HotWeather Concreting

1
Chapter 13

• Hot-Weather Concreting

2
Detrimental Hot-Weather Conditions

• Accelerate the rate of moisture loss and rate of
  cement hydration that occur at high temperatures.
• High ambient temperature (85 -90F)
• High concrete temperature
• FDOT 95F
• Low relative humidity

3
Hot-Weather and Fresh Concrete

• Increase water demand
• Accelerate slump loss (leading to adding water at
  the job site)
• Increase rate of setting resulting in placing and
  finishing problems
• Increase tendency for plastic shrinkage cracking
• Critical need for early curing
• Control entrained air content
• Long-term strength loss
• Increase potential for thermal cracking

4
Hot-Weather and Hardened Concrete

• Decrease strength from higher water-to-cement
  ratio
• Decreased durability due to cracking
• Increased permeability
• Non-uniform surface appearance
• Increased tendency for drying shrinkage cracking
• Reduced abrasion resistance from adding water too
  soon

5
Precautions to Avoid Problems

• Use materials and mix proportions that have a
  history in hot-weather conditions
• Cool the concrete or its ingredients
• Use concrete consistency that allows rapid
  placement and consolidation
• Reduce the time of transport, placing and
  finishing

6
Cont

• Schedule concrete placements to limit exposure to
  hot-weather conditions?
• Consider methods to limit moisture loss during
  placing and finishing?
• Apply temporary moisture-retaining films after
  screeding
• Organize a preconstruction conference to discuss
  the precautions required

7
Examples

• Temp of fresh concrete increases from 50 to 100F
• 33 pcy of water required to maintain a 3-inch
  slump
• results in a 12 to 15 reduction in compressive
  strength
• durability?????????

8
Examples

• Setting time can be reduced by 2 or more hours
  with an 18F increase in fresh concrete
  temperature
• Placing???
• Finishing??
• Cold Joints???
• Admixture - Which one?

9
Examples

• Rapid evaporation of water before sufficient
  hardening and after hardening
• plastic shrinkage cracking
• drying shrinkage cracking
• humidity
• Thermal cracking as concrete cools

10
Examples

• Air-entrainment in fresh concrete
• more AEA is required to produce the same amount
  of entrained air in hot weather

11
Examples

• Effect of high temperatures of fresh concrete on
  compressive strength.
• Figure 13-4
• higher temperatures lead to higher short term
  strengths but lower long term strengths

12
Cooling Concrete Materials

• Lower the temperature of concrete materials
  before mixing
• Water
• Easiest to cool
• Cooling water 4F lowers concrete temperature by
  1F
• Usually concrete temp lowered only by about 8F by
  cooling water.
• Using ice lowers the concrete temperature by 20F
• Vol. of ice should not replace more than 75 of
  mix water.
• Aggregates
• lower the temperature of the fresh concrete 1F
• coarse aggregate temperature needs to be reduced
  by only 2F
• Liquid nitrogen injected into the mixer reduce
  the concrete temperature by 40F or more.

13
Hot Weather Concrete Checklist

• Page 236 and 237 of your workbook
• 8 Recommended Items