Category Archives: Questions and Answers

What is the purpose of skin reinforcement for deep beams?

In BS8110, it states that secondary reinforcement should be provided for beams exceeding 750mm deep at a distance measured 2/3 depth from the tension face. Experimental works revealed that at or close to mid-depth of deep beams, the maximum width of cracks arising from flexure may be about two to three times larger than the width of the same crack at the level of surface where the crack originally forms.
The presence of crack is undesirable from aesthetic point of view. Moreover, it poses potential corrosion problems to reinforcement of deep beams. To safeguard against these crack formation, skin reinforcement is designed on the sides of deep beams to limit the formation of flexural crack widths. Though the principal function of skin reinforcement is to control crack width, it may be employed for providing bending resistance of the section.

What is the difference between epoxy grout, cement grout and cement mortar?

Epoxy grout consists of epoxy resin, epoxy hardener and sand/aggregates. In fact, there are various types of resin used in construction industry like epoxy, polyester, polyurethane etc.
Though epoxy grout appears to imply the presence of cement material by its name, it does not contain any cement at all. On the other hand, epoxy hardener serves to initiate the hardening process of epoxy grout. It is commonly used for repairing hairline cracks and cavities in concrete structures and can be adopted as primer or bonding agent.
Cement grout is formed by mixing cement powder with water in which the ratio of cement of water is more or less similar to that of concrete. Setting and hardening are the important processes which affect the performance of cement grout. Moreover, the presence of excessive voids would also affect the strength, stiffness and permeability of grout. It is versatile in application of filling voids and gaps in structures.
Cement mortar is normally a mixture of cement, water and sand. They are used as bedding for concrete kerbs in roadwork.

For column reinforcements, why is helical reinforcement sometimes designed instead of normal links?

The use of links for column design in Britain is very popular. However, in U.S.A. engineers tend to use helical reinforcement instead of normal links because helical reinforcement has the potential advantage of protecting columns/piles against seismic loads. Moreover, when the columns reach the failure state, the concrete outside hoops cracks and falls off firstly, followed by the eventual failure of the whole columns. The peeling off of concrete outside helical reinforcement provides a warning signal before the sudden failure of columns as suggested by G. P. Manning (1924). In addition, it can take up a higher working load than normal link reinforcement.
For instance, helical reinforcement is adopted in the design of marine piles in Government piers.
Note: Helical reinforcement refers to shear reinforcement which is spiral in shapes.

What is the difference in application between open stirrups and closed stirrups in concrete beams?

Open stirrups are provided principally to resist shear forces in concrete beams and they are applied in locations in which the effect of torsion is insignificant. U-shaped stirrups are placed in the tension side of concrete beams in which shear cracks would occur. However, when concrete beams are designed to resist a substantial amount of torsion, closed stirrups should be used instead.

For long slender structures like beams, propping is required after removal of formwork. Why?

After concreting, the time at which striking of formworks should not be too long, otherwise it would affect the colour of concreted structures. For long span concrete structures, when they have attained sufficient strength to support their self-weight, creep deflection may occur in these structures if propping is not provided after the removal of formwork.
Therefore, re-propping is carried out after removing formwork and these props should not be allowed to stand too long because creep loads may overstress them.
Note: Propping refers to provision of falsework to support slabs and beams during their gain in concrete strength after concreting.

General Specification for Civil Engineering Works (1992 Edition) Clause 15.09 specifies that tying wires for reinforcement adjacent to and above Class F4 and F5 finishes should be stainless steel wires. Why?

If plain steel tying wires are used for reinforcement adjacent to Class F4 and F5 finishes, it poses the problem of rust staining which may impair the appearance of exposed concrete surfaces. The rate of corrosion of plain steel tying wires is similar to normal steel reinforcement. However, for tying wires with very small diameter, upon long exposure it stands a high chance of rusting completely and these rust will stain the formwork and significantly affect the concrete finish. Therefore, stainless steel tying wires are specified for locations in the vicinity of high quality of finishes to avoid rust staining by corroded typing wires.
Note: Tying wires are wires used for fixing and connecting steel reinforcement bars.

Does the presence of rust have adverse impact to the bond performance of bar reinforcement?

In fact, the presence of rust in bars may not have adverse impact to the bond performance and it depends on the types of bar reinforcement under consideration.
For plain round bars, the rust on bars improves the bond performance by the formation of rough surfaces which increases the friction between steel and concrete.
However, for deformed bars, the same theory cannot apply. The presence of rust impairs the bond strength because corrosion occurs at the raised ribs and subsequently fills the gap between ribs, thus evening out the original deformed shape. In essence, the bond between concrete and deformed bars originates from the mechanical lock between the raised ribs and concrete. On the contrary, the bond between concrete and plain round bars derives from the adhesion and interface friction. With such differences in mechanism in bonding, the behaviour of bond between deformed bars and plain round bars in the presence of rust varies. Reference is made to CIRIA Report 147.

Can a concrete structure be completely free of expansion joints and contraction joints?

Consider that the concrete structure is not subject to the problem of differential settlement.
For contraction joints, it may be possible to design a concrete structure without any contraction joints. By using sufficient steel reinforcement to spread evenly the crack width over the span length of the structure, it may achieve the requirement of minimum crack width and cause no adverse impact to the aesthetics of the structure. However, it follows that the amount of reinforcement required is higher than that when with sufficient contraction joints.
For expansion joints, the consequence of not providing such joints may be difficult to cater for. For example, a concrete structure has the coefficient of thermal expansion of 9×10-6 /oC and a Young’s modulus of 34.5kN/mm2. With an increase of temperature of 20oC and it is restricted to free expansion, then the structure is subject to an axial stress of 6.21MPa. If the structure is very slender (e.g. concrete carriageway), buckling may occur. Therefore, the structure has to be designed to take up these thermal stresses if expansion joints are not provided. However, for water retaining structures, most of them are not affected by weather conditions because they are insulated from the water they contain internally and soil backfill that surround them. Therefore, it is expected that a smaller amount of thermal movement will occur when compared with normal exposed concrete structure.
Consequently, expansion joints may be omitted in this case with the view that the compressive stress induced by thermal expansion toughens the structure to limit the development of tensile stress.

Which type of bar reinforcement is more corrosion resistant, epoxy-coated bars, stainless steel bars or galvanized bars?

Based on the experiment conducted by the Building Research Establishment, it was shown that the corrosion resistance of galvanized steel was the worst among the three types of bar reinforcement. For galvanized steel bars, corrosion started to occur when a certain chloride content in concrete (i.e. 0.4% by cement weight) was exceeded. However, for epoxy-coated bars, they extended the time taken for cracking to occur when compared with galvanized steel bars.
The best corrosion resistant reinforcement among all is stainless steel. In particular, austenitic stainless steel stayed uncorroded even there was chloride contamination in concrete in the experiment. Reference is made to K. W. J. Treadaway (1988).

Can grout replace concrete in normal structure?

The mixture of cement and water alone cannot replace concrete (Longman Scientific and Technical (1987)) because:
(i) Shrinkage of grout is several times that of concrete with the same mass.
(ii) The effect of creep of grout is far more than that of concrete.
(iii) Heat of hydration of cement with water is more than normal concrete and this leads to the problem of severe cracking.