Cement Concrete Road (Pavement): Cost, Layers and Construction

What is a cement concrete road?

The cement concrete roads are in the form of monolithic slabs of cement concrete which serve two functions simultaneously, namely,  as the load-carrying base and as the wearing surface. According to the structural behavior,  the pavements can be classified as flexible pavement or rigid pavement.  Bituminous concrete is one of the best flexible pavement layer materials.  Various other types of bituminous roads are considered as flexible pavements. The cement concrete roads,  on the other hand,  are treated rigid pavements because of their rigidity.  

Cement   concrete road is a highly  rigid  surface  and hence,  for  the  success of  such roads, the  following  two  conditions  should  be  satisfied:

(i)      They  should   rest   on   non-rigid   surface   having   uniform bearing capacity.

(ii)      The combined thickness or depth of the concrete pavement and the non-rigid base should be sufficient to distribute the wheel load on a   sufficient area of the sub-base so that the pressure on the unit area remains within the permissible safe bearing capacity of the soil.

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The cement  concrete  roads  are    becoming popular because of the fact  that concrete  of desired  quality  can  be prepared by the moderm  techniques of  cement    concrete construction. 

Advantages of cement concrete roads:

Followings are the advantages of concrete roads:

• It can be designed more accurately for load distribution.
• It does not develop corrugations and hence, it grants a noiseless surface.
• It  has good visibility  at night  due  to  colour  contrast.
• It  has  low  cost  of  maintenance.
•  It  has  low  tractive  resistance  which   diminishes  the  cost of  running   of  vehicles.
•  It   has  more  useful   life  than  road  of  any  other -type  of construction.
• It  is  dustless  and  has  no  internal   attrition. (viii)   It   is   not  unduly  slippery.
•   It  is  possible  to  make  it  resistant  to  s_evere  stresses  and strains  caused  under  unfavourable  soil  and  temperature conditions  by placing  suitable  reinforcements.
• It  is  possible  to  make  use  of  old   concrete  road  as   a base or foundation  for  a new concrete road or for  further bituminous  treatment  at the  top.
•  It  is  practically  unaffected  by  weather   and  temperature.
•  It   permits   the  calculation  of  super-elevation   on  curves with   more  mathematical   accuracy   than   other   types  of roads.
•  It  requires  flat  camber  as  the  surface   is   impermeable.

  DISADVANTAGES OF CEMENT  CONCRETE ROADS

Following  are the  disadvantages   of  a cement  concrete  road:

(i) It   causes  the  surface   to   shine   in   severe   sunlight   and the  white  glare  reflected  from   surface   may  disturb  the concentration  of  driver  causing  serious  accident.

(ii)     It  does  not  permit  easy access to the subsoil  when trenches have  to  be  opened  to  locate  water   mains,  sewers  and electric  cables.

(iii)    It  is  difficult  to  repair  and  needs  expert  supervision.

(iv)    It  is   less   resilient  than   W.B.M.   or  bituminous  binder surfaces.

(v)     It is liable to crack, warp and twist.

(vi)    It is noisy under iron-tired traffic and vibrations are set up by heavy,  rapid-moving, and self-propelled vehicles.

(vii)   It proves to be very expensive in initial cost especially when a  suitable local aggregate  is not  available.

(viii)  It  reacts. more  readily  because of its  rigidity  to  stresses and  strains  caused  by  impact  and  may  therefore  crush under such  conditions .                                                                                                                                                                                             (ix)     It  requires  long  time to  protect,  cure  and  keep the  surface free  from  the  traffic  which may cause delay and difficulties in   business centres.

 COMPARISON BETWEEN BITUMINOUS CONCRETE ROADS  AND  CEMENT CONCRETE ROADS

It will be  interesting  to  summarise  the  two  important  types  of ad  surfaces   in   common  use,  namely,   flexible   pavement   in   the form  of  bituminous   concrete  roads   and   rigid   pavement   in   the form of  cement  concrete  roads.

Table  1   shows  the  comparison  of  these  types  of  roads  with respect  to  specific   items.

Table1. COMPARISON  BETWEEN BITUMINOUS   CONCRETE   ROADS  AND   CEMENT  CONCRETE  ROADS

No.	Item	Bituminous concrete road	Cement Concrete roads
1	Compaction	It   is   achieved   by  rolling with   usual   precautions.	It is   done  by  the  use  of vibrators.
2	Curing	Not   necessary.	Necessary.
3	Formwork	Not   necessary.	Required   for  edges  and near  joints.
4	Initial Cost	Less.	More
5	Joints	Note to be provided	To be invariably provided
6	Maintenance Cost	More	Less
7	Method of placing	The  premix  is  prepared  in   hot  mix  plant  and  it is  to  be  placed  in  hot condition.	The concrete is prepared by  mixing  cement, aggregates  and  water  and it  is  to  be  deposited before  cement  starts  to  set.  No  heating  is   required.
8	Opening  of underground trenches	Can be easily opened  up.	Difficult   to  open.
9	Opening to traffic	After  24   hours.	After  15   to  20   days.
10	Salvage value	Practically nil.	Can  serve   as   a   base  or foundation  for  the  new bituminous   surface.
11	Tractive resistance	More	Less
12	Useful life	Less	More
13	Utility	Less  durable  for heavy traffic, cost  of  repair  an maintenance  more.	More  durable  and  reliable for heavy  traffic.
14	Visibility at night	Bad	Good
15	Wear of surface	more	Less

METHODS  OF  CONSTRUCTION OF CONCRETE ROADS

The cement concrete roads can either be constructed in a single course or two courses.   In single course pavement, or two courses. In single-course pavement, the entire depth of concrete is composed of homogeneous material.  The concrete is laid in two courses or layers of equal or different depths with different concrete compositions in the two-course method.

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Following   are  the  three   methods   of  construction   of  cement concrete roads:

(1)      Alternate  bay  method

(2)      Continuous  bay  method

(3)      Expansion  joint  and  strip   method.

(1)  Alternate  bay  method: 

In this method,  if the road is of a single lane,  it is divided into suitable bays of 6  m  to  8  m  length, and the construction work is carried out in alternate bays as shown in fig.  below.

If the road is of the double lane;  the construction work is carried out in odd bays of one lane and even bays of the other lane as shown in fig.  3.  The construction of the next bays is commenced after the concrete laid in earlier bays dries out,   i.e.,   nearly after one week or so.

Following  are  the  advantages  of  this   method:

(i)       The  bays  which  have  been cured   can serve as additional working  space   for  the     preparation of concrete.

(ii)     The  construction  of joints  can  be easily carried out and its width can be suitably kept as desired.

The  method,   however,   suffers  from   the  following drawbacks:

i)       As the  construction  activity  is  spread  over  the full  width of road,   the  traffic  will   have  to  be completely  diverted.

ii)     During  rainy  season,  the  water  collects on  the surface  of bays  which  are  not  constructed.

(iii     It  requires  large  number of transverse  joints  which  may increase  the   construction  cost  and  reduce  the  smooth riding  quality  of the  surface.

iv)     It  requires   more  time   to  complete  the  work.

(2)   Continuous   bay   method:

 This   is  also  known   as  strip method or full-width method. The entire width of the road is constructed  continuously  from  one  end  to  the  other.  No  provision for  expansion joint   is   made.   However,   a  construction   joint   is provided   where   the   new   concrete   meets   the   previously   laid concrete.

This  method   is   suitable  for  roads having  width   not exceeding 4.5   m  and  it   is   very  simple   in   construction.   However,   it   does not  stand   a  high   temperature  variation   as  no  provision  is   made for  expansion   and   contraction.

It  also  requires  the   provision  of  a  temporary  diversion  road since  the  entire   road  width   is   under  construction.   This   method is   generally   not   favoured.

(3)  Expansion  joint  and  strip  method:

 In this method,  the road is divided into longitudinal strips and transverse bays by means of timber formwork as shown in fig 4. The joints are then suitably filled up with fillers like asphalt and finished so as to provide for the expansion of the concrete slab.

By this  method,   any width  of road  can be  carries  the traffic and  it  gives  better alignment  and  finish.  It  also  carries the traffic  during  construction   and  hence,   no temporary diversion aroad is necessary . There has been considerable impovement   in   the  technology of joints in concrte  roads and hence, most of the modern concrete roads are constructed by this method. 

   CONSTRUCTION     PROCEDURE     FOR      CEMENT CONCRETE  ROADS

Following  procedure  is  adopted  for  the construction  of cement concrete  roads:

• Preparation  of  subgrade  and  sub-base:  The subgrade is the natural soil on which the concrete slab is laid.  It must be cleaned,  shaped, and leveled.  It is properly and uniformly compacted with a roller.  It is brought to the true grade and profile.   It should be seen that the subgrade has  Uniform strength over its entire width and it extends at least  300  mm on either side of the width to be concreted.  If any local weak spots are found,   they should be removed and strengthened by placing new material which is compacted by hand tamping.  Where the subgrade consists of soft clay or at places where subsoil water trouble is anticipated, a suitable under-drainage system should be provided.

On the prepared subgrade, an insulating layer of 75  mm thick sand  is   provided.   It  is  thoroughly   wetted   so  that  it  may  not absorb  the  water  of  concrete.  It  also  allows   easy  creep  of  the concrete  slab  on  the top  of  layer.  Sometimes,  special  waterproof paper  is   used   instead   of  sand   layer   as  an  insulating   material. This  paper prevents  any absorption  of  water by the  subsoil  or by the  subgrade.

       If the subgrade  has  reasonable uniform  bearing capacity,  there rs  no necessity  of providing  sub-base  or foundation  course  to the concrete   road.  Depending  upon  the  type  of   soil    design  load, intensity  of  traffic  and  economic  consideration    the  decision  for providing  the sub-base  is  taken. The sub-base  serves  the following three  purposes:

(I)       It  provides a capillary cut-off  and thus the  damage caused due  to  mu de pumping is  prevented. 

(ii)        It  provides   a  strong   supporting   layer.

(iii)    It  reduces   the thickness of concrete  slab  and  thus  leads to  lower  cost  of  construction. 

The  sub-base  may consist  of W.B.M.,  granular  material,  stabilized soil  or  lean  cement  concrete  of proportion  (1   :  4 :  8).  The W.B.M. is  the most  popular. type  of  sub-base  which  is  generally adopted for  concrete   roads  in  our  country.

The subgrade  or sub-base,  as the  case  may  be,  is  completely checked  at  least  two  days  m  advance   of  concreting.

• Placing of forms:  The  forms  may  be  of  steel  or  timber. The  steel  form work are generally of mild steel  channel   sections  and their  depth is  equal  to  the   designed   pavement thickness.   They  are  usually   in 3m   lengths   except  on   curves   of  radius less  than  45   m  where shorter  sections  are  used.  They  are  fixed   in  position  by  3  spikes at the  back  of each  3  m  length.  When  the  consolidation  is to  be done  by  mechanical  means,  the  steel  forms are invariably  used as the  mechanical  appliance  can   move   on  the  top  of  these  forms along the  length  of road  and  carry out the  compaction of concrete. They  are  also  suitable  at  places  where deeper sections of  forms are required.

The timber forms are  dressed  on  side.  They  have a  minimum base  width  of  100  mm   for  slab  thickness  upto  200   mm  and  a minimum    base   width    of   150    mm   for  slab   thickness   over

200  mm.   The   depth  of  forms  is  equal   to  the  thickness  of  the slab.  These  forms  should   rest   upon   stakes   or  pegs  driven.  into the ground   at the  back  side  of side forms. They  should  be firmly nailed  to  the  stakes  to  resist  the  pressure  of  concrete.  The cross forms  are  also  fixed   at  the  ends  of  bays.  It. is  also  desirable  to provide  the  top  inside  corner   of  the  form  with  an  angle iron  to protect  it  during  construction.  They  should  be coated  with  boiled linseed  oil   before  placing of concrete  commences.

The forms should  be  set to the  exact  grade  and  alignment   at least 30m in advance of the point  where  concrete  is  placed.  It is  essential   that  the  forms  are checked before the concrete is placed because on them depends the alignment  of concrete  slab.

Watering  the  prepared subgrade or sub base:    After the forms  are  fixed,   the  prepared  surface   to receive concrete is made moist. But it should be seen that pools are not formed. If the subgrade is dry, it should be sprinkled with as much quantity of

water  as it can absorb.  It  is  advisable to wet the  surface  at least 12 hours in  advance  of  placing  the  concrete.   When  insulating   layer  of waterproof  paper is  provided, the moistening  of the surface prior to  placing of concrete   is   not  required.

(4)  Mixing and  placing of concrete: The ingredients of concrete are mixed in  proper proportions  in  a dry  state.  The mixing should generally  be done in a concrete mixer.  A measured quantity  of water is  added  to achieve  the  desired  water-cement  ratio  is obtained.

The  mixed  concrete  is  deposited  rapidly  on  the  subgrade  in layer  of  thickness  not  more than  50  mm  to  80  mm  or about  two or  three  times   the  size  of  aggregates.  The  concrete  should  be placed  over  the  entire  width  of  bay  in  successive  batches as  a continuous operation  and  the  topmost  layer  is  laid  about  1 O   mm higher  than  the  actual  profile  for further  tamping.   The top  layer should  also  be  laid  to  the  required  camber   and  gradient.

All  the  operations  from  the  time  when  water is added in the mixer  to  the  surface finishing  should  be  completed within  the setting time  of the  cement.  While the  concrete is  being  placed, it should be vigorously spliced  to  avoid  the formation of air pockets which  give  honeycomb concrete.  It  should also  be  seen  that the concrete   is   not   poured from   a   height,   but   it   is   gently  lowered and  placed  on  the  surface.

When  necessary,  the steel  mesh reinforcement  should  be placed as  indicated   on   the   drawings   before   the   placing   of  concrete commences.   The  reinforcement  should   be  securely  supported  in its correct position  and  care  should  be taken  to  see that  it is not displaced  during  the  concreting  operations.

(5)   Compaction   and  finishing:  After  the  concrete  is  placed in   its  position,   it  should   be  brought  in   its   proper  position  by a heavy  screed  or tamper fitted  with  suitable  handles.  The wooden tamper  is  at  least  75  mm  wide  and  its  underside  is  shaped  to the finished   cross-section   of  the   slab.  

Its   weight   is   about   10   kg/m and  it  should  have sufficient  strength  to  retain  its  shape  under  all the  working conditions.   Its  length  is  equal  to  length  of  bay plus 600   mm.  The  underside  of  tamper  is  provided   by  a metal  plate of  5  mm  thickness.  Fig.   6-4   shows   a  typical  wooden   tamper.

The  tamper  is  placed  on  the  side  forms  and  its  handles  are gripped   by  the  men  who   use  the  tamper   while  standing.   The tamper  is  used  across  the  bay.  It  is  moved   forward  in  a saw-rn motion  in  combination with  a series  of  lifts  and  drops  of  25  mm to  50   mm  with   a  forward  shift  of  25   mm.

At the transverse joints,    the tamper is stopped within   one metre of  the  joint,   lifted  up  and lased on the joint and drawn backward therefrom. The surplus concrete is removed by means of the shovel and thrown ahead of the joints. The surface is immediately inspected after this operation for high and low spots and defects, if any, are rectified.

The compaction can also prefable be done by mechanical vibrator which saves time and also gues a dense concrete.

The entire concrete surface is then floated longitudinally with a wooden float board  20  mm wide,  50  mm thick and  5 m long.  This board is provided with convenient handles at its ends.  While using it,  the two men sit on portable timber bridges that span the slab’s width.  The board’s lower surface is straight and it is placed with its longer side parallel to the longitudinal axis of the road slab.   It is then drawn back and forth in strokes of 600  mm and advanced slowly from one side of the slab to the other. 

The  purpose  of  this  operation  is  to  give  a   uniform  and even  surface  free  from   ransverse corrugations.  When  the  entire width  of  slab   is  floated,   the  two  bridges  are  moved  forward   so that  the  next  section  to  be  floated  overlaps  the  previous  one  by 1 m   to  1.20  m.

The   trueness    and   straightness   of   the   finished   surface   in longitudinal   direction   is   checked   by  a  straight   wooden   edge 75   mm  deep,   37.50  mm   wide   and  3   m  long  after  the  excess water  has  disappeared.  The  straight  edge  is  swung  from handles and irregularities  in  the  surface,  if any,  are rectified. 

Similarly, the trueness   of  surface  in  the   cross  direction   is  checked  and  the defects,  if  any,  are  removed.   The  straight  edging  and  re-floating are  continued  until  the  entire  surface  is  true  to camber and grade.

.              (6)   Belting,  brooming  and  edging:  If  necessary,  the  surface 1s   further   finished   by   a  rubber  or  canvas   belt   150   mm   to 300  mm  wide  and  of  sufficient  length   longer  than  the  width  of the  road.  It   is   fitted  with   handles  at  both  ends.  It  is  moved  in a  saw-in  motion  in  both the  transverse  and longitudinal  directions.

If   it   is   desired   to  have  a  rough   surface,.  the  brooming  is carried  by  a  fibre  broom  brush  after  belting.  The  broom  is came     ouII  d in  a direction  perpendicular  to the centre-lne  of the gentIY  put  in    such  a manner that a uniform  roughness  is  obtained.

The edges of the  slab  are  carefully  finishe.d  with   an  edging tool  before  the  concrete  becomes  hard  and  stiff.  The  expansion joints  are  also  rounded  off at  the  corners.

(7)  Curing:  After   12   hours or so,   the finished surface is covered with wet gunny bags for a  duration of  24  hours. The bags are then removed and  40   mm to  50  mm layer of sand is spread on the surface.  The sand layer is kept moist for 14  days. As an alternative,  the curing can also be carried out by ponding. The surface s divided into a  number of bays by forming small 5  mm high earthen ridges and the bays are filled with  40  mm depth of water and the surface is kept thoroughly wet for 14   days. The surface is cleaned and washed after curing.

(8)  Opening to traffic:   The expansion joints are suitably finished.  The edges of the road are provided with suitable shoulders of macadam, hard moorum, or bricks.  The edging protects the road slab. The road is then opened to traffic after 28 days of consolidation and finishing of concrete or when the concrete attains the required strength.

Equipment for Bituminous road:

You can read here: Equipment used for Bituminous Road construction

EQUIPMENTS FOR CEMENT CONCRETE ROADS

For the construction of cement concrete roads, the plants and equipment will be required for batching, mixing, placing, finishing and curing of the concrete.

Following are the commonly used machinery (Equipment) for cement concrete roads:

(1) Batching plant

(2) Concrete mixer

(3) Vibrating screed

(4) Internal vibrator

(5) Float

(6) Straight edge

(7) Belt

(8) Fibre brush

(9) Miscellaneous

Each of the above machinery used in the construction of cement concrete roads will now be described in brief.

• Batching plant:

The batching plant is a mechanical equipment for measuring, either by weight or by volume, the quantities of different ingredients required to make up each complete charge of a concrete mixer. The types of  batching plant in which the quantities of construction materials are measured by weight is called a weigh batcher.

If batching by volume is to be done, separate measuring boxes are provided for the different aggregates. Each box is provided with handles for convenient lifting and loading the mixer. The excess material after filling each box is removed by a straight edge.

• Concrete mixer:

The concrete mixers of various types and capacities are available in the market. It is either of tilting type or non-tilting type. They are generally provided with power-operated loading hoppers.

The concrete mixer consists of a drum with blades in the inside portion. The arrangement is provided to revolve or rotate the drum. The aggregates are collected in dry condition in hopper and they are then placed in the revolving drum of the mixer.

The aggregates together with water are rotated for a certain period. The mixing of the aggregates is thus carried out and the wet concrete thus discharged by the concrete mixer is consumed in 30 minutes.

Suitable arrangement is generally provided on the concrete mixer to allow only the required quantity of water to enter the mixer per batch. After use, the concrete mixer should be thoroughly washed and cleaned, If this precaution is not taken, cakes of hardened concrete will be formed inside the mixer. These cakes difficult to remove at a later stage and they considerably affect the efficiency of concrete mixer.

• Vibrating screed:

The vibrating screen is used for the compaction and finishing of concrete. It consists of wooden or mild steel screed with suitable handles. It is driven by vibrating units mounted thereon and propelled either electrically or by compressed air or by a petrol engine and made to travel on side forms.

• Internal vibrator:

This is also known as an immersion vibrator and it comprises of a vibrating head with suitable motive power either of compressed air, electricity, or of a petrol-driven engine. It is employed to ensure compaction of concrete along the forms to avoid any tendency in honeycombing at the edges of slab.

For roads exceeding 125 mm in thickness, the internal vibrators are provided at suitable spacings for compacting the concrete over the entire width of the slab in addition to the vibrating screed.

(5) Float: The longitudinal float is made of hard wood and it is provided with a handle. It is of 750 mm length and 75 mm width. It is used for smoothening the concrete.

(6) Straight edge: The straight edge is made of hard wood with mild steel plate at bottom. It is of 3 m length and 100 mm width. It is provided with two handles. It is used to check the finished pavement surface in the longitudinal direction.

(7) Belt: A canvas belt with two wooden handles at the ends is used for finishing the pavement surface before the concrete hardens. It is 250 mm in width and at least 600 mm longer than the width of the slab. For better working, a minimum of two belts should be provided.

(8) Fibre brush: A fibre brush is used to make marks across the pavement surface so that it becomes skid resistant. It is made from good quality hard fibres which project from the wooden brush of 450 mm length and 75 mm width. It is provided with a handle about 2 m long.

(9) Miscellaneous: In addition to the above main tools, various other miscellaneous tools and equipment required to finish up the work would be edging tools, spades, shovels, iron pans, water pots, rods, etc.

TRAVELLING PLANT

For large scale work, the loose concrete can be spread, compacted and finished by a travelling plant running slowly on the top of the steel side forms which are used as side shuttering for the concrete surfacing.

One plant is required for spreading the concrete in the required position and another for consolidating it and finishing its surface. The latter plant consists of a screed in its front, a vibrating beam in the center, and a finishing beam at its rear.

After the loose concrete is spread, the surface of the concrete is struck off by the front screed to the required thickness. The compacting unit consists of a surface vibrator and the vibrations compact the layer of freshly mixed and freshly spread concrete.

The finishing beam smoothens the surface which is compacted by the surface vibrator. If necessary, the hand float may be used for the proper finish after the mechanical finishing is over. The traveling plant permits the use of a stiff concrete mix and the consolidation of concrete is also better than when it is done by manual labor.

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