Mathematical modelling of crushed rock dust concrete: Performance using compressive strength

The paper gives out a mathematical model developed using linear regression statistical method to envisage the 28-day strength of CRD concrete, considering M20, M30 and M40 grades concrete and CRD replacement percentages of 0%, 10%, 20%, 30% and 40% by weight of cement. Strength results of M40, M30 and M20 grades concrete are used to develop the relationship between CRD content and compressive strength. The ratios of compressive strengths between CRD and control concrete (CC) have been related to CRD replacement percentage. The expression, derived is with strength ratios and not with experimental strength values. The mathematical equation developed is independent of the specimen parameters and may be applicable to all types of specimens. The model is considered as it involves non-dimensional variables and is independent of the specimen size, water to binder ratio (w/b) and grade of concrete.


INTRODUCTION
Concrete is an exceptional construction material is widely used in construction industry. Cement, being a key ingredient in concrete, consumes huge number of natural resources during the manufacturing process and is responsible for about 7% of all manmade CO2 emissions. The use of non-pozzolanic/ pozzolanic fillers (marble dust, granite dust and lime stone powder), as supplementary cementitious materials, can reduce the cement consumption in concrete production and control CO2 emissions from cement industry [1][2]. In an experimental study by Elayamany et al., it is revelled that, concrete produced with non-pozzolanic fillers has no negative effect on compressive strength compared to that of concrete with pozzolanic fillers. Moreover, concrete produced with nonpozzolanic fillers exhibited more resistance to segregation and bleeding than pozzolanic fillers. Marble dust, granite dust, crushed rock dust and lime stone powder commonly referred as quarry dust, is produced from stone processing industries and is an inert material, generally used for land filling, creating conservational problems [3,4]. Quarry dust is chemically inactive, but can affect the properties of the concrete in a positive way. Enhancement in the behaviour of concrete with the use of non-pozzolanic fillers is almost identical when compared with pozzolanic fillers in long term. While the addition of non-pozzolanic/ pozzolanic fillers beyond certain limit leads to the reduction in strength when compared with cement concrete [5].

Materials
Fine aggregate of maximum size 4.75mm with zone III (IS 383:1970) [6] was used. Locally available crushed stone coarse aggregate of maximum size 20mm as per IS 383:1970 [6] was used. As per IS 12269: 1987 [7], OPC 53 grade cement was used. Its consistency and specific gravity are 32% and 3.1. Fines of CRD with size less than 150µm is procured from the stone processing and crushing units and is shown in Fig. 1.

Figure1.
Image showing the CRD at a local stone crushing unit

Concrete Mix design
Confirming to IS 10262: 2009 [8], M20, M30 and M40 concrete mixes are designed as CC mixes. Table 1 shows the concrete mix proportions.

Test Procedure
The concrete ingredients are mixed homogenously by using an electric pan mixer. The freshly mixed concrete is transferred into cube moulds of size 100 mm, in accordance to IS 516: 1959 [9] and then cured for 28 days. The concrete cube was kept uninterrupted for one day and then de-moulded, to extend for water curing for 28 days. The compressive strength is conducted on UTM, having a capacity of 100 Tonnes and the values are used for developing a mathematical model using linear regression statistical method to forecast the 28-day compressive strength of CRD concrete. XRD meter Rigaku Miniflex 600 is used for testing the CRD patterns.

XRD of crushed rock dust
The SEM photograph and XRD outline of CRD are shown in Fig. 2. The XRD outline of CRD shows a peak intensity between 20 o and 30 o , represents the existence of quartz (SiO2). The composition of SiO2 and Al2O3 in CRD is 84.46%. The SEM photograph of CRD exhibited the presence of irregular and angular particles. The percentages of NaAlO2 and silicate in the CRD indicates the pozzolanic reactive nature. The sum of Al2O3, SiO2 and Fe2O3 in CRD is 87.86% satisfies the requirement of 70%, as per ASTM C 618 [10]. Table 2 shows the chemical compositions of CRD.

Modelling of experimental data
The 28-days compressive strength ratios of CRD concrete (fcrd) and CC (fcc) are given in Table 3. Since the same concrete with different CRD content is used for preparation of concrete testing specimens, the results are related to CRD replacement ratio and the compressive strength. The ratios of compressive strengths of the CRD concrete are identical and independent of grades of concrete. These ratios can be used to develop the generalized expression, which is free from the influence of grade of concrete and specimen parameters [11].
The relationship between the compressive strength ratios of CRD concrete and CRD content are shown in Fig.3. Equation (1)

CONCLUSIONS
The following conclusions are made from the CRD concrete are, 1. The use of CRD with 0%, 10%, 20%, 30% and 40% as partial replacement for OPC cement in producing concrete. 2. The XRD results of CRD, it is noticed that between 20 o and 30 o , showed the existence of quartz (SiO2). 3. From the experimental results, 20% CRD content in concrete showed enhanced results than CC.  (1) developed using statistical modelling is independent of specimen size and shape and can be related to all types of specimens and mix grades.