10. Retempering Concrete
Water additions are allowed if:
Maximum w/cm not exceeded
Maximum slump not exceeded
Maximum mixing/agitating time not
exceeded
Concrete remixed a minimum 30
revolutions
Not allowed for partial loads
11. Transporting and Handling
Delays
Prior planning to optimize productivity
Early stiffening and drying out
High temperatures/hot
weather/accelerators
Cold temperatures/cold weather/retarders
Segregation
Excessive shrinkage, honeycombing
21. Choosing Placement Method
Below, at, above ground level
Rapid without segregation or waste
Avoids cold joints
22. At and Below Ground Level
Largest volume concrete placements
Most methods are adequate
Pumps, cranes, conveyors, chutes…
Gravity is helpful
Watch for air loss in pumps
In this presentation we will be discussing the batching, mixing, transporting, and handling concrete. The discussion will begin with ordering concrete and the difference between performance and prescriptive specifications. The balance of the presentation will progress through the processes of producing concrete and delivering it to the jobsite.
The production and delivery of concrete are achieved in different ways. The basic processes and common techniques are explained in this chapter. ASTM C94/C94M, Standard Specification for Ready-Mixed Concrete, provides standard specifications for the manufacture and delivery of freshly mixed concrete. ASTM C94 provides three options for ordering concrete:
Option A is a performance based order. The purchaser designates the compressive strength of the concrete, while the concrete producer selects the mixture proportions needed to obtain the required compressive strength.
Option B is a prescription based order. The purchaser selects mixture proportions, including cement, water, and admixture contents.
Option C is a combined option. The purchaser designates the compressive strength and minimum cement content. The concrete producer selects the mixture proportions to comply with the requirements of the purchaser.
For all options, the purchaser should include requirements for slump and nominal maximum size of coarse aggregate.
Traditionally, the owner/agency develops the design requirements and establishes prescriptive provisions for the proportions of the concrete. These may include minimum cement content, and performance requirements for the concrete, such as the air content and strength. Contractors
are then directed by the specifications to order the concrete mixture from the producer in accordance with the prescriptive provisions provided by the specifications. This procedure allows the owner to control the concrete mixture design and proportions, while any risk associated with the performance of the mixture resides with the owner as well. This method may lead to confusion over responsibility and authority if a problem with the concrete performance arises. With the emergence of value engineering, design-build, performance specifications, and warranties, more owners are requiring the producer to develop the concrete mixture proportions based on performance criteria. This affords more flexibility and encourages innovation, but it also transfers more responsibility onto the concrete producer. Prescriptive specifications are not a guarantee of performance. Significant reductions in project cost may be realized by allowing concrete suppliers to optimize mixtures for performance properties. As trends continue to move away from prescriptive specifications toward performance specifications and warranties, concrete producers will become increasingly responsible for concrete mixture design and proportioning within the scope of their projects.
Batching is the process of measuring quantities of concrete mixture ingredients by either mass or volume and introducing them into the mixer. Most specifications require that batching be done by mass rather than by volume. Water and liquid admixtures can be measured either by volume or mass. Specifications generally require that materials be measured for individual batches within the following percentages of accuracy: cementitious material ±1% of each intermediate weighing in cumulative weigh batchers, aggregates ±2% of each intermediate weighing in cumulative weigh batchers (note that individual scales for aggregates are rare but they require 1%), batched water to ±1% of the total mixing water, and admixtures ±3% the desired quantity. The accuracy of scales and batching equipment should be checked periodically and adjusted when necessary. Scales are checked for accuracy by using a combination of certified test weights and product substitute loading. ASTM C94 requires that scales should be accurate to the larger of ±0.15% of the scale capacity of ±0.4% of the applied load in all quarters of the scale capacity. Water is typically measured through water meters, in volumetric tanks, or in scales that measure the mass. Chemical admixtures are typically charged into the mixture as aqueous solutions. Admixtures that cannot be added in solution can be either batched by mass or volume (generally in bag quantities) as directed by the manufacturer. Admixture dispensers should be checked frequently since errors in dispensing admixtures, particularly overdoses, can lead to problems in both fresh and hardened concrete.
All concrete should be mixed thoroughly until its ingredients are uniformly distributed. Mixers should not be loaded above their rated mixing capacities and should be operated at the mixing speed and for the period, either based on revolutions or time, recommended by the manufacturer. The rated mixing capacity of revolving drum truck mixers is limited to 63% of the gross volume of the mixer. Increased output should be obtained by using a larger mixer or additional mixers, rather than by speeding up or overloading the equipment on hand. If the blades of a mixer become worn or coated with hardened concrete, mixing action will be less efficient. If concrete has been adequately mixed, samples taken from different portions of a batch should have essentially the same strength, density, air content, slump, and coarse-aggregate content, with some allowance for testing variability. Concrete may be mixed at the jobsite in a stationary mixer or a paving mixer. Stationary mixers include both onsite mixers and central mixers in ready mix plants. They are available in sizes up to 9.0m3 (12 yd3) and can be of the revolving drum tilting or nontilting type, reversing drum, or the horizontal shaft revolving blade or paddle type. Many specifications require a minimum mixing time of one minute plus 15 seconds for every cubic meter (yard), unless mixer performance tests demonstrate that shorter periods are acceptable and will provide a uniform concrete mixture. Short mixing times can result in non-homogenous mixtures, poor distribution of air voids, poor strength gain, and early stiffening problems. The mixing period should be measured from the time all cement and aggregates are in the mixer drum, provided all the water is added before one-fourth of the mixing time has elapsed. Coarse aggregate should be charged initially to avoid head packs, or cement balls. Water then should be added uniformly with the solid materials, leaving about 10% to be added after all other materials are in the drum. When heated water is used in cold weather, this order of charging may require some modification to prevent possible rapid stiffening when hot water contacts the cement. In this case, addition of the cementitious materials should be delayed until most of the aggregate and water have intermingled in the drum. If supplementary cementing materials are used, they should be added with the cement. If retarding or water-reducing admixtures are used, they should be added in the same sequence in the charging cycle each time. Addition of the admixture should be completed not later than one minute after addition of water to the cement has been completed or prior to the start of the last three-fourths of the mixing cycle, whichever occurs first. If two or more admixtures are used in the same batch of concrete, they should be added separately to avoid any interaction that might interfere with the efficiency of the admixtures and adversely affect the concrete properties.
Ready mixed concrete is batched and mixed at a concrete plant and delivered to the project in a freshly mixed and unhardened state. This graphic illustrates a central mix ready mix plant.
Ready mixed concrete can be manufactured by any of the following methods: Central-mixed concrete is mixed completely in a stationary mixer and is delivered either in a truck mixer operating at agitating speed (top left), a truck agitator (bottom left), or a nonagitating truck (bottom right). Shrink-mixed concrete is mixed partially in a stationary mixer and completed in a truck mixer. Truck-mixed concrete is mixed completely in a truck mixer (top right). 70 to 100 revolutions of the drum or blades at mixing speed are usually required to produce the uniformly mixed concrete. The homogeneity of concrete is maintained after mixing and during delivery by turning the drum at agitating speed. Agitating speed is usually about 2 rpm to 6 rpm, and mixing speed is generally about 12 rpm to 18 rpm. Mixing at high speeds for one or more hours, along with the addition of water to maintain slump, can result in concrete strength loss, temperature rise, excessive loss of entrained air, and accelerated slump loss. When truck mixers are used, ASTM C94 (AASHTO M 157) also limits the time between batching and complete discharge of the concrete at the job site to 1½ hours. ASTM C94 (AASHTO M 157) also limits the number of drum revolutions to 300 times after introduction of water to the cement and aggregates, or the cement to the aggregates. With the use of specialized concrete mixtures, the limit on time and number of revolutions may be exceeded.
Mobile volumetric mixers batch concrete by volume and continuously mix concrete as the dry ingredients, water, and admixtures are fed into a mixing trough that is typically an auger system. The concrete must conform to ASTM C685 (AASHTO M 241). The concrete mixture is easily adjusted for quantities needed during project placement and varying weather conditions. These mixers are typically used to produce smaller quantities of concrete or to pro duce concrete that are avoided in truck mixers-–such as rapid setting or latex modified concrete.
Fresh concrete that is left to agitate in the mixer drum tends to stiffen before initial set develops. Such concrete may be used if upon remixing it becomes sufficiently plastic to be placed and compacted in the forms. ASTM C94 (AASHTO M 157) allows water to be added to the concrete when the truck arrives on the jobsite and the slump is less than specified providing the following conditions are met: (1) maximum allowable water-cement ratio is not exceeded; (2) maximum allowable slump is not exceeded; (3) maximum allowable mixing and agitating time (or drum revolutions) are not exceeded; and (4) concrete is remixed for a minimum of 30 revolutions at mixing speed or until the uniformity of the concrete is within the limits described in ASTM C94 (AASHTO M 157). Water should not be added to a partial load. Indiscriminate addition of water to make concrete more fluid should not be allowed because this increases the w/cm and lowers the quality of concrete. The later addition of water and remixing to retemper the mixture can result in marked strength reduction. If early setting becomes a persistent problem, a retarder may be used to control early hydration. Mixture adjustments are permitted at the jobsite. These adjustments are for air entrainment and the addition of other admixtures. For example, using a water reducing admixture to increase slump, followed by sufficient mixing.
Good advanced planning can help select the appropriate transporting method for concrete. Delays, early stiffening and drying, and segregation can all seriously affect the quality of the finished work and must be taken into consideration during transporting and handling of concrete. The objective in planning any work schedule is to produce the fastest work using the best labor force and the proper equipment for the work at hand. The greatest productivity will be achieved if the work is planned to optimize the productivity of personnel and equipment. Additionally, the equipment should be selected to reduce the delay time during concrete placement. 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 and compacted within 90 minutes after mixing unless hot concrete temperatures or high cement contents speed up hydration excessively. Admixture technology can considerably extend the delivery time to discharge if the project requires it. Less time is available during conditions that hasten the stiffening process, such as hot and dry weather, use of accelerators, and use of heated concrete. In turn, low concrete temperatures and low ambient temperatures may extend setting time well beyond the 90 minute time limit. Segregation is the tendency for coarse aggregate to separate from the sand-cement mortar. This results in part of the concrete having too little coarse aggregate and the remainder having too much. The former is likely to shrink more, crack, and have poor resistance to abrasion. The latter may be too harsh for full consolidation and finishing and is a frequent cause of honeycombing. The method and equipment used to transport and handle the concrete must not result in segregation of the concrete materials.
The tables on the next few slides summarize the most common methods and equipment for moving concrete to the point where it is needed.
There have been few major changes in the principles of conveying concrete over the last 75 years.
What has changed is the technology that led to development of more efficient machinery.
The wheel barrow and buggy, although still used, have evolved to become the power buggy (upper left); the bucket hauled over a pulley wheel evolved into the bucket and crane (bottom left); and the horse-drawn wagon is now the ready mixed concrete truck (right).
As concrete-framed buildings became taller, the need to hoist reinforcement and formwork as well as concrete to higher levels led to the development of the tower crane—a familiar sight on the building skyline today. It is fast and versatile, but its capacity is limited to one lifting point.
The first mechanical concrete pump was developed and used in the 1930s and the hydraulic pump was developed in the 1950s. The advanced mobile pump with hydraulic placing boom is probably the single most important innovation in concrete handling equipment. It is economical to use in placing both large and small quantities of concrete, depending on jobsite conditions. For small to medium size projects, a combination of truck mixer and boom pump can be used to transport and place concrete.
The conveyor belt is an efficient, portable method of handling and transporting concrete. A dropchute prevents concrete from segregating as it leaves the belt; a scraper prevents loss of mortar. Conveyor belts can be operated in series and on extendable booms of hydraulic cranes. Truck-mixer-mounted conveyor belts are also available.
The screw spreader has been very effective in placing and distributing concrete for pavements. Screw spreaders can place a uniform depth of concrete quickly and efficiently.
Shotcrete is concrete that is pneumatically projected onto a surface at high velocity. It may also be known as “gunite” and “sprayed concrete,” Shotcrete is used for both new construction and repair work. It is especially suited for curved or thin concrete structures and shallow repairs.
When choosing the best method for concrete placement, the initial consideration is the type of job, its physical size, the total amount of concrete to be placed, and the placement schedule. Further consideration will identify the amount of work that is below, at, or above ground level. This aids in selecting the concrete transporting equipment necessary for placing concrete at the required levels. Concrete must be moved from the mixer to the point of placement as rapidly as possible without segregation or loss of ingredients. The transporting and handling equipment must have the capacity to move sufficient concrete so that cold joints are eliminated.
The largest concrete volume placements on a typical job are usually either below or at ground level and therefore can be placed by methods different from those employed on a superstructure. Concrete work below ground can vary enormously-–from filling large-diameter bored piles or massive mat foundations to the intricate work involved in basement and subbasement walls. A crane can be used to handle formwork, reinforcing steel, and concrete. The concrete may be chuted directly from the truck mixer to the point needed. They must not slope greater than 1 vertical to 2 horizontal or less than 1 vertical to 3 horizontal. Long chutes, over 6 meters (20 ft), or those not meeting slope standards must discharge into a hopper before distribution to point of need. Belt conveyors are very useful for work near ground level. Since placing concrete below ground is frequently a matter of horizontal movement assisted by gravity, lightweight portable conveyors can be used for high output at relatively low cost. Alternatively, a concrete pump can move the concrete to its final position. Pumps must be of adequate capacity and must be capable of moving concrete without segregation. The loss of slump caused by pressure forcing mix water into the aggregates as the mix travels from pump hopper to discharge at the end of the pipeline must be minimal-–not greater than 50 mm (2 in). The air content generally should not be reduced by more than 2 percentage points during pumping. Air loss greater than this may be caused by a boom configuration that allows the concrete to freefall. In view of this, specifications for both slump and air content may be met at the discharge end of the pump.
Conveyor belt, crane and bucket, hoist, pump, or the ultimate sky-hook, the helicopter, can be used for lifting concrete to locations above ground level. The tower crane and pumping boom are the right tools for tall buildings. The volume of concrete needed per floor as well as boom placement and length affect the use of a pump; large volumes minimize pipeline movement in relation to output. The specifications and performance of transporting and handling equipment are being continuously improved. The best results and lowest costs will be realized if the work is planned to get the most out of the equipment.
In this presentation we discussed the batching, mixing, transporting, and handling concrete. The discussion began with ordering concrete and the difference between performance and prescriptive specifications. The balance of the presentation progressed through the processes of producing concrete and delivering it to the jobsite.
