Interlocking Concrete Pavers

Interlocking Concrete Pavers An Easy Approach for Road Construction

Interlocking Concrete Block Pavement (ICBP) has been extensively used in a number of countries for quite sometime as a specialized problem-solving technique for providing pavement in areas where conventional types of construction are less durable due to many operational and environmentalconstraints. ICBP technology has been introduced in India in construction, a decade ago, for specific requirement viz. footpaths, parking areas etc. but now being adopted extensively in different uses where the conventional construction of pavement using hot bituminous mix or cement concrete technology is not feasible or desirable. The paper dwells upon material, construction and laying of concrete block pavement as a new approach in construction of pavement using Interlocking Concrete Pavers Blocks.

Introduction to Interlocking Concrete Pavers

Concrete paver blocks were first introduced in Holland in the fifties as replacement of paver bricks which had become scarce due to the post-war building construction boom. These blocks were rectangular in shape and had more or less the same size as the bricks. During the past five decades, the block shape has steadily evolved from non-interlocking to partially interlocking to fully interlocking to multiply interlocking shapes. Consequently, the pavements in which non-interlocking blocks are used are designated as Concrete Block Pavement (CBP) or non-interlocking CBP, and those in which partially, fully or multiply interlocking blocks are used are designated as ‘Interlocking Concrete Block Pavement (ICBP).
CBP/ICBP consists of a surface layer of small-element, solid un-reinforced pre-cast concrete paver blocks laid on a thin, compacted bedding material which is constructed over a properly profiled base course and is bounded by edge restraints/kerb stones. The block joints are filled using suitable fine material. A properly designed and constructed CBP/ICBP gives excellent performance when applied at locations where conventional systems have lower service life due to a number of geological, traffic, environmental and operational constraints [1-8]. Many number of such applications for light, medium, heavy and very heavy traffic conditions are currently in practice around the world.

Advantages and Limitations

There are many distinct features of ICBP as compared to the conventional methods of pavement construction and hence make it a suitable option for application in the specified areas [7 & 10]. Some of these are:

• Mass production under factory conditions ensures availability of blocks having consistent quality and high dimensional accuracy.
• Good quality of blocks ensures durability of pavements, when constructed to specifications.
• ICBP tolerates higher deflections without structural failure and will not be affected by thermal expansion or contraction.
• ICBP does not require curing, and so can be opened for traffic immediately after construction.
• Construction of Interlocking concrete Pavers is labor intensive and requires less sophisticated equipment.
• The system provides ready access to underground utilities without damage to pavement.
• Maintenance of ICBP is easy and simple and it is not affected by fuel and oil spillage.
• Use of coloured blocks facilitates permanent traffic markings.
• ICBP is resistant to punching loads and horizontal shear forces caused by maneuvering of heavy vehicles
• Low maintenance cost and a high salvage value ensures low life cycle cost.

However, important limitations of the technique are the following:

• Quality control of blocks at the factory premises is a prerequisite for durable “ICBP”
• Any deviations of base course profile will be reflected on the “ICBP” surface. Hence extra care needs to be taken to fix the same.
• High quality and gradation of coarse bedding sand and joint filling material are essential for good performance.
• “ICBP” over unbound granular base course is susceptible to the adverse effects of poor drainage and will deteriorate faster. “ICBP” is not suited for high speed roads (speed above 60 km/h)

Physical Requirements

Since zero slump concrete is used in production of interlocking concrete pavers, the quality of blocks produced will depend upon various parameters like the capacity of compaction and vibration of machine, grade of cement used, water content, quality of aggregates used, their gradation and mix design adopted, additives used, handling equipment employed, curing method adopted, level of supervision, workmanship and quality control achieved, etc. Recommended grades of paver blocks to be used for construction of pavements having different traffic categories are given in Table 1 [9].

Application of ICBP Technology

Some of the proven areas where ICBP technology is being applied are listed below [9 & 10]:

1. Non-traffic Areas: Building Premises, Footpaths, Malls, PedestrianPlaza, Landscapes, Monuments Premises, Premises, Public Gardens/Parks, Shopping Complexes, Bus Terminus Parking areas and Railway Platform, etc.
2. Light Traffic: Car Parks, Office Driveway, Housing Colony Roads, Office/Commercial Complexes, Rural Roads, Residential Colony Roads, Farm Houses, etc.
3. Medium Traffic: Boulevard, City Streets, Small Market Roads, Intersections/Rotaries on Low Volume Roads, Utility Cuts on Arteries, Service Stations, etc.
4. Heavy and Very Heavy Traffic: Container/Bus Terminals, Ports/Dock Yards, Mining Areas, Roads in Industrial Complexes, Heavy-Duty Roads on Expansive Soils, Bulk Cargo Handling Areas, Factory Floors and Pavements, Airport Pavement, etc.

Shapes and Classifications

There are four generic shapes of paver blocks corresponding to the four types of blocks as below [9 & 10]:

1. Type A: Paver blocks with plain vertical faces, which do not key into each other when paved in any pattern,
2. Type B: Paver blocks with alternating plain and curved/corrugated vertical faces, which key into each other along the curve/corrugated faces, when paved in any pattern,
3. Type C: Paver blocks having all faces curved or corrugated, which key into each other along all the vertical faces when paved in any pattern and
4. Type D: ‘L’ and ‘X’ shaped paver blocks which have all faces curved or corrugated and which key into each other along all the vertical faces when paved in any pattern.

The generic shapes and groups of paver blocks identified to four types are illustrated in Figures 1 & 2.

Materials

The quality of materials, cement concrete strength, durability and dimensional tolerance of paving blocks, etc. is of great importance for the satisfactory performance of block pavements. These aspects and the block manufacturing process itself, which immensely affect the quality of paving blocks, have been outlined in the Indian Roads Congress Special Publications [9]. The Central Road Research Institute (CRRI) has prepared the specifications for ICBP [10]

Paving Blocks

The quality of materials, strength of cement concrete and durability as well as dimensional tolerances etc. are of great importance for satisfactory performance of block pavement. The recommended thickness of block and grades of concrete for various applications and specification for paving in which materials used for preparation of blocks, physical requirements, physical test methods, sampling and acceptance criteria has already been formulated in BIS Code [10].

Bedding and Joint Filling Sand

It is well established that if proper attention is not paid to the quality of bedding sand, and if the thickness of bedding sand layer is not uniform enough, serious irregularities in surface profile can result; excessive differential deformation and rutting can occur early in service life of the block pavement. The gaps in between two adjacent paving blocks (typically about 3 mm wide) need be filled with sand, relatively finer than the bedding sand itself. The desired gradation for the bedding and joint filling sands are given in Table 2 [9].
It is necessary to restrict the fines (silt and/or clay passing 75 micron sieve) to 10 percent, since excessive fines make joint filling very difficult. Similarly, it is not advised to use cement in the joint filling sand, which may not only make it difficult to completely fill the joints, but may also adversely affect the desired flexibility characteristics of the paving block layer. The joint filling sand should be advisably as dry as possible; otherwise complete filling of joints may be difficult.

Base and Sub base Materials

The engineering properties of base materials are the load spreading properties to disperse stresses to the subgrade and the desired drainage characteristics, having an important bearing on the performance of a block pavement. Although, local availability and economics generally dictate the choice of base material at the design stage, yet the commonly used materials considered suitable for base courses are unbound crushed rock, water-bound macadam, wet mix macadam, cement bound crushed rock/granular materials, and lean cement concrete/ dry lean concrete etc [11, 12, 13, 14 & 15]. In broad terms, wherever the subgrade is weak (having a CBR value below 5) use of bound granular materials, like, cement treated crushed rock, requiring a relatively thinner base, should be preferred while for high strength subgrades, unbound crushed rock may be used. The climatic and environmental factors also need be considered during the choice of a base material. Sub-base is essential where commercial traffic is expected. The quality of sub-base materials is inferior to the base materials and includes natural gravels, cement treated gravels and sands and stabilized subgrade materials. The quality of sub-base materials should be in conformance with IRC: 37 [16].

Construction of Interlocking Concrete Pavers for Pavement

Sequencing of operations

The sequencing of operations (Fig. 3) for construction of block pavement should be as follows [1 & 4]:

1. Installation of sub-surface drainage structures
2. Leveling and compaction of subgrade
3. Provision and compaction of sub-base course (where needed)
4. Provision and compaction of base-course and checking for correct profile
5. Installation of edge restraints
6. Provision and compaction of coarse bedding sand
7. Laying of blocks and interlocking
8. Application of joint sealing sand and compaction
9. Cleaning of surface
10. Filling any remaining empty portions in the block layer especially near edge restraint blocks with in situ concrete.

Construction of Sub–grade

This is the foundation layer over which the block pavement is constructed. Like in conventional pavements, the water table level should not be at a level of 600 mm or higher, below the subgrade level. It should be compacted in layers of either150 or 100 mm thickness guidelines [10]. The prepared subgrade should be graded and surface dressed to a tolerance of ± 20 mm of the design levels, and its surface evenness should have a tolerance of within 15 mm under a 3 m straight edge [10].

Construction of Base and Sub-base Layers

Base course and sub-base course are constructed in accordance with standard procedures contained in relevant IRC specifications like IRC:SP:49-1998, IRC:50-1973, IRC:51-1992, IRC:63-1976, IRC:19-1997 & IRC:37, 2001, [11, 12, 13, 14, 15 & 17]. When cement bound base is proposed it may be constructed using rolled lean concrete as IRC:SP-49. The quality control specified in IRC: SP-11 [16] shall apply. Constructing the lower layers to proper level and grade is very essential to maintain the top surface level and surface regularity of the block pavement surface.

Edge Restraint Blocks and Kerbs

Concrete blocks on trafficked pavements tend to move sideways and forward due to braking and maneuvering of vehicles. The tendency to move sideways has to be counteracted at the edges by special edge blocks and kerbs. The edge block should be designed and anchored to the base such that the rotation or displacement of blocks is resisted [7]. These are to be made of high strength concrete for withstanding the traffic wheel-load without getting damaged. These members should be manufactured or constructed in-situ to have at least a 28-day characteristic compressive strength of 30 MPa or flexural strength of 3.8 MPa. As far as possible the edge blocks should have vertical face towards the inside blocks. A few typical edge-blocks are also shown in Fig. 4. Where the space is not easily permitting the use of plate vibrators, jhurmut or manual compactor using small size plate rammer may be used. The road kerbs provided on edges of roads also serve the purpose of edge blocks. In case the kerbs are not provided, it has to be replaced by edge strips. In case of heavy traffic 150 mm x 150 mm plain cement concrete (M-25) may also be provided over dry lean concrete to give further confinement of blocks. In-between the edge-restraint blocks cement mortar (1: 6, cement: coarse sand) may be used in place of sand for sealing of blocks [7].

Placing and Screeding of Bedding Sand

The thickness of the sand bed after compaction should be in the range of 20-40 mm [10], whereas, in the loose form it should be 25 to 50 mm. It is preferable to restrict the compacted thickness to 20-25 mm to reduce the risk of any localized over-compaction, which would affect the final block surface level. Bedding sand should not be used to fill-up local depressions on the surface of a base or sub-base. The depressions if any, should be repaired with same base or sub-base material in advance before placing sand. The sand of specified gradation to be used, should be uniformly in loose condition and should have uniform moisture content. Optimum moisture content is that when sand is neither too wet nor too dry and has moisture of 6 to 8 percent. Requirement of sand for a day’s work should be prepared and stored in advance and covered with tarpaulin or polythene sheets. The processed sand so obtained is spread with the help of screed boards to the specified thickness. The screed boards are provided with nails at 2-3 m apart which when dragged gives the required thickness. The length of nail should take into account the surcharge to be provided in the uncompacted thickness. Alternatively, the screed can be dragged on edge strips kept on both sides as guides [7].

Laying of Blocks

Blocks can be laid generally by manual labour but mechanical aids like hand-pushed trolleys can expedite the work. Normally, laying should commence from the edge strip and proceed towards central line. When dentated blocks are used, the laying done at two fronts will create problem for matching joints in the middle. Hence, as far as possible, laying should proceed in one direction only, along the entire width of the area to be paved [4, 5, 6, 7, 8 & 10].
While locating the starting line, the following should be considered:

• On a sloping site, start from the lowest point and proceed to up-slop on a continuous basis, to avoid down-slop creep in incomplete areas.
• In case of irregular shaped edge restraints or strips, it is better to start from straight string line as shown in Fig. 5.
• Influence of alignment of edge restraints on achieving and maintaining laying bond.

Laying of block pavement at National Highway – 08 Jaipur-Kishangarh section for heavy commercial vehicles for truck-lay-byes on both side of pavement are shown in Figs 6 and 7.

Establishing the Laying Pattern

The blocks can be placed in different bonds or patterns depending upon the requirement, some popular bonds commonly adopted for block paving are [4]:
1. Stretcher or running bond 2. Herringbone bond 3. Basket weave or parquet bond The typical layouts of these bonds are given in Fig. 8.

Typical Pavement Composition

A typical compositions normally used in ICBP are given in Table 3 and a cross-section is shown in Fig. 9 [10].Block Pavements at Typical Locations:Essentially, there are three important aspects in detailing. These typical locations are [10]:Curves: It is necessary to cut the paving units to fit the edge restraints. Rectangular blocks of a similar or contrasting colour as an edging have been used to minimize the visual effects of small errors in block cutting. To avoid unsightly and potentially weak joints, it is often preferable to change the laying pattern at the curve. The curve itself can be installed in Herringbone bond and yet the pavement can revert to stretcher bond on the approaches [10].Pavement Intrusions: On some pavements, like in city streets, there could be several intrusions, like, manholes, drainage gulleys, etc. where coping with these intrusions with the pavement is desirable. Around intrusions, it is good practice to lay along both sides of the intrusion simultaneously so that closure is made away from the starting workface, rather than carrying the pavement around the intrusion to return to the original laying face to avoid accumulation of closing error [10].

Changes in alignment: Changes in alignment of a road pavement can some times be achieved by the use of special blocks. However, it is generally easier to choose a block that can be installed in Herringbone bond through simply cutting the blocks to fit into the edge restraints. Where aesthetic requirements of shape of the paving unit dictate the use of Stretcher bond, then only a 90o shape change in alignment can be achieved without cutting the blocks (Fig. 10). At intersections, if a Herringbone bond laying pattern is adopted, the block laying can proceed without the need for construction joints (Figs 11). An alternative to this is to install a shoulder (support) course of rectangular blocks between the main road and the side streets; this permits different laying patterns to be used in two roadways [10].

Compaction

For compaction of the bedding sand and the blocks laid over it, vibratory plate compactors are used over the laid paving blocks; at least two passes of the vibratory plate compactor are needed. Such vibratory compaction should be continued till the top of each paving block is in level with its adjacent blocks. It is not a good practice to leave compaction till the end of the day, as some blocks may move under construction traffic, resulting in the widening of joints and corner contact of blocks, which may cause spalling or cracking of blocks. There should not be delay in compaction after laying of paving blocks to achieve uniformity of compaction and retention of the pattern of laying. During vibratory compaction of the laid blocks, some amount of bedding sand may get filled up into the joints between them; the extent of sand getting filled up into the joints will depend on the degree of compaction of sand, i.e. the force applied by the compactor. Standard compactors may have a weight of about 90 kg, plate area of about 0.3 m2and apply a centrifugal force of about 15 kN, while heavy duty compactors may weigh 300-600 kg, have a plate area of about 0.5-0.6 m2 and apply a centrifugal force of 30-65 kN. Where the bedding sand is required to be compacted for heavy traffic block pavements, heavy-duty compactors should be used. After compaction by vibratory plate compactors, some 2 to 6 passes of a vibratory roller (with rubber coated drums or those of static weight less than 4 tonnes and nominal amplitude of not more than 0.6 mm) will further help in compaction of bedding sand and joint filling [4 & 10].

Laying and Surface Tolerances

While constructing the block pavement, the surface tolerances of individual layers may be observed as shown in Table 4 [4 & 10].

Conclusion

1. ICBP technology can provide durable and sustainable road infrastructure where construction and maintenance of conventional pavements are not cost effective.
2. ICBP is much cheaper than rigid (concrete) pavement designed for identical conditions. Compared to bituminous pavement for low traffic volumes and high strength subgrade, the initial construction cost of ICBP is likely to be equal to or marginally higher. For high traffic volumes and low strength subgrade, ICBP will be cheaper than flexible pavement.
3. Guidelines for use of Interlocking Concrete Block Pavement and Specification on Paver Blocks are published in Codes and available with Indian Roads Congress and Bureau of Indian Standards which are very useful for Indian industries and highway professions for adoption of block pavement technology.

Acknowledgments

Author is thankful to Dr. S. Gangopadhyay, Director, CRRI, for his kind permission to publish this paper and continuous encouragement. Sincere thanks are also due to Shri B.M. Sharma, Head, Pavement Evaluation Division for encouraging support to write this paper. The cooperation and valuable suggestions extended by Shri T. Muraleedharan, Retired Senior Scientist, Pavement Evaluation Division is gratefully also acknowledged.

Further reading

Introduction
This page details the procedure used to design a block or brick pavement for public highways, car parks, freight yards etc. In most circumstances, this procedure is not required for residential projects such as driveways or patios, unless there are special conditions, such as poor ground or exceptional loads.The design process outlined on this page is given in full in British Standard BS 7533:1992 'Guide for Structural Design of Pavements Constructed with Clay or Concrete Block Pavers', which should be consulted for full details, This page describes the basic procedure used by engineers to specify a flexible pavement constructed from either clay bricks or concrete blocks.	Cross-section showing all layers of a block pavement. Depending on usage, not all layers need be constructed
Before any pavement construction can be specified, it is necessary to quantify the value of 2 variables.... The CBR (California Bearing Ratio) of the sub-grade The expected traffic volumes and types over the design life of the pavement (usually 20 years) Sub-grade CBR See also sub-grades page
The CBR is usually quantified through onsite testing by qualified geo-technical engineers, but BS6677 gives a table of expected CBR's for typical British soils, which is reproduced opposite.It should be noted that when considering a project of some size, such as a car park or public highway, a thorough site investigation is essential to map any variations in CBR over the site. As CBR is partly dependent on soil type and moisture content, testing should be done in inclement conditions that will give a more accurate indication of the lowest expected CBR. The pavement should be designed to cope with the lowest measured CBR.	Typical CBR values for British Soils

If a site has a variable CBR, it may be possible to excavate 'soft spots' and replace with a suitable compacted fill material to bring up the CBR to an acceptable value, but this is a decision that can only be taken once all relevant data are collected and analysed, and often comes down to a judgement call made by the Resident Engineer based upon experience and local knowledge.Any sub-grade with a CBR of 5% or less will require a capping layer, sometimes known as a sub-grade improvement layer. This is constructed from approved granular or cement-bound material, laid and compacted in layers not greater than 200mm thick. A sub-grade with a CBR of 2% or less should have a 600mm thick capping layer; a sub-grade with a CBR of 2-5% should have a capping layer 350mm thick. To summarise this step in the design process........

Determining requirements of capping layer

Traffic volumes and types - Determining Sub-base This next stage of the design process considers the usage of the pavement, and first specifies a thickness for the sub-base before determining whether a road base will be required.To determine sub-base thickness, it is necessary to know if the pavement is to be used as an access road serving dwellings and/or commercial/industrial properties. If it is not to be used as an access road, then a standard sub-base 150mm thick is specified, although this should be increased to 225mm if no capping layer is present. On projects where the pavement is intended to be an access road, the thickness of the sub-base is to be 225mm, unless there is a capping layer present. The sub-base thicknes may be reduced if the number of premises served by the road is less than 20, or the number of standard axles expected to use the road each day is less than 200. A standard axle can be thought of as a single commercial vehicle with a weight of 8,200Kg making one pass over the pavement. This stage of the design process is summarised in the following table........

Determining requirements of sub-base layer

Road Base The next step is to determine whether a road base is required. This is dependent upon the cumulative number of standard axles anticipated over the design life of the pavement, and/or the existence of certain special conditions outlined below. A road base is defined as a layer or layers of bound material intended to give structural intergrity to a pavement; in this case, a road base, if required, will be constructed from Cement Bound Material (CBM) or dense base course bituminuous macadam (DBM).The cumulative number of standard axles is calculated to allow for anticipated traffic growth over the design life of the pavement and is measured in million standard axles (msa), usually rounded to one decimal place. Special conditions. If any of the following special conditions apply to the pavement, the traffic volume figure (in msa) calculated for the design life must be amended accordingly..... Channelised traffic If channelisation is predicted, the traffic volume figure should be multiplied by a factor of 3 to allow for concentrated loads imposed upon areas of channelisation. Most normal roads will not experience channelisation, but it can occur on any road given the right circumstances, such as at traffic controls or on steep gradients. Speed If speeds greater than 50Km/hr (30mph) are anticipated, the traffic volume figure should be multiplied by a factor of two to allow for the increased dynamic loads imposed on such a road. Frost heave A pavement constructed over a sub-grade susceptible to frost heave should have an overall thickness measuring not less than 450mm of non-susceptible material, ie, capping layer (if present) + sub-base + road base = 450mm min Moisture content Sub-base or road base materials that rely on a critical moisture content to ensure a stable compaction are only to be used under strict engineering supervision. Turning If a pavement is anticipated to be subject to frequent turning manouevres, especially by HGVs, the use of a road base should be considered. If none of the special conditions listed above apply and the traffic volume is below 0.5msa over design life, then the road base may be omitted. In all other cases, a road base must be constructed as indicated in the table below which gives the required thickness of two types of road base for a range of traffic volumes. The table also states the thickness of the bedding layer and thickness of the pavers to be used.

Road base, bedding and paver thicknesses

Bedding and Pavers As can be seen from the table above, the bedding layer thickness and paver thickness is specified according to usage and traffic volumes. Although the table shows only 60mm and 80mm blocks, 65mm clay pavers can be substituted for 60mm concrete blocks. Where there is a choice of block thickness, it's normal practice to select the thicker block unless there are good reasons to use 60mm or 65mm thick units.It should also be noted that in the case of exceptional loads, such as at ports, airports and heavy industrial applications, a 100mm thick block is also available and the construction layers ought to be individually designed to cope with the heavier-than-normal loads rather than relying on the procedure outlined above. In certain situations when laying flexible block paving over an impermeable or bound road base, special measures may need to be taken to ensure adequate drainage of the bedding layer. This is discussed more fully on the Blocks on Base Course page. And that's it - pavement designed! Now, it just needs to be constructed, following the principles and methods outlined on the Flexible Block Paving page.