cement. The variation was determined for covers 1,2, and 3cm. The study was done for two types of cement ASTM Type I and ASTM Type V. Read More: Ground Granulated Blast Furnace Slag in Concrete & its Advantages Fresh Concrete Properties with Ground Granulated Blast Furnace Slag Properties of Hardened Concrete with Blast Furnace Slag (GGBFS)