Steel Reinforcement Work in Building Construction

CUTTING AND BENDING OF REINFORCEMENT

Cold worked bars and hot rolled high yield bars shall not be straightened or bent again once having been bent. Where it is necessary to bend mild steel reinforcement projecting from the concrete, the internal radius of bend shall be not less than twice the diameter of the bar. Heating of bars is not permitted. All bending shall be done cold by the use of an approved bending machine. Only experienced steel benders and fixers shall be employed.

WELDING OF REINFORCEMENT

When fabricating reinforced steel frame and netting, if there is no specific requirements for designing to follow, theses standards will govern:
• For round and plain steel, welding is at all joint point
• For edged steel, welding is at all joint points on the two circumference lines. The other rest points will be welded at the middle on the line which skipped one line by one line alternately.
• For reinforced steel beam frame, welding is at all the joint points.

The welding points should meet these requirements:
• The surfaces shall be smooth without burns, interruption points, partial shrinkage and foam.
• Ensure all the welding height and length as the designing standard.

The welding joints shall be inspected by types and plots. Each plots consisted of 100 joints or 100 reinforced steel frame and netting. These plots will be checked as the followings principles:
• Each plot, 5% of the products but not less than 5 samples for checking the dimensions and 3 samples for checking pulling and 3 samples for checking bending.

FIXING REINFORCEMENT
GENERAL
The quantity, sizes, form, positions and spacing of all steel reinforcement bars shall be in accordance with the Drawings. All cutting and trimming of bars and wires shall be done cold with the used of approved manual or power operated cutters. Welding of reinforcement shall not be permitted except in special circumstances where approved by the Engineer. Reinforcement projecting from work already concreted shall not be bent out of its correct position for any reason, unless permitted by the Engineers, and shall be protected from deformation or other damage.

SPACERS
Spacer blocks shall be placed at regulars’ intervals to support first layers of reinforcement off the concrete blinding or bottom formwork. Spacers shall be securely fixed to the reinforcement at the time of placing. The number of spacers shall be the minimum necessary to maintain the reinforcement in its correct position. Spacers blocks shall be of standard cement mortar or such other composition as previously approved by the
Engineer. In slabs provided with two or more layers of reinforcement, the upper layers of steel shall be supported in position by the use of steel chairs.

LAP LENGTH FOR REINFORCED BAR
Binding works (two ends overlapped together) for reinforcement will follow the designing requirements. There is no binding points at the high-pressured or bending points. In the cross section of structure, the binding points will not exceed 25% of the total area of cross section for round bar and not exceed 50% for deformed bar

History of Raft Foundation

In early 70s, P.W.D, designed raft foundation as R.C.C. solid slab.The additional component of cut off walls on both sides U/s and D/s was considered necessary to take care of seepage and possible undermining of the raft due to seepage and the scour due to floods .

As the design approach was based on beams on elastic foundation, the entire bed was proposed to be provided with the flexible elastic media to ensure the elasticity of the bed. The raft was designed as one way slab spanning between two piers. It was observed that the raft was showing signs of cracks between pier and cut off walls. The arrangement was, therefore, subsequently changed by resting pier on raft over the cut off walls. The cut off walls when constructed monolithic with the raft, it was considered that the raft bed designed as two way slab (cut off walls on U/s & D/s and the piers provide fixity to the raft at all four edges). (Type designs issued in 1973).

The design of raft was then modified as channel type raft ( entirely as a one structural component. In channel type raft, the entire channel formed by cut off and raft acts as a monolithic component. The cut off walls in this case resist most of the shear forces and bending moments and the slab is designed for local conditions. This resulted in reduced thickness of raft slab and also the cut off walls and resultant saving in concrete was found quite substantial.

If heavy dewatering is involved, the quality of concrete for cut off wall becomes doubtful. In case of channel type raft, cut off wall is an important component and its quality has to be excellent. If this is not possible to be achieved then it is desirable to go for detached P.C.C. cut off walls which are supposed to act only for increasing seepage path and keeping soil below foundation raft in tact.

Such type designs are also evolved and issued by P.W.D. in 1996.

Bridge Deck Waterproofing Using Bitumen Membrane

Product:

The system uses a single-ply membrane with puncture resistance. The membrane is made from highly elastic SBS-modified bitumen and has a non-woven polyester reinforcement.

The upper side is self-protective with ceramic granules; the overlap is finished with a 10-cm polypropylene tissue. The lower side of the membrane is finished with a polypropylene film. The reinforcement is fully impregnated with SBS coating. The homogeneity between compound and impregnation-mixture gives the membrane its extremely high resistance against delamination.

Advantages:

• Light
• Increased resistance to cracking
• Exceptional adhesion on hydraulic concrete
• Easy to weld

Application

1. Concrete surface:

The concrete deck shall be conforming to relevant applicable standards. The concrete surface must be at least 2 weeks old before starting the waterproofing works. The surface must be in good condition, meaning that it must be clean, smooth and even, without any holes and gaps.

2. Surface preparation:

Before applying the primer, check the condition of the bridge deck surface:

• Checking the spots of hydrocarbon
• Checking if all dirt, dust, soil, clay and cement sauce has been removed.
• Thoroughly cleaning the surface, e.g by air pressure
• Verifying that the surface is well dry
• Checking for the gaps, holes, cavities

3. Membrane Installation

Apply the primer with brushes, rollers or spaying tools. Normally it takes about 2 hours for the primer to dry.

The membranes are rolled out, aligned and rolled up again for about ½ of their length. The membrane is welded completely on the surface by the use of high capacity torches.  Immediately the membrane is carefully pressed to the surface by the use of an iron roller to prevent the inclusion of air between the membrane and the surface. Make sure the cross directional overlaps are staggered. The width of the horizontal overlaps should be at least 12cm. The longitudinal overlaps should be 10cm.

Asphalt wearing course

The asphalt wearing course is put directly on the waterproofing. The minimum thickness is 5cm after compacting. The temperature of the asphalt may not exceed 180 Celcious degrees at pouring. During compacting the temperature not exceed 160 degree. Only trucks and vehicles involved with the asphalt wearing course is permitted to the bridge deck area. The contractor has to take care not to damage the  membrane.

How to Define the Concrete Surface Roughness

Equipment for concrete roughness measurement

Definition of the concrete surface roughness in accordance with the German Standard “Rili-DAStb”

• Defined Volume V (25 cm³ or 50 cm³)
• Dried kiln quartz sand 0.1 – 0.3 mm
• Round wooden disc (Ø 50 mm, thickness 10 mm)

Test:

Defined volume of quartz sand (I.e. 25 cm³) is to be placed onto the surface and distributed in a circling manner until the troughs are filled

RT = 40xV / p x d²

Roughness Definition by ICRI

The International Concrete Repair Institute (ICRI) has defined nine different guidelines for proper surface preparation and has developed profile replica blocks to give a visual point of reference for the user.

The nine profile replicas of the CSP standards can be obtained from ICRI.Each profile carries a CSP number ranging from a base line of 1 (nearly flat) through 9 (very rough).

Advantages and Applications of Post-Tensioning

There are post-tensioning applications in almost all facets of construction. In building construction, post-tensioning allows longer clear spans, thinner slabs, fewer beams and more slender, dramatic elements. Thinner slabs mean less concrete is required. In addition, it means a lower overall building height for the same floor-to-floor height. Post-tensioning can thus allow a significant reduction in building weight versus a conventional concrete building with the same number of floors. This reduces the foundation load and can be a major advantage in seismic areas.

A lower building height can also translate to considerable savings in mechanical systems and façade costs. Another advantage of post-tensioning is that beams and slabs can be continuous, i.e. a single beam can run continuously from one end of the building to the other. Structurally, this is much more efficient than having a beam that just goes from one column to the next.

Post-tensioning is the system of choice for parking structures since it allows a high degree of flexibility in the column layout, span lengths and ramp configurations. Post-tensioned parking garages can be either stand-alone structures or one or more floors in an office or residential building. In areas where there are expansive clays or soils with low bearing capacity, post-tensioned slabs-on-ground and mat foundations reduce problems with cracking and differential settlement.

Post-tensioning allows bridges to be built to very demanding geometry requirements, including complex curves, variable superelevation and significant grade changes. Post-tensioning also allows extremely long span bridges to be constructed without the use of temporary intermediate supports. This minimizes the impact on the environment and avoids disruption to water or road traffic below. In stadiums, post-tensioning allows long clear spans and very creative architecture. Post-tensioned rock and soil anchors are used in tunneling and slope stabilization and as tie-backs for excavations. Post-tensioning can also be used to produce virtually crack-free concrete for water-tanks.