Effect of Steel Fibers on Heat of Hydration and Mechanical Properties of Concrete Containing Fly Ash

This study investigated the effects of steel fibers on the fresh and hardened properties, and heat of hydration of concrete containing FA (Fly Ash). A total of 192 samples were cast comprising cubes, cylinders, and prisms, for six concrete mixes with varying contents of steel fibers by volume and a fixed content of FA i.e. 15% by weight of cement. The semi adiabatic setup was used to monitor temperature rise due to the heat of hydration in the concrete mixes for fourteen days. The use of FA increased workability, and decreased early compressive strength, tensile strength and heat of hydration of concrete. However, an increase in the compressive strength of FA concrete was observed by the addition of steel fibers up to 0.9% whereas a consistent increase in the splitting tensile strength and modulus of rupture was observed with the addition of the steel fibers from 0.4-1.8%. Further the test results showed that increasing steel fibers content decrease the evolution of heat due to hydration. It was concluded that the FA concrete with steel fibers can be used in precast industry and mass construction projects due to the improved mechanical properties and lower heat of hydration.


INTRODUCTION
(ASTM Class-F) has been widely used [4]. The use of FA in concrete has positively affected the mechanical and durability properties of concrete [5]. In the study of Malhotra [6] it was reported that FA has shown excellent durability properties against sulphate attack, and freezing and thawing cycles. In the study of Chindaprasirt et. al. [7] it was observed that drying shrinkage of the FA concrete was significantly reduced.
The sorptivity of concrete reduced by the replacement of cement with FA [8]. In the study of Naik et. al. [9] it was observed that FA concrete has given better result regions where formwork strips delay the further construction. Other supplementary materials like Silica Fume can improve the early strength of concrete but it will affect durability of concrete [19].
In some studies, it was observed that the addition of

MIX DESIGN AND DESIGNATIONS
A total of six concrete mixes were prepared for the study.

MATERIALS
The properties of the materials used in the preparation of the concrete mixes are provided in the following paragraphs. For this study locally available Lawrencepur brand sand was used as fine aggregate.
The water absorption and specific gravity of sand was 1.21 and 2.71% respectively (ASTM-C 128-15).
Whereas, the fineness modulus of sand was found to  Table   5. For partial replacement of cement ASTM C 618 FA was used in the study. The low calcium FA with particle size 25% retained on 45-micron sieve, Class-F ( Fig. 1) was obtained from Dirk® and the detailed properties [26] of FA are given in

MIXING, CASTING AND CURING
The concrete mixes were prepared in tilting drum type mixer by dry mixing sand and CA at first for one minute. Then binder (cement and/or FA) and steel fibers were added to the already mixed sand and CA, and mixed for another two minutes. Almost 90% of the required water was then added and mixed for further one minute to make it a homogeneous mixture. Finally, SP and the remaining water were added and mixed for two more minutes to get the desired concrete mix. The well mixed concrete was cast into cubes, cylinders and prisms moulds which were demoulded after 24 hours of casting. All concrete samples i.e. FA, OPC and steel fiber reinforced concrete were cured from 3-28 days. The cuboids for measuring heat of hydration in concrete were also cast at the same time and the temperature was recorded for 14 days.

HEAT OF HYDRATION
The heat of hydration in concrete is the result of reaction of cement with water. In this study, semi-adiabatic arrangement [27][28]

Benefits of Incorporating Induction Furnace Slag in Concrete as Replacement of Cement: A Case Study of Pakistan
The ball bearing effects due to the spherical shape of FA particles increases the workability of concrete. These spherical shaped FA particles reduces the internal friction of fresh concrete, thus increase the fluidity of concrete [29]. The specific gravity of cement is higher than the FA, and, when cement is replaced weight to weight with FA, it causes increase in binder (paste) volume of concrete that govern workability of concrete [30].

Compressive Strength
The

Splitting Tensile Strength
The  It can be seen in Fig. 6(b), when steel fibers ratio was increased in FA concrete, the tensile strength increases consistently; the increase in strength is the influence of steel fiber arresting cracking [31]. The MOR graph suggests that as the steel fibers ratio is increased, the flexural strength of the concrete will increase.
A 45% increase in the MOR value is observed when the steel fibers ratio is increased from 0.4-1.8% as compared to the FA15 mix. penetration of water in concrete is depicted in Fig. 8 and the results of water permeability are presented in Fig. 9.
It can be observed in Fig. 9 that penetration depth of water for OPC and FA15 mix is almost same. However, when the steel fibers are added the penetration depth of water is increased. Hence, it can be inferred that by the addition of the steel fibers the porosity of the concrete mix will increase which is in line with the study by Hwang and Kim [34].
The durability properties may be affected by the addition of high steel fiber contents in FA concrete due to increased porosity, and concrete can be susceptible to chloride and sulphate attacks.

Measurement of Heat of Hydration
The heat of hydration is produced as a result of a series of exothermic chemical reactions taking place in the concrete.
The temperature rise due to the heat produced in the concrete mix with respect to the time (hours) is shown in Fig. 10. The Initial temperature, peak temperature and the time to reach peak temperature for all the mixes are shown in

CONCLUSION
Based on the results described above following conclusions can be drawn: