The Manufacture of concrete paving blocks

1. Introduction

Concrete block paving is versatile, aesthetically attractive, functional, cost effective and requires little or no maintenance if correctly manufactured and laid. Most concrete block paving constructed in South Africa has performed satisfactorily but there are two main areas of concern: occasional failure due to excessive surface wear, and variability in the strength of blocks.

This technical note covers basic principles and aims to assist manufacturers, particularly new manufacturers, to produce a durable and consistent product.

It discusses: requirements for blocks; selection of materials; materials for manufacture; manufacturing equipment; propor-tioning of ingredients; manufacture; and quality control.

Scope is limited to blocks with a relatively small top surface area, ie about 50 blocks per square metre of paving. Flagstones are therefore not discussed.

2. Requirements for concrete paving blocks

2.1 Properties

• Blocks should meet the structural requirements for paving (specified in terms of block compressive strength).
• Blocks should be durable: they should be able to withstand abrasion, impact and chemical attack.
• Blocks should be of uniform dimensions to facilitate correct and easy placing and ensure good rideability.
• In some applications concrete blocks are required to be aesthetically attractive.

2.2 Specification requirements

The current specification giving the requirements for concrete paving blocks is SABS 1058:2002 Concrete paving blocks which is referred to in SABS 1200 MJ:1984 Standard specification for civil engineering construction: segmented paving. The specification requires that pavers comply with certain tolerances, and have a compressive strength of 25 MPa, for lightly trafficked situations, or 35 MPa, for more severe conditions or where a wheel load greater than 30 kN is encountered. In recent years there has been a trend in Europe, America and elsewhere to also, or alternatively, specify a flexural or tensile splitting strength requirement, but this has not yet been introduced in South Africa. It is increasingly being recognized that some form of
abrasion resistance test is also required.

Tests have shown that 25-MPa blocks are structurally adequate. Experience indicates however that strength levels should be increased to ensure a higher probability of
durability.

Block thickness is specified according to traffic and SABS 1200 MJ specifies standard thicknesses of 50, 60, 80, 100 and 120 mm. It is not normally economical to manufacture the last two sizes.

3. Selection of materials

3.1 Cement

Cement should comply with SABS EN 197-1. Strength class should be 42,5 or higher because blocks are required to develop good strength relatively quickly to permit early
delivery without excessive breakages. (A 32,5 strength class cement may be used if the blocks are to be subjected to elevated temperatures and/or good curing, eg steam
curing.) Note that it is illegal to sell cement not bearing the SABS mark.

3.2 Aggregates

General

Natural aggregates used in the manufacture of concrete paving blocks should meet the requirements for aggregates for concrete given in SABS 1083:2002 Aggregates from
natural sources – Aggregates for concrete. Slag aggregates may also be used if they can be shown to be physically and chemically sound. Waste materials, or materials not in demand, are often sought after as these are generally relatively cheap. But the use of such materials could be at the expense of quality or result in increased costs due to
the need to use higher cement contents to maintain quality. These materials might also create compaction difficulties which could adversely affect productivity and durability.

The performance of aggregates at the moulding stage and in the hardened block depends on the combined effects of particle size, grading, particle shape, and hardness. Each of these properties is discussed below.

Size

The recommended maximum nominal size of aggregate is 13,2 mm. However, the maximum size generally used is 9,5 mm. Smaller sizes (-4,75 mm) may be used to suit circumstances or may be specifically selected to obtain a particular surface texture. Generally, the use of coarse particles results in savings in binder provided the mix is properly proportioned. If coarse aggregate particles are too big, or if too much coarse aggregate is used in the mix, it may be difficult to achieve good compaction and acceptable surface texture.

Grading

Continuous grading will facilitate compaction. Guidelines for grading are given in Table 1. If a material is unsatisfactorily graded for use on its own, good grading may be achieved by blending two or more materials. This is particularly true of crusher sands.

Table 1: Recommended aggregate grading for making paving blocks

Particle shape

Because paving blocks are manufactured from semi-dry mixes, chunky particle shape will facilitate compaction. This property is more likely to be found with natural sands. On the other hand, good green strength is required as the units are extruded and handled straight after compaction in the mould. Here crusher sand is suitable because of its
elongated particle shape. It may therefore be beneficial to use a blend of natural sand (for easy compaction) and crusher sand (for green strength).

Hardness

For concrete subject to abrasion, SABS 1083 specifies that coarse aggregate should have a minimum 10% fines aggregate crushing test (FACT) value of 110 kN. Fine
aggregate, ie crusher sand, derived from such rock will be satisfactory. However, there is no specification requirement in SABS 1083 for fine aggregate. Sands containing large amounts of unsound weathered material should be avoided. Natural sands with a high silica content are suitable.

Sand and stone chip particles derived from rock types available commercially can differ considerably in hardness, eg 10% FACT of limestone is about 100 kN; that of andesite lava could exceed 450 kN. Abrasion resistance depends on many factors but the single most important factor is the degree of cementing of particles at the surface. Factors like surface texture, shape, cement content, compaction and curing are therefore important.

All aggregates complying with SABS 1083 should give satisfactory wear performance. But, assuming that aggregate particles are well cemented at the surface of the block, for most modes of wear the service life of block paving can be extended by using the harder aggregate types. To enhance wear resistance, selected aggregates may be used in a richer topping layer about 15 mm thick moulded simultaneously with the base concrete.

3.3 Pigments

Quality pigments are commercially available to add colour to paving blocks. Dosage, which will depend on the colour selected and the natural colour of the aggregate and
cement being used, is generally 5%, but not more than 10%, by mass of the cement. Experimentation may be required to determine the correct dosage because the
colour of the finished product in a dry state is influenced by density, curing and surface texture.

3.4 Chemical admixtures

Concrete paving blocks are manufactured from semi-dry mixtures which possess poor flow properties even under vibration. Using a water-reducing or plasticizing admixture
to improve compactibility may be cost-effective.

4. Manufacturing equipment

4.1 Batching equipment

To ensure that a uniform product is obtained, weigh-batching is recommended. For smaller projects, whole bags of cement should be used if the sizes of batch and mixer
are compatible.

4.2 Mixer

Because a semi-dry mixture is used to mould concrete paving blocks, effective mixing can be done with pan and trough mixers. Drum-type mixers are unsuitable.

The size of the pan mixer must be related to production so that batches are used up within a reasonable time, ie before work-ability is reduced by moisture loss or hydration of the cement.

4.3 Moulding machine

Unlike bricks and blocks used for masonry, paving blocks must be dense (fullest possible compaction to be achieved). Equipment must be capable of a high degree of compaction and satisfactory output. A combination of vibration and pressure is the most effective way of achieving compaction. Moulding pressure should be 10 MPa or more. Stationary plants using the pallet system are almost exclusively used as they are capable of providing the necessary high levels of vibration and pressure. Hand machines and “egg-layers” are unlikely to produce a satisfactory product unless more cement is used.

4.4 Curing chamber

Newly moulded blocks should be subjected to some form of curing. The form of curing ranges from the prevention of moisture loss to the use of elevated temperature and high
humidity, eg steam curing. Further details are discussed in section 6.5.

5. Proportioning of ingredients

Proportioning involves finding the best aggregate grading, aggregate:cement ratio and water content, for the specific blockmaking equipment and the way in which it is operated. Each of these aspects is discussed in the following sections.

5.1 Establishing a suitable grading

In general, the aggregate should be graded to permit full compaction of the mix with the least effort. If full compaction is not achieved, voids have a disproportionate effect on
strength: 1% voidage reduces strength by about 6%.

Good compaction will be facilitated by using aggregates which are continuously graded (and have good particle shape).

A grading envelope for aggregates which has been found suitable in South Africa is given in section 3.2. The envelope should be used for guidance only as it does not take particle shape into account; materials having a grading outside the suggested envelope may give satisfactory results. Generally, the poorer the shape the greater the voidage and, usually, the greater the amount of aggregate fines, fillers or cement that is required to reduce or eliminate voidage. Typical overall gradings of all solid particles are given in Appendix 1.

Small stone is often incorporated into the mix, resulting in a coarser grading and lower cement content. The stone content is usually between 10 and 25%. However, stone in the mix may change the surface texture and could interfere with compaction of the mortar between stone particles, particularly at acute angles in the block profile. Compressive strength could be reduced even though density may be increased. Finer graded mixes are generally preferred for coloured blocks.

The above information should provide a reasonable starting point for production. Examination and testing of the product should indicate what further adjustments are required to achieve the desired density and surface texture. (Density and the degree of cementing of particles may be assessed by cutting the block through at right angles to the plane of compaction with a diamond saw.)

5.2 Cement content

The cement content to achieve the required strength level will depend on the following factors:

• Type of cement
• Required rate of strength gain
• Degree of compaction and resultant voidage
• Ambient temperature

It is not practicable to obtain information on some of these parameters by simulating site practice in a laboratory. The only accurate method of establishing the optimum cement content is through a series of trials, using the machine intended for production, in which cement content is varied and the physical properties monitored.

5.3 Water content

The optimum moisture content (OMC) for moulding depends on the materials being used, quality of vibration, and moulding equipment. Generally, the coarser the particles are graded and the greater the compactive effort, the lower will be the OMC. Using a moisture content below OMC will hamper good compaction and may necessitate longer periods of vibration which in turn will reduce output. Lack of compaction will reduce durability. Using too much water will result in a reduction of density and may cause units to stick in the mould and thus make extrusion difficult, or cause deformation of the units after extrusion.

6. Manufacture

6.1 Batching

The different aggregate types should be stored separately and in such a way that they are well drained. They should be protected from rain so as to remain reasonably dry
before being mixed with the cement, thus ensuring that the OMC is not exceeded.

The cement, except when batched by whole bags, should be weighed to an accuracy of 1%. Aggregates should be weighed to an accuracy of 2%. It is essential to make allowance for moisture contained in the aggregates by adjusting the weight of aggregate batched.

6.2 Mixing

After batching, the aggregates and binder are discharged separately or simultaneously, together with pigment if required, into the mixer and thoroughly mixed before any additional water is added. Mixes having high fines contents may require longer mixing.

Based on measurement of the variable moisture content of the aggregates or visual assessment by experienced personnel, water is added to the mixture to bring it to OMC. Uniformity is important because differences in water content from batch to batch will result in differences in quality.

It must be noted that certain pigments, because of their particle shape, can have a significant effect on OMC.

After mixing, the mixture (and topping mix if required) is stored in hoppers ready for dispensing into the moulds.

6.3 Filling of moulds

Most production machines operate on one of two basic principles. In the first and more widely used system, an amount of mixture is progressively compacted under vibration until a predetermined height has been reached. In the second system, a gauged quantity of mixture is compacted for a set period. In both cases, variations in density will result if the gauged quantities are not consistent or the mixture is not uniformly distributed within the mould. These variations are over and above those due to variations in moisture content. Filling is usually facilitated by a period of previbration after which the moulds are “topped up” with a second filling of mixture. Where topping layers are used the “topping up” is done using a special topping mix. For very cohesive mixes some difficulty may be experienced when filling the moulds.

6.4 Compaction

The optimum period of vibration must be determined experimentally in the plant but is usually 3 to 12 seconds. Good compaction is more difficult to achieve in thicker blocks and those that have acute angles. For this reason concrete pavers with a thickness greater than 80 mm are seldom manufactured.

Frequency and amplitude of vibration should be optimised for the specific materials being used and the number of blocks being moulded per cycle.

6.5 Curing

As with all concrete products, the quality of concrete pavers is improved by water curing. However, it is not practicable to apply significant amounts of water onto newly moulded pavers or subsequently to subject them to water sprays or
immersion in water.

The phenomenon of efflorescence further complicates curing in practice. After moulding and on drying, soluble calcium hydroxide may migrate to the surface of the block,
particularly if it is porous. Here the calcium hydroxide combines with carbon dioxide from the atmosphere to form less soluble calcium carbonate which is white in colour. Known as lime bloom, it is particularly noticeable on coloured pavers. Other than for aesthetic reasons, lime bloom is not a problem and will disappear with time by normal weathering and the action of rainwater which is mildly acidic. It can also be removed by chemical treatment. Unfortunately, the severity of lime bloom increases with effectiveness of wet curing.

In practice, curing can be effected in one of three ways: moisture retention, steam curing and thermal insulation. These methods are discussed in the following paragraphs.

Moisture retention

This is the least sophisticated method; it is especially effective for blocks that are less dense and thus tend to lose moisture rapidly. Loss of moisture from newly made blocks
is prevented by wrapping them in plastic sheeting. However, if blocks are stored under moist conditions whereby condensation can occur (eg overnight when pavers are stored under plastic sheeting), lime bloom and staining (colour changes) can still occur.

Steam curing

Although steam curing adds to production costs it is particularly good for early strength development, provision of adequate wear resistance and prevention of lime bloom and staining. This system is unaffected by seasonal changes. (Cement extenders respond well to elevated temperatures.)

Thermal curing

Newly moulded paving blocks are placed in an insulated chamber for a period of 24 hours. Temperature is increased by the heat of hydration of the cement and no additional heat is provided. The method is therefore inexpensive. A constant temperature of 35°C or more can be maintained with this system. A high humidity is provided from moisture within the units and good curing can be achieved.

After initial curing by one of the methods described above, the pavers are sufficiently hard for stacking, packaging, etc. Shrinkwrapping helps retain moisture for extended
hydration but might promote lime bloom.

7. Quality control

Control measures should be implemented to:

(a) Monitor changes in materials so that timely changes can be implemented in manufacture.
(b) Routinely monitor the quality of the product for compliance with the specification.

Measures under category (a) include:

• Regularly monitor changes in grading and moisture content of aggregates.
• Monitor mix consistence.
• Measure wet density of blocks (by weighing). Causes for a change in density must be investigated.

The more routine quality/compliance tests under category (b) include:

• Check on dimensions of blocks.
• Test strength of blocks.
• Carry out abrasion tests at an appropriate age. (The majority of available tests are too severe for testing at an early age before the cement paste has matured or developed good bond with the aggregates – the abrasion test is more useful as a tool for formulating the optimum mix proportion and manufacturing procedures necessary to ensure good wear resistance.)

Appendix
Typical gradings for aggregate and cement combined.

Cement & Concrete Institute
PO Box 168, Halfway House, 1685

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, 1997, reprinted 1999, 2000, 2005.
©Cement & Concrete Institute