Free cutting of steel reinforced concretes

Abstract

Diamond core drills are widely used in drilling concrete materials in the construction and civil engineering industry. Steel reinforced concrete is a kind of very difficult-to-cut composite material. There are some difficult problems remaining unsolved in drilling steel reinforced concretes, especially finding good cobalt substitutes as bonds with free cutting abilities. Great progresses were made in new cobalt substitute matrixes and in new segment structure designs. But, just little progress was made in free drilling steel reinforced concretes by simply using a cobalt substitute based diamond core drill. The main objective of this thesis was to provide an insight into the role of the free cutting in the diamond drilling field and to find good solutions concerned with free cutting and life time. As attempts to differentiate the performances between cobalt based and cobalt replacement materials as bonds and to acquire the free cutting core drill with optimally prescribed cobalt substitutes, an extensive, comprehensive, and comparable investigation on the influences of various parameters, such as the bond type, the diamond selection (including quality, particle size, concentration, and coating), the core diameter, the concrete type (depending on the steel bar diameter and amount and the aggregate type), and the drilling machine power, on the free cutting ability of many different types of diamond core drills were conducted.<br />The penetration rates and the drilling ratios of different samples were measured, elaborated, and evaluated throughout this work. The worn surfaces of diamond segments of different samples were also observed, discussed, and analyzed. Besides, the drilling on different workpiece materials including plain concretes, steel reinforced concretes, and pure steel bars was compared, characterized, and discussed in order to better understand the drilling characteristics and the wear mechanisms.<br />The cutting performance of a diamond core drill is strongly influenced by the wearing process. It is proposed that under the same bond system, the higher the penetration rate, the higher the wear rate normally is.<br />The performance of a drill when drilling steel reinforced concretes is mainly controlled and can be evaluated by the distribution, the property (including the strength, hardness, and wearability), and the volume (relative to the bar diameter, bar amount, and segment thickness) of steel bars being cut. A well designed cobalt substitute based diamond core drill made with high quality and proper particle size and concentration of diamonds can obtain better free cutting action than the cobalt based diamond core drill when drilling steel reinforced concretes. The results and conclusions obtained in this work would be valuable technically and economically to diamond tool manufacturers and users.<br />Great efforts have been taken in investigating wearing and chipping mechanisms in different workpiece materials. Two new theoretical models of the material removal rate (MRR) are firstly proposed for the plain concrete (brittle material) drilling and for the pure steel (ductile material) drilling respectively. Based on these two models, a new comprehensive equation of the material removal rate is firstly deduced for the steel reinforced concrete (brittle + ductile material) drilling.<br />These models establish a clear relationship between the material removal rate and various influencing factors including the diamond size and concentration, operating parameters, and workpiece material properties.<br />Although MRRs are different from the materials being cut, they all increase with the diamond size, the load applied, and the rotation speed. But due to the distinctiveness of the diamond drilling, an overload will make diamond grits be pulled out and / or worn flat quickly and even cause a core drill motionless immediately. To maximize the MRR, an effective way is to select large and sharp diamonds.<br />The chip formation models and the MRR equations suggested in this research can be applied to explain some results and to calculate material removal rates in the diamond drilling of plain concretes, thin metals / steel bars, steel / glass reinforced concretes, rocks, stones, ceramics, refractories, and so on.