DERECHOS ECONOMICOS, SOCIALES Y CULTURALES

G.F.MASSON and R.T.L.ALLEN

3.1 Portland cements

The cements most frequently used in concrete repair work are ordinary Portland cement (OPC) and rapid-hardening Portland cement (RHPC), which should comply with BS 12(1). This recently published Standard classifies cements according to standard strength classes which are given in Table 3.1. Ordinary Portland cement (OPC) will be Class 42.5 and rapid-hardening Portland cement (RHPC) will be Class 52.5. BS 12(1) also includes requirements for initial setting time, soundness, loss on ignition, insoluble residue, sulphate and chloride.

A common misapprehension refers to setting times. The stiffening and hardening of cement paste is the result of the continuous chemical reactions of hydration; these start as soon as cement and water are mixed and continue at a gradually decreasing rate for a considerable period of time. Setting times are purely arbitrary states of stiffening that are determined by needle penetration tests on standard specimens of cement paste and defined in British Standard EN 196-3 (2); they do not represent any sudden change in properties, and they refer to cement paste only, not to mortar or concrete. Sulphate-resisting Portland cement (SRPC), complying with British Standard

Table 3.1 Strength requirements of BS 12:1991 Specification for Portland cement Compressive strength (determined in accordance with BS EN196-1)

Early strength Standard strength

Strength class 2 days 7 days 28 days

32.5N – ≥ 16 ≥32.5 ≤52.5

32.5R ≥10 –

42.5N ≥10 – ≥42.5 ≤ 62.5

42.5R ≥20 –

52.5N ≥20 – ≥52.5 ≤72.5

Compressive strength (determined in accordance with BS EN196-1)

Early strength Standard strength

Strength class 2 days 7 days 28 days

62.5N ≥20 – ≥ 62.5 –

Table 3.2 Typical proportions of principal constituents of Portland cement Tricalcium concrete, or deterioration has occurred due to sulphate attack. The principal difference between SRPC and OPC is in chemical composition: the maximum permitted content of tricalcium aluminate in SRPC is 3.5%, while no limit is given for OPC. Typical proportions of the principal constituents of present-day Portland cement are given in Table 3.2. It is important to remember that while SRPC is more resistant than OPC to attack by sulphate ions, there is little difference between the resistance of the two types of cement to acid conditions.

British ordinary, rapid-hardening and sulphate-resisting Portland cements are approximately equivalent to American Types I, III and V respectively (4).

White Portland cement complying with BS 12 may be used for reasons of appearance. It is made from selected raw materials and its composition is adjusted so as to reduce greatly the proportion of iron compounds in it. The proportion of tetracalcium aluminoferrite (C₄AF) is typically less than 1% in white cement compared with about 8% in OPC.

Coloured Portland cements are no longer available in Great Britain, but coloured mortars or concretes can be made by incorporating inorganic pigments complying with British Standard 1014 (5) in the mix. White cement is often necessary when pigments are used. If such a mix is correctly designed, all the aggregate particles will be coated initially with coloured cement paste and the colour will be governed by the cement and pigment. On exposure to the weather, however, the film of coloured cement on exposed particles will tend to disappear and the colour of the fine aggregate will become more evident. Consequently, it is advisable to choose a fine aggregate that has a colour resembling that desired.

CEMENTS AND AGGREGATES 35

3.2

High-alumina cement

High-alumina cement is sometimes used when rapid gain of strength is important, but its use for load-bearing structures is prohibited by current Codes of Practice (6). This is because of its susceptibility to ‘conversion’, which is a change of crystalline form with time that leads to a serious reduction in strength (7). This phenomenon occurs particularly when the concrete is stored under warm or in humid conditions. Carefully controlled mixtures of high-alumina and Portland cements produce rapid setting properties for non-structural applications such as temporary repairs or sealing leaks, but have little strength or long-term durability.

3.3 Aggregates

Aggregates should generally be natural concreting aggregates, complying with BS 882 and, unless proprietary pre-packed materials are used, the selection of type will depend largely on what is available within a reasonable distance from the site (8).

Generally speaking, coarse aggregates are used only in the larger repairs, and 10mm maximum sizes are used more frequently than they are in new construction. Rounded particles are preferable to very angular ones, because compaction is often more difficult in repair work than in new construction, but angular coarse aggregates may have to be used in some localities and it will seldom be worth paying higher prices for rounded coarse aggregates. Flaky and elongated particles should be avoided because they make the concrete difficult to compact.

With sands, particle shape has a greater effect on workability because of their greater surface area, so the use of rounded pit sands rather than crushed rock fines has distinct advantages by reducing the amount of water required, and hence the likelihood of shrinkage. Grading is important, however, in order to produce a dense concrete that can be compacted readily without segregation or bleeding, and it may be necessary to blend natural sands with crushed rock fines in order to get the best results. Sands containing excessive amounts of silt or clay should be avoided because their very great surface area increases the amount of water required and they interfere with the bond between cement and aggregate particles, and limits laid down in BS 882 should be adhered to (8). Building sands complying with BS 1199 or 1200 are not generally suitable for concrete repair work because of their finer gradings (9). The choice of sand may be affected by its colour, for reasons already stated.

Special aggregates may be needed in some circumstances. Lightweight aggregates to British Standard 3797 (10) are used occasionally in overhead work, because the thickness of mortar that can be built up in one layer, using

36 THE REPAIR OF CONCRETE STRUCTURES

normal aggregates, may be restricted by its weight. Lightweight fillers are used in some proprietary repair compounds for this reason. Specially hard metallic aggregates may be used when the work will be subject to severe abrasion, as in floors in heavy industrial premises.

References

1. British Standard 12:1991. Specification for Portland cement. British Standards Institution, London.

2. British Standard EN196–3. Methods of testing cement. Determination of setting time and soundness. British Standards Institution, London.

3. British Standard 4027:1991. Specification for sulfate resisting Portland cement.

British Standards Institution, London.

4. ASTM Standard C150–84. Standard specification for Portland cement. American Society for Testing and Materials.

5. British Standard 1014. Pigments for Portland cement and Portland cement products.

British Standards Institution, London.

6. British Standard 8110:1985. Parts I and II. Code of Practice for Structural Concrete.

British Standards Institution, London.

7. Neville, A.M. Properties of Concrete. Pitman, London.

8. British Standard 882:1983. Aggregates from natural sources for concrete. British Standards Institution, London.

9. British Standards 1199 and 1200:1976. Building sands from natural sources. British Standards Institution, London.

10. British Standard 3797:1990. Lightweight aggregates for masonry units and structural concrete. British Standards Institution, London.

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