Civil Engineering/Mass Concreting in Raft Foundation
QUESTION: I have a raft foundation having 4 meter depth and quite large plan. How can I do concreting of this raft?
I know that the raft will be divided into areas in the plan but do we also have to divide the raft vertically like for e.g first we fill 2 meter depth then the rest?
Is there any formulation to determine the actual and allowable difference in temperatures of concrete internally and outside?
ANSWER: Hi Ghulam,
This really depends on the capacity/capability of the contractor, but I would say that you will need to cast the raft in a minimum of 2 lifts and possibly in 3, in quick cycles.
You need to take all the precautions (low cement concrete, chilled water and aggregates, casting at night or in cooler times, insulation for all formwork and the top surface, etc) about heat of hydration and measure the temperatures between the core and the surface- the differential should not exceed about 15 degrees C.
Browse my past answers using keywords "heat of hydration, and thermocouples".
---------- FOLLOW-UP ----------
QUESTION: Thanks a lot for your instant reply.
But i am unable to find your past answer which u mentioned, can u provide me the link?
I have copied/pasted the Q&A below:
Subject: Concrete Temperature Control
Date Asked: 2013-08-21 03:24:50
Date Answered: today
I am working in Macao(China)and I am about to concrete some pile caps in the coming weekends! I work for i work for the constructor, and consulting is concerned with concrete temperature. They ask me to submit to theme one methodology to demonstrate the differential temperature control for the massive concrete!
So I told them that my company is going to try and demonstrate in a concrete block (not in situ) with 2 meters x 2 meters x 2 meters, for instance!
I can propose whatever I want to, so I need some advice on this temperature issue if you can kindly help me.
Thanks in advance.
The most important thing is to keep the differential temperature between the centre and the surface within certain limits (usually 20-25 degrees C) and to avoid centre temperatures above around 70-80 degrees C. As an example, here are detailed recommendations for thick walls:
4. THERMAL CONTROL METHODS
4.1 SHUTTERING WILL BE MADE UP OF 18MM MARINE PLY FOR BUNKER WALLS WITH POLYSTYRENE SHEETS ON STANDBY IF NEEDED.
4.2 SHUTTERING WILL BE LEFT FOR A LONGER PERIOD AS MUCH AS POSSIBLE: PLAN FOR 7 DAYS- MAY BE ABLE TO REDUCE THIS PERIOD.
4.3 USAGE OF ADMIXTURES TO KEEP DOWN THE W.C. RATIO IN SPECIFIC POURS ON BUNKERS. USE OF POZZALANA CEMENT OR pfa OR GGBS REPLACEMNET (IF AVAILABLE) TO REDUCE HEAT OF HYDRATION.
4.4 POSSIBLE USE OF ICE IN MIXING WATER TO MAINTAIN THE CONCRETE TEMPERATURE TO 30’ DURING SUMMER IF REQUIRED ON ADDITIONAL COST- NOT NECESSARY IF OTHER MEASURES ARE ADOPTED.
4.5 SHADES FOR SAND & AGGREGATES, SPRAYING OF WATER ON AGGREGATES PILES TO KEEP COOL
4.6 USE OF PORTABLE THERMOMETERS TO TEST THE TEMPERATURE OF CONCRETE BEFORE PLACING
4.7 USAGE OF THERMOCOUPLES ON THE INITIAL POURS AS TEST -. USE ON WALLS OR SLABS- IN PAIRS, ONE IN CENTRE AND ONE NEAR SURFACE.
4.8 CONSIDERATION OF DAY AMBIENT TEMPERATURE IN PLANNING- START POURS VERY EARLY MORNING OR LATE AFTERNOONS.
4.9 PREMIX SUPPLIER MUST BE ADVISED ON IMPORATNCE OF TEMPERATURE CONTROL OF CONCRETE
4.10 APPLICATION OF CURING COMPOUND AT ADDITIONAL COST TO BE ADVISED- USE POLYSTYRENE 50mm SHEETS AND 200mm THICK HESSIAN/GUNNY BAGS FILLED WITH SAWDUST.
4.11 MAXIMUM CORE TEMPERATURE SHOULD BE 70-80 DEG C; MAXIMUM DIFFERENTIAL BETWEEN CORE AND SURFACE SHOULD BE BETWEEN 15-20 DEG C FOR THE THICKER SECTIONS (WILL BE LESS FOR THINNER SECTIONS)