Concrete reinforcement

The conventional method using reinforcing steel consists of placing bars in positions one at a time in their final locations, and then tying the bars together as required to hold them in place. The bars are spliced in accordance with laps specified on the drawings. Bar sizes 44 mm and above are connected by exothermic welding.

Main advantages:

— Deviations are easier to spot and correct in the field;

— A higher built-in factor of safety, and thus can be considered more forgiving;

— Can be installed in all countries;

— Lighter rigging loads;

— Less pre-planning required.

Main disadvantages:

— Longer time to install;

— More material used due to lap slicing;

— Exothermic welding is a multi-step process requiring more quality control oversight.

Advanced methods

Prefabricated rebar module

Rebar installation by individual placement of bars is quite time consuming. Large quantities of rebar are needed in the base mat; containment wall and containment dome; and structural walls of the reactor and turbine buildings. Prefabricated rebar assemblies provided as modules for these areas can provide schedule savings.

Advantages:

— Assembly at grade versus elevation;

— Reduced schedule.

FIG. 22. Comparison of construction methods (the top shows the conventional rebar-form method, and the bottom part shows the advanced deck-plate method).

FIG. 23. Close up view of deck plate construction at Shin-Wolsong 1 and 2 auxiliary building, Republic of Korea.

Main disadvantages:

— More complex rigging and fit-up requirements;

— Very difficult to use in placements where there are many embedded parts.

Automated installation of rebar

Automated techniques for installing rebar represent an alternative approach that has recently been used to speed up the installation of rebar and provide consistent results for reinforced concrete structures at NPPs. One technique is the use of an automatic scaffold elevating horizontal rebar feeding machine that reduces labour requirements for rebar installation; another is the use of automation to assemble rebar according to instructions from a 3-D computer design model. Refer to Figs 25 and 26.

Rebar connections — tying

Historic construction methods used hand-tying or welding to join rebar. This was a very labour intensive and time consuming part of any large construction project. With a lack of skilled manpower in several areas of the world, alternative methods of tying rebar are required.

New tying machines are now available on the market. They allow one person to do the work of several people tying rebar by hand. These machines hold the crossed reinforcing bars; feed the tie wire; and wind, cut and tie in one action. The result is a strong, single-strand, double-wrap tie. This is a method that is easily acquired with minimal training.

FIG. 24. Deck plate construction at Shin-Wolsong 1 and 2 auxiliary building, Republic of Korea.

FIG. 25. Automatic elevating scaffolding and horizontal rebar feeding machine, Kashiwazaki-Kariwa Unit 6, Japan.

Rebar connections — coupling

The gripping technology provides for the development of full rebar strength and improved overall structural integrity in tension, compression stress-reversal and dynamic applications. This unique design allows the coupler to be smaller and more streamlined. From a construction point of view, it allows for easy and simple field installation.

All that is needed is a wrench, nut runner or impact wrench, depending on coupler size. The bolt heads will shear off when proper installation tightness has been reached, allowing for completely visual inspection.

Figure 27 shows rebar coupling and Fig. 28 shows a comparison of rebar tying and rebar coupling.

Rebar connections — mechanical splicing

For many years, the traditional method of connecting reinforcing bars has been with lap splicing. However, as many structural engineers, architects and specifiers have discovered, lap splicing has very few advantages and quite a few disadvantages when compared to mechanical splicing.

Mechanical splicing provides the assurance of maintaining load path continuity of the structural reinforcement, independent of the condition or existence of the concrete.

Advantages:

— Reduces rebar congestion and improves concrete consolidation;

— Improves steel-to-concrete ratio;

— Eliminates lap splices in high stress regions;

— Performs similar to a continuous piece of rebar;

— Speeds up construction.

FIG. 26. Automatic rebar assembly machine, Kashiwazaki-Kariwa Unit 7, Japan.

FIG. 27. Rebar coupling.

Disadvantages:

— Expensive to implement;

— Requires a good level of rebar expertise.

Rebar connections — welded cages

Conventional methods require construction personnel to manually place and tie each piece of reinforcing rod.

Bent bars at splice locations add congestion and cause difficulty in placing concrete. Pre-welded cages are an alternative.

Welded rebar cages (shown in Fig. 29) can be rapidly set and easily connected using splices at the common vertical edges of the cages. Forms are set around the dimensionally exact cages, and concrete is rapidly placed and vibrated over the full height of the hook-free bars of the grids.

Welded grids save labour in both cage fabrication and installation; eliminate congestion; speed up concrete placement and form setting; and expedite schedules. With ±3 mm manufacturing tolerance, grids provide accurate vertical bar location, making it possible to install a two-storey welded cage with vertical rebar couplers in less than 30 min.