Quality Control During Mixing/Placement
Drum Rotations of Transit Mixer/ Agitator
Another test method is to count the actual rotation of the ready mix truck drum rotations. Once the mix and water are placed in the drum the load should be delivered and discharged before the drum has revolved 300 times.
Over-mixing concrete damages the quality of the concrete. Over-mixing may:
• Grind the aggregate into smaller pieces,
• Increase the temperature of the mix,
• Separate the aggregates from the cement paste
• Lower the slump,
• Decrease air entrainment, and
• Decrease the strength of the concrete.
Also, over-mixing puts needless wear on the drum and blades of the transit mixer
Time
An important quality control test for concrete is to identify the length of time from when water is added to the mixture until the mixture is poured. The batch ticket has the elapsed time recorded between the mixture and the placement.
Concrete begins to stiffen as soon as the cement and water are mixed. However, the degree of stiffening that occurs in the first 30 minutes is not usually a problem. Concrete that is kept agitated generally can be placed within 1 1/2 hours after mixing.
Fresh concrete left to agitate in the mixer drum may be used; if upon remixing, it becomes sufficiently plastic to be compacted in the forms. A small amount of water may be added to remix the concrete provided the following conditions are met:
• Maximum allowable water-cement ratio is not exceeded,
• Maximum allowable slump is not exceeded,
• Maximum allowable mixing and agitating time (or drum revolutions) is not exceeded, and
• Concrete is remixed for at least half the minimum required mixing time or number of revolutions.
Concrete, mixed in a transit mixer, should be delivered and discharged within 1 1/2 hours. Excessive elapsed time may lead to substandard concrete and may be reason to reject the mix for placement.
Quality control during mixing ensures the specified components are mixed properly to yield the desired concrete mixture. The following is a list of various tests that check certain items for quality control during mixing and placement of concrete:
• time
• ready mix truck drum rotations
• admixture content
• slump tests
Drum rotation is used for two purposes: mixing and agitation. High speed rotation (on the order of 12 - 15 rpm) is used to mix PCC ingredients into a homogenous material. This type of mixing typically takes between 50 and 100 revolutions depending upon PCC characteristics and environmental factors. After this period of mixing, the PCC is usually required to meet at least 5 of the 6 homogeneity specifications. Samples for these specifications should be taken from widely separated portions but should also come from the middle 15 - 85 percent of the load so as not to be influenced by beginning and end of load abnormalities.
Low speed rotation (about 2 rpm) is used to agitate the PCC to
(1) maintain its homogeneity and
(2) prevent slump loss while in transit. Truck mixers are equipped with a revolution counter to help maintain tight control over the total number of drum revolutions. Mixing, which is generally short in duration, is usually planned for a specific time or place. If not mixing, truck mixers usually operate in the low speed agitation mode. Mixing is typically done using one of the following three methods (NRMCA, 2002):
• Mixing at the batching facility. The drum is turned at high speed (12 - 15 rpm) for about 50 revolutions while at the production facility, which allows for a quick check of batch characteristics. The PCC is then agitated (< 2 rpm) while in transit to the paving site.
• Mixing in transit. The drum is turned at medium speed (about 8 rpm) for 70 revolutions while driving to the job site. The PCC is agitated (< 2 rpm) until discharge.
• Mixing at the paving site. The PCC is agitated (< 2 rpm) while in transit to the paving site. Upon arrival, the PCC is mixed (12 - 15 rpm) for 70 to 100 revolutions, or about five minutes.
In general, short times between mixing and placement can better avoid the problems of premature hardening and slump loss that result from potential delays in transit. Regardless of the mixing mode, PCC is a perishable construction material. First, if it begins to set before being placed and consolidated it is of little use. Second, if it is mixed and agitated excessively it can loose its air entrainment or the effects of certain admixtures can diminish.
Another test method is to count the actual rotation of the ready mix truck drum rotations. Once the mix and water are placed in the drum the load should be delivered and discharged before the drum has revolved 300 times.
Over-mixing concrete damages the quality of the concrete. Over-mixing may:
• Grind the aggregate into smaller pieces,
• Increase the temperature of the mix,
• Separate the aggregates from the cement paste
• Lower the slump,
• Decrease air entrainment, and
• Decrease the strength of the concrete.
Also, over-mixing puts needless wear on the drum and blades of the transit mixer
Time
An important quality control test for concrete is to identify the length of time from when water is added to the mixture until the mixture is poured. The batch ticket has the elapsed time recorded between the mixture and the placement.
Concrete begins to stiffen as soon as the cement and water are mixed. However, the degree of stiffening that occurs in the first 30 minutes is not usually a problem. Concrete that is kept agitated generally can be placed within 1 1/2 hours after mixing.
Fresh concrete left to agitate in the mixer drum may be used; if upon remixing, it becomes sufficiently plastic to be compacted in the forms. A small amount of water may be added to remix the concrete provided the following conditions are met:
• Maximum allowable water-cement ratio is not exceeded,
• Maximum allowable slump is not exceeded,
• Maximum allowable mixing and agitating time (or drum revolutions) is not exceeded, and
• Concrete is remixed for at least half the minimum required mixing time or number of revolutions.
Concrete, mixed in a transit mixer, should be delivered and discharged within 1 1/2 hours. Excessive elapsed time may lead to substandard concrete and may be reason to reject the mix for placement.
Quality control during mixing ensures the specified components are mixed properly to yield the desired concrete mixture. The following is a list of various tests that check certain items for quality control during mixing and placement of concrete:
• time
• ready mix truck drum rotations
• admixture content
• slump tests
Drum rotation is used for two purposes: mixing and agitation. High speed rotation (on the order of 12 - 15 rpm) is used to mix PCC ingredients into a homogenous material. This type of mixing typically takes between 50 and 100 revolutions depending upon PCC characteristics and environmental factors. After this period of mixing, the PCC is usually required to meet at least 5 of the 6 homogeneity specifications. Samples for these specifications should be taken from widely separated portions but should also come from the middle 15 - 85 percent of the load so as not to be influenced by beginning and end of load abnormalities.
Low speed rotation (about 2 rpm) is used to agitate the PCC to
(1) maintain its homogeneity and
(2) prevent slump loss while in transit. Truck mixers are equipped with a revolution counter to help maintain tight control over the total number of drum revolutions. Mixing, which is generally short in duration, is usually planned for a specific time or place. If not mixing, truck mixers usually operate in the low speed agitation mode. Mixing is typically done using one of the following three methods (NRMCA, 2002):
• Mixing at the batching facility. The drum is turned at high speed (12 - 15 rpm) for about 50 revolutions while at the production facility, which allows for a quick check of batch characteristics. The PCC is then agitated (< 2 rpm) while in transit to the paving site.
• Mixing in transit. The drum is turned at medium speed (about 8 rpm) for 70 revolutions while driving to the job site. The PCC is agitated (< 2 rpm) until discharge.
• Mixing at the paving site. The PCC is agitated (< 2 rpm) while in transit to the paving site. Upon arrival, the PCC is mixed (12 - 15 rpm) for 70 to 100 revolutions, or about five minutes.
In general, short times between mixing and placement can better avoid the problems of premature hardening and slump loss that result from potential delays in transit. Regardless of the mixing mode, PCC is a perishable construction material. First, if it begins to set before being placed and consolidated it is of little use. Second, if it is mixed and agitated excessively it can loose its air entrainment or the effects of certain admixtures can diminish.
Comments
Post a Comment