1. Cross section of a slab on grade

Andrew DallasTechnical ManagerConslab Ltd.

2

Wearing Surface

Concrete Slab

DPM

Blinding layer

Subbase

Subgrade

Sub-Grade

• Should provide uniform support

• so hard or soft spots removed

• Consideration of settlement

– Due to differences in the subgrade

– Restraint from piles

– Could check for the stresses induced by rotations but

generally if you have differential settlement you will

have cracking.

Makes little change to the slab thickness

Significant cause of slab failures

Designed using the

Modulus of Subgrade Reaction “k”

Confirmation of Properties

• NZ: CBR or Benkleman beam

• TR34: Plate bearing test (750mm)

• For heavily loaded slabs we recommend having a geotechnical engineer recommend the k value

Source: Conslab from a report by OPUS

Source: Extract from TR34 4th Ed

Specification Clause

• the sub-grade or sub-base is tested using a Benkelman beam at 5m centres followed by a proof roll to identify weak spots. 90% of readings to be within the deflection limit and no reading greater than the maximum deflection limit.

CBR% 90% Deflection Limit mm Maximum Reading

3 4.1 5.0

4 3.1 4.0

5 2.5 3.5

10 1.2 2.0

15 0.9 2.0

Source: Conslab Contract Documents

– TESTING OF THE SUBGRADE

• A geotechnical investigation has been carried out by <insert name of company and report reference> and a copy of the report forms part of the contract documents. The target “K” value for the subgrade of this project is K = <insert value after discussing with geotechnical Engineer>. The slab design is to be based upon this value when preparing tender prices. Once the Contractor has been appointed, the K value is to be site checked in accordance with the following clauses. The slab design shall be modified if the K value varies significantly from that expected. The Contractor shall advise the Engineer within 5 days of the site testing and prior to placing any subbase.

• Subgrades in Cut

• The Contractor shall cut the site to approximately 150mm above Subgrade level and at this stage the Contractor shall arrange for the Geotechnical Engineer to carry out Benkelman Beam tests or other approved testing to determine the actual K value of the Subgrade and provide the results to the Engineer for approval. Following approval the final trimming to subgrade level shall be completed.

• Subgrade in Fill

• Where the site is to be filled then the compacted engineered fill material will be placed up to the design subgrade level. At this stage the Contractor shall arrange for the Geotech Engineer to carry out Beam testsor other approved testing to determine the K value of the subgrade. Provide the results to the Engineer for approval. Following approval the final trimming to Subgrade level shall be completed.

Specification clauses courtesy of Conslab, MSC Consulting Engineers and Jawa Consulting

Engineers

9

Wearing Surface

Concrete Slab

DPM

Blinding layer

Subbase

Subgrade

Sub-base

• Provides a working platform for construction

• Provides a level formation

• Transmits load to subgrade

• Granular material , min thickness 150mm

• Not used for enhancement of modulus of

subgrade reaction

• Proof roll to ensure no obvious soft spots

12

Wearing Surface

Concrete Slab

DPM

Blinding layer

Subbase

Subgrade

• Blinding layer

• Closes off any rough spots in the subbase

• Allows for finer tolerance on the subbase level

• Depth of Sand 0-5mm max

• Depth of PAP7 or GAP7 up to 20mm

Specification Clause

– SUB-BASE PREPARATION

• The basecourse shall be compacted in accordance with the <“Earthworks” or

“Siteworks”> Section of the General Specification.

• The basecourse layers will be suitable hardfill graded and compacted to provide a

closed even surface not prone to rutting.

• The top surface of the basecourse will be placed to a levels tolerance of +0mm or -

10mm. A certificate and levels plan is to be provided by a Registered Surveyor

engaged by the Contractor certifying that these tolerances have been achieved. A

copy of these reports is to be provided to the Engineer for review.

15

Wearing Surface

Concrete Slab

DPM

Blinding layer

Subbase

Subgrade

DPC is there to stop moisture rising

through the floor

Two layers for PT and jointless fibre

The second layer is simply to reduce friction

Specification Clause

– Damp Proof Membrane

• Under all ground floor slabs provide an effective DPM shown to have a water flow

resistance of not less than 90 meganewton seconds per gram.

• Side laps and end laps shall be taped and sealed in accordance with manufacturers

instructions

18

Wearing Surface

Concrete Slab

DPM

Blinding layer

Subbase

Subgrade

19

Concrete Mixes

• The key is consistency!

• Slump and set times

• Simplicity generally leads to success

• Avoid superplasticisers

• Minimum cement for abrasion

• Don’t get hung up on minimum slumps

• Consider how to get the concrete placed

• No air entrainer.

Need to consider the practical aspects:

• How are you going to get the concrete

into the slab?

• Leave panels off

• What is the access like for the finishing

equipment?

• Can you hang the finishing equipment

off columns? Access for the workers?

Specification Guide (not clauses)– The key is consistency – must allow controlled water

addition to trucks below the target slump

– Specify the strength required, any minimum cement

content.

– Any other durability requirement. Generally nothing

– Specify the testing required.

– Do not specify no pump mixes allowed

– Do not specify air entrainment (unless outside a

freezer roller door) or superplasticisers

– There are no such things a high grade or special grade

plants. “Concrete must come from a plant which hs

been audited as meeting the requirements of

NZS3104”

23

Wearing Surface

Concrete Slab

DPM

Blinding layer

Subbase

Subgrade

Abrasion

• Finishing, curing, strength in that order

• Around 300- 320kg cement

• toppings /dry shake

• Most commercial/industrial is 35-40 MPa 40 MPa will have adequate cement

Specification Clause Surface Finish

– Surface finish and surface tolerances need to be separated

Abrasion Requirement

The concrete is to have a minimum cement content of 325kg. The surface is to be

repeat power trowelled to a dense burnished finish and then water cured for 7

days.

(Information Note: Do not specify to AR1 or other finishes as per NZS3101 as there is

very limited capacity to measure this.)

Concrete Finishes

The class of finish to concrete surfaces shall comply with NZS3114. Floor tolerances

are specified under tolerances. Any defects to the concrete shall be made good at

the Contractors expense and in a manner agreed with the Engineer.

Specification Clause - CURING• The slab shall be water cured for a minimum of 7 days.

The contractor shall provide a method statement as to

how this will be achieved. Alternatively the Engineer may

approve curing using a membrane forming compound that

complies with the requirements of NZS3109:1997.

• Should a membrane curing system be approved then it

shall be compatible with future painted line markings or

any other surface coating required for this floor.