Comparative Analysis of Flexural Strength and Modulus Elasticity of Sustainable Concrete Using Supplementary Cementitious Material (SCM)

The use of Supplementary Cementitious Material (SCM) is widely used in production of sustainable concrete. Blended cements, incorporating SCM such as Pulverized Fly Ash (PFA) and Ground Granulated Blast Furnace Slag (GGBFS) have been widely used to reduce the cement contents and avoid adverse environmental impacts of CO2 produced during cement manufacturing. The analysis of various structural properties of concrete such as compressive strength, flexural strength and modulus of elasticity is important for its structural application. In this research, flexural strength of 100mmx100mmx500mm beams made from blended cement were tested under three curing conditions i.e. winter, summer and under water and the flexural strength was calculated using EN-12390-5 at the ages of 28 days and 56 days. For modulus of elasticity, concrete cylinders 150mmx300mm were tested as per procedure described in BS 1881-121(1983) at the age of 28 days. The compressive strength, flexural strength and modulus of elasticity for blended cement incorporating PFA and GGBFS has been increased under summer curing environment. The experimental values of Modulus of Elasticity are compared with the provision of BS 1881.


Introduction
The use of concrete in the built environment has been increased manifold over the last five decades. The annual production of concrete has reached about 25 billion tonnes each year at global level. This is twice the other material used in construction such word, steel, wood and plastics etc. The extensive use of natural resources in the manufacturing of cement has forced the researchers to explore the sustainable concrete by using of low water binder ratio (w/b) and use of Supplementary Cementitious Material (SCM) and fillers [1]. The cement produced with the addition of SCM is often called Binary Cement [2]. The use of binary cement incorporating Supplementary Cementitious Material (SCM) in concrete to replace cement, has been the major area of research during last 50 years, which was initially aimed at reducing the cost of concrete production. However, the enhanced awareness about the environmental degradation due to human activities towards the last quarter of 20 th century created the need for "Sustainable development and Sustainability in concrete production". [3]. It is generally believed that Green House Gases including CO2, NOx and SOx are mainly responsible for Global Warming, Climate Changes and other environmental issues. The cement manufacturing leads to heavy emissions of CO2 in the atmosphere as one tonne of CO2 is liberated during manufacturing of one tonne of cement [4]. The concrete technologists and researchers, have thus laid high emphasis over reducing the consumption of cement in concrete by exploring SCM like Pulverized Fly Ash (PFA), Ground Granulated Blast Furnace Slag (GGBFS), Rice Husk Ash ( RHA), Silica Fumes (SF). SCM have been used in various proportions as replacement of cement, to modify the properties of concrete in fresh and hardened forms. Several agricultural and industrial wastes are used for production of concrete to reduce the consumption of cement during this process [5][6]. SCM have also been used in High Performance Concrete (HPC) to achieve better workability, higher compressive strength and more durability of concrete [7][8]. Tehmina et al [9], presented a comprehensive review of the earlier research on the partial replacement of cement by various mineral admixtures such as Pulverized Fly Ash (PFA), Silica Fume (SF), Ground Granulated Blast Furnace Slag (GGBS), Metakaolin (MK), and Rice Husk Ash (RHA). They reported that besides compressive strength, tensile strength, flexural strength, and modulus of elasticity of concrete has been increased with the increase in replacement level of mineral admixtures.
The compressive strength of concrete is one of the basic mechanical property of concrete, which is used for determination of other properties like flexural strength, modulus of elasticity and creep etc. Various Building and Concrete Codes have suggested empirical equations to determine these mechanical properties of structural concrete in terms of 28 compressive strengths for standard cylinder or cube. Such equations are based on experimental and empirical evidences. The compressive strength of concrete is affected by many factors such as mix design, age, curing conditions etc. The curing conditions have however direct impact on the compressive strength of concrete if other factors remain unchanged [10].
The flexural strength of concrete is taken as function of 28 days compressive strength of concrete. Siddique [11] reported increase in the flexural strength of blended concrete by replacing the fine aggregates with FA in the range of 10-50% by weight. However very limited research is available on the flexural strength of GGBS blended cement [12]. The compressive strength and flexural strength of Class C Fly Ash up to 30% level of replacement of cement was observed almost the same or higher than the normal strength [13]. Bouzoubaâ, Zhang and Malhotra [14], used High Volume Fly Ash (HVFA), to study the mechanical properties and durability of blended concrete including flexural strength and modulus of elasticity and reported its better performance as compared to normal concrete. Solanki and Pitroda [15] performed flexural strength test on mini beams of size 100 mm x 100 mm x 500 mm. A concrete mix M20 grade was designed as per IS10262:2009 method. The water/cement ratio was 0.48 for all the mixes. Flexural strength test was performed at the age of 28 days. It was concluded that the 28 days flexural strength of concrete is increased up to 11.1 % with 20 % replacement level of PC by fly ash.
Modulus of Elasticity (E) of concrete also play key role in predicting the deflection in a structure. The elastic modulus of concrete is function of several parameters, hence theoretical modeling of concrete may not be possible to explain it [16]. The equation for E are based on empirical approaches from static and dynamic testing, are widely used by various Codes [17]. These Codes describe the E as function of compressive strength and are restricted to normal strength concrete having fc' up to 40MPa. Various changes have been proposed by researchers in the equation of E for normal concrete to cater with high strength Concrete and HPC. The role of aggregates sizes and aggregates interlocking becomes more important for High Strength Concrete and hence its E is also affected due to aggregate size. The researchers generally believe that the exiting equations of Codes cannot be used for HSC to determine Elastic Modulus [18][19]. Takafumi Noguchi et al [20] proposed equation to determine E, applicable to wide range of aggregates and admixture for various types of concrete, based on multiple regression model of the available test database. They observed that the E of both normal and high strength concrete varies with cube root of compressive strength as reported by European Code [21]. The modulus of elasticity of blended concrete with PFA and GGBS was observed as lower at the initial age as compared to normal concrete, however this at later ages of 28 days and 56 days, the blended concrete showed better results, which is attributed to the slow pozzolonic action of blended concrete [22]. Khatib and Hibbert [23], found that the early age strength gain of concrete containing GGBS decreases with increasing percentage of GGBS in concrete but the strength between 28 days and days increased as compared to the PC concrete. According to the findings of Kayali and Ahmed [18], if the total cementitious content of concrete is kept constant and it is cured for a limited time, there is a decrease in the compressive strength and modulus of elasticity of concrete containing PFA, compared to the PC only concrete and this decrease in strength is increased with the replacement level. The curing process also affects the properties of concrete made from ordinary cement or blended cement incorporating GGBS. The water curing was found more effective than heat curing [24], [26]. The slow steam curing of slag added concrete has gained strength more than water and air cured specimen [27]. In this research the mechanical properties of blended concrete incorporating PFA and GGBFS have been determined under three different curing condition which include compressive strength, flexural strength and modulus of elasticity.

Research Significance
Little research work is available on the mechanical properties specially modulus of elasticity and flexural strength of blended concrete with GGBFS and PFA under various curing conditions. This fact has been the major motivation for this experimental work. The research on flexural strength of GGBFS concrete has been done in limited scale. Hence the results of current research would add to the data of research in the area.

Ground Granulated Blast Furnace Slag (GGBS)
GGBS is a by-product obtained during the manufacture of iron in the blast furnace. GGBS is economically available in large quantities and suitable for production of large quantities of ready-mix concrete at site in precast product manufacturing. The granulated slag is dried and ground to a fine powder that is called GGBS. It is off-white in color and has a bulk density of 1200 kg/m 3 .

Pulverized Fly Ash (PFA)
PFA conforming to BS-EN 450-1(2012) [22], was used as binary cement component in the production of concrete. PFA used in the concrete is commercially available in the UK and is classified as CEM IV according to BS EN 197-1 (2011) [23].

Portland cement
Ordinary Portland cement (OPC) used conformed to BS EN 197-1 and was classified as CEM-I. The Portland cement was stored in the laboratory to avoid exposure to humidity.

Superplasticizer (SP)
High performance liquid superplasticizers conforming to BS-EN 934-2 was used, to achieve the required workability.

Aggregates
Graded natural sand with a maximum particle size of 5 mm and complying with the requirements of BS EN 12620-1 (2009) , was used as fine aggregate in the concrete mixes. Thames valley natural aggregates of lime stone were used as coarse aggregate in the concrete mixes. The maximum size of the aggregate used was 20 mm.

Concrete Mix Proportions
Trial mixes of concrete were designed to achieve an equal 28 days compressive strength of 40 MPa and the strengths of 10 MPa after 16 hours and 25 MPa after 38 hours to meet the practical requirement of post tensioned concrete beams. The trial mixes are shown in Table 1. To achieve a practical level of workability and cohesion, suitable for pumping, concrete was designed for a target slump of 200 mm. The overall maximum water/cement ratio was kept 0.45 and overall minimum cement content of 340 kg/m 3 . Superplasticizer was used to minimize water and cement contents to achieve low free w/c ratio.

Curing Environments
Engineering performance of concrete cured under three different regimes was recorded. The following three methods were chosen for curing the concrete which have a close resemblance with the onsite curing environment in the UK.

Summer Curing Environment (C1)
After casting concrete in the molds, it was stored for 24 hours at a laboratory temperature of about 20 ± 2 o C and covered with plastic sheets to minimize the loss of moisture. After 24 hours concrete was demolded and sealed in air-tight plastic bags so that there is no loss of moisture and stored at a laboratory temperature of 20 o C. This curing environment has been titled as C1 and shown in Figure 1a.

Winter Curing Environment (C2)
After casting concrete, it was stored for 24 hours with in the molds in the environmental chamber controlled at a temperature of 7 o C and 55 % relative humidity which resembles the normal winter temperature in the UK. Molds were covered with plastic sheets to minimize the loss of moisture. After 24 hours, concrete was demolded and sealed in the air-tight plastic bags to avoid any loss of moisture and stored in the environmental chamber controlled at 7 o C. Concrete cubes cured under the C2 curing environment as shown in Figure 1b.

Water Curing Environment (C3)
After casting concrete in the molds, it was stored at a laboratory temperature of 20 o C and was covered with plastic sheets. After 24 hours, concrete was demolded and was immersed in the water chamber controlled at a temperature of 20 ± 2 o C. Concrete stored under curing environment C3 is shown in Figure 1c.

Testing setup:
The flexural strength of the low carbon concrete was tested at the age of 28 and 56 days and compared with the Portland cement concrete. The flexural Test was carried out on 100 mm x 100 mm x 500 mm beams after curing for 28 days using the three different conditions as discussed above. Two specimens were tested of each concrete mix and each curing regime. In case of more than 10 % difference in the two results a third specimen was also tested. The flexural strength was calculated following the method described in EN 12390-5 (2009) using simple bending theory. The flexural strength test is shown in  The static modulus of elasticity of concrete specimens was determined following the procedure described in BS 1881-121(1983). A step loading was applied on a standard cylinder up to a maximum of one third of the failure load. The change in length of the test specimen was recorded at each load step, and converted to strain (ε). The load at each step was converted to stress (δ). The stress-strain relationship was established and its slope gave the static modulus of elasticity expressed in kN/mm 2 . A RUBICON-MAYES testing machine was used for determining the modulus of elasticity and it was operated by a computer program to apply the load cycles at a precise rate. The testing machine setup with the specimen during testing is shown in Figure 2. The modulus of elasticity test was carried out on 150 mm diameter and 300 mm high cylinder specimens cured for 28 days under the three curing conditions.   Table 2. The elastic modulus of elasticity will be 33.6 GPa according to BS-EN 1992-1 (2004) and 36.9 GPa according to BS-EN 1992-1 (2004). The modulus of elasticity determined in the laboratory is 39.8 GPa, which is slightly more than these estimates. In Table 3, the experimental values of modulus of elasticity, estimates of BS 8110-2 (1985) and BS EN 1992-1 (2004) are presented. It can be seen that the experimental values of modulus of elasticity are slightly higher than estimates of the codes of practices. The ratio of cylinder /cube strength is also presented in the Table 3, and is in the range of 0.75 to 0.84.