quantities for ordering building materials

1. Introduction

The quantities given in this pamphlet are approximate. They form a basis for a first estimate of quantities to be used on site.

The following notes apply generally.

  • Mix proportions in the tables are based on materials being batched in a loose state, ie poured into the batching container without being compacted
  • “Common cement” complies with SANS 50197-1. “Masonry cement” complies with SANS 50413-1 strength class 22,5X.
  • A bag of cement contains 50 kg of cement.
  • The volume of cement, when poured loosely into a container is approximately 33 l and it is recommended that only whole bags be used.
  • The capacity of a builder’s type 5 wheelbarrow com-plying with SANS 795 is about 65 l.Volumes of 130 l and 200 l are therefore equivalent to two and three barrowsful respectively.
  • The selection of good quality sand and stone is important.
  • Only sufficient water should be used to produce a workable mix of 60-100 mm slump. Too much mixing water will reduce the strength of the concrete.
  • The importance of curing cannot be overemphasised. For hardening to occur, cement must have access to water. Once the concrete is allowed to dry, no further strength development will take place. Preventing excessive loss of water is also important to avoid cracking and crazing and the possibility of a weak dusty surface. Cure for at least 7 days, and longer in cold weather when strength development slows down.

2. Materials quantities for concrete

The volume of concrete, for rectangular shapes such as foundation strips, floor slabs, etc, is thickness x length x breadth.

If each of these dimensions is measured in metres, the volume will be in cubic metres.

Quantities of materials for concrete suitable for hand com-paction (ie without the use of poker vibrators) are shown in Tables 1 a-d and do not include allowance for wastage.

Table 1a: 32,5N or R Common cement, 19 mm stone


Table 1b: 32,5N or R Common cement, 13,2 mm stone

Table 1c: 42,5N or R Common cement, 19 mm stone

Table 1d: 42,5N or R Common cement, 13,2 mm stone

Notes on mixes in Tables 1a – 1d:

  1. Low-strength (15 MPa) concrete is suitable for unreinforced foundations (single storey only); mass fill, infill concrete in masonry (only with13,2-mm stone).
  2. Medium-strength (25 MPa) concrete is suitable for unreinforced slabs, reinforced slabs and foundations, infill concrete in masonry (only with 13,2 mm stone).
  3. High-strength (30 MPa) concrete is suitable for reinforced concrete members and precast items such as concrete flagstones.

3. Mortar mix proportions

The proportion of each material in the mix should suit the type of work being done. Strength requirements and mix proportions recommended by C&CI are given in Table 2.

In general, the classes of mortars may be used as follows:

Class l
Highly stressed masonry incorporating high-strength struc-tural units such as might be used in multi-storey loadbearing buildings. Reinforced masonry.

Class ll
Normal loadbearing applications, as well as parapets, balus-trades, retaining structures, and freestanding and garden walls, and other walls exposed to possible severe dampness. In practice, Class II mortars are used for most applications. Although SANS 10249 refers to a Class III mortar, it is so seldom used that it has been omitted from Table 2.

Other proportions may be used if these can be shown by test to be satisfactory.

Table 2: Mortar strength requirements and mix proportions

4. Quantities of masonry units and mortar

The dimensions of units given in Table 3 are those of the commonly manufactured sizes.

NOTES:

  • The table is based on exact sizes of solid masonry units, with 10 mm thick bedding and vertical joints, and no wastage. For 15 mm thick joints, multiply the quantity of mortar required by 1,5; for 20 mm thick joints, multiply by 2.
  • Since no allowance is made for a number of factors which could influence mortar quantities, the following adjustments should be made:
  1. Allow for wastage which could typically range from 15-20%.
  2. For hollow units reduce mortar quantities by:

    Width of units, mm % reduction
    90 - 110 mm 20
    140 mm 30
    190 - 220 mm 40

  3. For units with perforations or holes increase mortar quantities by 15%.
  4. For units with frogs; frog laid face up (as required for structural walls), increase mortar quantities by 15%.
  5. Once the above adjustments have been made: for mortar ready-mixed and delivered into watertight containers on site, reduce quantities by 20% as against site-mixed mortars
  6. For under- or oversized units:
    Measure dimensions of 10 units and use the
    average for calculating the number of units per m2

Table 3: Quantities of masonry units and mortar


5. Materials for plaster

Quantities depend on the mix proportions, thickness of plaster and roughness of the background surface.

To calculate the approximate area that can be covered by a batch of plaster, use the following example:


Common cement using a 1:6 mix, ie 50 kg cement to 200 l sand.
From the table above, the yield = 154 l
For 15 mm plaster thickness and 20% wastage
= 154 ÷ 15 ÷ 1,2
= 8,5 m2

6. Materials for sand-cement floor screeds

Sand-cement screeds are essentially light-duty flooring elements and are suitable for:

  • Wearing surfaces of floors of utility rooms in domestic premises (eg store rooms, garages)
  • Floors covered with carpets, plastic tiles or linoleum, etc and subjected to relatively light traffic such as in offices, shops and hospitals.

Sand-cement screeds are not suitable for industrial premises.

Quantities of materials depend on the thickness of the screed. Using a mix consisting of 1 bag of cement and 130 l of coarse sand, and assuming a screed thickness of 25 mm,
quantities for 100 m2 of screed are 23 bags of cement and 3,0 m3 of sand.

The suggested wastage factor is 10%.

Appendix

Field test for quality of sand for mortar and plaster

This simple field test can be used to confirm that the sand yields a smooth, plastic and cohesive mix which does not require excessive amounts of water to reach a brick-laying or plastering consistence.

Where possible, the test should be carried out on a sample of sand before placing a final order.

The quantities used should be weighed out on a scale which is in good order, and the test carried out on a smooth imper-vious surface. It is also important that the sample used is fairly representative of the bulk supply.

Procedure


i) Weigh out the following amounts of material:


5 kg cement
25 kg of dry sand
5 kg (l) of water
1 kg (l) of water

ii) Mix the cement and sand to a uniform colour on a non-absorbent surface.
iii) Mix, in succession, each of the amounts of water (5 l and 1 l)


If 5 l is enough, the sand is of good quality
If 5 l +1 l is enough, the mix is of average quality

Only “good” or “average” sands are recommended for mortars and plasters.

 

Cement & Concrete Institute
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Tel (011) 315-0300 Fax (011) 315-0584
e-mail info@cnci.org.za website http://www.cnci.org.za
Published by the Cement & Concrete Institute, Midrand, 1996, reprinted 1997, 1999, 2001, 2003, 2006, 2007.
©Cement & Concrete Institute