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1. How can unreinforced concrete pavement function without mesh reinforcement?
For concrete carriageway, it is normally classified into two types: reinforced and unreinforced concrete pavement. The reinforcement in reinforced carriageway (in the form of mesh) is used for controlling cracking. Then one may query how unreinforced pavement can control cracking without the use of mesh reinforcement.
To answer this question, one should pay attention to the features of unreinforced concrete pavement. In accordance with Highways Standard Drawing No. H1109, an approximately 3mm wide groove with a depth of about one-third to one-fourth of slab thickness is designed with a regular spacing (normally 5m).
The grooves are designed to be too narrow for stones to fall into when the cracks are open due to concrete contraction. The sectional area in which the groove is located is a plane of weakness and thus this groove acts a potential crack-inducing device in which any potential cracks due to shrinkage and thermal contraction may form. Consequently, the cracks are formed at the base of the groove and thus it would not cause any unpleasant visual appearance on the exposed surface of unreinforced concrete pavement.
2. Why are concrete profile barriers designed with curved surface profiles?
Safety fencings are designed to contain vehicles in the carriageway in which they are traveling and prevent them from rebounding into the road and causing hazards. For normal fencing design, when vehicles crash into safety fencings, it will give way so as to absorb as much energy as possible, thus reducing the impact forces on the vehicles.
Moreover, it serves to realign the vehicles along the carriageway when vehicles hit on them. However, for concrete profile barriers they are not designed to absorb energy during vehicle crashing, but to hold the vehicles hitting on them.
In this connection, concrete profile barriers are designed with curved profiles so that vehicles can mount and go up partly on them, and yet they will not cause overturning of vehicles. Reference is made to Arthur Wignall, Peter S. Kendrick and Roy Ancil.
For shallow-angle crashing of cars, they would climb on the lower slope face of concrete profile barriers. On the other hand, when a car hits at a large angle to the barrier, the bumper collides with the upper sloping face of concrete profile barrier and the car rides upwards. This provides the uplift of the car whose wheels move up the lower sloping face of the barrier. It is not intended to lift the car too high, otherwise it may result in rolling. Since the friction between the wheels and barriers provide extra lifting forces, it is undesirable to design rough finish on these faces.
In essence, the kinetic energy of the car during collision is transformed to potential energy during its lifting up on profile barrier and finally converted back to kinetic energy when the car returns to the road.
Note: For details of concrete profile barriers, reference is made to HyD Standard Drawing No. H2101A.
3. Should joints of concrete kerbs be in line with the joints in concrete carriageway?
In normal practice, joints are provided in road kerbs to cater for concrete expansion and contraction. However, the location of joints in kerbs is not arbitrary and they should match with joints in concrete carriageway. Otherwise, it is very likely that cracks may form in concrete kerbs at location of pavement joints (Ministry of Transport (1955)).
4. Why is the slump specified in concrete carriageway comparatively low (30mm) when compared with normal concrete (75mm)?
The slump of concrete carriageway is purposely specified to be a relatively low value, i.e. 30mm. For concrete carriageway, traffic loads directly act on concrete pavement surface and therefore the surface strength is detrimental to its future performance.
In freshly placed concrete, segregation (may be in the form of bleeding) occurs within the mixture of cement paste and aggregates. The degree of resistance to segregation is related to workability of concrete. If substantial segregation is allowed to take place, then the relatively porous and weak laitance layer will be formed on the carriageway surface and the aggregates will concentrate in the bottom.
Hence, concrete which has insignificant bleed possesses a stronger surface layer and is more abrasion resistant. Consequently, a small slump value is specified to increase the wearing resistance of concrete and to achieve a suitable surface texture of concrete pavements.
Moreover, a low-slump concrete facilitates the use of slipforms when constructing the concrete pavement. With concrete of a low slump value, it still remains its compacted shape and is not liable to deform when the paving machines go away. However, if a high slump concrete is used instead, the pavement surface would drop and the edges may deform readily.
5. Can all utility detectors detect the depth of utilities?
In Hong Kong, underground utility detectors are normally divided into two types: electromagnetic detector and ground penetrating radar (GPR).
For electromagnetic detector, it can detect the signals emitted by metallic utilities themselves by passive mode. While in active mode, the detector has to pick up the signals through a transmitter and sonda connected directly to the non-metallic utility. Both the alignment and depth can be found in active mode while only alignment can be found in passive mode. The electromagnetic detector available in market can detect utilities up to depth of 3m.
For ground penetrating radar, it sends radio waves into the ground and receives signals from reflections from utilities. It has the advantage of locating both the depth and alignment of utilities. More importantly, it can detect both metallic and non-metallic utilities. However, it suffers from the disadvantage that it is quite expensive and interpretation of data is not simple. Reference is made to LD, DSD (2000).
For concrete carriageway, it is normally classified into two types: reinforced and unreinforced concrete pavement. The reinforcement in reinforced carriageway (in the form of mesh) is used for controlling cracking. Then one may query how unreinforced pavement can control cracking without the use of mesh reinforcement.
To answer this question, one should pay attention to the features of unreinforced concrete pavement. In accordance with Highways Standard Drawing No. H1109, an approximately 3mm wide groove with a depth of about one-third to one-fourth of slab thickness is designed with a regular spacing (normally 5m).
The grooves are designed to be too narrow for stones to fall into when the cracks are open due to concrete contraction. The sectional area in which the groove is located is a plane of weakness and thus this groove acts a potential crack-inducing device in which any potential cracks due to shrinkage and thermal contraction may form. Consequently, the cracks are formed at the base of the groove and thus it would not cause any unpleasant visual appearance on the exposed surface of unreinforced concrete pavement.
Fig. 6.1 Crack formed in un reinforced concrete pavement.
2. Why are concrete profile barriers designed with curved surface profiles?
Safety fencings are designed to contain vehicles in the carriageway in which they are traveling and prevent them from rebounding into the road and causing hazards. For normal fencing design, when vehicles crash into safety fencings, it will give way so as to absorb as much energy as possible, thus reducing the impact forces on the vehicles.
Moreover, it serves to realign the vehicles along the carriageway when vehicles hit on them. However, for concrete profile barriers they are not designed to absorb energy during vehicle crashing, but to hold the vehicles hitting on them.
In this connection, concrete profile barriers are designed with curved profiles so that vehicles can mount and go up partly on them, and yet they will not cause overturning of vehicles. Reference is made to Arthur Wignall, Peter S. Kendrick and Roy Ancil.
For shallow-angle crashing of cars, they would climb on the lower slope face of concrete profile barriers. On the other hand, when a car hits at a large angle to the barrier, the bumper collides with the upper sloping face of concrete profile barrier and the car rides upwards. This provides the uplift of the car whose wheels move up the lower sloping face of the barrier. It is not intended to lift the car too high, otherwise it may result in rolling. Since the friction between the wheels and barriers provide extra lifting forces, it is undesirable to design rough finish on these faces.
In essence, the kinetic energy of the car during collision is transformed to potential energy during its lifting up on profile barrier and finally converted back to kinetic energy when the car returns to the road.
Note: For details of concrete profile barriers, reference is made to HyD Standard Drawing No. H2101A.
3. Should joints of concrete kerbs be in line with the joints in concrete carriageway?
In normal practice, joints are provided in road kerbs to cater for concrete expansion and contraction. However, the location of joints in kerbs is not arbitrary and they should match with joints in concrete carriageway. Otherwise, it is very likely that cracks may form in concrete kerbs at location of pavement joints (Ministry of Transport (1955)).
4. Why is the slump specified in concrete carriageway comparatively low (30mm) when compared with normal concrete (75mm)?
The slump of concrete carriageway is purposely specified to be a relatively low value, i.e. 30mm. For concrete carriageway, traffic loads directly act on concrete pavement surface and therefore the surface strength is detrimental to its future performance.
In freshly placed concrete, segregation (may be in the form of bleeding) occurs within the mixture of cement paste and aggregates. The degree of resistance to segregation is related to workability of concrete. If substantial segregation is allowed to take place, then the relatively porous and weak laitance layer will be formed on the carriageway surface and the aggregates will concentrate in the bottom.
Hence, concrete which has insignificant bleed possesses a stronger surface layer and is more abrasion resistant. Consequently, a small slump value is specified to increase the wearing resistance of concrete and to achieve a suitable surface texture of concrete pavements.
Moreover, a low-slump concrete facilitates the use of slipforms when constructing the concrete pavement. With concrete of a low slump value, it still remains its compacted shape and is not liable to deform when the paving machines go away. However, if a high slump concrete is used instead, the pavement surface would drop and the edges may deform readily.
5. Can all utility detectors detect the depth of utilities?
In Hong Kong, underground utility detectors are normally divided into two types: electromagnetic detector and ground penetrating radar (GPR).
For electromagnetic detector, it can detect the signals emitted by metallic utilities themselves by passive mode. While in active mode, the detector has to pick up the signals through a transmitter and sonda connected directly to the non-metallic utility. Both the alignment and depth can be found in active mode while only alignment can be found in passive mode. The electromagnetic detector available in market can detect utilities up to depth of 3m.
For ground penetrating radar, it sends radio waves into the ground and receives signals from reflections from utilities. It has the advantage of locating both the depth and alignment of utilities. More importantly, it can detect both metallic and non-metallic utilities. However, it suffers from the disadvantage that it is quite expensive and interpretation of data is not simple. Reference is made to LD, DSD (2000).
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