Goal of ACI 306 Cold Weather Concreting
The main goal of ACI 306 is to assure concrete serviceability and durability to satisfy the intended service requirements when it is properly produced, placed, and protected.
What Is Cold Weather?
Cold weather is defined as the atmospheric average daily temperature falls below 40°F [-4°C] for more than three successive days.
This situation has significant importance while placing, finishing, curing, and protecting concrete against the effects of cold weather. Since the Atmospheric situation can change rapidly in the winter months, good concrete practices and proper planning are critical.
ACI 306 Objectives/ Intent of Cold Weather Concreting
- Normalize set of concrete
- Limit rapid temperature change
- Prevent damage from freezing at early stages
- Provide protection consistent with the serviceability of the structure
Why Consider Cold Weather?
For Proper cold weather concrete, it is required to understand the effect of cold weather on the properties of concrete.
1. The Effect in Plastic State
Temperature falls below about 25°F [-4°C]. If concrete freezes in the plastic stage, its estimated strength will be reduced by more than 50% and its durability will be adversely affected.
Concrete should be protected from freezing until it attains a mini-mum compressive strength of 500 psi [3.5 MPa], which is about two days after placement for most concrete maintained at 50°F [10°C].
2. Effect on Hydration of Cement
The heat of the hydration process of cement has a significant effect of lower temperature, which results in a slower setting and rate of strength gain.
A good rule of thumb is that a drop in concrete temperature by 20°F [10°C] will approximately double the setting time. The slower the rate of gain of strength by concrete the estimated time of removal firms will be more. It is ultimately lead to delay in project work.
3. Concrete Contact with Water
Concrete should air-entrained if it is in contact with water and exposed to cycles of freezing and thawing, even if only during construction.
Newly poured concrete cured by ponding and should be protected from cycles of freezing and thawing until it has attained a compressive strength of at least 3500 psi [24.0 MPa].
4. Temperature Effect
As the heat of hydration is a chemical reaction of cement with water and it generates heat. Newly poured or cast concrete elements should be adequately insulated to retain this heat and thereby maintain favorable curing temperatures.
A large temperature difference of concrete surface and inner portion of concrete mass should be prevented as cracking may result when this difference exceeds about 35°F [20°C]. The gradual removal of Insulation or protection to avoid thermal shock.
Potential Problems for Freshly-Mixed Concrete in Cold Weather
- Delayed set times
- Frozen sub-grade
- Ice in bottom of forms
- Plastic shrinkage cracking
ACI 306 – Cold Weather Concrete
Setting Time of Concrete at Various Temperatures (Delay)
|Temperature||Approximate Setting Time|
|100 ºF (38 ºC)||2 Hours|
|90 F (38 ºC)||3 Hours|
|80 ºF (38 ºC)||4 Hours|
|70 ºF (38 ºC)||6 Hours|
|60 ºF (38 ºC)||8 Hours|
|50 ºF (38 ºC)||11 Hours|
|40 ºF (38 ºC)||14 Hours|
Read More: Ultrasonic Pulse Velocity Test On Concrete
ACI 306 Cold Weather Concrete
Specification for Cold Weather Admixture Systems (ASTM C-1622 / C-1622M-05 Standard)
- Determine the placement temperature of concrete according to ASTM C 1064/C1064M
- The more massive the concrete section, the less rapidly it loses heat.
In case, when water from concrete evaporates too quickly, the crack may occur.
Plastic shrinkage occurs when the rate of evaporation of surface moisture exceeds the rate at which the rising bleed water can replace it.
How to Avoid Plastic Shrinkage Cracks
- Accelerate the set (different from high-early concrete)
- Use micro-fibers
- Use temporary evaporation control
- Use evaporation retarder
- use poly/plastic sheeting
- Cut joint as soon as feasible-DO NOT WAIT!!
- Use a high quality curing compound, sooner rather than later
- Use curing/insulating covers
- Protect & Cure!!
Principles of Cold Weather Concreting (ACI 306)
Concrete protected from freezing until it attains a compressive strength of at least 500 psi (3.5MPa) will not be damaged by exposure to a single freezing cycle (Powers 1962). Where a specified concrete strength should be attained in a few days or weeks, planning and protection may be required to maintain the concrete temperature
Under certain conditions, CaCl2should not be used to accelerate setting and hardening because of increased chances of corrosion of metals embedded in the concrete or other adverse effects.
Required for Durability
Concrete exposed to freeze/thaw while saturated requires lower w/cm than required for strength
1. w/cm .50 (4,000 psi) moderate to severe freeze/ thaw
2. w/cm .45 (4,500psi) deicing salts
3. w/cm .40 (5,000 psi reinforced concrete subject to brackish water, seawater or deicing chemicals
How to Place Concrete in Cold Weather?
The following process is recommended by aci cold weather concrete pouring.
The recommended Temperature of concrete while placing concrete is shown below.
1) The RMC provider can help in reducing protective measure in cold weather concreting by heating the mixing water and/or the aggregates and furnish concrete in accordance with the guidelines in ASTM C 94.
aci concrete temperature requirements as given in table,
|Section Size, Minimum dimension, Inch (mm)||Concrete Temperature as Placed|
|Less than 12 (300)||55ºF (13ºC)|
|12 – 36 (300 – 900)||50ºF (10ºC)|
|36 – 72 (900 – 1800)||45ºF (7ºC)|
2) Cold weather concrete temperature should not exceed these recommended temperatures by more than 20°F [10°C]. More water is required when concrete has a high temperature, has a higher rate of slump loss, and is more susceptible to cracking.
3) In cold weather, Setting time and strength gaining of concrete is slower which typically delays finishing operations and form removal. The admixture of concrete can help can accelerate the rate of setting and strength gain.
4) ASTM C 494—Types C (accelerating) and E (water-reducing and accelerating) conforming to an accelerating chemical admixture are commonly used in the wintertime. A Calcium Chloride is one of common admixture for accelerating setting, but should not exceed a maximum dosage of 2% by weight of cement.
5) The percentage of fly ash or slag can be limited in cold weather but this may not be possible if the mixture has been specifically designed for durability. A proper prediction and analysis is required for an economically viable solution with the least impact on the ultimate concrete properties.
7) Try to place concrete at the lowest practical slump of concrete as this reduces bleeding and setting time. Restrict finishing while the bleeding process is continuing, wait for the process to be finished.
8) Proper preparation should be done before starting concreting. Make surface clear of snow, ice, and dust and the temperature of surfaces and metallic embedment’s in contact with concrete should be above freezing.
9) Concrete protecting material should be ready both during and after placement, from early age freezing and to retain the heat generated by cement hydration. Protecting materials like tarps and insulated blankets, as well as straw covered with plastic sheets, are commonly used measures.
10) More focus on corner and edges which are more susceptible to heat loss. For safety reasons and to prevent carbonation of newly placed concrete surfaces fossil-fueled heaters in enclosed spaces should be vented.
11) Make sure concrete is not set in the plastic stage as this causes plastic shrinkage cracks. Subsequently, Concrete should be thoroughly cured
12) At freezing temperature water curing is not suitable. Try to utilize membrane-forming curing compounds or impervious paper and plastic sheets for concrete slabs.
Cold Weather Concreting Plan
1. Transportation & Placement
1. Schedule deliveries to minimize truck waiting times
2. Tie temperature measurements to action if temps drop below allowable minimum
3. Means for thawing, heating or insulating subgrade and forms
2. Protection (Different for different temp ranges)
1. Blankets, enclosures (lumber, plastic sheeting, vents, hardware), means of heating (vents, fuel)