ASSIGNMENT SUBMITTAL & IDENTIFICATION
Student Number: 2 1 7 0 0 1 3 7 3
Surname & Initials: FORTUIN RD Program:
Concrete Technology IV
Subject Code: B E T 4 2 A I
Assignment Number 1
Due date: 2 0 1 8 1 0 0 3
Lecturer: Mr. R Gopinath
Table of Contents
TOC o “1-3” h z u SELF COMPACTING CONCRETE3INTRODUCTION3CONSTITUENT MATERIALS3SELECTION CRITERIA5MIXTURE PROPORTIONS5
METHODS USED IN PRODUCTION AND PLACING6
TEST METHODS USED FOR ASSESMENT THE PERFORMANCE OF SCC7IMPALA NO.16 SHAFT LARGE SCALE PROJECT12DESIGN OF CONCRETE131 PAGEREF _Toc463955489 h 9DESIGN OF PROPOSED MIXES13 PAGEREF _Toc463955491 h 10MIXING AND TRANSPORTING OF CONCRETE14 PAGEREF _Toc463955492 h 10PLACING OF CONCRETE16 PAGEREF _Toc463955496 h 12PROTECTION AND CURING OF CONCRETE20 PAGEREF _Toc463955504 h 17REFERENCES22
QUESTION 1INTRODUCTIONConcrete has been one of the most essential materials in the construction industry , this is due to the different properties it may possess based on the method of production and many other reasons such as the various materials used to produce the concrete availability. Developments have also lead to the existence of the many types concrete used world-wide. Selection of a specific type of concrete for use in any project depends on the challenges faced in the particular scenario, therefore different factors will be considered.
For the purpose of our study, elaboration will be made on the following types of concrete together with recent large scale use and the challenges involved.
Re-Design of Concrete Mix
SELF COMPACTING CONCRETESelf-compacting concrete (SCC) is a flowing concrete mixture that is able to consolidate under its own weight. The highly fluid nature of SCC makes it appropriate for placing in problematic conditions and in sections with congested reinforcement. Use of SCC can also help minimize hearing-related damages on the worksite that are induced by vibration of concrete. Another advantage of SCC is that the time required to place large sections is considerably reduced.
Such concrete should have a moderately low yield value to ensure high flow ability, a moderate viscosity to resist segregation and bleeding, and must maintain its homogeneity during transportation, placing and curing to ensure adequate structural performance and long-term durability.
For SCC, it is generally required to use superplasticizers in order to obtain high mobility. Adding a large volume of powdered material or viscosity modifying admixture can eliminate segregation. The powdered materials that can be added are as follows; Fly ash, Silica Fume, Limestone Powder, Glass Filler and Quartzite Filler. Self-compatibility is largely affected by the characteristics of materials and the mix proportions, it becomes necessary to produce a procedure for mix design of SCC.
CONSTITUENT MATERIALSMixture proportions for SCC differ from those of usual concrete, in that the former has more powder content and less coarse aggregate. SCC incorporates high range water reducers (HRWR, super-plasticizers) in larger amounts and frequently a viscosity modifying agent in small doses.
Aggregates for concrete are divided into two types as follows:
3886835128905 Sand < 4.75mm
Sand < 4.75mm
00 Sand < 4.75mm
Sand < 4.75mm
3783330217170Grit 4.75mm to 12.5mm
00Grit 4.75mm to 12.5mm
33108909207500385699013335 Gravel > 12.5m
00 Gravel > 12.5m
Fig. 1: Types of aggregates
Between the various properties of aggregates, the important ones for SCC are the shape and gradation. Many researchers have been able to produce self-compacting concrete with locally available aggregate. It is observed from these studies that self-compact ability is reachable at lower cement (or fines) content when rounded aggregates are used, as compared to angular aggregates.
In the case of SCC, rounded aggregates would provide a better flowability and less blocking potential for a given water-to-powder ratio, compared to angular and semi-rounded aggregates. The presence of flaky and elongated particles may give rise to blocking problems in confined areas, and also increase the minimum yield stress.
SCC consistently incorporates chemical admixtures. A high range water reducing admixture (HRWRA) and sometimes, viscosity-modifying agent (VMA). The HRWRA helps in achieving excellent flow at low water contents and VMA reduces bleeding and improves the stability of the concrete mixture.
An effective VMA can also bring down the powder requirement and still give the required stability. Moreover, SCC almost always includes a mineral admixture, to improve the deformability and stability of concrete.
Polymer fibres can be used to improve the stability of SCC, as they help prevent settlement and cracking due to plastic shrinkage of the concrete.
Steel or long polymer structural fibres are used to modify the ductility/toughness of the hardened concrete. Their length and quantity is chosen depending on the maximum size of aggregate and on structural requirements.
If they are used as a substitute for normal reinforcement, the risk of blockage is no longer applicable but it should be emphasised that using SCC with fibres in structures with normal reinforcement significantly increases the risk of blockage.
Where recycled water, recovered from processes in the concrete industry is used, the type/content and in particular any variation in content of suspended particles should be taken into account as this may disturb batch to batch uniformity of the mix.
SELECTION CRITERIAApplication area
SCC may be used in pre-cast applications or for concrete placed on site. It can be manufactured on site, batching plant or in a ready mix concrete plant and transported to site by truck. It can then be placed either by pumping or pouring into horizontal or vertical structures. In designing the mix, the size and the form of the structure, the dimension and density of reinforcement and cover should be taken in consideration. These aspects will all influence the specific requirements for the SCC.
Due to the flowing characteristics of SCC it may be difficult to cast to a fall unless contained in a form. SCC has made it possible to cast concrete structures of a quality that was not possible with the existing concrete technology.
Due to the high content of powder, SCC may show more plastic shrinkage or creep than ordinary concrete mixes. These aspects should therefore be considered during designing and specifying SCC. Current knowledge of these aspects is limited and this is an area requiring further research. Special care should also be taken to begin curing the concrete as early as possible.
The workability of SCC can be characterised by the following properties:
Filling ability – filling ability measured by slump flow and flow time.
Passing ability – is the difference between slump flow and J-ring flow diameter.
Segregation resistance – used to assess the resistance of self-compacting concrete to segregation
A concrete mix can only be classified as Self-Compacting Concrete if the requirements for all three characteristics are satisfied.
MIXTURE PROPORTIONSInitial mix composition:
In designing the mix it is most suitable to consider the relative proportions of the key components by volume rather than by mass.
Indicative typical ranges of proportions and quantities in order to obtain self-compactability are given below. Further modifications will be necessary to meet strength and other performance requirements.
Water/powder ratio by volume of 0.80 to 1.10
Total powder content – 160 to 240 litres (400-600 kg) per cubic meter.
Coarse aggregate content normally 28 to 35 per cent by volume of the mix.
Water: cement ratio. Typically, water content does not exceed 200 litre/m3.
The sand content balances the volume of the other constituents
Generally, it is advisable to design conservatively to ensure that the concrete can maintain its specified fresh properties in spite of anticipated variations in raw material quality. Some variation in aggregate moisture content should also be expected and allowed for at mix design stage.
Adjustment of the mix
Laboratory trials should be used to verify properties of the initial mix composition. If necessary, adjustments to the mix composition should then be made. Depending on the apparent problem, the following courses of action might be appropriate:
using additional or different types of filler, (if available);
modifying the proportions of the sand or the coarse aggregate;
using a viscosity modifying agent, if not already included in the mix;
adjusting the dosage of the superplasticizer and/or the viscosity modifying agent;
using alternative types of superplasticizer (and/or VMA), more compatible with local materials.
adjusting the dosage of admixture to modify the water content, and hence the water/powder ratio.
METHODS USED IN PRODUCTIONMixing
There is no requirement for any specific mixer type. Forced action mixers, including paddle mixers, freefall mixers, including truck mixers, and other types can all be used. The mixing time necessary should be determined by practical trials. Generally, mixing times need to be longer than for conventional mixes. Time of addition of admixture is important, and procedures should be agreed with the supplier after plant trials. If the consistence has to be adjusted after initial mixing, then it should generally be done with the admixtures.
Before placing SCC, It should be confirmed that reinforcement and formwork are arranged as planned. The formwork must be in good condition but no special measures are essential to prevent grout loss. Contractors may wish to reflect possible advantages of pumping from the bottom of formwork. If concrete is placed by skip, attention should be paid to the closure of the gate. For forms in excess of 3 m in depth, the full hydrostatic head should be taken into consideration. This may require modification of the formwork design and/or the SCC.
Though it is easier to place SCC than ordinary concrete, the following rules are advised to minimise the risk of segregation:
limit the vertical free fall distance to 5m
limit the permissible distance of horizontal flow from point of discharge to10 m.
As we have to use different superplasticizers, fillers and some new and rare materials like nanoslica in the production of S.C.C, it is difficult to use for economic reasons in developing countries. Although using S.C.C can considerably decrease the required manpower, but this is not that much significant like it is in developed countries because manpower is relatively cheap in developing countries.
TEST METHODS USED FOR THE ASSESSMENT ON THE PERFORMANCE OF SCC
The methods presented here are devised specifically for SCC.
A concrete mix can only be classified as SCC if the requirements for all the following three workability properties are fulfilled.
Passing ability, &
Filling ability: It is the ability of SCC to flow into all spaces within the formwork under its own weight. Tests, such as slump flow, V-funnel etc, are used to determine the filling ability of fresh concrete.
Passing ability: It is the ability of SCC to flow through tight openings, such as spaces between steel reinforcing bars, under its own weight. Passing ability can be determined by using U-box, L-box, Fill-box, and J- ring test methods.
Segregation resistance: The SCC must meet the filling ability and
ability with uniform composition throughout the process of transport and placing.
Test methods to determine workability of SCC are:
Slump flow test
V Funnel Test
L Box Test
U Box Test
Fill Box Test
Slump flow test and T50cm test on Self Compacting Concrete
The slump flow test is used assess the horizontal free flow of in the absence of obstructions. It was first developed in Japan for use in assessment of underwater concrete. The test method is based on the test method for determining the slump .T diameter of the concrete circle is a measure for the filling ability of the concrete.
Assessment of test:
This is a simple, rapid test procedure, though two people are needed if the T50 time is to be measured. It can be used on site, though the size of the base plate is somewhat unwieldy and level ground is essential. It is the most commonly used test, and gives a good assessment of filling ability. It gives no indication of the ability of the concrete to pass between reinforcement without booking, but may give some indication of resistance to segregation. It can be argued that the completely free flow, unrestrained by any foundries, is not representative of what happens in concrete construction, but the test can be profitably be used to assess the consistency of supply of supply of ready-mixed concrete to a site from load to load.
Fig: Accessories for Flow cone Flow table Slump test
Fig: Slump flow test and T50cm test
V funnel test and V funnel test at T 5 minutes on SCC
The equipment consists of a v shaped funnel as, show in Fig. An alternative type of V-funnel, the O funnel, with circular. The test was developed in Japan and used by Ozawa et al. The equipment consists of V-shaped funnel section is also used in Japan. The described V-funnel test is used to determine the filling ability (flow ability) of the concrete with a maximum aggregate size of 20mm. The funnel is filled with about 12 liter of concrete and the time taken for it to flow through the apparatus measured. After this the funnel can be refilled concrete and left for 5 minutes to settle. If the concrete shows segregation then the flow time will increases significantly.
Assessment of test:
Though the test is designed to measure flow ability, the result is affected by concrete properties other than flow. The inverted cone shape will cause any liability of the concrete to block to be reflected in the result-if, for example there is too much coarse aggregate. High flow time can also be associated with low deformability due to a high paste viscosity, and with high inter-particle friction. While the apparatus is simple, the effect of the angle of the funnel and the wall effect on the flow of concrete is not clear.
Fig: V Funnel test Apparatus
L Box Test on Self Compacting Concrete
This test is based on a Japanese design for under water concrete, has been by Peterson. The test assesses the flow of the concrete and also the extent to which it is subjected to blocking by reinforcement. The apparatus is shown in the figure. The apparatus consist of rectangular section box in the shape of an „L?, with a vertical and horizontal section, separated by a movable gate, in front of which vertical length of reinforcement bar are fitted. The vertical section is filled with concrete, and then the gate lifted to let the concrete flow into the horizontal section. When the flow has stopped, the height of the concrete at the end of the horizontal section is expressed as a proportion of that remaining in the vertical section. It indicates the slope of the concrete when at rest. This is an indication passing ability, or the degree to which the passage of concrete through the bars is restricted. The horizontal section of the box can be marked at 200mm and 400mm from the gate and the times taken to reach these points measured. These are known as the T20 and T40 times and are an indication for the filling ability. The section of bar con be of different diameters and are spaced at different intervals, in accordance with normal reinforcement considerations, 3x the maximum aggregate size might be appropriate. The bar can principally be set at any spacing to impose a more or less severe test of the passing ability of the concrete.
Assessment of test:
This is a widely used test, suitable for laboratory and perhaps site use. It asses filling and passing ability of SCC, and serious lack of stability (segregation) can be detected visually. Segregation may also be detected by subsequently sawing and inspecting sections of the concrete in the horizontal section. Unfortunately there is no arrangement ton materials or dimensions or reinforcing bar arrangement, so it is difficult to compare test results. There is no evidence of what effect the wall of the apparatus and the consequent „wall effect? might have on the concrete flow, but this arrangement does, to some extent, replicate what happens to concrete on site when it is confined within formwork. Two operators are required if times are measured, and a degree of operator error is inevitable.
791845topFig: L Box test Apparatus
U box test method on SCC
The test was developed by the Technology Research Centre of the Taisei Corporation in Japan. Sometime the apparatus is called a “box shaped” test. The test is used to measure the filing ability of self-compacting concrete. The apparatus consists of a vessel that is divided by a middle wall into two compartments; an opening with a sliding gate is fitted between the two sections. Reinforcing bar with nominal diameter of 134 mm are installed at the gate with centre to centre spacing of 50 mm. this create a clear spacing of 35 mm between bars. The left hand section is filled with about 20 liter of concrete then the gate is lifted and the concrete flows upwards into the other section. The height of the concrete in both sections is measured.
Assessment of test:
This is a simple test to conduct, but the equipment may be difficult to construct. It provides a good direct assessment of filling ability-this is literally what the concrete has to do- modified by an unmeasured requirement for passing ability. The 35 mm gap between the sections of reinforcement may be considered too close. The question remains open of what filling height less than 30cm is still acceptable.
Fig: U box test Apparatus
Fill box test method for SCC
This test is also known as „Kajima test?. The test is used to measure the filling ability of self-compacting concrete with a maximum aggregate size of 20 mm. the apparatus consists of a container (transparent) with a flat and smooth surface. In the container are 35 obstacles are made of PVC with a diameter of 20mm and a distance center to center of 50mm, see figure. At the top side is a put filling pipe (diameter 100mm height 500mm) with a funnel (height 100mm). The container is filled with concrete through this filling pipe and difference in height between two sides of the container is a measure for the filling ability.
Assessment of test:
This is a test that is difficult to perform on site due to the complex structure of the apparatus and large weight of the concrete. It gives a good impression of the self-compacting characteristics of the concrete. Even a concrete mix with a high filling ability will perform poorly if the passing ability and segregation resistance are poor.
Fig.: Detail of fill box empty & filled with concrete
RECENT LARGE SCALE PROJECT USING SELF-COMPACTING CONCRETE
Impala Platinum Mine No. 16 Shaft Project
Platinum-miner Impala Platinum’s Number 16 shaft, comprised of a main shaft and a ventilation shaft with the associated winder buildings, a bulk air cooler, personal tunnel and service duct. The main shaft headgear was a concrete structure that was unique in the following ways:
Its the tallest concrete headgear in the world at a height of 108.3m above bank level. The overall height from the foundation is 132m.
It is the first headgear slide in South Africa that was pumped, using a Putzmeister stationary pump.
The project received first place in the civil engineering category of the Concrete Society of Southern Africa’s (CSSA) Fulton awards for excellence in the use of concrete in five categories: civil engineering projects, building projects, design aspects, construction techniques, and aesthetic appeal.
Fig: Self compacting concrete used that doesn’t need any external vibration for full compaction and filling of formwork.
This noval approach necessitated the design of a special concrete mix (SCC) to enable the concrete to be pumped vertitally upwards, using special additives supplied by Chryso admixtures
The headgear utilised the following quatities of raw materials:
Rebar used: 1300t of high-tensile steel
Concrete placed in slide: 7000m3
Concrete placed in floors (ten levels): 700m3
QUESTION 2DESIGN OF PROPOSED MIXESGeneral
The Contractor shall design the mixes which he proposes to use in the Works to meet the specified criteria including the requirements for durability, hot and cold weather, temperature control and curing of concrete.
The Contractor shall submit full details of all the mixes he proposes to use to the Engineer not less than 35 days prior to the intended use of the concrete (refer to SC 14.6.3).
Concrete mixes shall also comply with the following requirements:
(a) The PFA content of cementitious materials used for the manufacture of all structural concrete shall be at least 30% by mass.
(b) The maximum cementitious content shall exceed 400 kg/m3
(c) The water/cement (w/c) ratio shall be the minimum consistent with adequate workability taking due account of any free water and water absorbed in the aggregates. The Contractor shall take into account that this requirement may require the inclusion of a water reducing agent in the mix. The workability shall be consistent with ease of placing and proper compaction having regard to the presence of reinforcement and embedded items. However, the concrete shall not exhibit signs of bleeding nor of an excess of mortar being brought to the surface after thorough vibration.
(d) The fine aggregate portion shall be well graded and conform to the grading given in SANS 1083.
(e) Plastic shrinkage shall be limited such that cracking does not occur along reinforcement with minimum cover
(f) The drying shrinkage of the concrete determined in accordance with BS 1881 shall not be greater than 0.05%.
MIXING AND TRANSPORTING OF CONCRETEConcrete Batching and Mixing PlantsConcrete for the Works shall be batched and mixed in one or more central plants unless the Engineer agrees to some other arrangement.
Batching and mixing plants shall be modern efficient equipment complying with the requirements of BS 1305 and capable of producing a uniform distribution of the ingredients throughout the mass. If the batching plant proposed by the Contractor does not fall within the scope of BS 1305, it shall be tested in accordance with BS 3963 and shall have a mixing performance within the limits of Table 5 of BS 1305.
All mixing operations shall be under the control of an experienced Engineer. Operators shall not interfere with automatic operations of the batching and mixing plant.
The nominal drum or pan capacity of the mixer shall not be exceeded. The turning speed and the mixing time shall be as recommended by the manufacturer, but in addition when water is the last ingredient to be added, mixing shall continue for at least one minute after all the water has been added to the drum or pan for a mixer of 1.5 m3 or less. The mixing time shall be increased by 20 seconds for each additional cubic metre or part thereof.
If an extender has not been pre-blended with the cement, the mixing time shall be at least 3 minutes after the last ingredient has been added to the drum or pan.
The blades of pan mixers shall be maintained within the tolerances specified by the manufacturer of the mixer and the blades shall be replaced when it is no longer possible to maintain the tolerances by adjustment. Mixers shall be fitted with an automatic recorder and printer registering in triplicate the number of batches discharged for each grade of concrete produced. One copy of the printout shall accompany each batch of concrete to the place of deposition and shall be submitted to the Engineer’s Representative.
After mixing for the required time, each batch shall be discharged completely from the mixer before any materials for the succeeding batch are introduced.
Batching of Materialsa) General
Cements and aggregates shall be batched separately by mass. Water may be measured by mass or volume.
The mechanism for transfer of materials to the drum shall be maintained in good order at all times such that loss of material does not occur.
If cumulative weigh bins are employed an automatic cut out shall be incorporated to prevent the discharge of the following material component before the previous component has been correctly weighed.
The Contractor shall provide standard test weights at least equivalent to the maximum working load used on the most heavily loaded scale and other auxiliary equipment required for checking the satisfactory operation of each scale or other measuring device. Tests shall be made by the Contractor at monthly intervals or as otherwise determined by the Engineer and shall be carried out in his presence. For the purpose of carrying out these tests, there shall be easy access for personnel to the weigh hoppers. The Contractor shall furnish the Engineer with copies of the complete results of all check tests and shall make any adjustments, repairs or replacements necessary to ensure satisfactory performance.
Mixers which have been out of use for more than 30 minutes shall be thoroughly cleaned before any fresh concrete is mixed. The mixer shall be lined with mortar of the same proportions as the concrete before mixing of concrete is commenced.
The Contractor shall measure and record the moisture content of the aggregate as frequently as may be required in order to control the water content of the concrete as required by the Specification. Such measurement frequency shall not be less than twice per shift and may necessitate the use of automatic moisture recording devices.
b) Cement and Cement Extenders
Cement and cement extenders shall be batched by mass to within an accuracy of ± 1% unless supplied in standard bags in which case the mass of concrete in each batch shall be adjusted for the use of whole bags.
c) Water and Admixtures
Mixing water and admixtures for each batch shall be measured either by mass or by volume to within an accuracy of ± 1%.
The weighing and dispensing mechanisms shall be maintained in good order at all times.
When the correct quantity of water, determined as set out in the Specification and after due allowance for moisture contained in the aggregates, has been added to the mix, no further water shall be added, either during mixing or subsequently.
All aggregates shall be weighed separately to within an accuracy of ± 2%.
The mass of aggregates measured shall if necessary be adjusted to allow for the moisture content of each class. The frequency of adjustment will be dependent on the variation in moisture content of the aggregates, particularly the sand and adjustment shall be made as often as necessary to provide concrete of consistent quality.
Transport of ConcreteThe concrete shall be discharged from the mixer and transported to the Works by means which shall prevent adulteration, segregation or loss of ingredients, and which shall ensure that the concrete is of the required workability at the point and time of placing. All proposed means of transporting concrete shall require the acceptance of the Engineer.
The time elapsing between mixing and placing a batch of concrete shall not exceed 60 minutes. If the placing of any batch of concrete is delayed beyond this period, the concrete shall not be placed in the Works.
PLACING OF CONCRETEApproval for PlacingConcrete shall not be placed in any pour to any part of the Works until the Engineer ‘s acceptance has been given in writing, and the Contractor shall give the Engineer at least 24 hours’ notice of his intention to place concrete in any pour.
When the Contractor has completed all preparatory work such as concrete surface preparation, formwork, reinforcement, and the like, he shall request the Engineer for acceptance to commence concreting at a particular time and shall provide every facility necessary to enable the Engineer to inspect prepared surfaces, formwork, reinforcement and built in parts and shall not place the mortar layer or concrete until he has obtained the written acceptance of the Engineer.
If concrete placing is not commenced within 4 hours of the Engineer ‘s acceptance, the Contractor shall again request written acceptance as specified above.
Placing ProceduresThe concrete shall be deposited as nearly as possible in its final position. It shall be placed so as to avoid segregation of the concrete and displacement of the reinforcement, other embedded items, or formwork. It shall be brought up in layers not exceeding the lesser of 500 mm or the vibrating length of the vibrator needle in compacted thickness, unless otherwise permitted or directed by the Engineer, but the layers shall not be less than four times the maximum nominal size of aggregate in thickness.
Placing of concrete shall proceed in such a way that shrinkage and thermal stresses are minimized. Layers shall not be placed so that they form feather edges nor shall they be placed on a previous layer of the same pour which has taken its initial set without first treating this as a construction joint. In order to comply with this requirement, a layer may be started before completion of the preceding layer and where necessary the thickness of the layers shall be reduced to meet this requirement.
When concreting closed circuits, placing shall proceed from one or more points on the periphery in both directions at the same time, so that closing junctions are always made between newly poured faces. The point from which placing of concrete is to commence shall be agreed with the Engineer so that, if an emergency should occur which prevents the layer being completed, the construction joint will be formed in a structurally favorable position.
All the concrete in a single bay or pour shall be placed in a continuous operation. It shall be carefully worked around all obstructions, irregularities in the foundations and the like so that all parts are completely full of compacted concrete with no segregation or honeycombing. It shall also be carefully worked around and between reinforcement, embedded steelwork and similar items to be embedded in the pour and in particular those which protrude above the surface of the completed pour, taking particular care to avoid displacing or distorting such items.
All work shall be completed on each batch of concrete before its initial set commences and thereafter the concrete shall not be disturbed before it has set hard. No concrete that has partially hardened during transit shall be used in the Works and the transport of concrete from the mixer to the point of placing shall be such that this requirement can be complied with.
Concrete shall not be placed during rain which is sufficiently heavy or prolonged to wash mortar from coarse aggregate on the exposed faces of fresh concrete. Means shall be provided to cover the concrete and to remove any water accumulating on the surface of the placed concrete. Concrete shall not be deposited into such accumulations of water. The surface of the concrete shall be protected from damage by rain until the concrete has reached sufficient strength to prevent marking of the surface.
When concrete is discharged above its place of final deposition, segregation shall be prevented by the use of chutes, downpipes, trunking, baffles or other appropriate devices.
Forms for walls, columns and other thin sections of significant height shall be provided with openings or other devices that will permit the concrete to be placed sequentially at different levels and positions in such a manner that will prevent segregation and accumulations of hardened concrete on the formwork or reinforcement above the level of the placed concrete.
Interruptions to PlacingIf concrete placing is interrupted for any reason and the duration of the interruption cannot be forecast or is likely to be prolonged, the Contractor shall immediately inform the Engineer and take the necessary action to form a construction joint at the place of stoppage in the manner that will least impair the durability, appearance and proper functioning of the concrete. The Contractor shall eliminate as far as possible feather edges and adversely sloping top surfaces and shall thoroughly compact the concrete already placed. All work on the concrete shall be completed while it is still plastic and it shall not thereafter be disturbed until it is hard enough to resist damage.
Sufficient equipment and materials to comply with this requirement shall be readily available at all times during concrete placing.
Before concreting is resumed after such an interruption the Contractor shall cut out and remove all damaged or uncompacted concrete, feather edges or any other undesirable features and shall leave a clean sound surface against which the fresh concrete may be placed.
If it becomes possible to resume concrete placing without contravening the Specification and the Engineer approves to resumption, the new concrete shall be thoroughly worked in and compacted against the existing concrete so as to eliminate any cold joints.
Requirements using Sliding FormworkWhen using sliding formwork the following additional requirements shall apply:
The Contractor shall take all the necessary measures to ensure continuity of operations. All the necessary lighting and standby equipment for mixing, hoisting, placing and compaction must be provided and all the materials required for completing each structure must be ready on Site before casting commences.
Concrete shall be cast in uniform layers along the formwork so that the top surface of the concrete does not differ by more than 150 mm at any part of the formwork. In addition the level of the concrete shall never be more than 300 mm below the top of the sliding panel. The working platform must be kept clean and no concrete which has partially dried out may be swept into the formwork.
The concrete shall be compacted during and immediately after placing. Care shall be exercised not to damage or disturb previously placed concrete. To ensure proper bonding of successive layers, new layers shall not be placed after the previous layer has started to set.
If during the period of sliding, the ground or air temperature is expected to fall below 2°C, the Contractor shall submit to the Engineer details of his proposed method to prevent freezing of the fresh concrete. Permission to begin sliding will not be granted until the Engineer is satisfied that effective measures have been taken to overcome the detrimental effects of low temperature conditions.
Compaction of ConcreteConcrete shall be fully compacted by acceptable means during and immediately after placing. It shall be thoroughly worked against the formwork, around reinforcement, tendons, ducts and embedded fittings without displacing or distorting them and into corners in order to form a solid mass free from voids.
The concrete shall be free from honeycombing and planes of weakness and successive layers of the same lift shall be thoroughly bonded together by appropriate vibration techniques.
Concrete shall not be subjected to disturbances by vibration within 4 to 24 hours after compaction. Unless otherwise agreed to by the Engineer, concrete shall be compacted by means of internal vibrators. Internal vibrators shall operate at a frequency of between 90 and 250Hz, or to the Engineer ‘s acceptance.
The Contractor shall ensure that vibrators are operated at pressures or voltages not less than those recommended by the manufacturer in order that the compactive effort is not reduced. Vibration shall be applied by experienced operators only, who shall be under the direct supervision of an experienced concrete Engineer at all times.
A sufficient number of vibrators shall be operated to enable the entire quantity of concrete being placed to be vibrated for the necessary period and, in addition, standby vibrators shall be available for instant use at each place where concrete is being placed. The vibrators shall be inserted into the concrete to penetrate the layer underneath at regular spacing which shall not exceed the distance from the vibrator over which vibration is visibly effective and in any case shall not exceed four times the nominal diameter of the vibrator. Vibration shall be continued at any one section until no more air bubbles come to the surface.
Vibration shall not be applied by way of reinforcement nor shall vibrators be allowed to touch formwork or reinforcement or other embedded items as far as is practicable.
Special attention shall be given to the compaction of concrete around anchorage zones and behind anchor plates and in all places where high concentrations of reinforcing steel or cables occur.
In such cases where the placing and compaction of concrete is difficult, a mix containing smaller size aggregate may be used, but only with the acceptance of the Engineer and after a mix containing such aggregate has been designed and tested.
Whenever vibration is applied externally, the design of the formwork and positioning of vibrators shall be such that efficient compaction and avoidance of surface blemishes is ensured.
Re-vibration of concrete prior to the initial set may be required to overcome settlement cracks in reinforced concrete.
Hot or Cold Weather RequirementsThe Contractor shall prevent damage to concrete arising from exposure to extreme temperatures, and shall maintain in good working order all equipment required for this purpose.
In the event that conditions become such that even with the use of the appropriate equipment the requirements cannot be met, concrete placing shall immediately cease until such time as the requirements can again be met, except when sliding work is in progress.
Prevention of Plastic Shrinkage CracksThe Contractor shall take whatever measures are necessary to prevent plastic shrinkage cracking in the concrete. Particularly on dry windy days or hot sunny days the Contractor shall make provision for fine spraying of the concrete surface with water as soon as it has taken its initial set or covering of the concrete with plastic sheeting. It may be necessary to change the aggregates or the concrete mix proportions. In order to deal with shrinkage cracking it may also be necessary to change the time at which, or the manner in which, floating is carried out.
If plastic shrinkage cracking occurs, the cracks shall be closed up by re-vibrating the concrete while the concrete is still in a plastic state. Once the cracks have been closed, the concrete shall be kept thoroughly wet, or covered with plastic sheeting for at least a further three hours.
PROTECTION AND CURING OF CONCRETEProtection of Fresh ConcreteFreshly placed concrete shall be protected from rainfall and from water running over the surface until it is sufficiently hard to resist damage from this cause.
In particular, the Contractor shall protect the surfaces against which water will flow at high speed (e.g. spillway chute) by covers until the concrete has gained sufficient strength to prevent damage when rain or hail falls on such surfaces.
While concrete is setting it shall not be subjected to any stress or vibrations due to traffic or any other cause. Where necessary suitable walkways, equipment paths and barrow paths, independently supported, shall be provided over concrete work. Such walkways and paths shall not be supported by the steel reinforcement.
Concrete placed in the Works shall not be subjected to any structural loading until it has attained at least its characteristic strength. If the Contractor desires to impose structural loads on newly-placed concrete, he shall make at least 3 sets of 3 test cubes and cure them in the same conditions as the concrete they represent. These cubes shall be tested in sets of 3 at suitable intervals in order to estimate the time at which the characteristic strength is reached.
Curing of ConcreteConcrete shall be continuously protected during hardening from loss of moisture and from the development of temperature differentials within the concrete sufficient to cause cracking. Proper curing of all exposed surfaces of concrete (including joint surfaces) is essential in order to achieve durable, impermeable concrete. The methods used for curing shall not cause damage, discoloration or marring of any kind to the concrete.
Curing of all concrete shall be continued for at least 14 days or until the concrete is covered by later construction.
When the temperature of the concrete falls below 5°C, the curing period shall be extended by the period during which the temperature of the concrete was below 5°C. The curing process shall commence as soon as the concrete is hard enough to resist damage from the process, and in the case of large areas of continuous pours shall commence on the completed section of the pour before the rest of the pour is finished.
Details of the Contractor’s proposals for curing concrete shall be submitted to the Engineer for agreement before the placing of concrete commences in the Works.
Loss of MoistureAs soon as exposed concrete has hardened sufficiently, it shall be protected against loss of moisture by application of one or more of the following curing methods as agreed by the Engineer for each type of structure:
Sprinkling or spraying with water such that the entire surface is kept constantly wet.
Ponding of water on exposed surfaces.
Covering with moisture retaining materials, such as sand, cotton or jute mats, kept constantly wet.
Covering with waterproof plastic sheets or curing paper firmly held down and particularly at the edges.
Retaining the formwork in position.
In addition, when using sliding formwork the concrete shall be protected against the weather and rapid drying out by means of a 4 m wide skirt attached without gaps being formed to the lower perimeter of the formwork and hanging over the working platform. The skirt shall consist of hessian in summer months but in winter months canvas or other suitable material shall be used. The skirt shall be weighted at the bottom to prevent it flapping around in windy conditions. If instructed by the Engineer, the Contractor shall, in addition to the curing provisions set out above provide a suitable form of shading to prevent the direct rays of the sun reaching the concrete surfaces for at least the first four days of the curing period.
REFERENCESJapan Society of Civil Engineers, ‘Recommendations for Design and Construction of Antiwashout Underwater Concrete’, Concrete library of JSCE, 19 (1992) 89 p.Ozawa, K., Sakata, N., Okamura, H., ‘Evaluation of Self-Compactibility of Fresh Concrete Using the FunnelTest’, Concrete Library of JSCE, (25) (June 1995) 59-75.Bartos, P.J.M., ‘An appraisal of the Orimet Test as a Method for On-site Assessment of Fresh SCC Concrete’, Proceedings of International Workshop on Self-Compacting Concrete, (Japan, August 1998) 121-135.Hajime Okamura,Masahiro Ouchi;self_compacting concrete;Journal of advanced concrete technology;Vol 1,5-15,April 2003
EFNARC;Specification and Guidelines for SELF-COMPACTING CONCRETE;february2002
Resilient infrastructure, June 1-4 2016, Michael L.J. Maher, Ph.D., P.Eng. Golder Associates Ltd., Canada John B. Hagan, P.Eng. Golder Associates Ltd., Canada
SELF COMPACTING CONCRETE WITH TESTS. Available from: https://www.researchgate.net/publication/315706742_SELF_COMPACTING_CONCRETE_WITH_TESTS accessed Oct 03 2018.