Cold Weather Concreting

Cold Weather Concreting

Cold weather can have damaging effects on freshly placed concrete. Both setting time and rate of strength gain are slower in cold weather, and if the concrete freezes during the first few days of curing, it will suffer reduced strength and weather resistance, and increased moisture permeability. When it is necessary to work in cold weather, certain precautions must be taken to assure the quality of the finished concrete.

Cold weather is defined as a period when the mean daily temperature drops below 40°F for more than three consecutive days. On commercial projects, heated enclosures are often provided to protect concrete and masonry work during cold weather. Although this is not usually done on residential work because of the expense, the following protective measures can and should be taken.

• For slabs and other flatwork such as driveways, sidewalks, and patios, reduce the amount of mixing water so that the concrete has a slump of 4 in. or less. This will minimize bleeding of mix water to the surface and decrease the time until initial set.
• Use air-entrained cement or an air-entraining admixture even if the concrete will not be exposed to freeze-thaw cycling in service.
• Use either an extra bag of cement per cubic yard of concrete, a high-early-strength cement (Type III), or a non chloride set accelerator to develop strength faster.
• If you are ordering from a ready-mix supplier, specify heated concrete with a minimum temperature.
• Remove ice and snow from inside forms and thaw frozen sub grade before concrete placement.
• If you are mixing concrete on site, store ingredients in a heated area if possible, and use heated water for mixing.
• Reduce the time between mixing and placing as much as possible to reduce heat loss. Work with smaller batches if necessary.
• Delay form removal as long as possible to minimize evaporation and to reduce damage to formed surfaces caused by premature form stripping.
• Wrap protruding reinforcing bars with insulation to prevent heat drain.

Whenever you can schedule concrete pours during milder weather, it is best to do so, but in some climates this is impractical. When cold weather concreting cannot be avoided, quality does not have to be sacrificed if proper precaution is exercised.

Concrete Mix Designs

Concrete Mix Designs

For work requiring more than one cubic yard of material, concrete is usually ordered from a ready mix supplier for delivery to the job site. The supplier will need to know the minimum compressive strength, the maximum aggregate size, and any special requirements such as air entrainment for added freeze-thaw durability. The supplier will then select a mix design that is appropriate for your needs. If you are mix-ing small batches of concrete on site, you will need to understand the basic principles of concrete mix design yourself. The proportion of dry ingredients and the ratio of water to cement are the two most important factors.

Cement and aggregates provide strength, durability, and volume stability in concrete, but too much or too little of one in relation to the other reduces quality.

• Lean or oversanded mixes with low cement content and high aggregate proportions are harsh and have poor workability.
• Fat or undersanded mixes with high cement content and low aggregate proportions are sticky and expensive.

Within the range of normal concrete strengths, compressive strength is inversely related to water content. That is, the more water you use, the lower the concrete strength. But increasing water content increases fluidity and workability. Since water is required for workability, and since workability is required for high quality concrete, the low water requirements for strength and high water requirements for workability must be balanced. The ratio of water to cement is the weight of water divided by the weight of cement. Water cement ratio affects the consistency of a concrete mix. The consistency, in turn, affects how easily the concrete can be poured, moved around in the forms, compacted, and finished. Up to a point, a mix with more water is easier to work with than one that has less water and is therefore stiffer. Too much water, though, will cause the ingredients to separate during the pouring, placing, and handling and will destroy the integrity of the concrete. Too much water also lowers strength, increases the porosity and water permeability of the cured concrete, and makes it more prone to shrinkage cracking. The trick is to use enough water to make the fresh concrete workable, but not so much that it creates weak or porous structures.

Characteristics and Performance of Concrete

Concrete is a fluid mixture of cement, aggregates, and water which can be formed into different shapes and cures to a hard and durable construction material. Masonry is construction of natural building stone or manufactured units such as brick or concrete block.

All building materials expand and contract. Concrete and other cement based products shrink permanently, and clay products expand permanently with changes in moisture content. Both materials (as well as wood, metal, glass, and plastics) expand and contract reversibly with changes in temperature. Since concrete and masonry are brittle, if construction does not accommodate this expansion and contraction, cracking and water penetration can result. Flexible anchorage and the installation of control joints in concrete and concrete masonry and expansion joints in clay masonry allow this natural expansion and contraction to occur without damage to the construction.

Concrete can be used as a structural and a finish material in slabs, walls, paving, and retaining walls. Masonry can be used as a structural system, as a veneer, or as a paving system and can be used to build fire places and retaining walls. Concrete and masonry are strong in compression but require the incorporation of reinforcing steel to resist tensile and bending stresses. Masonry veneers can be constructed over many types of structural frames and backing walls. Concrete and masonry also provide fire resistance, energy efficiency, and durability.

Fire Resistance: Concrete and masonry are noncombustible —they will not burn. This is a higher level of protection than mere fire resistance. Wood can be injected with chemicals to make it resistant to fire damage for a longer period of time than untreated wood, but ultimately wood becomes fuel for the fire. Steel is noncombustible, but it softens and bends when subjected to the high heat of a fire. In commercial construction, steel structural members must be protected from fire by sprayed on mineral coatings, layers of gypsum board, plaster, or masonry. The highest level of protection and the highest fire protection ratings are associated with concrete and masonry.

Durability: Concrete and masonry are durable against wear and abrasion and weather well for many years with little or no maintenance. Wood is highly susceptible to moisture damage and requires protective coatings to prolong service life. Properly designed and constructed concrete and masonry will provide many years of service to the home owner without any additional investment of time or money.

Energy Efficiency: For centuries the thermal performance characteristics of masonry have been effectively used in buildings. Large masonry fireplaces used during the day for heating and cooking were centrally located within a structure. At night, the heat stored in the masonry radiated warmth until dawn. In the desert South west of the United States, thick adobe masonry walls provided thermal stability. Buildings remained cool during the hot summer days, and heat stored in the walls was later radiated outward to the cooler night air. Until recently, however, there was no simple way of calculating this behavior.

Masonry

Masonry

Masonry is defined as the art of construction in which building units, such as clay bricks, sand-lime, bricks, stones, Pre-cast hallow concrete blocks, concrete slabs, glass bricks, combination of some of these building units etc are arranged systematically and bonded together to form a homogeneous mass in such a manner that they can with stand point to other loads and transmit then through the mass without fail or disintegration.

Masonry can be classified into the following categories.

1. Stone masonry

2. Brick masonry

3. Hallow block concrete masonry

4. Reinforced masonry

5. Composite masonry

These can be further sub-divided into varies types depending upon workmanship and type of materials used.

Sampling of Concrete

Sampling of Concrete

Critical decisions, often involving very high potential costs, are made on the basis of concrete test results. Correct sampling is paramount to the validity of these test results but is an aspect of testing that is frequently overlooked and often carried out by untrained people. It is therefore essential that the sampling is done correctly and is representative of the concrete delivered.

After the truck-mixer has re-mixed its delivery on site allow at least the first one-third of a m3 of concrete to be discharged prior to taking any samples. Take at least 4 incremental samples from the remainder of the load avoiding sampling the last cubic meter of concrete. Thoroughly re-mix this composite sample either on a mixing tray or in the sampling bucket and proceed with the required testing.

Describe the recommended sampling methods for ready mixed concrete in British code. Using a standard scoop, this can collect about 5kg of normal weight concrete. Each load of concrete to be tested should be nominally divided into a number of scoopfuls.

The Standard method: To ensure that the concrete is representative of the whole load is standard sample consists of scoopfuls taken from at least four different parts of the load and collected in buckets. The scoopfuls should be taken at equally spaced intervals; the scoop being passed through the whole width and thickness of the stream in a single movement. The first and the last 1/6th

portion of the discharge should be disregarded as unrepresentative. This is then thoroughly re-mixed on a non-absorbent surface before carrying out any individual test. This operation is necessary to even out any variation between individual scoopfuls and to counteract any segregation that may have occurred in transporting the sample from the sampling point to the testing area.

The Alter native Method: An alternative method of sampling concrete for slump testing from

a truck-mixer before the majority of the load has been discharged is permitted. This enables the concrete to be tested before being placed. When this alternate method is used, an initial discharge of at least 0.3 m3 is made before a sample of six scoopfuls is collected from the

moving stream; The sample is then r e-mixed on a non-absorbent surface and split into two equal parts. Each part is then tested or slump, with the average of the two tests recorded as the test result. This method of sampling is only applicable to the slump test. Concrete sampled by this method must not be used to make cubes for compliance testing, as it will produce erroneous results.

Heavyweight Concrete

Heavyweight Concrete

Heavyweight concrete is mainly used for radiation protection. The critical properties of a heavyweight concrete are:

• Homogeneous density and spatial closeness of the concrete
• Free from cracks and honeycombing
• Compressive strength is often only a secondary criterion due to the large size of the structure
• As free from air voids as possible
• Keep shrinkage low

Composition

• Aggregate

–       Use of barytes, iron ore, heavy metal slags, ferrosilicon, steel granules or shot

• Cement

–       Allow for hydration heat development when selecting the cement type and content

• Water content

–       Aim for a low water/cement ratio

• Workability

–       To ensure a fully closed concrete matrix, careful consideration should be given to the placing (compaction).

• Curing

–       Allowance must be made in the curing method for the high heat develop-ment due to the likely large mass of the structure.