Understanding Nominal and Design Mixes


Cement concrete in India on large scale is being used since the last about 70 years. In the early days the following nominal ratio by volume for concrete were specified.

Cement

:

Sand

:

Aggregate

1

:

2

:

4

Correspond to M-15 Grade

1

:

1.5

:

3

Correspond to M-20 Grade

1

:

1

:

2

Correspond to M-25 Grade

IS : 456-2000 has recommended that minimum grade of concrete shall be not less than M-20 in reinforced concrete work. Design mix concrete is preferred to nominal mix. If design mix concrete cannot be used for any reason on the work for grades of M-20 or lower, nominal mixes may be used with the permission of engineer-in-charge, which however is likely to involve a higher cement content.

Accordingly all concrete of above M-20 Grade for RCC work must be of design mixes. The code allows nominal mix for RCC work of M-20 Grade, but what shall be the nominal mix, the reader will find from the following table that it is better to adopt design mix, rather than to go for M20 nominal mix which is too cumbersome to determine a fixed nominal mix value.

Nominal mixes as per IS : 456-2000 if fine aggregate is of Zone II as per IS : 383-1970.

Grade of Concrete

As per IS:383-1970 Maximum size of graded coarse aggregate

Mix Ratio by Weight

Max W/C Ratio

Max cement: Aggregate ratio by mass

Cement

Fine Aggregate

Coarse Aggregate

M-20

10

1 : 1.8 : 2.7

0.60

1.5

M-20

20

1 : 1.5 : 3.0

0.60

1.5

M-20

40

1 : 1.3 : 3.2

0.60

1.5

Proportions by weight can be converted to proportions by volume, by dividing with the bulk density of the materials available for use at site. The bulk density of cement may be taken 1.44 kg/lit.

The above nominal mixes are worked out for Zone II fine aggregate. As per IS: 383-1970 there are three more zone of sands. Therefore, the total nominal mixes shall be 12 for 10, 20 & 40 mm maximum size of coarse aggregate.

Thus, it could be seen that nominal mixes cannot have a fix conventional proportions such as 1:2:4 or 1:1.5:3, but may vary according to maximum size of coarse aggregate and grading of fine aggregate. Hence nominal mixes are also needed to be designed according to the sizes of aggregates available at site. However, the ultimate aim must be to get the specified properties of concrete.

As per IS: 456-2000, volume batching may be allowed only where weight batching is not practical and provided accurate bulk densities of materials to be actually used in concrete have earlier been established. Allowance for bulking shall be made in accordance with IS: 2386(Part 3). The mass volume relationship should be checked as frequently as necessary.

The exposures of Indian Construction sites at most places are Moderate for which IS: 456-2000 specified that minimum grade of concrete for reinforced concrete should be M25. Accordingly for durability consideration the structural concrete must not be below M-25 grade. The high strength benefits obtained should be taken into account in the design consideration of the concrete structure.

If for practical purpose, we go deeper than we will find that for all reinforced concrete structures we must have concrete from design mixes.

In the IS: 456-2000 there is nothing mentioned of 1:1:2 ration for M-25 grade of concrete. Concrete of above M-20 must be design mixes. If one takes 1:1:2 ratio then the cement content comes to 528 kg/m3. Where as IS: 456-2000 on page 19 clause 8.2.4.2 mentioned that OPC in excess of 450 kg/m3 should not be used.

The concrete surfaces of the structure exposed to severe rain, alternate wetting and drying such as RCC OH water tank comes to severe exposure environment for which the minimum grade of concrete shall be M-30, minimum cement content 320 kg/m3 and maximum free W/C ration 0.45. The following table will show the compression of nominal and design mixes for RCC work.

Materials : OPC 43-grade, River sand of Zone II and 20 mm graded crushed stone aggregate. Specific gravity of sand and aggregate 2.65. Workability of design mixes 50±10mm slump.

Grade of Concrete

Mix. Free W/C ratio

Min. Cement content kg/m3

Nominal mixes by weight C:S:A

Design mixes by weight C:S:A

Saving in cement

M-20

0.55

300

1:1.5:3

Cement= 392kg/m3

1:2.22:3.48 Cement= 327kg/m3

65 kg/m3

M-25

0.50

300

1:1:2

Cement= 528kg/m3

1:1.93:3.17

Cement= 360kg/m3

168 kg/m3

M-30

0.45

320

1:1.67:2.84

Cement= 400kg/m3

Note: For high strength concrete plasticizer/superplasticizer should be used which will reduce water and with the same W/C ratio reduction in cement content.

From the above table it can be calculated in nominal mixes of M-20 and M-25 how much extra cement is used in the construction, its total cost and how much CO2 is emitted in the production of this extra cement.
When a mix is referred for designing, it is design for target strength. For example M-30 (by Vol. ratio) is design for:

30 + 1.65 x 6 = 39.9 N/mm2 at 28 days age

The above is design target strength of the consultant Laboratory. When this mix is used at construction site, its concrete shall have strength as per table 11 of IS : 456-2000.

30 + 4 = 34 N/mm2 at 28 days age

For starting the work a construction site cannot weight for 28 days. Therefore according to various literatures, if at 7 days its strength is about 65% (22 N/mm2 ) the work may be started. However in all the cases 28 days cube compressive strength shall alone be the criterion for acceptance and rejection of the concrete.

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Types of dams: Introduction and classification


 A dam is a hydraulic structure of fairly impervious material built across a river to create a reservoir on its upstream side for impounding water for various purposes. These purposes may be Irrigation, Hydro-power, Water-supply, Flood Control, Navigation, Fishing and Recreation. Dams may be built to meet the one of the above purposes or they may be constructed fulfilling more than one. As such, it can be classified as: Single-purpose and Multipurpose Dam.

Different parts & terminologies of Dams:

  • Crest: The top of the dam structure. These may in some cases be used for providing a roadway or walkway over the dam.
  • Parapet walls: Low Protective walls on either side of the roadway or walkway on the crest.
  • Heel: Portion of structure in contact with ground or river-bed at upstream side.
  • Toe: Portion of structure in contact with ground or river-bed at downstream side.
  • Spillway: It is the arrangement made (kind of passage) near the top of structure for the passage of surplus/ excessive water from the reservoir.
  • Abutments: The valley slopes on either side of the dam wall to which the left & right end of dam are fixed to.
  • Gallery: Level or gently sloping tunnel like passage (small room like space) at transverse or longitudinal within the dam with drain on floor for seepage water. These are generally provided for having space for drilling grout holes and drainage holes. These may also be used to accommodate the instrumentation for studying the performance of dam.
  • Sluice way: Opening in the structure near the base, provided to clear the silt accumulation in the reservoir.

Illustration of dam-parts in a typical cross section

  • Free board: The space between the highest level of water in the reservoir and the top of the structure.
  • Dead Storage level: Level of permanent storage below which the water will not be withdrawn.
  • Diversion Tunnel: Tunnel constructed to divert or change the direction of water to bypass the dam construction site. The hydraulic structures are built while the river flows through the diversion tunnel.

CLASSIFICATION OF DAMS

Dams can be classified in number of ways. But most usual ways of classification i.e. types of dams are mentioned below:
Based on the functions of dams, it can be classified as follows:
  1. Storage dams: They are constructed to store water during the rainy season when there is a large flow in the river. Many small dams impound the spring runoff for later use in dry summers. Storage dams may also provide a water supply, or improved habitat for fish and wildlife. They may store water for hydroelectric power generation, irrigation or for a flood control project. Storage dams are the most common type of dams and in general the dam means a storage dam unless qualified otherwise.
  1. Diversion dams: A diversion dam is constructed for the purpose of diverting water of the river into an off-taking canal (or a conduit). They provide sufficient pressure for pushing water into ditches, canals, or other conveyance systems. Such shorter dams are used for irrigation, and for diversion from a stream to a distant storage reservoir. It is usually of low height and has a small storage reservoir on its upstream. The diversion dam is a sort of storage weir which also diverts water and has a small storage. Sometimes, the terms weirs and diversion dams are used synonymously.
  2. Detention dams: Detention dams are constructed for flood control. A detention dam retards the flow in the river on its downstream during floods by storing some flood water. Thus the effect of sudden floods is reduced to some extent. The water retained in the reservoir is later released gradually at a controlled rate according to the carrying capacity of the channel downstream of the detention dam. Thus the area downstream of the dam is protected against flood.
  3. Debris dams: A debris dam is constructed to retain debris such as sand, gravel, and drift wood flowing in the river with water. The water after passing over a debris dam is relatively clear.
  4. Coffer dams: It is an enclosure constructed around the construction site to exclude water so that the construction can be done in dry. A coffer dam is thus a temporary dam constructed for facilitating construction. These structure are usually constructed on the upstream of the main dam to divert water into a diversion tunnel (or channel) during the construction of the dam. When the flow in the river during construction of hydraulic structures is not much, the site is usually enclosed by the coffer dam and pumped dry. Sometimes a coffer dam on the downstream of the dam is also required.
Based on structure and design, dams can be classified as follows:
  1. Gravity Dams: A gravity dam is a massive sized dam fabricated from concrete or stone masonry. They are designed to hold back large volumes of water. By using concrete, the weight of the dam is actually able to resist the horizontal thrust of water pushing against it. This is why it is called a gravity dam. Gravity essentially holds the dam down to the ground, stopping water from toppling it over.

Gravity dams are well suited forblocking rivers in wide valleys or narrow gorge ways. Since gravity dams must rely on their own weight to hold back water, it is necessary that they are built on a solid foundation ofbedrock.
Examples of Gravity dam: Grand Coulee Dam (USA), Nagarjuna Sagar (India) and Itaipu  Dam (It lies Between Brazil and Paraguay and is the largest in the world).

  • Earth Dams: An earth dam is made of earth (or soil) built up by compacting successive layersof earth, using the most impervious materials to form a core and placing more permeable substances on the upstream and downstream sides. A facing of crushed stone prevents erosion by wind or rain, and an ample spillway, usually of concrete, protects against catastrophic washout should the water overtop the dam. Earth dam resists the forces exerted upon it mainly due to shear strength of the soil. Although the weight of the this structure also helps in resisting the forces, the structural behavior of an earth dam is entirely different from that of a gravity dam. The earth dams are usually built in wide valleys having flat slopes at flanks (abutments).The foundation requirements are less stringent than those of gravity dams, and hence they can be built at the sites where the foundations are less strong. They can be built on all types of foundations. However, the height of the dam will depend upon the strength of the foundation material.
    Examples of earthfill dam: Rongunsky dam (Russia) and New Cornelia Dam (USA).
  • Rockfill Dams: A rockfill dam is built of rock fragments and boulders of large size. An impervious membrane is placed on the rockfill on the upstream side to reduce the seepage through the dam. The membrane is usually made of cement concrete or asphaltic concrete.

    In early rockfill dams, steel and timber membrane were also used, but now they are obsolete. A dry rubble cushion is placed between the rockfill and the membrane for the distribution of water load and for providing a support to the membrane. Sometimes, the rockfill dams have an impervious earth core in the middle to check the seepage instead of an impervious upstream membrane. The earth core is placed against a dumped rockfill. It is necessary to provide adequate filters between the earth core and the rockfill on the upstream and downstream sides of the core so that the soil particles are not carried by water and piping does not occur. The side slopes of rockfill are usually kept equal to the angle of repose of rock, which is usually taken as 1.4:1 (or 1.3:1). Rockfill dams require foundation stronger than those for earth dams.
    Examples of rockfill dam: Mica Dam (Canada) and Chicoasen Dam (Mexico).

  • Arch Dams: An arch dam is curved in plan, with its convexity towards the upstream side. They transfers the water pressure and other forces mainly to the abutments by arch action.An arch dam is quite suitable for narrow canyons with strong flanks which are capable of resisting the thrust produced by the arch action.The section of an arch dam is approximately triangular like a gravity dam but the section is comparatively thinner. The arch dam may have a single curvature or double curvature in the vertical plane. Generally, the arch dams of double curvature are more economical and are used in practice.
    Examples of Arch dam: Hoover Dam (USA) and Idukki Dam (India).
  • Buttress Dams: Buttress dams are of three types : (i) Deck type, (ii) Multiple-arch type, and (iii) Massive-head type. A deck type buttress dam consists of a sloping deck supported by buttresses. Buttresses are triangular concrete walls which transmit the water pressure from the deck slab to the foundation. Buttresses are compression members. Buttresses are typically spaced across the dam site every 6 to 30 metre, depending upon the size and design of the dam. Buttress dams are sometimes called hollow dams because the buttresses do not form a solid wall stretching across a river valley.The deck is usually a reinforced concrete slab supported between the buttresses, which are usually equally spaced.In a multiple-arch type buttress dam the deck slab is replaced by horizontal arches supported by buttresses. The arches are usually of small span and made of concrete. In a massive-head type buttress dam, there is no deck slab. Instead of the deck, the upstream edges of the buttresses are flared to form massive heads which span the distance between the buttresses. The buttress dams require less concrete than gravity dams. But they are not necessarily cheaper than the gravity dams because of extra cost of form work, reinforcement and more skilled labor. The foundation requirements of a buttress are usually less stringent than those in a gravity dam.
    Examples of Buttress type: Bartlett dam (USA) and The Daniel-Johnson Dam (Canada).
  • Steel Dams: Dams: A steel dam consists of a steel framework, with a steel skin plate on its upstream face. Steel dams are generally of two types: (i) Direct-strutted, and (ii) Cantilever type . In direct strutted steel dams, the water pressure is transmitted directly to the foundation through inclined struts. In a cantilever type steel dam, there is a bent supporting the upper part of the deck, which is formed into a cantilever truss. This arrangement introduces a tensile force in the deck girder which can be taken care of by anchoring it into the foundation at the upstream toe. Hovey suggested that tension at the upstream toe may be reduced by flattening the slopes of the lower struts in the bent. However, it would require heavier sections for struts. Another alternative to reduce tension is to frame together the entire bent rigidly so that the moment due to the weight of the water on the lower part of the deck is utilised to offset the moment induced in the cantilever. This arrangement would, however, require bracing and this will increase the cost. These are quite costly and are subjected to corrosion. These dams are almost obsolete. Steel dams are sometimes used as temporary coffer dams during the construction of the permanent one. Steel coffer dams are supplemented with timber or earthfill on the inner side to make them water tight. The area between the coffer dams is dewatered so that the construction may be done in dry for the permanent dam.
    Examples of Steel type: Redridge Steel Dam (USA) and Ashfork-Bainbridge Steel Dam (USA).
  • Timber Dams: Main load-carrying structural elements of timber dam are made of wood, primarily coniferous varieties such as pine and fir. Timber dams are made for small heads (2-4 m or, rarely, 4-8 m) and usually have sluices; according to the design of the apron they are divided into pile, crib, pile-crib, and buttressed dams.The openings of timber dams are restricted by abutments; where the sluice is very long it is divided into several openings by intermediate supports: piers, buttresses, and posts. The openings are covered by wooden shields, usually several in a row one above the other. Simple hoists—permanent or mobile winches—are used to raise and lower the shields.
  • Rubber Dams: A symbol of sophistication and simple and efficient design, this most recent type of dam uses huge cylindrical shells made of special synthetic rubber and inflated by either compressed air or pressurized water. Rubber dams offer ease of construction, operation and decommissioning in tight schedules.
  • These can be deflated when pressure is released and hence, even the crest level can be controlled to some extent. Surplus waters would simply overflow the inflated shell. They need extreme care in design and erection and are limited to small projects.
    Example of Rubber type: Janjhavathi Rubber Dam (India).