Structural Lightweight Concrete is a versatile building material typically with unit weights from 90 to 125 pounds per cubic foot and compressive strengths from 2,500 psi to more than 8,000 psi. Since it is generally 20% to 40% lighter than normalweight concrete, a structure’s dead load can be reduced, its foundation costs lowered, and it’s concrete and rebar lessened. Structural lightweight concrete also resists fire better than normalweight concrete because of its lower thermal conductivity and its lower coefficient of thermal expansion.
The secret to high quality lightweight structural concrete lies in the aggregate used to produce the mix. Norlite is a ceramic material produced by expanding and vitrifying select shales, in a rotary kiln. The process produces a high quality ceramic aggregate that is structurally strong, durable, environmentally inert, light in weight, and highly insulative. It is a natural, non-toxic, absorptive aggregate that is dimensionally stable and will not degrade over time. The use of Norlite and other quality components in a mix will provide a concrete that offers all of the advantages derived from being light in weight while not sacrificing performance of the concrete.
It is recommended that our Norlite customers follow a few handling and-stockpiling procedures in order to maintain the proper gradations. Cone stockpiling should always be avoided to prevent segregation. Stockpiles should be arranged so that stocking and removal operations may be conducted without having machinery rolling onto or over the inventoried aggregate. The method of removing material from a stockpile is also of importance. Handling equipment should be worked straight into the stockpile as far as practically possible. The adjacent areas may then be worked in the same manner. Stockpiles should never be worked around the edges one load at a time.
Norlite involves no great difficulties in the batching and mixing operations. This lightweight aggregate has a higher moisture absorption capability than sand and gravel stone. Because the moisture content varies more sharply with atmospheric conditions than the natural aggregates, the moisture content of the Norlite should be known at the time of mixing. This will allow the proper calculation for free water and yield. Experience has shown that Norlite in a dry condition will absorb 5% to 8% moisture within the first two or three minutes of mixing the water and lightweight aggregate as recommended below. After this initial water demand has been met, there is no appreciable absorption. For this reason saturation of the lightweight aggregate to its maximum absorption is not necessary (with the exception of pumping). The following mixing procedures are recommended:
1. Charge the mixer with approximately 2/3 of the total mixing water
and all of the lightweight aggregate. Allow high speed mixing for a period
of 2 to 3 minutes.
There is little or no difference in the techniques required for placing lightweight concrete from those used in properly placing normalweight concrete. The most important consideration in handling and placing concrete is to avoid segregation of the coarse aggregate from the mortar matrix.
The basic principles required for a good lightweight concrete placement are:
A well-proportioned lightweight concrete mixture can generally be placed,
screeded, and floated with less effort than that required for normalweight
concrete. Over vibration
Satisfactory floor surfaces are achieved with properly proportioned quality materials, skilled supervision, and good workmanship. The quality of the finishing will be in direct proportion to the efforts expended to ensure that proper principles are observed throughout the finishing process. Finishing techniques for lightweight concrete floors are described in ACI 302.1R.
Slump is an important factor in achieving a good floor surface with
lightweight concrete and generally should be limited to a maximum of
5 in. (125 mm). A lower slump of about 3 in. (75 mm) imparts sufficient
workability and also maintains cohesiveness and body,
Surface preparation before troweling is best accomplished with magnesium or aluminum screeds and floats, which minimize surface tearing and pullouts.
A satisfactory finish on lightweight concrete floors can be obtained as follows:
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