Perlite concrete aggregate combined with Portland cement, air entraining agent, and water produces an ultra lightweight concrete that is used for insulating roof decks and lightweight floor fills, insulating structural rood decks, curtain wall systems and for a variety of permanent insulating applications. An air-entraining agent is used to improve the workability and to control water content and insulation value. Perlite concrete may be more accurately defined as concrete containing a minimum of 20 cubic feet of perlite concrete aggregate per cubic yard (0.74 m³ perlite aggregate per cubic meter concrete.)

Mixing Perlite Concrete

While perlite insulating concrete is mixed in the same equipment and by similar methods as sand-gravel concrete, there are some considerations that must be given to the order of addition of materials and to the time of mixing to assure correct concrete properties and adequate yield. The following mixing information for transit mixing is presented as a guide to the manufacturer to eliminate some of he questions that may arise in the field and to ensure successful construction with perlite concrete. When mixing perlite concrete on-site, in mechanical mixers, please refer to Publication 32-87, Perlite Insulating Concrete, Information for Contractors---available from The Perlite Institute (www.perlite.org).

Transit Mixing

When transit mixing perlite concrete for the first time, it is suggested that the perlite manufacturer be consulted for suggestions as to the correct mixing time and procedure. These may vary with the type and age of locally available equipment.

The mixing procedure used by many transit operations is as follows:

1. Determine the load of the mixer.
2. Add to the mixer the correct amount of water for the load.
3. Add the correct amount of air entraining agent and cement to the mixer and mix until a slurry is formed.
4. Add the required number of bags of perlite concrete aggregate.
5. Do not rotate the drum during transit.
6. Upon arriving at the job site, mix at top speed for not less than 3 nor more than 5 minutes.
7. Discharge in normal manner after checking wet density for conformance to specification range.
8. Rotate the drum at idle speed when discharging last of load to assure complete discharge of concrete.
9. Do not wash out drum between loads, use the same trucks throughout a job.
10. NOTE: When mixing perlite concrete it is generally suggested that 100-150 revolutions of the transit mixer drum will give good perlite concrete. This varies with the age and effectiveness of equipment.

If possible, the drum should be charged with water, air entraining agent and cement at the batching plant, and the perlite aggregate should be added at the job site, mixing at high speed until the desired density and consistency is reached.

Care must be taken to ensure proper mixing time for the perlite insulating concrete. Using the correct amount of water as specified, the perlite concrete should have a slump of approximately 7 inches (18 cm) when properly mixed and may appear too wet by normal concrete practices. However, if the mix appears too dry or too stiff, the mixing time has probably been too short. Continue mixing until the desired plasticity is reached. Extra water and under mixing may reduce the yield.

The first truck load of perlite concrete will usually discharge about one third of a yard sort since this quantity will coat the inner walls of the truck drum. However, this will not occur in subsequent loads because the walls will be coated sufficiently. After discharging the last load, 7-10 gallons (26-38 liters) of water may be added to the revolving drum to wash out this coating and this mixture may be dumped and blended with concrete from the previous load. This will not impair the perlite concrete properties because the excess water bleeds out without loss of cement.

Wet Density

The wet density of perlite insulating concrete as poured is important in the control of the physical properties of the dry concrete and the yield achieved. The strength and the insulating value depend on the wet density. Heavier weight means greater strength and less insulation. Therefore, the architect or engineer usually specifies the wet density of the perlite concrete with a plus or minus 2 lb/ft³ (32 kg/m³) range.

Air Entraining Agent

The use of the correct amount of air entraining agent of the proper concentration is of extreme importance in successful perlite concrete construction. The air-entraining agent produces countless tiny air bubbles in the concrete which reduces the density, increase the yield and contributes to the insulation value of the dry concrete. Air entrained concrete is also more resistant to water absorption.

Care should be taken in mixing perlite concrete to ensure that he proper amount of agent is used. Excess air entraining reduces the strength of the concrete. An even distribution of agent is also essential to produce the desired concrete characteristics. For further information on air entraining, contact the Perlite Institute(www.perlite.org.), your ready mix company, or your local perlite expander.

Placement of Perlite Concrete

Perlite concrete may be placed through the use of crane and bucket or by pumping. The preferred method is by pumping. A progressive cavity pump is recommended, as this type of pump will not place undo pressure on the perlite. In addition, pump hose should be a minimum of 3 inches in diameter. Hose kinking should be avoided. Hoses should be tied off at each joint. For further information on pumping, refer to the Perlite Institute’s (www.perlite.org) literature on "Pumping Equipment and Air Entraining Agents for Perlite Concrete" .

Yield

Yield is defined as the ratio between the volume of the wet perlite concrete as discharged from the mixer and the volume of perlite concrete aggregate used in the mix. The mix proportions included in the specifications are based on extensive field and laboratory tests and are established for 100% yield. If the correct quantities of material and mixing procedure are used, a 100% yield should result. Yield may vary however as it is effected by job site conditions included mixing time, fineness of aggregate, pumping equipment, height to be pumped, length of hose, etc.

Loss of Yield: