Concrete Supply, Placement and Testing on Civil Projects

Ready mixed concrete is the material a civil contractor uses most confidently and controls least. It is ordered by phone, arrives in a truck, goes in the hole, and four weeks later a laboratory sends a number that either passes or does not. In between are a dozen decisions — how long the truck waited, whether water was added, how it was compacted, whether it was cured, where the sample was taken — and every one of them affects the number.

Concrete non-conformances are among the most common on civil projects, and the great majority are process failures rather than material failures. The supplier delivered what was ordered; something between the chute and the cylinder went wrong.

This guide covers ready mixed concrete as a controlled process: what the specification is actually asking for, how supply works, what the pre-pour check must catch, how testing works and what to do when a break comes back low. Precast elements are covered separately in our guide to precast concrete supply, delivery and erection.

Why concrete generates so many non-conformances

• The result arrives long after the work. By the time a low break is known, the element is buried, loaded or built on.
• It cannot be undone. Unlike most defects, a bad pour is removed and replaced, and removal is often harder than the original placement.
• The variables are invisible. Water added on site, over-vibration, segregation and poor curing all produce concrete that looks identical to good concrete.
• The pressure is always in one direction. Everyone on site wants the pour to keep moving, and every shortcut that makes it move faster makes the concrete worse.
• Sampling and testing are themselves error-prone, and a low result may reflect a badly made cylinder rather than bad concrete.
• Responsibility is split between supplier, contractor and testing laboratory, which is why a low result immediately becomes an argument.

Reading the concrete specification

Concrete is specified by more than a strength number, and the other parameters are where the cost and the compliance sit.

Parameter	What it controls	Where it bites
Characteristic strength	Structural capacity	The headline number, and the one everyone quotes
Exposure classification	Durability in the environment	Drives minimum strength, cement content and cover; marine and wastewater are severe
Cover to reinforcement	Durability and fire	A site control, measured, and a common non-conformance
Slump or consistency	Workability at placement	The parameter site staff are most tempted to alter
Maximum aggregate size	Placement in congested reinforcement	Determines whether the mix can actually get around the steel
Cement or binder type	Durability, heat, sulfate resistance	Supplementary materials change strength gain rate
Special requirements	Shrinkage, heat of hydration, water tightness, finish	Trial mixes and additional testing, with lead time

Two specification items catch tenderers out. Special-class concrete, where the mix must be designed and trialled and approved before use, carries weeks of lead time. And exposure classification frequently drives a higher grade than the structural design needs, which changes the rate you should be pricing.

Where the client’s specification adds requirements on top of the standard — additional testing frequency, restricted supplementary materials, particular curing methods — those are cost items, and they belong in the price rather than in a later claim. Our guide to preparing civil works cost estimates covers how these get missed.

Supply: the plant, the order and the docket

• Plant certification. Most authority specifications require supply from a plant with recognised third-party certification. Confirm the specific plant, not the company.
• Mix registration or approval. Many specifications require the mix design to be submitted and accepted before use. Do this early.
• Batching distance and travel time. There is a limit on the time between batching and discharge, and a plant an hour away in traffic may not be able to comply. This is a supplier selection criterion, not a delivery detail.
• Supply rate. A large pour needs trucks arriving at a rate the plant can sustain, and the plant is also supplying other customers. Book the pour, confirm the rate, and know what happens if a truck breaks down.
• The delivery docket is the primary record — mix identification, batch time, quantity, and any water added. Keep every one of them; they are the evidence in any later dispute.
• Water added on site must be recorded on the docket and is limited. Adding water to make a stiff load placeable is the single most damaging thing done routinely on Australian civil sites, and it is usually done because someone did not want to reject a load.
• Rejecting a load is a decision that must be available to the site engineer without an argument. Agree in advance who can reject and on what basis.
• Admixtures added at the plant or on site, per the supplier’s instruction.
• Supplier’s technical support. A good supplier will attend a significant pour. Ask.

The pre-pour: the checklist that prevents most problems

The pre-pour inspection is a hold point in almost every civil ITP, and it is the last moment anything can be fixed. Our guide to quality management plans and ITPs covers how hold points work; this is what the concrete one has to cover.

• Foundation — level, prepared, proof-rolled or inspected as required, and not softened by rain since.
• Formwork — dimensionally correct, clean, released, tight against grout loss, and adequately braced. Formwork is engineered temporary works when it is significant, and formwork failure during a pour is a serious safety event.
• Reinforcement — correct bar sizes, spacing, laps, and quantity against the drawing.
• Cover — bar chairs and spacers of the right height, at the right spacing, of a type suited to the exposure. This is the most common cause of a durability non-conformance and it takes minutes to check.
• Cast-in items — conduits, ferrules, dowels, waterstops, holding-down bolts, lifting inserts — in position and secured, checked against the drawings and the shop drawings of whatever connects to them.
• Joints — construction joint prepared, waterstop continuous and correctly positioned.
• Access for placement — how the concrete actually reaches every part of the element.
• Plant on site and working — vibrators, and a spare vibrator, because a failed poker mid-pour is a defect in the making.
• Curing materials on site before the pour starts, not ordered during it.
• Weather forecast for the pour and the following day.
• Contingency — what happens if the supply stops, and where the emergency construction joint goes if it does. Decide before, not during.
• Testing arranged — the laboratory booked and attending.
• Hold point released in writing by whoever the ITP nominates, before any concrete is ordered.

The emergency joint decision is the one nobody makes in advance, and it is the reason a stopped pour becomes a structural problem instead of an inconvenience.

Placement, compaction and the things that go wrong

• Free fall and segregation. Dropping concrete a long way separates the aggregate from the paste. Use a tremie, chute or pump line into deep elements.
• Layer thickness. Place in layers the vibrator can penetrate, and vibrate into the layer below to knit them together.
• Compaction. Systematic vibration at a regular spacing, vertical insertion, withdrawn slowly. Under-vibration leaves honeycombing and voids; over-vibration segregates.
• Never move concrete with a vibrator. It is the most common misuse and it segregates the mix.
• Congested reinforcement — the mix and the aggregate size must suit the bar spacing, and where they do not, the concrete will not fill.
• Pumping changes the mix requirements and the practicalities; pump line blockages are a pressure hazard and a stoppage risk.
• Cold joints from a delay between loads, which is a defect and which is why supply rate matters.
• Formwork movement and grout loss during the pour, monitored by someone whose job that is.
• Concrete burns. Wet concrete is caustic; skin contact causes serious chemical burns that develop hours later, and personal protective equipment for this is not optional.
• Silica. Cutting, grinding and drilling cured concrete generates respirable crystalline silica, which is a regulated exposure — see our guide to noise, vibration and dust management.

Hot weather, cold weather and rain

• Hot weather accelerates setting, reduces workability, increases the temptation to add water, and drives plastic shrinkage cracking. Concrete temperature limits are specified, and mitigation includes early starts, night pours, chilled water or ice, shading, and windbreaks.
• Plastic shrinkage cracking is driven by evaporation from the surface, which is a function of air temperature, concrete temperature, humidity and wind. Wind is the underrated one; a hot dry windy day is the worst combination and it can crack a slab within hours.
• Cold weather slows strength gain, which delays formwork stripping and loading, and in alpine areas freezing fresh concrete is a genuine risk.
• Rain during or shortly after placement damages the surface and can raise the water content. Have covers on site.
• The decision to postpone. Have a documented basis for deciding not to pour, so the decision is made on the forecast rather than on the pressure in the room.

Australian conditions make hot weather concreting the normal case rather than the exception across much of the country, and it should be a standing part of the method rather than a response. Weather delay and its contractual treatment is covered in our guide to extension of time and delay claims.

Finishing and joints

• Finish type as specified — broom, float, trowel, exposed aggregate — and for pavements, the surface texture and skid resistance requirement.
• Do not finish bleed water into the surface. Working water back into the top layer produces a weak, dusting, scaling surface, and it is the most common finishing defect.
• Timing. Finishing operations have a window, and it moves with the weather.
• Construction joints at planned locations, prepared for the next pour — roughened, clean, with reinforcement continuing through.
• Contraction and expansion joints at the specified spacing, cut or formed, with the right sealant and backing.
• Saw cutting timing is critical: too early and the edge ravels, too late and the slab has already cracked. This window is short and weather-dependent.
• Dowels and tie bars aligned. Misaligned dowels lock a joint and crack the slab either side of it.
• Levels and tolerances, surveyed and recorded.

Curing: the cheapest quality step and the most skipped

Curing has more effect on the durability of the finished element than almost any other site decision, and it is done after the interesting part is over, by whoever is left. That combination is why it gets skipped.

• Start immediately. The critical period is the first hours, particularly for surfaces exposed to sun and wind.
• Methods — water curing, wet hessian, plastic sheeting, curing compound, leaving formwork in place — each specified for particular situations and not freely interchangeable.
• Curing compound compatibility. Some compounds prevent adhesion of later toppings, membranes or sealants. Check before spraying.
• Duration is specified and is longer for higher exposure classifications and for mixes with high supplementary cementitious content.
• Record it. Curing is a conformance item and “we cured it” is not a record.
• Poor curing shows up as surface scaling, dusting, reduced abrasion resistance, cracking, and reduced cover-zone durability — which is exactly the layer protecting the reinforcement.

Testing: sampling, cylinders and what the results mean

• Who tests. Testing is normally by a laboratory with recognised accreditation for the specific test methods. Check the accreditation covers what they are doing.
• Sampling frequency is specified, usually per volume, per element, per day or per mix, whichever gives more samples.
• Where the sample is taken matters — normally at the point of discharge — and taking it from the wrong point makes the result meaningless.
• Cylinder making, handling and curing is where most bad results originate. Cylinders left in the sun, knocked, poorly compacted or transported badly will break low regardless of the concrete.
• Initial curing on site before collection has requirements, and a cylinder box on site that meets them is a cheap investment.
• Slump testing at delivery, and its role in accepting or rejecting a load.
• Air content where air entrainment is specified.
• Concrete temperature at delivery in hot weather.
• Cover meter surveys on completed elements where specified.
• Early-age testing to allow stripping or loading, which is a different sample from the acceptance test.

The single highest-value control is watching the cylinders being made and stored. A contractor who takes cylinder handling seriously eliminates a large share of the low results the industry attributes to concrete quality.

When a result comes back low

A low break is a non-conformance and it must be raised as one. What it is not is an automatic demolition order.

• Raise the NCR immediately and notify the superintendent. Concealing a low result is the version of this that ends careers.
• Check the acceptance criteria. Concrete acceptance is statistical, and a single result below the characteristic strength does not necessarily fail the assessment. Understand what the standard and the specification actually require before conceding.
• Check the sample. Was the cylinder made, cured and transported correctly? A documented handling failure is a legitimate basis to question the result.
• Check the companion result from the same sample.
• Investigate the placement records — docket, batch time, water added, weather, curing.
• In-situ assessment where the designer requires it — core testing, non-destructive testing, or in-situ load assessment. Cores are the usual next step and they have their own correction factors and interpretation rules.
• The designer decides acceptance. Whether the element is adequate for its actual loading is a design assessment, not a contractor’s or a supplier’s opinion.
• Outcomes range from accept as is, accept with a load restriction or additional protection, strengthen, or remove and replace.
• The commercial position follows the cause. If the concrete as supplied did not comply, that is the supplier’s; if the placement or curing was deficient, that is yours; if the specification or design was inadequate, that is a variation. The records determine which, which is why the docket and the pre-pour record matter. Our guide to formwork, falsework and steel reinforcement covers what that pre-pour record has to capture.

Handling this well is a reputational moment. The non-conformance process itself sits in your quality system; the concrete-specific point is that a contractor who raises a low break themselves, with the records already assembled and a proposed investigation, is treated completely differently from one whose client finds it in the test reports.

Records, conformance and payment

• Delivery dockets, filed against the element they went into.
• Pre-pour inspection records with the hold point release.
• Pour records — volume, times, weather, plant, personnel, any incident.
• Test certificates from the laboratory, matched to the pour.
• Curing records.
• Survey records for levels and dimensions.
• Cover survey results where required.
• NCRs and their close-out.

Missing test records delay payment and delay practical completion, because conformance documentation is normally a precondition to both. Assembling this at the end of a job is far harder than filing it as you go — see our guide to practical completion, defects liability and the final claim.

Pricing concrete work honestly

• Supply at the correct grade for the exposure classification, not the structural grade alone.
• Delivery charges — small load fees, out-of-hours fees, and waiting time, which on a slow pour is substantial.
• Pumping where access requires it, including the pump, the operator, and the priming and cleaning.
• Wastage — over-order, spillage, and part loads.
• Formwork, including its design where it is significant temporary works, and stripping.
• Reinforcement supply, cutting, bending, fixing, chairs and spacers.
• Placement labour and plant, including vibrators and a spare.
• Finishing, at the specified finish, including any specialist trade.
• Curing materials and the labour to apply and maintain them.
• Joints — saw cutting, sealant, dowels, waterstop.
• Testing at the specified frequency, which the specification may make the contractor’s cost.
• Hot weather measures — early starts, night work penalty rates, ice or chilled water.
• Trial mixes and mix approval for special-class concrete.
• Weather standby and abandoned pours.

What a tender response should show

• The nominated supplier and plant, with certification status and travel time to site.
• The pre-pour hold point in the ITP, with what it covers and who releases it.
• Placement method for the significant elements, including access and pump arrangements.
• Hot weather concreting measures as a standing method, not a contingency.
• The curing regime by element type.
• Testing arrangements — laboratory, accreditation, frequency, and cylinder handling on site.
• The response to a low result, showing you have a process rather than an argument.
• Supply continuity — what happens if the plant fails mid-pour, and where the emergency joint goes.

Checklist

• Is the concrete grade set by the exposure classification as well as the structural design?
• Is special-class concrete identified, with trial mix and approval time in the programme?
• Is the supplying plant certified as the specification requires, and the mix registered?
• Is travel time from plant to site within the allowable limit, including traffic?
• Is the supply rate confirmed for the largest pour, with a contingency if it stops?
• Is the emergency construction joint location decided before the pour starts?
• Has the pre-pour hold point been released in writing before ordering?
• Are cover spacers the correct height, spacing and type for the exposure?
• Are cast-in items checked against the drawings and any connecting shop drawings?
• Is a spare vibrator on site?
• Are curing materials on site before the pour starts?
• Is anyone authorised to reject a load, and does the crew know it?
• Is added water recorded on the docket every time?
• Are hot weather measures planned — timing, temperature, wind and evaporation?
• Are cylinders made, stored and transported correctly, and is site curing adequate?
• Is the laboratory accredited for the specific test methods?
• Is saw cutting timing planned and someone responsible for it?
• Are dockets filed against the element they went into?
• Is there a defined process for a low break, including who raises the NCR?
• Is conformance documentation being assembled as you go, not at completion?

Sources and further reading

This guide is general information for Australian civil construction businesses and is not engineering advice. It deliberately states no concrete grades, exposure classification requirements, cover values, slump limits, aggregate sizes, temperature limits, batching-to-discharge times, water addition limits, curing durations, sampling frequencies, acceptance criteria or core correction factors: those are set in the Australian Standards for concrete structures, for the supply of concrete, and for the sampling and testing of concrete, and in the specification of the road, rail or water authority for the project, and they differ between exposure classifications, element types and jurisdictions. Take the applicable requirements from those documents and the project specification. Whether a concrete element with a low test result is acceptable is a design assessment for the project designer. Formwork of significance is engineered temporary works. Wet concrete causes serious chemical burns, and cutting or grinding cured concrete generates respirable crystalline silica, which is subject to regulated workplace exposure controls.

• Australian Standards for concrete structures, for the specification and supply of concrete, and for methods of sampling and testing concrete, referenced throughout and particularly in §02, §09 and §10. These set exposure classifications, cover, mix requirements, sampling frequency, cylinder preparation and statistical acceptance criteria; no values are reproduced here.
• Road, rail and water authority specifications referenced in §02 and §03 impose additional and sometimes more stringent requirements than the Standards, including plant certification schemes, mix registration, restricted materials, testing frequency and curing methods. These differ between authorities and must be read for the specific project.
• Laboratory accreditation referenced in §09 is scope-specific; verify the laboratory’s accreditation covers the particular test methods being used on the project.
• Workplace exposure standards for respirable crystalline silica referenced in §05, and the control measures required, are set in the model work health and safety regulations as enacted in each jurisdiction, together with the associated code of practice.
• Related TenderBuilt guides carrying the primary-source detail referenced above: quality management plans and ITPs, noise, vibration and dust management, precast concrete supply and erection, extension of time and delay claims, preparing civil works cost estimates and practical completion and the final claim.