Assessment of stone dust in production of high performance Concrete

Abstract

This study evaluated the suitability of stone dust in the design and production High Performance Concrete (HPC). Stone dust from the quarry was sampled, tested and used to produce HPC mix. Structural design was done using the new HPC in comparison with conventional concrete classes currently in the market (Class 25, 30 and 35). Comparison was done using Cost Benefit Analysis (CBA) taking into consideration project full cycle costs. Economic evaluation of HPC was done using Internal Rate of Return (IRR) and Net Present Value (NPV). The research addressed two problems; improving the quality and strength of concrete which results in more economical structural member sizes when used in design. This in effect increases working (lettable) space per floor in high-rise buildings. Use of stone dust which is a by-product of stone crushing and rarely used as a construction material will enhance environmental conservation. Environmental conservation was achieved in two ways; use of the rarely used by product of stone crushing process and reduced or eliminated use of river sand which over time has led to river degradation. Laboratory tests were done to establish the properties of the designed concrete, compressive strength, and modulus of elasticity. Structural analysis using BS 8110:1997 was done for a 10 storey office building to establish structural member sizes. Members obtained from concrete classes 25, 30, 35 and the new compressive strength from HPC (Class 80) were compared. Analysis was done for structural members’ sizes and area freed as a result of designing with HPC as well as the weight of reinforcement steel used. To justify the use of HPC, CBA was used to compare high initial capital investment and benefits resulting from increased work space created. The minimum class of concrete used in design was limited to class 25 N/mm2 which formed the base of the design. The research shows that it is possible to manufacture high strength concrete using locally available stone dust. The concrete made using stone dust sampled from a quarry in Athi River, Kenya, achieved a nominal strength of 87.6 N/mm2 at a water cement ratio of 0.32. A plasticizer and water reducing admixture was used to reduce water cement ratio in the concrete mix and improve workability. Structural analysis of a 10 storey structure with columns spaced at 8 meters center to center was designed using the four classes of concrete (25, 30, 35 & 80) and results compared. There was a reduction of column sections sizes from 0.9 m wide to 0.50m (over 45%) when concrete class changes from class 25 to class 80 creating over 3% of the total space area per floor. Cost benefit analysis using Net Present Value (NPV) and Internal Rate of Return (IRR) presented business case for the use of HPC. Using concrete class 80, the IRR was calculated at 3% and NPV being 8% more than total initial investment. The steel reinforcement increased by 8.64%using class 30, 11.68% using class 35 and reduced by 8.37% at class 80. The base class for comparison was class 25 which is commonly used for structural design in Kenya. This study provides useful information to design Engineers and Architects and informs future design of reinforced concrete structures.