7.
7 Poor Curing 167
Table 7.11 Compression and tension lap lengths [29]
Tension splice (Class B) mm (in.)
Bar Compression splice mm Concrete strength (MPa)
size (in.) 20 25 30 35 40 45
10 M 330 (13) 473 (19) 423 (17) 390 (15) 390 (15) 390 (15) 390 (15)
15 M 470 (19) 670 (26) 599 (24) 547 (22) 506 (20) 474 (19) 446 (18)
20 M 570 (23) 816 (32) 730 (29) 666 (26) 617 (24) 577 (23) 544 (21)
25 M 740 (29) 1319 1179 1077 997 (39) 932 (37) 879 (35)
(52) (46) (42)
30 M 870 (34) 1564 1399 1277 1183 1106 1042
(62) (55) (50) (47) (41) (41)
35 M 1050 (41) 1868 1671 1525 1412 1321 1245
(74) (66) (60) (56) (52) (49)
Note: Multiply above values by 1.5 for epoxy-coated bars, 1.3 for top reinforcement, 1.7 for epoxy
top reinforcement
combination, at the surface and the heat generated during setting. Hence, to prevent
this loss of water, curing is required. Concrete must be kept moist till it gains suffi-
cient strength.
Well-cured concrete has better surface hardness and will better withstand surface
wear and abrasion. Concrete also becomes more watertight with better curing pre-
venting moisture and chemicals from entering into the concrete, thereby increasing
durability and service life of the concrete.
The most common methods of curing are as follows:
1. By Continuous Ponding or Frequent Applications of Water to Concrete Surface
This method includes continuous ponding of finished concrete surface in
water. The water curing can also be done by frequent spraying of water or
covering the concrete surface constantly with wetted gunny bags (burlap), sand,
earth, straw, or sawdust.
2. Membrane Curing of the Concrete
Liquid membrane-forming curing compounds are provided to prevent exces-
sive loss of water from the concrete by evaporation. These compounds are usu-
ally made from waxes, natural and synthetic resin, and solvents of high volatility
at atmospheric temperature. White or grey pigments are mostly added to these
compounds so as to provide heat reflection and to make the compound visible.
For application purposes, manufacturer’s instructions must be followed.
3. Covering the Surface with an Impermeable Material
The concept of covering the fresh concrete surface with impermeable materi-
als is the same as that of liquid membrane, i.e. to prevent loss of water by evapo-
ration. These impermeable materials usually consist of polyethylene sheets or
fibre-reinforced paper. After applying water to concrete, the polyethylene sheet
is placed to cover the fresh concrete as soon as possible.
4. Steam Curing
In this method, steam is used as a source of curing to increase the curing tem-
perature of concrete that increases its rate of development of strength. The basic
