In-situ concrete can consist of various elements each with their own complexities but there are some important commonalities with all in-situ concrete works that should be followed.
The quality aspects of in-situ concrete can be split into the following three stages:
- Pre-pour planning and conformance checking
- Concrete placement conformance checking
- Post pour conformance checking
As almost all permanent works concrete on the project references the RMS B80 specification, this spec will be the reference point on this tip sheet.
Pre Pour Planning
- Make sure the proposed concrete mix has been approved by Sub IC.
Even if a mix is in the RMS Approved Mix Register it still needs to be approved for use on the project.
- The designed rate of rising for the formwork (for pier and wall pours) must be determined. Be aware of B80 formwork HPs.
- What method is being used to place the concrete – a concrete pump, kibble or directly out of the chute?
- Access to the concrete pump, is a dual feed possible when required?
- The amount of surface area required to be Must be aware of weather conditions.
- The supply rate of concrete should be agreed between the supervisors, the engineer and the concrete finishers in advance of the
- Start times should preferably be early in the morning to ensure concrete temperatures are low, supply from the concrete plant is consistent and the ambient temperature does not affect the
- Planning should also ensure that enough concrete vibrators are available, vibrating screeds are checked and working, the type of curing has been agreed and organised, and the right amount of manpower is
- Check the weather and make a decision before the batcher starts Do not pour if rain is expected.
Pre Pour Conformance Checking
- Ensure conformance checking is carried out at various stages of the process in line with NCX-ITP-B80-CON and related VCs.
- Ensure the formwork/falsework has been installed as per the approved design drawings and the relevant specs/code.
- Check that the reinforcing is in accordance with the design drawings including the cover, lap lengths, positions and bar
- Always receive design approval if you need to deviate from the approved design via the FCR process.
- Any welding of reinforcing bars must comply with RMS D&C B204.
- Ensure cast-in items are installed and surveyed prior to
- Ensure all checks are completed by the responsible Engineer and that all the B80 VCs are properly completed and submitted to Sub IC via the relevant Hold Points.
- Does the Concrete Supervisor and 50% of the workforce hold an RMS B80 Grey Card?
Concrete Placement Conformance Checking
The first thing to check is the concrete delivery docket for the following:
- Correct concrete for project and
- Correct mix code (mix design).
- The time since batching is within the specification requirements (B80 spec).
- The maximum allowable water than can be added on site without compromising the water-cement
- The driver should have also written on the docket how much water he has added at the slump
- The concrete shall be field tested in accordance with the specification which includes slump, compressive strength, concrete temperature and air content (when required).
Vibration during concreting is extremely important to ensure the concrete achieves the desired density and compressive strength. As a general rule vibration should be done in a systematic manner. The vibrator shall be left in the concrete until air voids have stopped rising to the concrete surface. It usually takes 7 to 15 seconds for entrapped air to be expelled.
Surface vibration can be achieved with vibrating truss screeds suitable for pours with large surface areas such as bridge decks. These only provide vibration to the top 80mm (approx.), enough to ensure the cover zone is adequately vibrated and a level surface finish is achieved.
A rule of thumb for the amount of poker immersion vibrators required is one vibrator per 1 to 2 trucks per hour. Concrete pokers should be placed in the concrete quickly but removed slowly so that voids are not left in the concrete.
Care must be taken not to over vibrate the concrete. Leaving a poker in the same spot for too long can cause segregation of the concrete, however, it is worse to under vibrate than to over vibrate concrete.
The type of finish and surface texture of the concrete will be detailed on the design drawings and specification.
- Non-exposed footings require a wood float
- Bridge decks require a wood or “magnesium” floating the surface (or steel trowel).
- Concrete finishes to deck surfaces shall be carried out in accordance with RMS B80.
Initial finishing involves screeding the concrete to the correct level, bull floating and then leaving it to set.
Bleed water should appear on the surface and it is important to either let this dry up or remove it before final finishing commences.
- Mixing bleed water with the surface paste will weaken it resulting in it wearing out and becoming This is not permitted.
- Once bleed water dries up and the concrete can support a person’s weight, with only a slight marking to the surface, the final finishing can
The use of aliphatic alcohol finishing aid, sprayed onto the finished concrete surface is recommended to reduce plastic shrinkage cracking, especially on hot or windy days. Final finishing involves floating, trowelling, edging, jointing or patterning the concrete such as brooming in a textured finish.
Once the concrete surface has been finished it must be cured. Curing means to cover the concrete so it stays moist as concrete doesn’t harden properly if it is left to dry out and can crack.
- Concrete is cured by either applying extra water to the surface of the concrete or stopping water loss from the
- Water curing, steam curing or using a chemical curing compound are all permitted by the B80
- The duration required for curing is detailed in the specification but is usually a minimum of 7
- If using a curing compound check whether it is compatible with any future surface coatings such as anti-graffiti
- Make sure the curing compound is applied as per the manufacturer’s recommendations and forms a continuous unbroken Compounds must comply with AS3799.
Post-Pour Conformance Checking
It is important to carry out an “as constructed” survey to ensure that the requirements have been met. Remedial measures may be required. Concrete strength results should be checked for conformance.
- Seven-day results are used to flag early issues with
- Three-day tests may be required to confirm formwork stripping
Surface finish should be inspected for excessive blowholes, honeycombing, chipping and other damage.
A site crack inspection needs to be completed on all concrete surfaces in a timely manner prior to backfilling with documented crack positions and surface widths recorded for any cracks formed that are:
- >0.05mm for precast structures
- >0.3mm for in-situ concrete structures
These may require remedial works in accordance with design approval.
- Poor concrete quality due to a mix of non- conformance issues caused by water-cement ratio, material gradings, slump and All of these issues should be solved by performing trial mixes.
- Inexperienced concrete workers and finishers with poor
- Non-compliant curing or delays in curing can cause cracking and poor
- Overworking the surface and the inappropriate use of aliphatic alcohols as finishing aids causes delamination and loss of the textured
- Strict quality control is required by engineers on site during concrete pours to ensure concrete meets slump, temperature and age
- Good planning is essential for a successful concrete
- Ensure access is adequate to get concrete trucks to the pump/kibble as quickly as
- Ensure pre-pour checks are carried out progressively during construction to ensure rebar and cast-ins are checked properly while still
- Formwork should be properly designed and validated to ensure it has been built correctly and pour rates of rising should be strictly adhered
- Do not wait until the concrete truck is ready to discharge before visually checking the Carry out a visual check either at the pump hopper or before it backs up to the pump. Identifying problem slumps early will reduce delays in concrete supply.
- If in doubt stop the pour and check/test the
- Plan the pour meticulously to prevent any possible issues