ABSTRACT
The placement of the concrete underwater was experimentally modeled as shown in Figure 2. The steel mold-diameter 600 mm and 600 mm height-was totally filled with water, and made sure there was no leakage or buckling that may occur. 700 mm height and 20 mm diameter two injecting PVC pipes were vertically soaked inside the water; the distance between the two pipes was 300 mm. The soaked height was 600 mm, the same height of the mold, and 100 mm was left above the
1 INTRODUCTION
Two-stage concrete (TSC) is defined as concrete produced by placing the coarse aggregate in the place of destination then grouting the cavities (voids) of the coarse aggregate with a special mixture (grout) under pumping tubes extended to the bottom of the form, to fill the voids between aggregate particles, Figure 1. The aggregate that is used in (TSC) should be washed, free of surface dust and fines, the void content of the aggregate should be as optimum as possible. The grout that is used in (TSC) normally consists of ordinary Portland cement and well graded sand, the flow of the grout around the aggregate is essential (Abdelgader 1999). The cost of two stage concrete is less than the cost of normal concrete by nearly 40%. This is related to the reduction of cement content by 30% for the same compressive strength. The main material difference when compared also to normal concrete is that the aggregates are gap-graded with only fine sand commonly less than 2 mm and coarse aggregate usually greater than 20 mm used, thus cutting out the need to use expensive gravels. In addition, there is no need for compaction or vibrating of concrete (Abdelgader 1995). The drying shrinkage of two stage concrete is lower than that of ordinary concrete, due to the contact points between the large aggregate particles. Figure 1. Two stage concrete.
