#rcc structure

Honeycombing refers to a structural defect related to a RCC Structure. The areas of the concrete surface where the coarse aggregate are clearly shown known as honeycombed surface that provides an appearance of honey bees nest.

If honeycombed surface is ignored, the RCC structure fails to function properly according to the design (structurally fragile). Besides, it enables penetration of damaging agents like impure water and air through the existing voids which impact the stability of the structure considerably.

Honeycombing happens for the following reasons :-

1. Concrete mix does not remain integral. 2. Existence of more percentage of larger size of aggregate in concrete resists concrete to fill narrow spaces among the reinforcement rods. 3. The workability of concrete is inadequate. 4. Inadequate compaction to concrete. 5. Imperfect vibration throughout concreting. 6. Steel congestion does not permit concrete to flow toward all corner. 7. Concrete is already set prior to placing. 8. High free fall of concrete at the time of pouring

9. Form work is not water-resistant or inflexible. 10. Inappropriate detailing and/or fixing of steel 11. Inferior proportion of cement to water that can minimize the workability of concrete.

Guidelines to get rid of Honeycombing in Concrete.

All concrete batches should be integrated; examine the concrete production/cohesiveness frequently. If it is possible to develop “ball” form the fresh concrete, concrete mix is defined as cohesive.

Concrete workability should meet the placement requirement, as for instance, a lightly reinforced column can contain 75mm slump, 150 mm slump is necessary for a highly reinforced column.

Make sure that exact compaction is provided for placed concrete, vibrators should be detached since large air bubbles stop to step out (over vibration will cause bleeding). The sizes of the vibrator needle should remain as 25 mm, 40 mm and 60 mm according to RCC sections.

Concrete should be exhaustively consolidated and fully functional around the reinforcement, around implanted fixtures as well as the corners of the formwork. Precautions should be taken throughout vibration otherwise honey combing may occur.

The strength of concrete is decreased by 30% due to 5% voids in concrete.

Cover to formwork, Pins and spaces bars to layers of reinforcement should be provided to maintain exact compaction.

Concrete should maintain Slump prior to placing. Besides, initial slump, concrete should be designed to detain slump untill the time it is set.

The height from where the concrete is dropped should be minimum, if possible concrete bucket with canvas pipes, concrete hose pipe, should be provided to minimize the concrete free fall height.

Formwork should be water resistant; cement grout should not be wasted at the time of placing concrete.

Steel detailing and fixing should be provided to allow smooth flow of concrete across all corners and depths. To get rid of steel congestion, special concrete formulations like self-compacting concrete, concrete with lower maximum aggregate size (12.5mm) etc should be used.

The concept of Clear cover and nominal cover is seen in the Reinforced concrete structures.

Reinforced concrete structures have been main structural materials for over a century and still the most popular material for public structures all over the world. The concrete structure is durable enough to keep its shape for a long time.

The reinforced concrete structure refers to the members like beams, boards, columns, and roof trusses etc. which are consisted of concrete and steel bars.

In these structures, the steel bars are covered by concrete but their mechanical properties will still lose due to the fire to destroy the whole structure.

In this design of reinforced concrete structures, the provided reinforced is inserted in the concrete upto a particular distance from the face of the member as there are some reasons.

Video Source: CE&T-Civil Engg & Technology

Generally, the following four types of steel bars are utilized in concrete structure : Types of Steel Reinforcement used in Concrete Structures -

1. Hot Rolled Deformed Bars: It is extensively used in regular RCC structures. Hot rolling is performed in the mills by providing deformations on the surface i.e. ribs with the purpose of developing bond with concrete.

The stress - strain curve presents a separate yield point accompanied with a plastic stage in which strain is raised without raising the stress. It is succeeded by a strain hardening stage. It comprises of typical tensile yield strength of 60,000 psi.

2. Mild Steel Plain bars: These belong to plain bars and do not contain ribs on them. These are suitable for small projects where cost is a vital factor. Since the plain bars can’t be secured properly with concrete therefore hooks are arranged at the ends. In this type of steel, stress - strain curve also presents a separate yield point accompanied with a plastic stage in which strain is raised without raising the stress. It is succeeded by a strain hardening stage. Normal tensile yield strength is 40,000 psi.

3. Cold Worked Steel Reinforcement: Cold worked reinforcement is provided if hot rolled steel bar encounters process of cold working. Cold working is done by twisting or drawing the bars at room temperature. It properly reduces the Plastic Stage in the Stress-Strain curve, even though it provides more control on the size and tolerances of bars. As there is no plastic stage, ductility becomes lower as compared to Hot Rolled bars. It is mostly effective for the projects where low tolerances and straightness are vital factors.

The stress – strain curve does not present a separate yield point since the plastic stage is completely discarded. Yield point is obtained by drawing a line parallel to the Tangent Modulus at 0.2% strain. Yield stress stands for the point where this line overlaps the stress – strain curve.

It is defined as 0.2% proof stress. If yield stress is obtained at 0.1% strain it is defined as 0.1% proof stress. Normal tensile yield strength is 60,000 psi.

4. Prestressing Steel: Prestressing steel is applied in the form of bars or tendons which are constructed with several strands. The application of tendons / strands is very common since these can be easily placed in different profiles. Prestressing strands are built up with various wires (usually with 2, 3 or 7 wire strands). Normal seven wire strand comprises of six wires spun around the seventh wire with marginally a greater diameter, consequently a helical strand is created.

These wires are cold drawn and contain very high tensile ultimate strength (normally 250,000 - 270,000 psi). Due to extreme tensile strength, the concrete is prestressed properly even after experiencing short term and long term losses. These are applied as prestressed concrete in bridges or prestressed slabs in buildings. Prestressing steel also comes as non-bonded strands encased in PVC sheath. It is applied in Post-Tensioning of members. Prestressing strands can also be applied as Low Relaxation Strands which show low relaxation losses after prestressing. These are normally applied in prestressing members with large distances./p>

Because of cold drawing method, plastic stage in this type of steel is removed. Thus stress – strain curve does not present a separate yield point. Yield point is obtained at 0.1% or 0.2% proof stress. The design of prestressed concrete is not based on yield stress rather it relies on the ultimate strength; therefore the property of interest in this type of steel is the ultimate strength.