All About Construction

What are the functions of different components of a typical expansion joint? In a typical expansion joint, it normally contains the following components: joint sealant,  joint filler, dowel bar, PVC dowel sleeve, bond breaker tape and cradle bent. Joint sealant : it seals the joint width and prevents water and dirt from entering the joint and  causing dowel bar corrosion and unexpected joint stress resulting from restrained  movement. Joint filler : it is compressible so that the joint can expand freely without constraint.  Someone may doubt that even without its presence, the joint can still expand freely. In fact,  its presence is necessary because it serves the purpose of space occupation such that even if  dirt and rubbish are intruded in the joint, there is no space left for their accommodation. Dowel bar : This is a major component of the joint. It serves to guide the direction of  movement of concrete expansion. Therefore, incorrect direction of placement of dowel b

In the design of elastomeric bearings, why are steel plates inserted inside the  bearings? For elastomeric bearing to function as a soft spring, the bearing should be allowed for  bulging laterally and the compression stiffness can be increased by limiting the amount of  lateral bulging. To increase the compression stiffness of elastomeric bearings, metal plates  are inserted. After the addition of steel plates, the freedom to bulge is restricted and the  deflection is reduced when compared with bearings without any steel plates under the same  load. Tensile stresses are induced in these steel plates during their action in limiting the  bulging of the elastomer. This in turn would limit the thickness of the steel plates. However, the presence of metal plates does not affect the shear stiffness of the elastomeric  bearings.

What are the three major types of reinforcement used in prestressing? (i) Spalling reinforcement Spalling stresses are established behind the loaded area of anchor blocks and this causes  breaking away of surface concrete. These stresses are induced by strain incompatibility  with Poisson’s effects or by the shape of stress trajectories. (ii) Equilibrium reinforcement Equilibrium reinforcement is required where there are several anchorages in which  prestressing loads are applied sequentially. (iii) Bursting Reinforcement Tensile stresses are induced during prestressing operation and the maximum bursting stress  occurs  where the stress trajectories are concave towards the line of action of the load.  Reinforcement is needed to resist these lateral tensile forces.

Is stainless steel really stainless in construction application? Stainless steel refers to alloy steels with more than 10.5% of chromium and consists of  several groups like austenitic, ferritic, martensitic etc. Austenitic stainless steel is normally  used in structural applications because of its high corrosion resistance. Austenitic and  ferritic types of stainless steel cover about 95% of stainless steel applications. Stainless  steel is not stainless although it is corrosion resistant under a wide range of conditions. A passive layer of chromium oxide is formed on stainless steel’s surface which renders it  corrosion resistant. This chromium oxide layer acts as a stiff physical barrier to guard  against corrosion and makes it chemically stable. Moreover, when this layer is damaged, it  can perform self repairing where there is a sufficient supply of oxygen. However, stainless  steel will still corrode by pitting in marine environment where chloride attack occurs.  Therefore, a

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 appl

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 .

Is it good if increase concrete cover beyond contractual  specification (i.e. 40mm to 70mm), shall engineers accept the proposal? In contractual aspect, based on the requirement of General Specification of Civil Engineering Works (1992 Edition), the tolerance of concrete cover is between +5mm and –5mm and engineers should not accept sub-standard work because they do not possess the authority to change the acceptance criteria. In case engineers consider contractor’s proposal acceptable in technical point of view, consent has to be sought from the employer regarding the changes in acceptance criteria. From technical point of view, the effect on cracking due to an increase in concrete cover should be considered. In general, there are three main parameters which govern crack width, namely tensile strain at the point considered, the distance of longitudinal bar to the concerned point and the depth of tension zone. For the second factor, i.e. proximity of longitudinal bars to point of