Specialty Portland Cement Concretes:
New generations of specialty concretes have improved one or more aspects of performance and allow for greater flexibility in highway design and construction. High-performance concrete typically has compressive strengths of at least 10,000 psi.
Today, ultra-high-performance concretes with formulations that include silica fume, quartz flour, water reducers and steel or organic fibers have even greater durability and compressive strengths up to 30,000 psi. These enable construction with thinner sections and longer spans .
Latex-modified concrete overlays have been used for many years to extend the life of existing, deteriorating concrete bridge decks by the Virginia DOT, which pioneered the use of very early strength latex-modified concretes for this application.
In high-traffic situations, the added costs of the concrete have been more than offset by savings in traffic-control costs and fewer delays for drivers (Sprinkel, 2006). When the air temperature dips below 40oF, costly insulation techniques must be used when pouring concrete for highway projects.
By using commercially available admixtures that depress the freezing point of water, the U.S. Cold-Weather Research and Engineering Laboratory has developed new concrete formulations that retain their strength and durability at temperatures as low as 23oF.
Compared to insulation techniques, this innovation has significantly decreased construction costs and extended the construction season in cold weather regions (Korhonen, 2004).As useful as these and other specialty concretes are nanotechnology and nano engineering techniques, which are still in their infancy have the potential to make even more dramatic improvements in the performance and cost of concrete.
The Superpave Design System:
Due to premature failures of hot-mix asphalt pavements in the early 1980s, a well funded congressionally mandated crash research program was conducted.Its aim is to improve our understanding of asphalt pavements and their performance.
The seven-year Strategic Highway Research Program (SHRP), was managed by the National Research Council. It developed a new system of standard specifications, test methods and engineering practices for the selection of materials and the mix proportions for hot-mix asphalt pavement.
The new system has improved matches between combinations of asphalt binder and crushed stone and the climatic and traffic conditions on specific highways. State departments of transportation (DOTs) spend more than $10 billion annually on these pavements, so even modest improvements in pavement durability and useful life can lead to substantial cost savings for agencies and time savings for motorists (TRB, 2001).
SHRP rolled out the Superpave system in 1993. It took years for individual states and their paving contractors to switch to the new system. It represents a significant departure not only in design but also in the procedures and equipment used for testing.
Each state DOT had to be convinced that the benefits would outweigh the modest additional costs of Superpave mixes, as well as the time and effort to train its staff and acquire necessary equipment.
Bituminous roads:
Bituminous roads are constructed with varying aggregate sizes and compositions.This type of construction is adopted as a wearing course or a surface course. In heavy traffic roads bituminous construction are also adopted for base course.
Compared to cement concrete surfacing, bituminous surfacing takes very less time to use after construction.Black top construction is in large use in developing countries like India where cement is in great demand as a construction material. Further stage construction can be done effectively.
Types of Bituminous Construction:
There are various types and methods of construction of bituminous pavements. The following construction techniques are in use:
1.Surface dressing or painting
2.Interface treatments
3.Premix materials like,
a)Bituminous bound macadam
b)Carpet
c)Bituminous concrete
d)Sheet asphalt or rolled asphalt
e)Mastic asphalt.
Bituminous binders adopted in the above methods are straight run bitumen, road tar, cutback or emulsion. The choice of type of binder is based on the type of construction, availability of material, climatic conditions, equipment, etc.
For example, bitumen and tar require heating to bring them to a proper viscosity for their use. Use of this type is called hot-mix technique. On the other hand cutback and emulsion do not require heating as that of bitumen or tar. Use of this material is called cold-mix technique.Based on the technique of mixing and construction, the bituminous constructions are classified as,
Road-mix methods.
Central-plant mix methods.
Premix materials construction depends on the gradation.They are classified as open-graded and dense-graded.
Bitumen construction method for base course are grouted or penetration macadam (either fully-grouted or semi-grouted) and bitumen bound macadam.Different bituminous constructions employed in surface course of pavements are surface dressing, grouted or penetration macadam and seal coat and bituminous premix surfacing.
Surface Dressing:
1.Functions and Applications:
Main functions of the surface dressing are:
i)To provide a dust free pavement surface over the base course.
ii)To provide full stability to the base course and pavement structure as a whole.
iii)To prevent infiltration of surface water and thereby form as a water proof layer.
In water-bound macadam roads the binding material, soil slurry, is removed easily by fast moving vehicles.They expose the stone pieces and also make them loose.In such situations a surface dressing by bitumen for a thickness of 2 to 3 cm improves the surface performance better and adds to the overall stability of the structure.
2.Materials Requirements:
The bitumen binder may be bitumen or tar or cutback.For a WBM road or renewal of black top surface, the weight required for 10 sq.m of area are given in Table.
Table: Binder requirement
Base course type
Binder requirement in kg per 10 sq. m. area
Single or first coat
Second coat
Bitumen
Tar
Cutback
Bitumen
Tar
Cutback
WBM
17 to 19.5
17 to 22
19 to 22
10 to 12
12 to 15
12 to 15
Renewal of black to surface
10 to 12
12 to 17
10 to 12
10 to 12
12 to 15
12 to 15
3.Construction Procedure:
Surface dressing can be done on an existing road surface as a black top surface or as a new WBM road. In the case of new WBM road surface, the metals are properly rolled after sprinkling water and then the dressing is applied.In case of an old road or existing WBM, the surface is reconditioned to have the design camber and defects rectified.
The surface is cleaned with wire brush such that the aggregates are exposed atleast 1.25 cm. Surface dressing is done in one or two coats depending on the necessity.Over the prepared surface a uniform layer of bituminous binder is applied as given in Table .
In general 2 kg of binder per square meter surface is sufficient. In order to avoid bleeding no excess binder should be used.After spraying, binder c
hippings of 12 mm thick (for first coat) of 12 mm size of 0.14 to 0.15 cm of materials for 10 sq.m area is used. For second coat 9 mm thick of 9 mm size of 0.09 to 0.11 cm of material for 10 sq.m area is applied.
Rolling is started after this from the edges towards the center longitudinally with an overlapping not less than half of the width of rolling wheel. After completion of half of the surface the rolling is started from the other edge. The rolling is continued till all the particles are properly interlocked. This forms the final rolling in case of single coat.
In the case of additional coat, the binder is applied to the prepared surface and chippings spread as per requirement and rolled finally.After ensuring proper cross-sections and 24 hours after rolling the road may be opened to traffic.
Interface Treatment:
1.Functions and Applications:
Depending on the condition of the existing base the following interface treatments are recommended.
1. Prime coat is the first coat of bituminous surfacing done on an existing previous texture base.
2. Tack coat is the bituminous surfacing done over an existing cement concrete top or already existing black top surface road.
3. Seal coat is a final thin coat over certain previous bitumen pavements.
The functions of a seal coat are:
i)To increase the skin resistance of the surface ii) To make the surface water proof and iii) To increase the life of the surface.
Penetration of grouted machadam is the process by which the bitumen is spread over the compacted aggregate surface to allow the bitumen to fill voids and binding the aggregates together. When the bitumen penetrates fully it is called full grout.
Generally the above treatments are given to existing WBM road surfaces. It is essential that adequate bond should exist between the newly laid bituminous layer and the existing surface at the interface.
2.Construction Procedure:
The existing surface is lightly brushed and reconditioned. Priming coat is applied if required.
Additional course aggregates are spread with proper protection of edges. The profile of the cross section is checked with template. A 10 ton roller is used to roll the aggregates when dry.
Rolling is continued till crushing of aggregates are ensured. The profile of the dry-rolled surface is checked for its correctness. Over the dry and compacted surface, the specific binder (bitumen) is uniformly spread either with pressure distributor or hand sprayer.
For a compacted surface of one sq.m with a thickness of 5 cm and 7.5 cm, 50 kg and 68 kg bitumen respectively are required. After this process, chippings or key aggregates are spread and rolled. Key aggregates of 10 to 12 mm size is used. Volume of material needed varies from 0.15 to 0.18 m3 respectively.
Over this surface a premixed sand bitumen or surface dress type seal coat is applied after a few days or immediately.
Premixed Method – Bitumen Bound Macadam:
1.Functions:
Bitumen Bound Macadam are used in the following areas:
i)In hilly regions e they resist frost action better than normal base courses.
ii)In regions of high water table, they act as impervious cut off.
iii)By using them, superior quality aggregates may be replaced by medium quality aggregates.
The binder and aggregates are mixed prior to placement and the finished thickness is kept from 5 to 7.5 cm. Size of aggregate varies with the thickness. For a 7.5 cm thick, the maximum size is limited to 37 mm.
2.Construction Procedure:
The existing surface is reconditioned to proper cross-section and the surface is cleaned. On the prepared and cleaned surface, a thin layer of binder (prime or teak coat) is applied on a 10 sq.m surface and 4.0 to 7.5 kg of binder is to be used for black top surface or 7.5 to 10 kg for untreated WBM surfaces.
Aggregates of proper gradation and binder are separately heated at about 120°C and then mixed in a mixture. The mixture is placed on the already prepared surface and uniformly spread for the required thickness with rakes. The cross-section is again checked.
Rolling is done as early as possible after placing premixed material with a 8 to 10 tonne roller.The rolling is started from the edges and processed towards the center with uniform overlapping. Wheels of the roller should be kept wet while rolling so as to avoid sticking of mixed material on the wheels.
Premixed Method-Bituminous Carpet:
1.Materials:
Bituminous carpet is a premix prepared from 10 to 12 mm size chippings with sand and bitumen binder. Thickness varies from 2 to 2.5 cm. It is used for surface coarse layer. Sand-bitumen carpet mix containing no coarse aggregate is used as a wearing course.
2.Construction Procedure:
The undulations and the pot-holes, if any on the existing road are reconditioned properly with pre-coated aggregates. In case of old bitumen or WBM road, a tack coat is applied on the surface. If the surface is made out of soft aggregate, a prime coat is applied.
Bitumen binder is heated to the specified temperature and tack or prime coat is applied. Chippings and bitumen are heated separately to the specified temperature and then mixed so as to form a homogeneous mix.
The mix is spread over the prepared surface with suitable rakes. The cross-section is once again checked before rolling. At a stretch, the pre-mixed material is laid for a length of 15 meters and rolled.
Wheel surfaces are kept wet with the help of wet gunny bags or sprinkling water to avoid adhesion of mix on the wheels. Rolling is continued till it stopped showing wave in front of the roller.In low rainfall areas, a premixed medium coarse sand is applied on the carpet.
For a 10 sq.m. area, 6.83 kg of binder and 0.06 m3 of sand are required. In high rainfall areas, a liquid seal sprayed with a layer of chippings applied over the carpet. Chippings of 6 mm size of 0.091 m3 per 10 sq.m area are used. Then the chippings are rolled with a light roller.The pavement may be allowed for traffic after 24 hours of the final rolling.
Premixed Method-Bituminous Concrete:
1.Functions and Materials:
Bituminous concrete surfacing has the following functions:
i)It provides a resilient, dust-free water proof, smooth road surface.
ii)It resists heavy and mixed vehicles in a better way.
iii)It has high durability and riding qualities as that of cement concrete pavements.
In this method coarse and fine aggregates along with bitumen binder is used to give a dense mix. Thickness depends on the traffic intensity and on the quality of base course.Aggregates and binders are separately heated and then mixed till all the aggregates are coated with bitumen. Mastic asphalt is also used as a binder.
Bituminous concrete surfacing is done in two layers:
i) Binder course
ii) Wearing course
The coarse to fine aggregate ratio is 2:1. Binder used is 7.5 % of the mix by weight.
2. Construction Procedure:
The existing base course is reconditioned atleast one week before laying the binder course. Then the bitumen course layer will be laid.This is also a hot-mix process.
The hot mix is collected and spread over the prepared surface. The camber and thickness of bitumen layer is checked.The placed concrete is rolled by a roller at a speed not more than 5 km/hour. The number of passes required to attain the final desired thickness depends on the thickness.
The initial rolling is done using a 8 to 12 tonne roller followed by fixed wheel pneumatic roller of
15 to 30 tonnes.The wheels are kept damp with water or wet gunny bags.
Cement concrete Roads:
Portland cement concrete pavements is formed with a relatively rich mixture of Portland cement, sand and coarse aggregate laid as a single course.Cement concrete roads are preferred by every user because of excellent riding surface and pleasing appearance
Construction of cement concrete pavement may be dealt in two parts, viz., construction of concrete pavement slabs and construction of joints.
Necessity of Base:
A cement concrete pavement is constructed with or without a base. If a sufficiently strong subgrade is available the concrete slab can be directly constructed on the subgrade, otherwise a base course has to be provided.
Apart from soil subgrade, other factors which are considered for providing a base is Design load, Traffic volume and Economic considerations.
Various purpose of providing base beneath the cement concrete pavement are:
1.To perform as a strong supporting layer.
2.To prevent the damage due to mud pumping by acting as a capillary cut off.
3.To reduce the thickness requirements of slab.
Materials:
Materials used in concrete for highway construction are coarse aggregate, fine aggregate (sand), cement and water. In order to modify some of its properties certain additives and admixtues are added.
1.Coarse Aggregate:
It consists of natural, crushed stone, gravel or air cooled iron blast furnace slag .Very angular, flaky, elongated or splintery aggregates give a concrete of low workability which should be avoided. To make a quality concrete, crushed aggregates of poor shape should also be avoided. To get a quality concrete, the desirable limits of important properties of coarse aggregate are:
i)Aggregate crushing value= 30 % max
ii)Aggregate impact value= 30 % max
iii)Los Angeles abrasion value= 16 % max
iv)Soundness:
Average loss in weight after 10 cycles = 12 % max in sodium sulphate
18 % max in magnesium sulphate
v)Gradation in two separate size ranges*
a)5 to 2.5 cm~ 50 %
& 2.5 to 4.75 mm~ 50 %
b)40 to 20 mm~ 50 %
& 20 to 4.75 mm~ 50 %
(* Proper proportioning to get the minimum voids)
2.Fine Aggregate:
This may be a natural sand, crushed stone sands or a combination. When volumetric batching is done, bulking of sand due to the presence of moisture should be accounted. Fine aggregates should not have more than 45 % retained between any two consecutive sieves.
Fineness modulus should not be less than 2.3 and not more than 3.1. It should be free from organic impurities.
3.Cement:
Ordinary Portland cement is generally used.The cement should comply with standard specifications prevalent in the country.High-alumina cement may be used in making speedy repairs to roads. Rapid- hardening cement may be used when it is necessary to open traffic with the minimum delay.
In regions of alternate freezing and thawing weather air-entering cement may be used.Irrespective of the type of cement used, the cement should be tested in advance so that 7 and 28 days tests can be completed before the cement is used.
4.Water:
It should be clean and clear. Should be free from salt, oil, acid, injurious alkali and vegetable matter or other substances harmful to the finished concrete.
5.Concrete:
It has to be proportioned to obtain a minimum modulus of rupture of 35 kg/cm2 on field specimens after 28 days curing or to develop a minimum compressive strength of 280 kg/cm2 at 28 days or higher values as per design.
Modes of Construction:
There are two methods of construction of cement concrete slabs:
i)Alternate bay method
ii)Continuous bay method
1.Alternate Bay Method:
This method involves constructing a bay or one slab in alternate succession leaving the next or intermediate bay .The next construction is done after time gap of one week or so.
For example, the alternate bays X,Y and Z (Figure) are constructed at one stretch. Others, viz., X’, Y’ and Z’ are constructed after one week. This technique provides additional working convenience during the laying of slabs. Provision of construction joints are easier.
Modes of construction of cement concrete road
This mode of construction has the following setbacks:
i)More number of transverse joints have to be provided and thereby increasing the cost.
ii)Possibility of collection of surface water on the base or subgrade and thereby disturbing the base or subgrade.
iii)Diversion of traffic is needed as the construction is done on alternate bays covering the entire width.
2.Continuous Bay Method:
In the continuous bay method, X, Y, Z, etc are done at a stretch in sequence. Construction joints are however provided at the end of day’s job.In general the second method is preferred as construction of one half of the pavement can be constructed while the other half is being used by traffic.
Construction Procedure of Pavement Slab:
Sequence of construction of the pavement slab consists of the following steps:
1.Preparation of subgrade and base
2.Placing of forms
3.Installation of joints
4.Batching of aggregates and cement
5.Mixing and placing concrete
6.Consolidation and finishing concrete
7. Cutting of concrete
1.Preparation of Subgrade and Base:
The subgrade or base should be prepared complying with the following conditions:
i)No soft spots are present in the subgrade or base.
ii)Subgrade or base should be uniformly compacted and extended about 30 cm, on either side of the width of pavement to be concreted.
iii)Subgrade or base should be adequately drained.
iv)Plate load test conducted on the subgrade should yield a minimum modulus of subgrade reaction of 5.5 kg/cm3.
Steps to be followed before placing concrete:
i)The subgrade is prepared and checked atleast two days in advance of concreting.
ii)Th subgrade or base is kept in moist condition at the time of concrete so as to avoid the subgrade absorbing water from concrete. If necessary, the subgrade may be saturated well with water for 6 to 20 hours prior to placing of concrete.
iii)Water proof paper may be spread on subgrade if concrete to be placed directly on the subgrade. In this case no saturation of subgrade is necessary.
2.Placing of Forms:
Wooden or steel forms are used. Wooden forms have minimum base width of 10 cm for 20 cm slab thickness and of 15 cm for slabs over 20 cm thickness. Forms are joined neatly and are set with exact grade and alignment. Forms are rigidly fixed such that during the entire operation of concreting they should not deviate more than 3 mm from straight edge of 3 m length.
Steel forms commonly used are straight 3 m sections. They are aligned vertically and horizontally by slip joints and held in position by three or more steel stakes. These stakes are of 2.5 cm diameter, 45 to 75 cm long driven at intervals at back of form. Forms are available in heights of 15 to 30 cm and with corresponding base width over a similar range. These are made of 4 to 6 mm steel plate.
3.Installation of Joints:
Extreme care should be taken in all operations connected with joints. Face of transverse joints should be straight, perpendicula
r to the center line of pavement and also perpendicular to the surface of the finished slab. Load transfer devices like dowel bars used in expansion joint should be aligned and placed accurately. There should be free movement of slab ends in longitudinal direction.
4.Batching of Aggregates and Cement:
Based on the design concrete mix, the proportion of ingredients like coarse aggregate and fine aggregate are proportioned by weight in a weight batching plant. These are placed in the hopper of the mixer along with the necessary quantity of cement.
Cement is measured by the bag which measures 50 kg. All batching of material is done on the basis of one or more whole bags of cement taking the unit weight of cement as 1440 kg/m3. The mixing is done in batch mixer.
Mixer performs well such that there is uniform distribution of materials and mixed material is uniform in color and homogeneous. The batch of cement, fine aggregate and coarse aggregates are added to the mixer. The water for mixing is introduced into the drum within the first 15 seconds of mixing. After all the materials are placed in the mixer, the mixing is to be started within one- and-a half minutes.
5.Mixing and Placing Concrete:
The ingredients are mixed in required quantity for immediate use and is deposited on the subgrade or base. The concrete should be deposited to the correct depth and width of pavement section within the formwork. The operation of placing concrete should be continuous. Care should be taken to see that no seggregation of materials takes place in any stage of the operation.
6.Consolidation and Finishing:
Concrete is spread uniformly by shovels with re-distribution wherever needed. Needle vibrator is used for compaction. Surface of the pavement is compacted either using a power-driven finishing machine or using a vibrating hard screen. Hard compaction may be resorted to where the width of slab is very small and at corners.
After placing and operating concrete, the crown and the cross-section of the pavement has to be done with the required grade. Concrete is further compacted parallel to carriageway center line by using a longitudinal float.
After the longitudinal floating is done and the excess water gets disappeared the final grade and level are tested with the straight edge. Before the concrete gets hardened, the surface is belted with a two-ply canvas belt.
After belting, the pavement is given a broom finish with fiber broom brush. Before the concrete develops initial set, the edges of the slab are finished with edging tool.
7.Curing of Concrete:
It is very important to ensure proper curing of the finished concrete.
Following are the methods usually adopted:
i)Earth cover kept wet
ii)Hay or straw cover kept wet
iii)Cover of wet felt mats or cotton mats
iv)Saw dust kept wet
v)Continuous sprinkling of water
vi)Surface application of calcium chloride or sodium sulphate
v)Cover by water-proof paper or impervious membrane.
Forms are not removed until concrete has set for atleast 12 hours. Special devices are used to pull the forms without damaging the pavement. At the end of curing period the surface grooves in longitudinal and transverse joints are cleaned and filled with sealing compound. After the clearing the road surface, the road is open to traffic.