Scope : Interlocking Concrete Block Pavements have been extensively used in a number of countries for quite sometime. Considering their advantages and potential for use, the guidelines have been prepared for the design and construction of such pavements, giving the suggested applications, design catalogues, construction practices and specifications for their use.
5.1. Suggested Design Procedure
Design procedures have been developed by agencies abroad based on successful performance, or mechanistic principles. They cover a variety of roads ranging from the lightly trafficked to the heavily trafficked. In the absence of research in India, it is recommended that the catalogue of designs given subsequently may be used.
5.2. Lightly Trafficked Pavements
Pedestrian side-walks, footpaths, cycle tracks, car parks and malls are lightly trafficked. In such situations, the pavement can consist of blocks 60 mm thick laid over a sand bedding 20-30 mm and .a base course 200 mm thick. The base course can be in WBM/WMM/crushed stone/soil-cement. This design can be adopted for the range of subgrade soils met with in India. A typical cross-section is given in Fig. 5.
5.3. Block Pavements Subjected to Commercial Traffic
City streets and highway sections subjected to commercial traffic (trucks and buses) require a heavier section. Though design methods based on empirical approach and mechanistic behaviour are available, enough work has not been done in India to evolve the country's own design procedure. In the absence of such knowledge, the ad-hoc design catalogues based on international experience as given in Table 1 are suggested for adoption. A design life of 20 years can be considered for determining the repetitions of standard axles.
5.4. For block pavements for industrial applications like container yard and port wharf and roads and warehouses the following thickness is recommended, based on international experience:
Block
Sand Bedding
Hydraulically bound base
Granular sub-base (out of whichthe bottom 150 mm is a drainage layer)
: 100 mm
: 30-50 mm
: 300 mm
: 300 mm
6.1. General
The quality of materials, cement concrete strength, durability and dimensional tolerances, etc. are of great importance for the satisfactory performance of block pavements.
TABLE 1 : DESIGN CATALOGUE FOR PAVEMENT THICKNESS
|
Traffic and Road Type |
Subgrade CBR (%) |
||
|
Above 10 |
5-10 |
||
|
Cycle Tracks, Pedestrain Footpaths |
Blocks |
60 |
60 |
|
Commercial Traffic |
Blocks |
60-80 |
60-80 |
|
Commercial traffic Axle |
Blocks |
80-100 |
80-100 |
|
Commercial traffic Axle |
Blocks |
80-100 |
80-100 |
Notes: 1. Thickness of layers given above are in mm.
2. Granular sub-base should have at least 150 mm layer at the bottom which is drainable.
3. A Typical cross-section is given in Fig.6.
4. If the subgrade soil has a CBR of less than 5, it should be improved by suitable stabilization technique to bring the CBR value to 5.
5. MSA denotes repetitions in million standard axles * in case of road having inadequate drainage or heavy rainfall areas (above 1500 mm per annum)
Block manufacturing process itself, which immensely influences the quality of paving blocks, are broadly outlines in the subsequent paragraphs. The desired engineering properties/ joining sand layer beneath the block, the base course and sub-base material are also described.
6.2. Salient Mix Design Aspects
The commonly used processes for the manufacture of pre-cast cement concrete paving units require dry, low-slump mixes. The desired characteristics of the mix are as under:
Water/cement ratio
Water content of the mix
Quantity of cement in mix
: 0.34 to 0.38
: 5 to 7% of total mix
: Generally not less than 380 kg/m3 depending on the equipment being used for block making. Upper limit of cement shall not be more than 425 kg/m3. Flyash also can be used in the mix, replacing Ordinary Portland Cement to an extent of 35 per cent.
The above values are for general guidance only. The actual mix design has to be made to suit each individual requirement.
Aggregate/cement ratio : 3:1 to 6:1
Aggregates : Should be sound and free from soft or honeycombed pieces.
The proportion of coarse aggregate in the mix is typically 40 per cent and the fine aggregate (sand) 60 per cent. The size of coarse aggregate should lie between 6 mm and 12 mm and the gradation should be in the recommended range for cement concrete mixes in general.
Strength : In general terms, the paving block must have adequate strength to withstand handling, construction stresses and effects of traffic, though the strength as such is not considered a vital factor in the satisfactory performance of a block pavement. However, it is suggested that the minimum compressive strength of a single block should be above 30 MPa.
Addition of Pigments : To provide the desired colour to paving blocks, appropriate type and amount of pigments are added during mixing, in the form of powder or slurry. Although organic pigments render brighter colours than inorganic pigments, the former are adversely affected by the alkaline environment of concrete and do deteriorate with time. Inorganic pigments, mostly metal oxides, are more durable and hence preferred for consistency and purity. Saturation of colour takes place with a pigment volume of around 5 to 9 per cent of cement content. Pigments should be finer than cement (fineness value between 2 and 15 m2/gm). For the same slump, addition of pigments requires increase in mixing water, which in some cases may lead to lowering of flexural and compressive strength of concrete; therefore, suitable adjustments in mix proportions may become necessary.
Other Additives : Under special circumstances, super-plasticizers at around 0,4 per cent of cement by weight may be added for high early strength. Water repellant admixtures of calcium stearate are sometimes used to reduce water absorption. Air entraining agents, when added to the mix, cause some reduction in the needed amount of cement. Further reduction is achieved by substituting part of the cement with blast-furnace slag or.pozzolanas like flyash; besides reducing cost, these also control "efflorescence" (surface deposition of salts as a result of water movement upwards.