Vincent T. H. CHU

When a concrete pavement is constructed over a box culvert, it is important to match the location of transverse joints with boundary of box culvert. Otherwise, it is likely that full-depth transverse cracks would develop on the pavement slabs just above the location of boundary of box culvert. In case a layer of granular sub-base is introduced to place under the pavement slabs, the sub-base layer serves as crack-arresting layer and the possibility of development of transverse cracks in concrete pavement is reduced.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Longitudinal joints are installed in concrete pavement to prevent differential settlement between adjacent concrete panels. Moreover, it serves to control cracking from stresses caused by volumetric changes of concrete owing to moisture and thermal gradients. In essence, the joint contains tie bars to enhance efficient load transfer between adjacent concrete panels.

Sometimes keyway joint are designed in longitudinal joint to improve the performance of the joint. Though the installation of keyway joint does not appear to increase the load transfer efficiency of longitudinal joint, it proves to help reduce the deflection of concrete pavement. Keyways are not recommended for thin slab (less than 250mm thick) because of the difficulty in construction. Moreover, keyways are prone to failure in thin concrete slabs where they are too large or too close to slab surface.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Tie bars are deformed rebars or connectors used for holding faces of rigid slabs in contact to maintain aggregate interlock. Tie bars are not load transferring device. For instance, tie bars are used in longitudinal joints in concrete pavement.

Dowel bars are smooth round bars which mainly serve as load transfer device across concrete joints. They are placed across transverse joints of concrete pavement to allow movement to take place. Where movement is purposely designed for longitudinal joints, dowel bars can be adopted.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

For unreinforced concrete pavement, the contraction joint is an approximately 3mm wide groove with a depth of about one-third to one-fourth of slab thickness and a regular spacing of normally 5m. The grooves are designed such that they are too narrow for stones to fall into when the cracks are open due to the contraction of concrete. The groove location is a plane of weakness and the groove acts as a potential crack-inducing device where any potential cracks due to shrinkage and thermal contraction may form will be confined to the base of the groove. It will not cause any unpleasant visual appearance on the exposed surface of unreinforced concrete pavement.

The above-mentioned contraction joints can be designed as unsealed. These grooves are very narrow so that stones can hardly get into these grooves even when the joint undergoes contraction. The fine particles or grit entering into the groove are likely to be sucked out by the passing vehicles. The joints can be self-cleansing and it may not be necessary to seal the joints for fear of attracting the accumulation of rubbish and dirt

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

A longitudinal joint consists of a tie bar placed at the mid-depth of a concrete pavement and it is not intended for joint lateral movement. Then one may doubt the reasons of placing longitudinal joints in concrete pavements. In fact, longitudinal joints are normally designed at a regular spacing e.g. 4.5m to accommodate the effect of differential settlement of pavement foundation. When uneven settlement occurs, the tie bars in longitudinal joints perform as hinges (Ministry of Transport (1955)) which allow for the settlement of concrete carriageway. Moreover, it also serves to cater for the effect of warping of concrete due to moisture and temperature gradients by permission of a small amount of angular movement to occur so that stresses induced by restrained warping can be avoided.

Dowel bars are provided in longitudinal joints for the following reasons:
(i) In case of the occurrence of uneven settlement between adjacent panels, it helps to maintain a level surface by transfer of loads through dowel bars.
(ii) Keep the longitudinal joints close.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

If the construction of concrete carriageway is carried out in summer, expansion joints may not be necessary as suggested by Arthur Wignall, Peter S. Kendrick and Roy Ancil. Expansion of concrete carriageway is mainly due to seasonal changes with an increase in temperature from that during construction to the ambient temperature (i.e. the temperature in summer). However, if the construction of concrete carriageway takes place in summer, the concrete carriageway will undergo contraction in the following winter, thus the space available in contraction joints can accommodate the future expansion in the next summer.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Let’s take an example to illustrate the sequence of closing and opening of joints (Ministry of Transport (1955)). For instance, an unreinforced concrete carriageway is constructed in winter. When temperature rises in the following summer, the section between expansion joints will expand as a whole single element resulting in the closure of expansion joints. This section of concrete pavement will move outwards from the mid-point
between the expansion joints. In the next winter, each bay (i.e. concrete pavement between adjacent contraction joints) of concrete contracts about the midpoint of its length with opening of contraction joints.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

The purpose of plastic sleeve or debonding agent around dowels bars in expansion joints is to minimize the frictional resistance between the bar and its surrounding concrete. This is the reason why plain round bars are usually used instead of deformed bars which provide mechanical interlock with concrete and hence it hinders the free movement of the dowel bar.

Both bituman-based paint (debonding agent) and plastic sleeve could serve the purpose of reducing friction between dowel bar and surrounding concrete. From practical point of view, the use of plastic sleeve (e.g. PVC dowel sleeve) around dowel bars can well prepared off-site and manufactured well in advance, thus saving the time of construction.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

The two main principal functions of joint sealant are to minimize the entry of surface water and prevent the ingress of incompressible material from entering the joint. Other minor function of joint sealant is to reduce the possibility of corrosion of dowel bar by the entrance of de-icing chemicals.

Water entry into joints is undesirable because it leads to the softening of subgrade and pumping of subgrade fines under heavy traffic. However, it is impractical to maintain a completely watertight pavement structure. In fact, vacuum tests show that no sealants could provide 100% watertight seal. The current philosophy to combat water ingress into joints is only to minimize but not to completely prevent water from entering the pavement structure. Instead, a permeable subbase is designed to remove water from the pavement.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

There are four generic types of joint sealant with high performance. Their properties are highlighted in the following table:

Acrylic	Polysulfide	Polyurethane	Silicone
Accommodate 12% movement.	Poor recovery in high cyclic movements	Accommodate 50% movement.	Accommodate 50% movement.
Exhibit shrinkage upon curing	Exhibit excellent chemical resistance	Excellent bonding, can be used without primer	Excellent low temperature movement capability
Solvent-based.	Good performance in submerged conditions.	Good UV resistance	Excellent UV and heat stability

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.