Self help guide to readymix

Lafarge Readymix aims to be right first time, every time, delivering the correct product at the correct time to its customers. Nevertheless, it is important that the customers check delivery tickets carefully before any readymix concrete is discharged. The few minutes spent doing it properly could avoid a time-consuming, costly problem later. The procedure is particularly relevant on a large site where there can be several concerting gangs expecting deliveries of concrete with different specifications.

Delivery Notes

Check that the delivery note shows the correct quantity and grade of concrete being delivered, together with essential details of the concrete specification required for the particular job. Another important piece of information is the time of batching the load. This will indicate whether the transport time was normal in relation to the distance traveled or whether there has been an excessive delay between dispatch from the batch plant to site.

Time limits

Establishing a time limit on the acceptance of concrete as workable and suitable for placement is not straightforward. The rate of change in the properties of concrete depends on the mix constituents, including any special additives, the ambient temperature and general weather conditions. At what stage the concrete becomes unusable also depends on the nature of the job. As a result, the acceptance time period could be less than half an hour or more than six hours and the decision on acceptability is based more on experience than on an objective test.

In an attempt to overcome the reliance on operator judgment, some contract specifications lay down a maximum permitted time between batching and discharging. After which the concrete is deemed to be unsuitable. Unfortunately this can often create another problem.

During a pour, a load may be rejected due to the expiry of the arbitrary time limit but, in the time required to organize a replacement load, the pour site may experience a cold joint problem. In this case, the quality of the rejected load may still have been workable and the better solution could have been to accept the load and continue the pour without interruption. Conversely, the situation can arise in hot weather where a contactor finds that a concrete mix has stiffened unacceptably before the specified time period has lapsed.

The delivered mix may still be within the original tolerance limits, but the concrete is either too stiff or too wet and the originally specified workability is no longer valid. In such instances, Lafarge Readymix should be notified and the arrangements made to supply a concrete mix with workability suitable for the new conditions.

Control through workability

The workability of a mix is generally considered to be the key quality control indicator for production control purposes at the readymix batching plant and also for the on-site placement. Only when workability is satisfactory are the other properties of the concrete also likely to be correct. Each mix is designed to have a specific workability, which impacts on all the main characteristics of a concrete: with the control of workability being largely based on the free water/cement ratio, this influences final strength and durability, the quality of the surface finish and the ease with which the concrete is transported and compacted.

It is therefore essential that the correct level of workability is specified and maintained within the relevant tolerance. SANS method 861 specifies that the acceptable slump is within a tolerance of 25mm or 30% of the target slump.

When monitoring concrete workability as the primary quality control indicator, it is important to realize that the repeatability of the test results are very dependant on using a methodical testing procedure. Results obtained by two different people on the same concrete sample can differ significantly, especially if the correct test procedures are not rigorously followed.

These are three basic forms of cracking that occur in concrete slabs:

• Plastic shrinkage cracking
• Plastic settlement cracking
• Drying shrinkage cracking

The advantages associated with pumping readymix concrete are numerous. From a time and money saving perspective, it often cannot be matched. When it comes to placing concrete in difficult situations, pumping is generally the only viable option.

Reasons to use the Lafarge pumping service

Quality

Easy placement of high quality concrete of uniform consistency. 
Pumped concrete is flowable yet highly cohesive. 
Pumped concrete is formulated to allow pumping without segregating and with minimal bleeding.

Saving time

The site or the element to be concreted may be difficult to reach. Pumps can bypass this problem quickly
Pumps place concrete at higher speeds than alternatives. 
For high rise projects, placing concrete at various levels is quick and easy.

Saving money

Pumping reduces plant costs. 
Pumping reduces labour costs.

Convenience

Lafarge Readymix pumping removes many of the handling and logistical problems associated with working with large amounts of concrete. 
Pumping means concrete is placed directly where needed, easily and efficiently. 

Pumped concrete in residential applications

Removes the need to build ramps to wheelbarrow concrete onto first floor decks. 
Removes the need to break down garden walls and destroy established gardens to allow for access to Readymix trucks or other concrete material delivery vehicles.

Tenders and quotations

If it is decided that the concrete will have to be pumped, there will be a cost for setting up the pump adjustments to mix proportions and the cost of concrete to accommodate pumping.

Acceptance and confirmation

Confirm the date and time of your order when the quotation is accepted, or arrange to confirm these details later - at least 72 hours in advance to avoid cancellation fees.

Pre-pumping site inspection

To plan for a pump job, a Lafarge Readymix representative will inspect the site to check:

• Access for the pump and truck mixer to manoeuvre on site.
• Equipment needed to complete the job as efficiently as possible.
• Whether the operation will be done using a truck-mounted boom pump or a static pump.
• Whether there is adequate water supply and washout area on site.
• Safety aspects on site.
• Special site requirements- some residential areas have curfews on construction work.
• That the customer is aware of the pump priming requirements.

Note: if Lafarge considers that the site is unsafe, corrective action may be required before the pour is undertaken.

Communication

There must be close communication between the contractor, the Lafarge pumping team and the Lafarge batch plant.

Access

A truck mounted pump and mixers need good access to the site. A truck mixer loaded with 6m³ of concrete weighs 30 tons. The vehicle is 8m long and 2,8m wide. A pump would require a maximum space of 12m in length and 9m width.

Washout area

A washout area should be provided so that truck chutes can be cleaned and the pump and pipeline segments washed out. Wash water should not be allowed to drain into the sewage or storm water systems. If there is no place to dispose of washout water, prepare a sandbagged area.

Safety

Safety aspects are particularly important when pumping. High pressures are used to force the concrete through the pipeline. All staff not directly involved should keep clear of the area when pumping is taking place. No one should work underneath the boom, in case of boom failure.

Finishing equipment

More equipment for compacting and finishing will be needed because the concrete is discharged faster. A rough guideline is one vibrator for each 10m³ per hour of concrete placed. Pumping capacity varies according to the pump specifications and application. A guideline is between 20m³ and 60m³ per hour, depending on the project requirements.

Larger or more complicated pours

Large pours may require more than one pump. High rise buildings may entail the installation of a static pump and a pipeline, which will be left in place throughout the building process. In these cases, the pre-pumping site inspection will be arranged well in advance and the engineer may need to be involved. This needs to be made known to Lafarge at the time of the tender.

Pours over long distances can be handled by Lafarge Readymix. Distances of up to 200m are not uncommon, however it should be remembered that the Lafarge Technical Department will need to adjust the mix design for this purpose.

Concrete placing and compaction has to be right first time. The degree of success will not be fully known until the hardened concrete can be inspected. At this stage it will be difficult and expensive to correct any defects. Important aspects of managing the placement process are the selection of suitable equipment and correctly trained staff.

Productivity

A placement method that transports and places the concrete in a single operation is the most productive approach.

The concreting team needs to know what action to take if the appearance of any concrete is abnormal. While being focused on productivity, they must also know that it cannot be achieved at the cost of quality.

Dropping concrete

Developments in concrete technology allow concrete to be dropped much further than before. Whereas traditional construction methods for columns involved numerous construction joints, it is now possible - with the involvement of the contractor, specifier and Lafarge Readymix - to design concrete to pour a 15 meter high column in a single lift. A special concrete has to be designed to meet this special demand. Also, should such a pour be undertaken it is important to increase the strength of the formwork to cope with the increased hydrostatic pressure of the concrete.

Dropping inappropriate concrete over a long distance or allowing it to fall through congested reinforcement will produce honeycombing and a poor standard of surface finish.

Careful placing

It is important to discharge the concrete as close as possible to where it is required in order to reduce the risk of segregation, which could lead to honeycombing or unsightly grout lines. The concrete should be evenly spread before compaction. Unless there is careful control of concrete layers, thicker sections may be inadequately compacted. Where it is not possible to place the concrete in its final position (e.g. in the corner of a wall), it should be placed as close as possible and then made to flow into place with a poker vibrator. The concrete must still be compacted afterwards.

Defing compaction

Compaction of concrete is the process of removing air bubbles that have become entrapped during mixing and discharge of the concrete from the drum or other handling equipment. If the air is not removed the appearance, strength and durability of the concrete will be inferior. Entrapped voids or pockets of air have a natural tendency to rise, but in general concrete is not sufficiently fluid to allow air to escape.

The role of compaction is to make concrete more fluid so that entrapped air is able to rise to the surface. Properly compacted concrete is dense and impermeable. The structure being built is correspondingly stronger and more durable. Generally, every 1 % of air that is trapped in the concrete results in the loss of 5 % of design strength.

Compaction methods:

Poker vibrators

The fundamental principle of compaction is to use the most powerful vibrator that can be conveniently handled. The usual method of imparting vibration is by using poker vibrators. However, even large diameter pokers have a limit to the radius of their compacting effect and need to be inserted in an overlapping pattern so that all the concrete is properly vibrated. This is particularly important with walls where reinforcement makes it difficult to position the poker closer to the form face. In this case, insertions must be made at closer centres to ensure the important cover zone will be adequately compacted.

When the first concrete is placed, the lower vibrators should already be running so that the removal of air is progressive from the bottom upwards. It is a mistake to place a thick layer of concrete before inserting the poker as the compaction of the top layer would prevent air trapped down from escaping. Vibration should continue until no further air bubbles appear on the surface. It is never possible to achieve 100% compaction and some small blow-holes will always be evident on a vertical concrete surface.

External vibrators

It may not be practical to use poker vibrators on slender columns and walls containing congested reinforcement. In such cases, external vibrators clamped to the form work will work best. It is essential that the formwork is sufficiently robust to transmit the compactive force to the external vibrator, in addition to the hydraulic load of the concrete.

Beam vibrators

Beam vibrators are used to compact thin slabs providing sufficient concrete coverage can be maintained. However, at the edge of the slab, much of the energy will have been dissipated and proper compaction will require the use of poker vibrators aswell.

How compaction affects quality

Lack of proper compaction results in:

• Poor appearance: as a result of air voids and honeycombing.
• Reduced strength: every 1% of air causes drop in strength of between 5 and 6%. In other words, 5% under compaction produces around 25-30% loss of concrete making it vulnerable to deterioration form:
• Freeze/thaw effects.
• Sulpahtes in solution.
• Increased rates of carbonation, particularly where chlorides are present.
• Corrosion of reinforcement steel.

Note: Air voids produced in concrete by an air en-training agent should not be confused with entrapped air. Entrapped air exists in randomly distributed pockets and has relatively large bubble sizes. Entrained air consists of tiny, discreet bubbles dispersed throughout the concrete that have a beneficial on durability by allowing the concrete to expand without any adverse structural effects.

Over vibration

As compaction continues, the air pockets become fewer and the quality of the concrete is enhanced. As a general rule, it is not possible to over-compact concrete. In any event, the possible consequences of over vibration are less serious than the real problems arising from stopping compaction prematurely.

Exceptions:

• Very high workability concrete: segregation of the sand and aggregate may occur.
• Inappropriate mix or difficult aggregate: if a mix is prone to segregation, the aggregate may sink leaving a layer at the top of deep lifts that is deficient in coarse aggregate. This is rarely a problem.
• Exposed aggregate architectural finishes: in the case of over-vibration, the aggregate deficient top layer may simply have to be removed and replaced with a quantity of fresh concrete.

Correct specification, mix design and careful placing generally minimize any segregation problems caused by over-vibration.

Procedure

• Concrete can be ordered telephonically via our central orders departments in the various regions.
• Place orders at least 24 hours in advance.
• Confirm your order by 12h00 the day before delivery.
• All account customers must fax an official order number to our Central Orders Department.
• Same day orders will be accommodated if our delivery schedule allows it and the available time is suitable for the customers needs.
• Special concrete mix requirements can be met, subject to the outcome of trials in our labs by our technical team.
• When placing orders, customers will be asked about the application as well as the strength of concrete they require.
• If help is required to specify the concrete mix design, please contact the local Lafarge Technical Department. 
  

After hours and weekend deliveries

• After hours pours should be arranged with the Lafarge District Sales Manager.
• After hours pours will incur extra costs.
• Requirements for pours on Sundays should be discussed with the Lafarge Districts Sales Manager.
• Sunday deliveries will also incur extra costs. 
  

Cancellations

• Cancellations for concrete orders must be done by 15h00 at the latest on the day prior to the pour.
• Late cancellations will incur extra cost. If the concrete has been loaded before cancellation, the customer will be charged for the full load of concrete. 
  

Payments

• Orders being paid for in cash will only be confirmed once payment has been received.
• Payments by cheque to the value of R5000 and over must be bank guaranteed.
• Internet payments must be done at least 48 hours in advance.
• Customers should fax the proof of payment through to the Lafarge Central Orders Department at least four working hours before dispatch of the order is required. 
  

Part load charges

• Loads less than 4.5m³ will attract a part load charge.
  

Pumping

• The Lafarge concrete pumping service needs to be booked one week in advance.
• At least 24 hour notice should be given to cancel a pump booking. Cancellation of a pump on the same day will incur a cancellation fee. 
  

Quality control

The Lafarge Technical Department conducts random quality control testing on all Readymix concrete that the company delivers.

Terms and Conditions

Download a copy of our trading terms and conditions

Before starting a project, a decision should be taken between using Readymix or Site mix. There are numerous benefits of using Lafarge Readymix compared with mixing concrete on site. Some of the more important ones are:

Technical

Lafarge Readymix has the resources and technical expertise to provide a range of mixes assuring the user of the right mix design for the application. This is more difficult with small batching operations.

Flexibility

Readymix concrete is better equipped to cope with changes in weather conditions or the construction programme and maintain the projects schedule.

Time

The ease of using ready mixed concrete means that one delivery may be efficiently distributed to various positions on site. Site mixing tends to be time-consuming compared with the speed of discharge of Lafarge Readymix concrete, especially when larger volumes are involved.

Quality control

The quality of the finished concrete should always be of primary concern. Lafarge Readymix delivers a guaranteed product which reduces the risk of quality problems. Product from batching plants, particularly the smaller less sophisticated units, is exposed to a higher risk of failure or lack of consistency.

Access

Some sites have insufficient space to set up a batch plant: ready mixed concrete provides the perfect solution. Where access is also restricted, Lafarge Readymix offers a pumping service to move the concrete across difficult distances and heights.

Cost effective

Ready mixed concrete is generally the cost effective solution when all cost factors of mixing concrete on site are taken into account. These include the total cost of materials, storage, wastage and theft, hiring and operation of the necessary plant, labour and supervision of an on-site batch plant.

Environmental

• Noise: the noise level of a batching plant can become a problem if the site is in a residential area. Lafarge Readymix is able to discharge the concrete at a rate of at least 1m³ every two minutes and is therefore only on-site for a short period of time.
• Pollution: batch plants have to be well managed to prevent contamination or blockage of storm-water drains.
• Clean up: environmental concerns are reduced when raw materials are not stored on site and a major clean-up operation is avoided at the end of the project.

The Association

Lafarge Readymix is a member of the South African Readymix Association (SARMA) which represents all the reputable Readymix companies in the country. SARMA sets standards and guidelines with regard to the protection of the environment, safety and quality issues. All members are expected to abide by them and are audited regularly to ensure they meet the necessary requirements.

The method and issues associated with the cutting and sealing of joints in concrete floors are summarized bellow.

Cutting

Standard method of cutting

Sawing should be performed by experienced personnel who are generally best qualified to determine the correct time to saw. The correct time for sawing is when sawing causes only slight raveling. If no raveling occurs, it might be too late to prevent some uncontrolled cracking. This will generally be within 24 hours after casting. Cut joints a minimum of one quarter of the slab thickness (i.e. 25mm deep for a 100mm slab thickness). Limit rectangular panel length to no more than one and a half times the short side; no more than 30 times the depth of the slab and do not exceed 4,5m panels.
During late sawing, the concrete may crack ahead of the saw before the joint is completed. When this happens, sawing of that particular joint should be discontinued, and the next joint sawn immediately. The crack will act as a joint and should be routed and sealed. Failure to discontinue sawing of a joint when cracking occurs ahead of the saw will almost certainly result in spalling of the concrete between the joint and the crack. Every attempt should be made to saw joints in succession. This procedure will limit joint opening and improve floor performance. However, if uncontrolled cracking occurs repeatedly, changes in procedure will be required. This may involve sawing every second or third joint first and returning later to saw the intermediate joints.

Sawing of contraction joints

If contraction joints are sawn while the forms are in place, the cut cannot be extended to the slab edge. Uncut edges should be sawn after the forms have been removed. Contraction joints should be cut to a depth of one quarter of the thickness of the slab to ensure that the cracks from bellow the saw cuts
The joints should be cleaned immediately after sawing, and all sawing residue removed from the slab surface. Contraction joints should be sawn before uncontrolled cracking occurs, but not so early that it causes dislodgement of coarse aggregates or excessive raveling of joint edges
Manufacturers of saw blade should be consulted about the proper choice of blades for the aggregates being used. An unsuitable choice of was blades can result in slow cutting and excessive blade wear.

Joint sealing

Preparations for sealing

Prior to sealing, damaged joint edges (if any) must be repaired with either an epoxy or a cementitious based repair mortar specially developed for this application. Joint surfaces must be clean and free of residue and any other foreign material. Cleaning can be done with water or compressed air, sandblasting or high-pressure water blast, depending on the joint surface condition and the sealant manufacturer's recommendations.
Proper cleaning is essential in order to obtain a joint surface that will not impair bond or adhesion with a field-moulded sealant. Surfaces must be dry when the sealant

Installation of liquid sealant

If it is recommended that this type of sealant should be used

There are two types of liquid sealant: a two-part system and a single-part system. Normally the single-part system would be used only for small jobs as it is more expensive.
When mixing a two-part system, it is critical that the components are mixed strictly in accordance with the manufacturers recommendations. If this step is not followed the sealant could possibly not set, which will result in the sealant having to be removed
Apply the recommended primer to the joint faces in a thin film by brush or airless spray equipment. The primer should completely wet the surface and should be track-free prior to installation of the flexible bond-breaking cord
The bond-breaking cord should be examined before sealing and reapplied if necessary. It should not be stretched or twisted during installation. If the bond-breaking cord is left out, the joint could fail due to poor joint design
Masking tape should be taped on either side of the joint to ensure that the sealant does not dirty the new floors. Installation of liquid sealants should be such that the sealant will adhere to the concrete and be free of voids. The depth to which the sealant is applied should be half the breadth of the joint. For example, if the joint was 5mm wide, the depth to which the sealant is applied would be 2,5mm. Should this depth be exceeded, the sealant would not be able to stretch as the joint moves and could delaminate from the sides. In extreme cases it may even de-bond from the sides of the joint. Low joints should be spot treated to bring within specification. Immediately after the sealant is applied it should be tooled to promote bonding with the joint faces. Excess material on the surface should be removed and the slab surface left in a clean condition. A smooth surface can be achieved by wetting the finishing tool with a soupy water solution
The curing period of the liquid sealant should be in accordance with the manufacturers recommendations

Installation of preformed seals

Preformed neoprene compression seals should be installed without twisting, distortion or deliberate stretching. The faces of the joint should be surface dry and the atmospheric and pavement temperatures should be above 0oC at the time of installation.

Lafarge Aggregates and Readymix have high standard SANS accredited laboratories in its region of operation to carry out rigorous quality assurance testing on all its products. The routine tests are conducted in accordance with SANS standards.

The Lafarge laboratories also provide an unequalled service to customers who may need assistance to comply with any other established standards or with problem solving. Successful testing requires good sampling procedures. These are outlined below:

Sampling concrete from a readymix truck (SANS Method 861-2:2004)

Purpose

To lay down a procedure to achieve consistent representative samples. If the sampling is not done correctly, the test results will be incorrect and invalid.

Equipment

Corrosion resistant metal scope. 
Shovel and wheelbarrow or other suitable container.

Procedure

• Take the samples from the stream of concrete being discharged from the truck's mixing drum. Do not sample the first and last 10% percent of the load.
• Hold the sampling scoop as close to the discharge point as possible. The concrete should not be allowed to freefall more than 500mm.
• Take at least nine samples at equally spaced intervals during the discharge.
• Mix the nine samples to ensure the overall sample is uniform.
• During all stages of sampling, handling and transporting, protect the sample from water, excessive heat and segregation.

Slump testing (SANS Method 862-1:2004)

Purpose

The slump test is a measure of the workability of a concrete mix.

Equipment

Slump mould (‘cone') with a base diameter of 200mm (+-2mm), a top diameter of 100mm (+-2mm) and a height of 300mm (+-2mm). The mould must have a smooth inside surface and be clean. 
A mixing tray, flat steel plate, corrosion resistance scoop and a round tamping rod (nominal diameter of 16mm and a length of 600mm +-2mm).

Procedure

• Moisten the cone, place it on a steal plate and stand on the foot pieces.
• Scoop the sample into the cone in three layers, each about a third of the height of the cone. Tamp each layer 25 strokes with the tamping rod, spread evenly over the whole surface area of each layer.
• Make sure the rod passes into the second and third layers into the layer below. When tamping the last layer, ensure the surface of the concrete remains above the top of the cone.
• Use the tamping rod in a rolling/sawing motion to scrape off the excess concrete.
• Without disturbing the cone, clean off any concrete sticking to the outside of the cone.
• Slowly and carefully lift the cone vertically upwards and off the concrete.
• The aim is to avoid knocking the concrete sideways. The entire process from filling the cone to removing it should be carried out within two and a half minutes.
• Immediately after removing the cone, the slump measurement must be taken. Holding the tamping rod horizontally across the cone, measure the distance between the bottom of the tamping rod and the highest point of the concrete to the nearest 5mm. this shows how far the cone of concrete collapsed or ‘slumped' after the mould was removed.
• The SANS specification allows a tolerance of 25mm or 30% above or below the design target slump.

Making and curing test cubes (SANS Method 861-3:2004)

Purpose

Test tubes of concrete are used to check the strength of the concrete at various stages.

Equipment

The moulds will either be 100mm square or 150mm square +-1% in compliance with SANS Method 861. Normally made from steal or another non-absorbent material, the mould is constructed so that the sample is not damaged when the mould is removed. The mould must be oiled with a good quality mould release agent to prevent the samples from sticking to the sides .
A tamping rod (the sane as the rod used for the slump test) plus a steal float/trowel.

Procedure

• Mix the sample of concrete thoroughly in a mixing tray/wheelbarrow or other suitable container.
• Fill about one third of the mould with the sample.
• Using the round end of the tamping bar, tamp each layer uniformly all over, especially in the corners. Tamp each layer at least 45 times for 150mm cubes and 20 times for 100mm cubes
  After tamping each layer, tap two faces of the mould with a rubber mallet five times to collapse any voids in the concrete.
• Remove surplus concrete with a steel float, scrapping the top of the concrete cube level with the top of the mould.
• Cover the mould with a damp cloth and a plastic sheet. Once the mould is covered, it should be stored in the shade, sheltered from the wind and any vibration on site until the test cube has reached sufficient strength to be stripped from the mould without damage. (typically 12 to 18 hours).
• A test consists of making a set of at least four cubes, preferably six: one (or three) for crushing at seven days and three for crushing at 28 days. On larger projects, SANS method 1200g specifies at least one set of cubes to be taken each day and from at least 50m3 of each grade of concrete placed.

Storing cubes

Handle the mould with care, keeping them protected extreme cold and wet conditions. The cubes should be stripped from the moulds within 12 to 18 hours of casting, marked and put into a curing tank. New cubes may be fairly soft and should be handled carefully.

Part the sides of the mould by tapping gently with a hammer and lift off. Do not forget to number the cubes before putting them into the curing tank.

Check that the tanks thermostat is set between 22 and 25oC and that the cubes are completely covered by the water. Arrange that the cubes are taken to the laboratory to check for crushing strength after the specified 7 day and 28 day curing period.

Using Readymix concrete for smaller projects can save time, effort and ultimately money. Some of the benefits of Lafarge Readymix concrete are outlined below.

The possible problems with multiple suppliers:

Mixing concrete on site means dealing with multiple material suppliers and the associated:

• Stress
• Work stoppages owing to late deliveries
• Work stoppages, overtime and down time
• Additional costs

Lafarge Readymix offers you:

• One reputable supplier
• A competitive price
• Scheduling assistance
• On-time deliveries

Mess, storage space and raw materials

Using Readymix reduces the mess associated with building as well as the space required for stockpiling materials.

The use of Readymix helps prevent:

• Clean-up costs for the garden and house interior
• Complaints from neighbours
• The house becoming a neighbourhood eyesore
• The cost of rubble removal
• Excess space being used for stockpiling materials
• The need to get council permit for stockpiling of materials on the pavement and the expenses involved

In addition Lafarge Readymix will help you to:

• Reduce handling costs of materials
• Eliminate wastage
• Improve profitability
• Ease planning and scheduling
• Ensure guaranteed strength
• Limit haulage costs by eliminating multiple material suppliers
• Shorten construction time

Labour

The use of Lafarge Readymix can reduce the number of labourers required on site, who would normally be used for mixing (by hand or machine) and handling the concrete

Equipment hire

• Dealing with Lafarge Readymix will eliminate the need to hire a concrete mixer, resulting in considerable savings
• In addition, a Lafarge mixer truck has a higher discharge rate than a concrete mixer, resulting in better productivity

Delivery

Lafarge Readymix will deliver concrete to where it is needed, usually directly into the pour site, saving both money and time

South African Readymix Association (SARMA)

Lafarge Readymix is a member of the South African Readymix Association (SARMA) which represents all the reputable Readymix companies in the country. SARMA sets standards and guidelines with regard to the protection of the environment, safety and quality issues. All members are expected to abide by them and are audited regularly to ensure they meet the necessary requirements.

Ambient temperature has a significant effect on the setting time of concrete. The recommended practices during cold weather are outlined below:

Introduction

The chemical combination of cement and water is known as hydration, which produces a strong binding medium for the aggregates in concrete.

The hydration reaction is exothermic, resulting in the liberation of heat. The rate of hydration (and the amount of heat liberated) determines the setting time of concrete.

This is dependant upon the composition and fineness of the cement, as well as the ambient conditions, particularly temperature.

Temperature effect

In cold ambient temperatures, concrete is very likely to have an extended setting time due to a slower rate of hydration. In such cases, the following points should be considered:

• The timing of finishing, such as power-floating of floor slabs, will be critical. In cold conditions, the concrete will bleed for a longer period. Therefore, it will take longer for the concrete to stiffen before power-floating and other finishing operations can start.
• Cutting of joints in slabs will also be critical. With a slower rate of strength gain, it may be necessary to delay cutting of joints to prevent stones from being plucked from the concrete. Conversely, care must be taken not to cut joints too late, since the concrete may have shrunk sufficiently to start forming cracks by then.
• Striking times of form work should be extended.
• Plastic shrinkage cracks may occur due to the longer period before the concrete sets.
• Cubes should be cured in water that is constantly maintained between 22 and 25oC.

Experience has shown that delayed setting has no effect on the structural integrity of the concrete. In fact, a higher compressive strength is ultimately achieved due to the extended hydration of the cement.

Measures to improve early strenghts

In order to reduce or eliminate problems with lack of early strengths in cold weather, the following should be considered:

• Use of a higher grade concrete is strongly recommended.
• Because of their slower setting and strength gain, the use of high percentages of extenders such as blast-furnace slag is not recommended.
• Insulating the structure or the concrete surface.
• Increasing the curing time prior to removal of formwork.
• The use of an accelerated admixture, where permissible.

This Lafarge Readymix guide is intended to help the first time house builder to cast foundations. It is applicable to foundations for brick walls around a house, extensions of an existing house or for a new house.

THE National Homebuilders Association Council's (NHBRC) requirements for house foundations:

In stable soil conditions, the minimum width of strip foundations for single storey buildings should be 500mm for external walls and 400mm for internal walls (for a house with a tiled or sheeted roof). If the house will have a reinforced concrete roof, the minimum width is 750mm for external walls and 600mm for internal walls.

The minimum depth of the foundation must be 200mm. the strength of the concrete used for unreinforced strip footing must be 10MPa or better. Reinforced foundations (e.g. for double storey buildings) would require 25MPa.

Excavation

Trim the sides of the excavation to avoid having extra holes on the side faces being filled with concrete, which would lead to requiring more concrete than had been ordered.

After excavation measure the actual depth and width of the trench, and calculate the required volume of concrete using these actual figures.

Ordering

• The strength of concrete for foundations will range from 10MPa to 25MPa depending on the structure to be built on the foundation.
• Lafarge will design a concrete mix specifically for house foundations which can contain blends of OPC and slag or fly ash.
• The volume of concrete ordered from the Lafarge Readymix Orders Department should be according to the actual measured figures of the trenches (normally the theoretical calculated volume plus 10% for over excavation) plus an additional 10% to allow for wastage and unforeseen circumstances.

Placing

• Clear the excavated material away from the edges of the trench to prevent it falling back into the trench and contaminating the concrete.
• To prevent loose soil falling into trenches due to the weight of the wheelbarrows, place planks along the edges with soft bedding such as fill material.
• Wet the excavation before placing the concrete.
• Compact the concrete to remove air bubbles. This will maximize density and strength.

Curing

Fresh concrete needs to be protected from drying out. Spray water over the foundation as soon as the concrete has set or alternatively the concrete can be covered with plastic sheeting.

Note: when concrete looses moisture either by evaporation or absorption by the surrounding soil, it shrinks. If the concrete is not free to move, it will crack.

When can bricks be laid on the foundation?

Forty-eight hours after placing the concrete, build the first 500mm of brickwork to above the ground level. Then, wet the concrete thoroughly and cover with soil. Seven days after placing the concrete, the walls can be built to roof level. At this stage, the concrete mixes will have gained 60% of the designed strength.