ABSTRACT: Stone base has been established as a veritable material for base course construction in road pavements. Recent research has shown that it can be used as an all-in concreting aggregate. Tests carried out at University of Uyo have showed that such a concrete can attain compressive strengths in excess of 20N/mm2 at 28 days. This project reports the results of an investigation into the strength properties of superplasticised stone base concrete with particular reference to 1:6 nominal mix which is equivalent to 1:2:4 of conventional concrete. A total of 48 cubes with varying water cement ratios were cast. 12 of these cubes were used as control while 36 were dosed with Conplast SP430 at 1,2 and 3 litres/100kg of cementitious material respectively. Slump, saturated density and compressive strength were determined. The result of the investigation revealed that the strength of superplasticised stone base concrete is greater than normal stone base concrete. The result also showed that 3 litres/100kg of Conplast SP430 was overdosed at 0.6 water cement ratio. The saturated density of the cubes was found to be independent of the age of curing and dosage of superplasticier. The highest strength of 26.89N/mm2 was obtained at water cement ratio of 0.6 with 3 litres of superplasticier per 100kg of cement.

CHAPTER ONE

  1. INTRODUCTION

Concrete is one of the most versatile construction materials that have been widely used in the world. This is seen in the construction of innovative concrete bridges, buildings, highways, airport runways, dams, pavements etc. Concrete which is a composite material, made from the combination of aggregates, cement and water possesses different strength depending on the constituents making it. At the turn of the 20th century, concrete strength was in the range of 13.8 to 2oN/mm2, by 1960s it was in the range of 27.6 – 41.4N/mm2.

Recent advances in the study of concrete behaviour has resulted in the use of new materials which can produce concrete of higher strength than the conventional cement-aggregate concrete but at a low cost. One of such advancement is the replacement of fine and coarse aggregate with stone base. Stone base which was hitherto known and used as a base coarse in the construction of road is a crushed stone (rock) with sizes ranging from 0 – 60mm and above. Stone base has long played an important role in road construction but little or nothing is known yet about it in concrete production, including its strength properties.

Stone base aggregate produces concrete that has a lower strength when compared to the conventional fine/coarse aggregate concrete, having the same cement content and water-cementitious ratio. This is due to the angularity of the particles and poor grading i.e. presence of too much of fine or coarse particles. This results in a high water- cementitious ratio leading to a high workability but reduced strength. This can be improved by the use of water reducing admixture at a low cementitious ratio.

With this admixture, there is the possibility of obtaining a concrete with compressive strength greater than that of the conventional concrete of the same mix design but without the admixture. Since stone base is cheaper than normal concrete made with fine and coarse aggregate, it therefore becomes imperative to investigate the use of stone base as aggregate in the production of superplasticised concrete.

1.1       IMPLICATION OF THE STUDY

About three-quarters of the volume of concrete is occupied by aggregate; this reveals that aggregate plays a very important role in the determination of strength, durability and structural performance of concrete. From the stand point of cost, there is a remarkable difference in price between a tone of stone base and a tonne of granite chippings and river sand which are the constituents of the conventional concrete. The presence of large particles may however pose a serious problem especially in sections where reinforcement is needed. Also the presence of too much fine or coarse aggregate will reduce the strength of the concrete. It is believed that the addition of certain admixture such as superplasticiser will influence the concrete property by maintaining a high workability at a low water-cementitious ratio thereby increasing the strength.

1.2       OBJECTIVE OF THE STUDY

This study investigated the strength properties of superplasticised stone base concrete through the following objectives:

  • To determine the variation in strength of stone base concrete at different water cementitious ratio.
  • To ascertain whether the use of superpasticiser will have any effect on stone base concrete.
  • To examine the effect of different dosages of superplasticiser on stone base concrete.
  • To examine and compare the cost of producing superplasticised stone base concrete and stone base concrete without the admixture.

1.3       SIGNIFICANCE OF THE STUDY

This study will be of immense benefit to people working in the construction industry i.e the engineers, contractors, builders and even the civil engineering consultant. The cost of producing conventional concrete has been on the high side in recent times due to increase in cost of granite chippings, being the only material (aggregate) that is widely known and used. The result of this study will therefore enhance the use of superplasticiser and stone base aggregate in production of concrete with high strength at a very low cost, especially mass concrete (foundation concrete) where reinforcement is not needed.

1.4       SCOPE STUDY

The study was limited to the following:

  • The type of cement used was Eagle cement. A brand of Ordinary Portland Cement (OPC) which is a type one (1) cement.
  • The superplasticiser type used was High Performance Conplast SP430, a Sulphonated Napthalene Formaldehyde (SNF) base admixture.
  • Nominal concrete mix of 1:2:4 was examined.
  • 150mm x 150mm x 150mm concrete cube moulds were used.
  • Dosages of superplasticiser used were 1, 2 and 3 litres per 100kg of cementitious materials.