VACUUM CONCRETE: TECHNIQUES, EQUIPMENTS & ADVANTAGES

Water-cement ratio is detrimental for concrete. We always try to restrict the water-cement ratio in order to achieve higher strength. The chemical reaction of cement with water requires a water-cement ratio of less than 0.38, whereas the adopted water-cement ratio is much more than that mainly because of the requirement of workability. Workability is also important for concrete, so it can be placed in the formwork easily without honeycombing.
After the requirement of workability is over, this excess water will eventually evaporate leaving capillary pores in the concrete. These pores result into high permeability and less strength in the concrete. Therefore, workability and high strength don’t go together as their requirements are contradictory to each other.
Vacuum concrete is the effective technique used to overcome this contradiction of opposite requirements of workability and high strength. With this technique both these are possible at the same time.
In this technique, the excess water after placement and compaction of concrete is sucked out with the help of vacuum pumps. This technique is effectively used in industrial floors, parking lots and deck slabs of bridges etc. The magnitude of applied vacuum is usually about 0.08 MPa and the water content is reduced by upto 20-25%. The reduction is effective upto a depth of about 100 to 150 mm only.

Technique and Equipments for Vacuum Concrete:

The main aim of the technique is to extract extra water from concrete surface using vacuum dewatering. As a result of dewatering, there is a marked reduction in effective water-cement ratio and the performance of concrete improves drastically. The improvement is more on the surface where it is required the most.
Mainly, four components are required in vacuum dewatering of concrete, which are given below:

1. Vacuum pump
2. Water separator
3. Filtering pad
4. Screed board vibrator

Vacuum pump is a small but strong pump of 5 to 10 HP. Water is extracted by vacuum and stored in the water separator. The mats are placed over fine filter pads, which prevent the removal of cement with water. Proper control on the magnitude of the water removed is equal to the contraction in total volume of concrete. About 3% reduction in concrete layer depth takes place. Filtering pad consists of rigid backing sheet, expanded metal, wire gauge or muslin cloth sheet. A rubber seal is also fitted around the filtering pad as shown in fig.1. Filtering pad should have minimum dimension of 90cm x 60cm.

Fig. 1: Vacuum dewatering of concrete

Advantages of vacuum concreting:

• Due to dewatering through vacuum, both workability and high strength are achieved simultaneously.
• Reduction in water-cement ratio may increase the compressive strength by 10 to 50% and lowers the permeability.
• It enhances the wear resistance of concrete surface.
• The surface obtained after vacuum dewatering is plain and smooth due to reduced shrinkage.
• The formwork can be removed early and surface can be put to use early.

Fig. 2: Effect of vacuum dewatering of concrete

The advantages of dewatering are more prominent on the top layer as compared to bottom layer as shown in fig. 2 above. The effect beyond a depth of 150mm is negligible.

TYPICAL CONSTRUCTION JOINTS IN BEAMS AND COLUMNS

Construction joints are required when the concreting has to be stopped for the day or more than 30minutes. In such case, typical construction joints shall be provided so that bond is maintained between set concrete and fresh concrete. Below are some images showing correct method of construction joints to be provided in columns, beams and beam-column junction. To know about what is construction joint, read Joints in Concrete Construction.

1. Construction Joint in Column

Following figure shows correct method of providing construction joint in column. Construction joint in column shall not be provided with smooth surface or inclined surface. The top surface of the column should be rough with parts of coarse aggregates being seen.

Fig: Typical Construction Joint in RCC Column

2. Construction Joint in Beams and Beam-Column Joint:

Following figure shows the typical construction joint to be provided in beams and beam column joints.

Fig: Typical Construction Joint in Beams and Beam-Column Joint

The arrow symbol shows the direction of concreting, tick mark shows the correct method of providing construction joint while cross-mark shows the wrong method.

JOINTS IN LIQUID RETAINING CONCRETE STRUCTURES

1. MOVEMENT JOINTS:

There are three types of movement joints.
(i) Contraction Joint: It is a movement joint with deliberate discontinuity without initial gap between the concrete on either side of the joint . The purpose of this joint is to accommodate contraction of the concrete. The joint is shown in Fig. 1 (a).

Fig. 1(a)

A contraction joint may be either complete contraction joint or partial contraction joint. A complete contraction joint is one in which both steel and concrete are interrupted and a partial contraction joint is one in which only the concrete is interrupted, the reinforcing steel running through as shown in Fig. 1(b) .

Fig. 1

(ii) Expansion Joint: It is a joint with complete discontinuity in both reinforcing steel and concrete and it is to accommodate either expansion or contraction of the structure. A typical expansion joint is shown in Fig.2.

Fig. 2.

This type of joint requires the provision of an initial gap between the adjoining parts of a structure which by closing or opening accommodates the expansion or contraction of the structure.
(iii) Sliding Joint: It is a joint with complete discontinuity in both reinforcement and concrete and with special provision to facilitate movement in plane of the joint . A typical joint is shown in Fig. 3.

Fig. 3.

This type of joint is provided between wall and floor in some cylindrical tank designs.
2. CONTRACTION JOINTS
This type of joint is provided for convenience in construct ion. Arrangement is made to achieve subsequent continuity without relative movement. One application of these joints is between successive lifts in a reservoir wall. A typical joint is shown in Fig. 4.

The number of joints should be as small as possible and these joints should be kept from possibility of percolation of water.

3. TEMPORARY JOINTS

A gap is sometimes left temporarily between the concrete of adjoining parts of a structure which after a suitable interval and before the structure is put to use, is filled with mortar or concrete completely as in Fig.5(a) or as shown in Fig.5 (b) and (c) with suitable jointing materials. In the first case width of the gap should be sufficient to al low the sides to be prepared before filling.

Fig: 5 (a)

Fig. 5(b)

Fig. 5(c)

SPACING OF JOINTS IN LIQUID RETAINING CONCRETE STRUCTURES:

Unless alternative effective means are taken to avoid cracks by al lowing for the additional stresses that may be induced by temperature or shrinkage changes or by unequal settlement, movement joints should be provided at the following spacing:-
(a) In reinforced concrete floors, movement joints should be spaced at not more than 7.5m apart in two direct ions at right angles. The wall and floor joints should be in line except where sliding joints occur at the base of the wall in which correspondence is not so important .
(b)For floors with only nominal percentage of reinforcement (smaller than the minimum specified) the concrete floor should be cast in panels with sides not more than 4.5m.
(c) In concrete walls, the movement joints should normally be placed at a maximum spacing of 7.5m. In reinforced walls and 6m in unreinforced walls. The maximum length desirable between vertical movement joints will depend upon the tensile strength of the walls, and may be increased by sui table reinforcement . When a sliding layer is placed at the foundation of a wall , the length of the wall that can be kept free of cracks depends on the capacity of wall sect ion to resist the friction induced at the plane of sliding.
Approximately the wall has to stand the effect of a force at the place of sliding equal to weight of half the length of wall multiplied by the co-efficient of friction.
(d)Amongst the movement joints in floors and walls as mentioned above expansion joints should normally be provided at a spacing of not more than 30m between successive expansion joints or between the end of the structure and the next expansion joint ; al l other joints being of the construct ion type.
(e) When, however, the temperature changes to be accommodated are abnormal or occur more frequently than usual as in the case of storage of warm liquids or in uninsulated roof slabs, a smaller spacing than 30m should be adopted that is greater proportion of movement joints should be of the expansion type). When the range of temperature is small, for example, in certain covered structures, or where restraint is small , for example, in certain elevated structures none of the movement joints provided in small structures up to 45mlength need be of the expansion type. Where sliding joints are provided between the walls and either the floor or roof, the provision of movement joints in each element can be considered independently.

JOINTS IN CONCRETE CONSTRUCTION

Joints in concrete building construction are construction joints, expansion joints, contraction joints and isolation joints. They prevent cracking of concrete. Types of joints in concrete are described below:

Construction Joints:

Construction joints are placed in a concrete slab to define the extent of the individual placements, generally in conformity with a predetermined joint layout.
They must be designed in order to allow displacements between both sides of the slab but, at the same time, they have to transfer flexural stresses produced in the slab by external loads.
Construction joints must allow horizontal displacement right-angled to the joint surface that is normally caused by thermal and shrinkage movement. At the same time they must not allow vertical or rotational displacements. Figure 1 summarizes which displacement must be allowed or not allowed by a construction joint.

Expansion joint

The concrete is subjected to volume change due to many reasons. So we have to cater for this by way of joint to relieve the stress. Expansion is a function of length. The building longer than 45m are generally provided with one or more expansion joint. In india recommended c/c spacing is 30m. The joints are formed by providing a gap between the building parts.

Contraction Joints

A contraction joint is a sawed, formed, or tooled groove in a concrete slab that creates a weakened vertical plane. It regulates the location of the cracking caused by dimensional changes in the slab. Unregulated cracks can grow and result in an unacceptably rough surface as well as water infiltration into the base, subbase and subgrade, which can enable other types of pavement distress. Contraction joints are the most common type of joint in concrete pavements, thus the generic term “joint” generally refers to a contraction joint. Contraction joints are chiefly defined by their spacing and their method of load transfer. They are generally between 1/4 – 1/3 the depth of the slab and typically spaced every 3.1 – 15 m

Isolation Joints

Joints that isolate the slab from a wall, column or drainpipe

Isolation joints have one very simple purpose—they completely isolate the slab from something else. That something else can be a wall or a column or a drain pipe. Here are a few things to consider with isolation joints:

• Walls and columns, which are on their own footings that are deeper than the slab subgrade, are not going to move the same way a slab does as it shrinks or expands from drying or temperature changes or as the subgrade compresses a little.

Even wooden columns should be isolated from the slab.

• If slabs are connected to walls or columns or pipes, as they contract or settle there will be restraint, which usually cracks the slab—although it could also damage pipes (standpipes or floor drains).
• Expansion joints are virtually never needed with interior slabs, because the concrete doesn’t expand that much—it never gets that hot.
• Expansion joints in concrete pavement are also seldom needed, since the contraction joints open enough (from drying shrinkage) to account for temperature expansion. The exception might be where a pavement or parking lot are next to a bridge or building—then we simply use a slightly wider isolation joint (maybe ¾ inch instead of ½ inch).
• Blowups, from expansion of concrete due to hot weather and sun, are more commonly caused by contraction joints that are not sealed and that then fill up with non-compressible materials (rocks, dirt). They can also be due to very long unjointed sections.

Very long unjointed sections can expand enough from the hot sun to cause blowups, but this is rare.

• Isolation joints are formed by placing preformed joint material next to the column or wall or standpipe prior to pouring the slab. Isolation joint material is typically asphalt-impregnated fiberboard, although plastic, cork, rubber, and neoprene are also available.
• Isolation joint material should go all the way through the slab, starting at the subbase, but should not extend above the top.
• For a cleaner looking isolation joint, the top part of the preformed filler can be cut off and the space filled with elastomeric sealant. Some proprietary joints come with removable caps to form this sealant reservoir.
• Joint materials range from inexpensive asphalt-impregnated fiberboard to cork to closed cell neoprene. Cork can expand and contract with the joint, does not extrude, and seals out water. Scott Whitelam with APS Cork says that the required performance is what determines the choice of joint materials. How much motion is expect, exposure to salts or chemicals, and the value of the structure would all come into play—and of course the cost.

Polyethylene foam isolation joint material comes in various colors. C2 Products

• At columns, contraction joints should approach from all four directions ending at the isolation joint, which should have a circular or a diamond shaped configuration around the column. For an I-beam type steel column, a pinwheel configuration can work. Always place the slab concrete first and do not install the isolation joint material and fill around the column until the column is carrying its full dead

SEALING MATERIALS FOR JOINTS IN CONCRETE

There are three types of joints in concrete construction, viz. construction joint, expansion joint and contraction joints. Learn more about Joints in Concrete Structures.

Materials for joints in water retaining structures and water tight structures

(1) Materials for joints in water retaining structures and water tight structures for sewage and effluent treatment shall be resistant to aerobic and anaerobic microbiological attack and resistant to attack by petrol, diesel oil, dilute acids and alkalis.
(2) Materials for joints in water retaining structures for potable and fresh water shall comply with the requirements of BS 6920.

Joint filler

Joint filler shall be firm, compressible, single-thickness, non-rotting filler. Joint filler for joints in water retaining structures and watertight structures shall be non-absorbent.

Bitumen emulsion

Bitumen emulsion for joints in water retaining structures and watertight structures shall comply with BS 3416. Bitumen emulsion for surfaces against which potable or fresh water will be stored or conveyed shall comply with BS 3416, type II.

Fig: Joints in concrete structures

Joint sealant

(1) Joint sealant shall be a grade suited to the climatic conditions of Hong Kong and shall perform effectively over a temperature range of 0°C to 60°C. Joint sealant for exposed joints shall be grey.
(2) Joint sealant other than cold-applied bitumen rubber sealant shall be:
(a) A gun grade for horizontal joints 15 mm wide or less and for vertical and inclined joints,
(b) A pouring grade for horizontal joints wider than 15 mm.
(3) Polysulphide-based sealant shall be a cold-applied two-part sealant complying with BS 4254. Polysulphide-based sealant for expansion joints in water retaining structures and watertight structures shall have a transverse butt-joint movement range of at least 20%.
(4) Polyurethane-based sealant shall be a cold-applied two-part sealant complying with the performance requirements of BS 4254.
(5) Hot-applied bitumen rubber sealant shall comply with BS 2499, type N1.
(6) Cold-applied bitumen rubber sealant shall be of a proprietary type.
(7) Joint sealant for joints in water retaining structures and water tight structures shall be as stated in Table-1.
(8) Primers and caulking material for use with joint sealant shall be of a proprietary type recommended by the joint sealant manufacturer.
(9) Different types of joint sealant and primers that will be in contact shall be compatible.

Table-1: Joint sealant for water retaining structures and water tight structures

Structure for retaining / excluding	Type of joint	Type of joint sealant
Sewage	All joints	Polyurethane based
Other than sewage	Expansion joint	Polyurethane based or Polysulphide based
	Horizontal joints other than expansion joints	Hot applied bitumen rubber, Polysulphide based or polyurethane based
	Vertical and inclined joints other than expansion joints	Polysulphide based, polyurethane based or cold-applied bitumen rubber

Bond breaker tape

Bond breaker tape shall be of a proprietary type recommended by the joint sealant manufacturer and approved by the Engineer. The tape shall be a polyethylene film with adhesive applied on one side and shall be the full width of the groove.

Bearing strip for sliding joints

Bearing strip for sliding joints shall consist of two plastic strips of a proprietary type approved by the Engineer. The strips shall be resistant to all weather conditions and to chemicals to which the structure will be subjected without impairing the reaction, durability or function of the strips.
The strips shall be of a type that will not require maintenance after installation. The strips shall be capable of withstanding a vertical load of at least 300 kN/m² and shall have a maximum coefficient of friction of 0.3 under a constant shearing force.

Waterstops or water stoppers

Waterstops, including intersections, reducers and junctions, shall be of a proprietary type approved by the Engineer. Waterstops shall be natural or synthetic rubber or extruded polyvinyl chloride and shall have the properties stated in Table-2.

Table-2: Properties of waterstops or waterstoppers

Property of water stops	Rubber waterstops	PVC waterstops
Density	1100 kg/m3 (+/-5%)	1300 kg/m3 (+/-5%)
Hardness	60 – 70 IRHD	70 – 90 IRHD
Tensile strength	>/= 20 N/mm2	>/= 13 N/mm2
Elongation at break point	>450%	>285%
Water absorption	<5% by mass after 48 hours immersion	<0.15% after 24 hours immersion
Softness number	-	42 – 52

While principles of concrete joints remains same, references may also be made to ACI 224.3R-95 Joints in Concrete Construction and IS:3414 – 1968 – Indian Standard Code of Practice for Design and Installation of Joints in Buildings (Reaffirmed in 2010).
Read More:
1. Joints in Concrete Construction
2. Joints in Liquid Retaining Structures

STAGES OF FRACTURE (CRACKING) IN CONCRETE

The stages of cracking (fracture) in concrete:
There are three stages of cracking or fracture in concrete. When we describe the cracking mechanisms, it is important to differentiate between the mode of crack initiation and how this occurs at the microscopic level, and the subsequent paths of propagation and the eventual macroscopic crack pattern at the engineering level.
Although some discontinuities exist as a result of the compaction process of fresh concrete, the formation of small fissures or micro-cracks in concrete is due primarily to the strain and stress concentrations resulting from the incompatibility of the elastic moduli of the aggregate and paste components.
Following are the Stages of Concrete Cracking:
Stage I: Even before loading, intrinsic volume changes in concrete due to shrinkage or thermal movements can cause strain concentrations at the aggregate–paste interface. Within this stage localized cracks are initiated at the microscopic level at isolated points throughout the specimen where the tensile strain concentration is the largest. This shows that these cracks are stable and, at this load stage, do not propagate
Stage II: As the applied load is increased beyond Stage I, initially stable cracks begin to propagate. There will not be a clear distinction between Stages I and II since stable crack initiation is likely to overlap crack propagation and there will be gradual transition from one stage to another. This is shown in Fig.1. During Stage II the crack system multiplies and propagates but in a slow stable manner in the sense that, if loading is stopped and the stress level remains constant propagation ceases.

The extent of the stable crack propagation stage will depend markedly upon the applied state of stress, being very short for ‘brittle’ fractures under predominantly tensile stress states and longer for more ‘plastic’ fractures under predominantly compressive states of stress.
Stage III: This occurs when, under load, the crack system has developed to such a stage that it becomes unstable and the release of strain energy is sufficient to make the cracks self-propagate until complete disruption and failure occurs. Once Stage III is reached failure will occur whether or not the stress is increased. This stage  starts at about 70–90 per cent of ultimate stress and is reflected in an overall expansion of the structure as signified by a reversal in the volume change behaviour. As stated above, the load stage at which this occurs corresponds approximately to the long-term strength of concrete.

EXCAVATION HAZARDS- THEIR EFFECTS AND PREVENTION

Different types of hazards are associated with excavation of soil. These hazards should be identified and preventive measures should be taken to avoid any accident at construction sites. The following table highlights hazards associated with excavation of soil during construction, their types, effects and preventive measures.

TYPE OF EXCAVATION	TYPE OF HAZARD	EFFECT OF HAZARD	PREVENTIVE MEASURES
Pit excavation upto 3m	Falling into pit	Personal injury	Provide guard rails / barricade with warning signal Provide atleast two entries / exits Provide escape ladders
	Earth collapse	Suffocation /breathlessness Buried	Provide suitable size of shoring and strutting if required. Keep soil heaps away from the edge equivalent to 1.5m or depth of pit whichever is more. Don’t allow vehicles to operate too close to excavated areas. Maintain atleast 2m distance from the edge of cut. Maintain sufficient angle of repose. Provide slope not less than 1:1 and suitable depth of excavation in all soils except hard rock. Battering / benching the sides.
	Contact with buried electric cable Gas / Oil Pipelines	Electrocution Explosion	Obtain permission from competent authorities prior to excavation if required. Locate the position of buried utilities by referring to plant drawings. Start digging manually to locate the exact position of buried utilities and thereafter use mechanical means.
Pit excavation beyond 3m	Same as above plus flooding due to excessive rain / underground water	Can cause drowning situation	Prevent ingress of water Provide ring buoys Identify and provide suitable size dewatering pump or well point system
	Digging in the vicinity of existing building / structure	Building / structure may collapse Loss of health and wealth	Obtain prior approval of excavation method from local authorities. Use under-pinning method. Construct retaining wall side by side.
	Movement of vehicles / equipments close to the edge of cut.	May cause cave-in or slides. Person may get buried.	Barricade the excavated area with proper lighting arrangements. Maintain atleast 2m distance from edge of cut and use stop blocks to prevent over-run. Strengthen shoring and strutting.
Narrow deep excavations for pipelines etc.	Same as above plus frequent cave-in or slides	May cause severe injuries or prove fatal.	Battering / benching of sides. Provide escape ladders.
	Flooding due to hydro-static testing	May arise drowning situation	Same as above. Bail out accumulated water. Maintain adequate ventilation.
Rock excavation by blasting	Improper handling of explosives	May prove fatal	Ensure proper storage, handling and carrying of explosives by trained personnel. Comply with the applicable explosive acts and rules.
	Uncontrolled explosion	May cause severe injuries or prove fatal.	Allow only authorized persons to perform blasting operations. Smoking and open flames are to be strictly prohibited.
	Scattering of stone pieces to atmosphere	Can hurt people	Use PPEs like goggles, face mask, helmets etc.
	Entrapping of persons / animals	May cause severe injuries or prove fatal	Barricade the area with red flags and blow siren before blasting.
	Misfire	May explode suddenly	Do not return to site for atleast 20min or unless announced safe by designated person.
Piling work	Failure of pile-driving equipment	Can hurt people	Inspect piling rigs and pulley blocks before the beginning of each shift/
	Noise pollution	Can cause deafness and psychological imbalance.	Use personal protective equipments like ear plugs, muffs etc.
	Extruding rods/casing	Can hurt people	Barricade the area and install sign boards Provide first aid
	Working in the vicinity of live-electricity	Can cause electrocution / asphyxiation	Keep sufficient distance from live electricity as per relevant standard codes. Shut off the supply if possible. Provide artificial / rescue breathing to the injured.