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Beating The Heat

Because of technology advancement through research, construction of masonry structures now occurs year round. In recent years, the majority of research has been on cold weather construction — the assumption being that contractors already know warm and hot weather techniques. But what do you really know about hot weather

Because of technology advancement through research, construction of masonry structures now occurs year round. In recent years, the majority of research has been on cold weather construction — the assumption being that contractors already know warm and hot weather techniques. But what do you really know about hot weather masonry construction?

According to the ACI 530.1-02/ASCE 6-02/TMS 602-02, the Specification for Masonry Structures, section 1.8D, when the ambient air temperature exceeds 100 degrees F, or exceeds 90 degrees F with a wind velocity greater than 8mph, special construction techniques are required to ensure that the masonry units have the proper bond and materials strengths for which they were designed. If you are not familiar with some of the techniques involved, read on.

Temperature Range
(Ambient)*
Preparation Requirements
(Prior to Commencing Masonry Work)
Construction Requirements
(While Masonry Work is in Progress)
Above 115ºF (46.1ºC)

or

Above 105ºF (40.6ºC) with a wind velocity greater than 8 mph (12.9 km/hr)
Shade materials and mixing equipment from direct sunlight.

Maintain sand piles in a damp loose condition

Provide necessary conditions and equipment to produce mortar and grout having a temperature below 120ºF (48.9ºC).
Use cool mixing water for mortar and grout

Maintain temperature of mortar and grout below 120ºF (48.9ºC)

Flush mixer, mortar and grout transport container and mortar boards with cool water before they come into contact with mortar ingredients or mortar

Maintain mortar consistency by retempering with cool water

Use mortar within 2 hours of initial mixing
Above 100ºF (37.8ºC)

or

Above 90º F (32.2º C) with a wind velocity greater than 8 mph (12.9 km/hr)
Maintain sand piles in a damp, loose condition.

Provide necessary conditions and equipment to produce mortar and grout having a temperature below 120º F (48.9ºC)
Maintain temperature of mortar and grout below 120º F (48.9º C)

Flush mixer, mortar and grout transport container and mortar boards with cool water before they come into contact with mortar ingredients or mortar

Maintain mortar consistency by retempering with cool water

Use mortar within 2 hours of initial mixing

Temperature Range
(Ambient)*
Hot Weather Protection Requirements
(Newly Completed Masonry Construction)
Above 100 F (37.8 C)

or

Above 90 F (32.2 C) with a wind velocity greater than 8 mph (12.9 km/hr)
Fog spray all newly constructed masonry until damp, at least three times a day until the masonry is three days old

Mortar and Grout
The most important materials that need special attention are the mortar and grout. Mortar temperatures and properties, masonry unit temperatures and properties, wind velocity and relative humidity can all have adverse effects on the performance of mortar in masonry.

As the temperature of the mortar increases, many physical properties change. For example, workability is reduced and more water must be added to maintain a proper workability. As the temperature increases, both initial and final set occur earlier. Also, depending on the surface characteristics, temperature and moisture content of the masonry units, moisture kiss from the mortar due to suction takes place much more rapidly.

Rapid water loss due to evaporation and suction reduces the amount of water available for hydration of the cement. Since hydration of cement is necessary for normal strength development of mortar, a reduction of strength development may occur under rapid drying conditions. Evaporation removes moisture more rapidly from the surface of mortar joints resulting in weaker mortar on the surface.

Covering the walls immediately after construction will effectively slow the rate of water loss from the masonry while the application of a fog spray during the first 72 hours can reduce the effects of hot, dry and windy weather. Strength development in masonry subjected to early dry-out often can be reactivated by spraying the masonry with water.

Grout also sets quicker in higher ambient temperatures. However, the effects are primarily influenced by the temperature of the wall into which the grout is placed. Though this influence disturbs placement procedures, it has no adverse effect on final strength.

Clay and Concrete Masonry Units
When considering the influence of masonry units on construction during hot weather, one should recognize that the absorption of the units might vary depending on their exposure at the site. Units heated and dried by the sun will absorb more water from the mortar and grout than units kept shaded.

High absorption units can contribute to rapid dry-out of mortars in hot weather. Wetting of high absorption fired clay units prior to use will reduce this tendency to dry out the mortar. Concrete masonry units should not be wetted before use, but concrete masonry can be covered with wet burlap or water sprayed after walls are constructed to assure adequate curing moisture. Though not recommended, a light mist can also be applied to the units prior to installation.

Planning and Construction
Planning, preparation and procedures for hot weather masonry construction should focus on assuring that mortar remains workable from the time of mixing until placement of units and that sufficient moisture remains in the masonry units to provide for normal strength development of the mortar. Available methods of achieving these goals include scheduling construction to avoid hot, mid-day periods, optimizing selection and preparation of masonry materials, protecting constructed masonry from sun and wind, damp curing masonry by covering with wet burlap, or fog spraying with water.

Materials
The materials used in hot weather masonry construction are no different than those for any other weather. All materials should conform to the appropriate ASTM material standards. Commonly in hot weather, retarders are used to delay the set time of mortar. They do not, however, reduce evaporation rates. Though this is common, it is not recommended. Any admixtures or retarders should only be used when specified and only when its compatibility with other ingredients has been confirmed by laboratory tests.

Cool water should be used to mix mortar and grout. Ideally, the water should be stored in the shade in a light-colored, open barrel to maximize cooling from surface evaporation. Water from long hoses exposed to the sun should not be used. When exposed to sunlight, long hoses act as water heaters. When practical, ice may be added to the mix water. Complete melting of the ice must take place before the water comes into contact with the other mortar or grout ingredients.

Equipment used to mix, transport and store mortar and grout needs to be flushed thoroughly with cool water immediately before use. Mortar can absorb heat from metal mixers, wheelbarrows or mortar pans, and lose water to wooden mortarboards. Protection

The use of windscreens has been shown to be effective in protecting against the drying effects of wind during extremely hot weather. Materials commonly used for windscreen protection are canvas and synthetic coverings (reinforced polyethylene and vinyl). Under severe drying conditions, research has shown that the daily application of a fog spray to the surface of the walls for a period of about three days, covering of the walls with polyethylene plastic, or both, dramatically improves flexural bond strength over walls not similarly protected. The tops of all walls not otherwise protected should be covered with a weather resistive membrane extending a minimum of two feet (0.6m) down on both sides to prevent water from entering the masonry.

Workers responsible for installing protection should have full knowledge of the type of windscreen being used, and how to properly erect, adjust, brace, anchor and dismantle the complete assemblage. Windscreens must be properly erected, braced and anchored so that the assemblage is safe from wind loads and uplift. Protection should include measures to safeguard workers from injuries caused by high winds.

Recommendations
Hot weather masonry construction and its attendant quality control require some additional attention to construction practices and protection. Attention should be directed to the following details as well as those normally watched.

1. Implement the hot weather construction and protection requirements of the appropriate practice when daytime temperatures are forecast to exceed 100F (37.8 C) or 90 F (32.2 C) with a wind velocity greater than 8 mph (12.9km/h). Check local weather reports before the start of each day and periodically measure air temperature and wind speed during the day.

2. Receive, store and protect construction materials in ways that prevent water from entering the materials. Cover or shade mortar materials; protect water, sand and cement from exposure to direct sunlight. When shelters are constructed to shade materials, support covers so they do not come into direct contact with the materials being protected and sufficient air circulation is allowed to take place.

3. Do not exceed a maximum mortar or grout temperature of 120 F (48.9 C). Check mortar temperature after mixing and before use. Use cold water to mix the mortar or grout. Ice may be added to the water, but complete melting must take place before mixing with other materials. Sprinkling can be used to cool sand piles.

4. Fog spray all newly constructed masonry until damp, at least three times a day until the masonry is three days old. Cover walls with polyethylene plastic sheeting to prevent moisture loss from the masonry to the atmosphere.

5. At the end of the day, protect the top surfaces of all masonry to prevent moisture, such as rain, from entering the masonry. Cover the top surfaces such that the protection extends a minimum of 2 feet (0.6m) down all sides of the masonry.

Masonry construction in hot weather requires careful planning to minimize the affects of the dryness, temperature and wind. Taking simple precautions and monitoring both the weather and the materials being used will assure the strength and integrity of the masonry project is maintained.







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