PNRC 82 / APP 167 - Quality of In-situ Reinforced Concrete Works at an Early Age
Table of Contents
Introduction
This article would introduce the content of PNRC 82 – “Quality of In-situ Reinforced Concrete Works at an Early Age”.
This practice aims to ascertain earlier the quality of in-situ RC works and sets out an alert system for RSE and RC to take early follow-up action.
In addition, the standard for HST under BS EN 12504-2:2021 would be introduced.
BD Approval Condition for RHT
Under item 6 of section 17(1) of the BO (as shown below), a condition will be imposed, when giving the first approval of superstructure plans involving the use of different concrete grades in the structure, that RHT should be carried out to structural
concrete elements with higher concrete grade at an early concrete age.
Section 17(1) of the BO - Conditions for Building Works
The detailed content in cl.17 of the BO would be separately discussed.
This clause of BO empowers the BA impose conditions in approval of plans or consent, qualified supervision should be provided by the RBP and the RC.
If the conditions for building works are violated
Under cl.40 Offence of BO, any person not complying condition according to 17(1) shall be guilty of an offence –
Requirements for RHT at an Early Concrete Age
Qualified Supervision by TCP
RSE determines the structural concrete elements to be subject to RHT.
All RHT should be carried out by RC-TCP-T1 who should be trained in the use of rebound hammers.
RSE-TCP-T3 should witness at least 10% of the RHT conducted by RC
RHT results should be record in a standard form attached in PNRC 82 and should be kept on site.
Testing frequency and location
| RC elements | Min. Frequency |
|---|---|
|
Beam of higher concrete than adjoining |
10% |
|
Vertical elements of higher concrete than adjoining |
10% |
|
Transfer Beam |
50% |
|
#Vertical elements of higher concrete grade than base provided by “pile cap” / “raft foundation” / “transfer structure”. |
50% |
If vertical elements are with kickers, RHT should be conducted to the kickers in addition to the vertical elements.
Testing date
Seventh and tenth day after they are cast
If RHT cannot be conducted between the seventh and tenth day due to, for example, public holiday, RHT should be carried out on the following working day.
Alert Level for Follow-up Actions
| Concrete | RHT readings |
|---|---|
|
With PFA / ggbs |
<55% of the minimum characteristic strength of concrete |
|
Without PFA / ggbs |
<65% of the minimum characteristic strength of concrete |
Follow-up Actions when reaching Alert Level
Examine the record (approved plans, concrete order .. etc.) to identify any irregularities.
Further ascertain the quality of concrete (further RHT / coring test).
Enhance the site supervision for the whole process of concrete works.
If found necessary, report to BD and suspend the concrete works.
Alternative to RHT - Maturity Method
As an alternative to RHT, the quality of in-situ concrete at early age may be assessed by the maturity method on the seventh day after they are cast.
What is Maturity Method?
The maturity method is a technique to account for the combined effects of time and temperature on the strength development of concrete. The method provides a relatively simple approach for making reliable estimates of in-place strength during construction.
It requires securing temperature sensors within the casted section before concrete placement and then estimate its strength by comparing to the calibration curve.
The standard for HST under BS EN 12504-2:2021 as required by PNRC
A test location should be about 300mm x 300mm.
Grind the surfaces until they are smooth and free of loose material unless they are smooth-formed surfaces.
Perform at least five impacts on the reference anvil to ensure the readings are within ± 3 unit value.
If the impact has crushed a near-to-surface void, discount the result.
Take a minimum of nine valid reading. (Although the standard form having 13 box per row)
No two impact points are closer than 25mm and noe are within 25 mm of an edge.
The result shall be taken as the median of all the readings.
If more than 20% of all the readings differ from the median by more than 25%, the entire result shall be discarded.
THIS DOCUMENT IS FOR REFERENCE PURPOSES ONLY.