Civil Engineering Blog: 2015

Friday, March 6, 2015

Guidelines on use of Ready Mixed Concrete

GUIDELINES ON USE OF READY MIXED CONCRETE

1. Temperature of concrete:

Concrete is not recommended to be placed at a temperature above 40 oC
and below 5o C without proper precaution as laid down in IS: 7861 (Pt.I or
pt. II as the case may be ). IS:7861 pt. I deals with hot weather concreting
and Pt.II deals with cold weather concreting.

1.1 Hot weather concrete:

Any operation of concreting done at atmospheric temperature above 40 oC may be put under hot weather concreting. In the absence of special precautions as laid down under IS: 7861 (Pt.I), the effect of hot weather may be as follows:

a) Accelerated setting: A higher temperature of fresh concrete results in a more rapid hydration and leads to reduced work ability accelerated setting. This reduces the handling time of concrete.

b) Reduction in strength: Concrete mixed, placed and cured at higher temperature normally develops higher early strength than concrete produced and cured at normal temperature but at 28 days
or later the strength are generally lower.

c) Increased tendency to crack: Rapid evaporation may cause plastic shrinkage and cracking and subsequent cooling of hardened concrete would introduce tensile stresses.

In order to avoid harmful effect of hot weather concreting IS: 7861 (Pt.1) recommends that temperature of ingredients should be controlled so that the temperature of produced concrete is lower. Mixing water has the greatest effect on lowering of temperature of concrete. The use of chilled water/ flaked ice in mixing produces adequate reduction in concrete temperature.

In order to control the temperature of concrete and to avoid adverse effect of hot weather, it is desirable to limit the maximum temperature of concrete as 35 oC to keep margin for increase in temperature during transit.

1.2 Cold weather concreting:

Any concreting operation done at a temperature below 5 oC is termed as cold weather concreting. IS: 7861 (Pt.II) recommends special precautions to be taken during cold weather concreting.

In the absence of special precautions, the effect of cold weather concreting may be as follows:

a) Delayed setting:- When the temperature is falling to about 5oC or below, the development of strength of concrete is retarded compared with development at normal temperature. Thus, the
time period for removal of form work has to be increased.

b) Freezing of concrete at early stage:- The permanent damage may occur when the concrete in fresh stage is exposed to freeze before certain pre-hardening period. Concrete may suffer irreparable loss in its properties to an extant that compressive strength may get reduced to 50% of what could be expected for normal temperature concrete.

c) Stresses due to temperature differentials:- Large temperature differentials within the concrete member may promote cracking and affect its durability adversely.

In view of above, it is desirable to limit the lowest temperature of
concrete as 5o C

Jointed Plain Concrete Pavement

Jointed plain concrete pavement (JPCP, Figure 1) uses contraction joints to control cracking and does not use any reinforcing steel. Transverse joint spacing is selected such that temperature and moisture stresses do not produce intermediate cracking between joints. This typically results in a spacing no longer than about 6.1 m (20 ft.). Dowel bars are typically used at transverse joints to assist in load transfer. Tie bars are typically used at longitudinal joints.

Properties

Crack Control

Contraction joints, both transverse and longitudinal

Joint Spacing

Typically between 3.7 m (12 ft.) and 6.1 m (20 ft.). Due to the nature of concrete, slabs longer than about 6.1 m (20 ft.) will usually crack in the middle. Depending upon environment and materials slabs shorter than this may also crack in the middle.

Reinforcing Steel

None.

Load Transfer

Aggregate interlock and dowel bars. For low-volume roads aggregate interlock is often adequate. However, high-volume roads generally require dowel bars in each transverse joint to prevent excessive faulting.

Other Info

A majority of U.S. State DOTs build JPCP because of its simplicity and proven performance.

- Source at: http://www.pavementinteractive.org/article/jointed-plain-concrete-pavement

Why dowel bar is provided in road pavement joints?

Dowel bars are short steel bars that provide a mechanical connection between slabs without restricting horizontal joint movement. They increase load transfer efficiency by allowing the leave slab to assume some of the load before the load is actually over it. This reduces joint deflection and stress in the approach and leave slabs.

Dowel bars are typically 32 to 38 mm (1.25 to 1.5 inches) in diameter, 460 mm (18 inches) long and spaced 305 mm (12 inches) apart but it depend on each country's standard code because it can be different from each other. Specific locations and numbers vary by state, however a typical arrangement might look like Figure 1. In order to prevent corrosion, dowel bars are either coated with stainless steel (Figure 2) or epoxy (Figure 3). Dowel bars are usually inserted at mid-slab depth and coated with a bond-breaking substance to prevent bonding to the PCC. Thus, the dowels help transfer load but allow adjacent slabs to expand and contract independent of one another. Figure 3 shows typical dowel bar locations at a transverse construction joint.

Figure 2. Stainless steel-clad dowel bars/ (Epoxy Coating on Ends Only)

Figure 3. Dowel bars in place at a construction joint- the green color is from the epoxy coating.

Source: http://www.pavementinteractive.org/article/dowel-bar/

Why reinforcement in road pavement?

Jointed reinforced concrete pavement (JRCP, see Figure 1) uses contraction joints and reinforcing steel to control cracking. Transverse joint spacing is longer than that for JPCP and typically ranges from about 7.6 m (25 ft.) to 15.2 m (50 ft.). Temperature and moisture stresses are expected to cause cracking between joints, hence reinforcing steel or a steel mesh is used to hold these cracks tightly together. Dowel bars are typically used at transverse joints to assist in load transfer while the reinforcing steel/wire mesh assists in load transfer across cracks.

Properties