Name: concrete placement for tight spaces
Uploaded: 2024-11-07T14:48:35.000Z
Duration: 1 min 29 s
Channel: JoVE

Preplaced aggregate concrete is produced by initially placing gap-graded coarse aggregate that is sufficiently wet within the formwork and compacting it.
The remaining voids, constituting about one-third of the total volume, are then filled with mortar comprising Portland cement, pozzolan, fine aggregates, water, and a fluidizing aid.
Alternatively, a cement and fine sand mortar, blended in a high-speed colloid mixer, can be used.
The mortar is introduced under pressure through slotted pipes about 1.5 inches in diameter spaced at regular intervals. The pipes are then gradually removed as the mortar level rises.
External vibration at the top level of the mortar can improve the finish of exposed surfaces.
Preplaced aggregate concrete exhibits uniformity with reduced segregation and is used in constructing nuclear shields and underwater projects.
Water-retaining structures, large monolithic blocks, and repair works also use preplaced aggregate concrete due to its low shrinkage and reduced permeability.
In large construction projects, before applying mortar, circulating chilled water around the aggregates controls the heat of hydration, and in cold conditions, circulating steam warms up the aggregates.

preplaced aggregate concrete

Preplaced aggregate concrete is ideal for construction environments that are not easily accessible. The process begins by properly wetting the gap-graded coarse aggregates to remove the dirt, then placing it in the form and compacting it. Voids are filled with a mortar mix pumped under pressure through slotted pipes. This mortar typically consists of Portland cement, pozzolan, fine aggregates, water, and a fluidizing aid. The pozzolan helps reduce bleeding and segregation while improving the fluidity of the mortar, whereas the fluidizing aid delays the setting of the mortar. A cement-fine sand mortar is also used. It is prepared in a high-speed colloid mixer to keep the cement suspended until the mortar is pumped. As the mortar rises to the top, the pipe is gradually withdrawn. Applying external vibration to the top layer of the mortar can enhance the finish of the exposed surfaces.
Preplaced aggregate concrete is favored for its uniformity and reduced risk of segregation, which is essential for constructing nuclear shields and underwater projects. Due to its low shrinkage and permeability, preplaced concrete is used in water-retaining structures, large monolithic blocks, and repair works. In large-scale construction, refrigerated water is circulated around the aggregates before adding the mortar to control the heat from cement hydration. In contrast, steam is circulated to prevent frost damage in cold conditions.

Preplaced Aggregate Concrete

Preplaced aggregate concrete is produced by initially placing gap-graded coarse aggregate that is sufficiently wet within the formwork and compacting it.
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Education

JoVE Core

Civil Engineering

Unit 1

Mixing, Placing and Curing Concrete 

KEY TERMS AND CONCEPTS

concrete placement for tight spaces

Preplaced aggregate concrete is used when a construction site is hard to reach. The process starts by wetting gap-graded coarse aggregates. This helps remove dirt from the stones. The aggregates are then placed in the form and compacted.

Next, a mortar mix is pumped under pressure through slotted pipes to fill the spaces between the aggregates. The mortar usually contains Portland cement, pozzolan, fine aggregates, water, and a fluidizing aid. The pozzolan reduces bleeding and segregation and also improves mortar flow. The fluidizing aid delays the setting of the mortar.

A cement and fine sand mortar can also be used. It is mixed in a high-speed colloid mixer so the cement stays suspended until pumping. As the mortar rises to the top, the pipe is slowly withdrawn. External vibration can be applied to the top layer to improve the finish of the exposed surface.

This method is valued for its uniformity and its lower risk of segregation. That makes it useful for nuclear shields and underwater projects. Its low shrinkage and low permeability also support water-retaining structures, large monolithic blocks, and repair work. In large-scale construction, refrigerated water may be circulated around the aggregates to limit heat from cement hydration. In cold conditions, steam may be circulated to help prevent frost damage.

Concrete Placement for Tight Spaces

mixing concrete

Concrete Mixer Types and Capacity

Concrete mixing creates a uniform blend of cement, aggregates, and paste. The goal is to coat the aggregates evenly with cement paste. Mixers used for this job are usually grouped into two main types: batch mixers and continuous mixers. Batch mixers handle one load at a time. They mix the materials thoroughly, discharge the batch, and then receive the next one. Continuous mixers work differently. They take in a steady flow of ingredients, mix them at a constant rate, and discharge the concrete...

mixing time

Concrete Mixer Charging and Timing

Concrete mixer charging and timing are key to making a uniform concrete mix with the needed strength. Mixing begins only after all ingredients are inside the mixer. The usual charging order starts with 10% of the water, then the solid materials are added steadily, followed by 80% of the water. The last portion of water is added later, within the first quarter of the total mixing period. The minimum mixing time depends on mixer capacity. Mixers with up to 1 cubic yard of capacity need at least...

ready mixed concrete

How Ready-Mixed Concrete Is Delivered

Ready-mixed concrete is prepared at a centralized plant and then delivered by truck to a construction site. It arrives ready for placement, which makes it different from concrete mixed entirely on site. Ready-mixed concrete includes three main forms: central-mixed, truck-mixed, and shrink-mixed. Central-mixed concrete is fully mixed at the plant and carried to the site in agitator trucks. These trucks rotate at 2 to 6 rpm, which helps keep the concrete in motion during transport. Truck-mixed...

pumped concrete

Concrete Pumping for Hard-to-Reach Sites

Concrete pumping moves large amounts of concrete through pipes to places that are hard to reach. It is used for sites such as deep foundation pits and other inaccessible locations. The system includes a hopper, a pump, and pipelines that carry the concrete to the placement point. Direct-acting pumps use a piston to move the concrete. Concrete enters the pump through the inlet valve by gravity and by suction created as the piston moves. The forward movement of the piston then pushes the...

placing concrete

Concrete Placement Methods and Compaction

Concrete placement methods focus on keeping the mix in a uniform shape and position. The concrete is placed as close as possible to its final location to reduce segregation, which is the separation of ingredients. It is then fully compacted to remove trapped air. The next layer is added while the layer below is still plastic, or soft enough to shape. The rate of placing and compacting concrete is kept equal. Concrete is laid in uniform layers, and hand shoveling is avoided. Moving concrete...

vibrating concrete

Concrete Compaction with Vibrators

Concrete compaction with vibrators helps newly poured concrete settle inside formwork and around reinforcement bars. The goal is to remove trapped air pockets and create a dense concrete mass. Mechanical vibration is a key part of that process. One common method uses an internal vibrator, also called a poker vibrator or immersion vibrator. It is quickly inserted through the full depth of the fresh concrete and slightly into the layer below, which is still in a plastic state. The vibrator is...

shotcrete

Shotcrete for Vertical and Overhead Surfaces

Shotcrete is a concrete spraying method used on vertical and overhead surfaces. It uses mortar or small aggregate concrete with a high cement content. The material is sprayed at high velocity, and the impact helps compact it so it can cling without sagging. Builders place shotcrete in layers until it reaches about 4 inches thick. The quality depends strongly on the operator’s skill in managing the nozzle. Careful nozzle control helps the material land evenly and build up properly. Shotcrete...

Preplaced aggregate concrete is used when a construction site is hard to reach. The process starts by wetting gap-graded coarse aggregates. This helps remove dirt from the stones. The aggregates are then placed in the form and compacted. Next, a mortar mix is pumped under pressure through slotted pipes to fill the spaces between the aggregates. The mortar usually contains Portland cement, pozzolan, fine aggregates, water, and a fluidizing aid. The pozzolan reduces bleeding and segregation and...

finishing concrete

Concrete Surface Finishing Steps

Concrete surface finishing begins right after the concrete has been placed and consolidated. The first step is screeding, which levels the surface by removing extra material until it is even with the top of the formwork. A screed is a straight edge used for this leveling step. After screeding, a bull float or darby is used to smooth the surface further. These tools help lower high spots, fill low areas, and push larger aggregate particles deeper into the concrete. This preparation should...

curing of concrete

How Curing Improves Concrete Strength

Curing helps concrete gain strength after it has set. It keeps the concrete moist and at the right temperature so cement hydration can continue. Hydration is the process where cement reacts with water in the water-filled capillary pores and forms the products that fill spaces in the paste. Dry air can interrupt this process by pulling water from the concrete surface. Concrete with a water-cement ratio below 0.5 can also lose water through self-desiccation, which means internal water is used up...

curing methods

Concrete Curing for Moisture Retention

Concrete curing for moisture retention helps fresh concrete keep the water it needs as it hardens. For members with a small surface-to-volume ratio, the forms are often oiled and moistened before the concrete is placed. These forms may stay in place for a long time to reduce moisture loss. If the form material can be wetted, it can also be kept damp. When forms are removed early, the concrete member is moistened and then covered with polythene sheets. Large horizontal concrete surfaces in dry...

accelerated curing of concrete

How Concrete Gains Strength Faster

Concrete can gain strength faster when heat and extra moisture speed up cement hydration. This faster hydration helps the concrete build early strength sooner than normal curing. The process is used when quicker hardening is needed after casting. One common method is steam curing. Concrete products may move through a chamber on a conveyor belt, or they may be encased in plastic so steam at atmospheric pressure can circulate around them. Steam curing starts with moist curing for about 3 to 5...

key terms and concepts

Learn how preplaced aggregate concrete fills hard-to-reach forms with pumped mortar, low segregation, and uses in underwater, repair, and water-retaining projects.

Concrete Placement for Tight Spaces | JoVE K12

JoVE Core Civil Engineering offers a foundational exploration of essential construction materials such as bricks, stones, steel, wood, and concrete, along with fluid mechanics topics like flow over immersed bodies, open channel flow, and dimensional modeling. It also covers key surveying concepts, including leveling, traverse adjustments, curve design, topographic mapping, and geospatial technologies (GPS & GIS). Science-in-action videos showcase laboratory research on materials, fluids, and surveying practices in real-world engineering systems.

civil engineering

Concrete Placement for Tight Spaces

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