A Program Management Approach : Constructability Challenges

“You can use an eraser on the drafting table or a sledge hammer on the construction site.” - Frank Lloyd Wright

Focus Challenges

1.?????Mass Foundation Concreting

o??Concrete Mix Design & Supply

o??Weather & Climate

o??Site Logistics

2.?????Concrete Pumping

3.?????Formwork Systems

4.?????Structural Steel Erection

o??Construction Sequence

1.?????Mass Foundation Concreting

Many high-rise buildings are supported by thick mat foundations which require mass concrete castings.?Concrete consolidation, mix design, site logistics and heat of hydration control are the major challenges to construct these raft foundations.?

Case Study, CCTV Headquarters, Beijing, China.?Possibly the largest mass concrete pour ever completed was for the China Central Television (CCTV) Headquarters project in Beijing, China.?CCTV has four interconnected building elements consisting of two inclined towers, a base building that joins the towers at the bottom, and a cantilevered overhang that joins the towers at the top.?China State Engineering and Construction Corporation (CSCEC) was the contractor and Turner was its construction management consultant.?CSCEC successfully completed this two phase continuous mass concrete pour.?The first phase was for 31,000 cubic meters (for one of the inclined towers, Tower II) and was cast between 21 December 2005 and 24 December 2005; the second for 41,000 cubic meters (for the other inclined tower, Tower I) and was cast from 28 December 2005 to 30 December 2005.?The mat foundations on CCTV are integrated with the piling system.?It acts as homogeneous pile caps and its weight counteracts the overturning moment forces caused by the cantilevered overhang structure.?The overall weight of rebar in both tower mat foundations is approximately 14,000 tons.

The concrete mix design for these pours was thoroughly evaluated by the CSCEC Technical Department team.?Flow-ability issues were a major concern, specifically as it relates to the elimination of any potential for blockages in the pump pipelines.??Thus, fly ash was included in the mix design to insure the workability of the concrete mix.?Mechanically, fly ash particles are small and effectively fill voids.?Because they are hard and round, they have a ball bearing effect that allows concrete to be produced with less water, resulting in more durable and workable concrete.?An added benefit is that fly ash chemically lowers the heat of hydration caused by concrete’s exothermic reaction, preventing rapid heat gain which is detrimental to concrete durability and causes concrete cracking.?The performance of this mix was tested during smaller mat pours in other basement locations prior to the two big pours.

Particular design constraints needed to be considered.??Some of these included low temperatures, workability, and the congestion of rebar.?Due to the cold climate, naphthalene antifreeze, an early strength high efficiency water reducer, was included in the concrete mix.

?As with most large volume concrete castings, concerns over large variances in the temperature gradient from core to surface had to be minimized.??The temperature was controlled by the combination of insulating blankets and close monitoring of the concrete temperature at intermediate levels.?Temperatures were monitored though a series of temperature sensors, that were fastened to the rebar frame prior to the concrete casting at numerous locations, 23 temperature survey points for Tower I, and 17 temperature survey points for Tower II. All sensors were cable connected to computers in the temperature monitoring center, at which point data was automatically recorded at half hour intervals.

?With all preparations in place, the first continuous mass concrete foundation casting occurred under Tower II. Tower II was selected as the first pour since it is the smaller of the two pours (comparatively speaking) and logistically more convenient to the primary truck routes around the site.??The pour began at 8:00 PM on December 21, 2005 and continued until 3:00 AM on December 24, 2005.?A total of 31,000 cubic meters of concrete were placed during this fifty-five hour duration and outside air temperatures were as low as -10o C.??Labor and supervision forces were split into two 12 hour shifts (8:00 PM to 8:00 AM, 8:00 AM to 8:00 PM) of approximately 400 persons per shift.?A total of 15 stationary concrete pumps (Zoom Lion HBT100 and HBT80) and 1 concrete pump boom truck were allocated for Tower II.??Eleven stationary pumps were staged at grade level along the east bank (20+ meters above the pour), with the remaining stationary pumps and boom truck located at an intermediate basement staging area on the south bank (10 meters below grade) at the southwest corner of Tower II.???

?The concrete was pumped down into the mat through fixed hard piping extending from each stationary pump and a flexible line from the boom truck.?All pipes were insulated to reduce the chances of freezing.??Each of the 15 pipe lines ran vertically down along the face of basement excavation then horizontally across the top row of mat reinforcement.??Concrete was cast progressively starting from the west side of the mat moving east.??Piping was installed in sections such that as the concrete pour progressed east towards the staging area the sections could be removed to progress with the pour sequence.

?The pours for both towers were designed to slope from the starting point on the west, towards the east to aid and control the progression of the pour.?Pours varied between 4.5 to 7 meters deep. To reduce segregation of aggregate in both tower foundation pours, certain areas down within the multiple layers of rebar were designated and blocked off as safe crawl space access to control the concrete pour and vibration in the lower depths.?To insure safety within these crawl spaces, access was clearly highlighted with fluorescent safety tape, and a mandatory registry (sign-in) was put in place for each individual entering and exiting the crawl space. For both tower foundation pours, approximately 160 concrete mix trucks (drum trucks with a capacity of 6 to 10 cubic meters) were continuously cycled between the batch plant (1 hour away) and the project site, spending about 20 minutes per cycle onsite to unload concrete.

The second continuous mass concrete foundation pour was for Tower I of the structure.?This pour began at 11:00 AM on December 28, 2005, and was completed by 8:00 PM on December 30, 2005.?The operation took fifty-seven consecutive hours for a total concrete pour of 41,000 cubic meters, by a labor force of approximately 450 persons per shift with 2 shifts (8:00 PM to 8:00 AM, 8:00 AM to 8:00 PM).??

?A total of 20 stationary concrete pumps (Zoom Lion HBT100 and HBT80) and 1 mobile concrete boom were used for Tower I.?Eleven concrete pumps were located at grade on the east bank (20+ meters above the pour)?staging area above the excavation, 3 concrete pumps on the west bank, 6 concrete pumps on the south bank (10 meters below grade), and 1 mobile concrete boon assisting as needed.?The concrete was pumped down into the matt through fixed hard piping just as it was for the Tower II pour.?The pour progressed steadily from north to south.??

?During subsequent temperature monitoring of the completed pours, some gratifying results were recorded, further emphasizing the high standards of performance for these works.?For example, the accuracy of the calculated concrete core temperature in comparison to the actual temperature was considerably high.?The structural design engineer’s calculation for the concrete core temperature was 50.38o C, whereas the actual concrete core temperature at its highest reading was about 53o C.?The acceptable gradient difference, per design, between the concrete core temperature and the surface temperature could not exceed 25o C.?The actual surface temperature was measured to be an average of 30o C, well within design tolerances.?Surface temperatures were controlled by the layers of insulating blankets placed on the surface.

?Large Pour Considerations.?To successfully cast a large raft foundation (normally more than 15,000 cubic meter) continuously, the following must be carefully considered and managed:

·???????Mix Design.?Slump retention should be good to provide flexibility for continuous pumping. A viscous (“sticky”) mix will wear out the pumps, causing pump break down and potentially cause a blockage.?The concrete mix should also be low heat. Performing a full height mockup simulating the actual pace of placement is advisable to accurately predict the peak temperature and temperature differential at the actual casting. If the ambient temperature is high, it is critical to lower the temperature at the time of placement.

·???????Concrete Supply.?A large pour will require more than one batching plant and, in most cases, more than one concrete supplier. For the sake of consistency, the raw material, which includes cement, aggregates, and admixtures, should come from the same sources and uniform quality control procedures be applied to all plants.?It is recommended that an independent testing and verification agency, contracted directly by the owner, be retained as an expert third party during construction.?They should be empowered by the owner and have the authority to inspect any portion of the concrete works.

·???????Site Logistics.?Traffic management and the requisite permits for continuous truck access into and out of the site is paramount. An adequate number of reliable high performance pumps are required to continuously and evenly distribute the concrete over a large area.?Access to the pumps should be made to allow duel feed of concrete mixer trucks for continuous pumping, and it is recommended that at least one (1) pump remain unused and on standby, in the event another breaks down.

·???????Weather Protection.?What happens if it rains??Such a question might not even be asked in Dubai, but in Malaysia, where it could rain torrentially each day during the rainy season, weather must be accounted for when planning a mass concrete pour.?A properly designed weather shelter to protect the concrete placement activities from rain is absolutely essential (and relatively commonplace in the construction industry today).

?Case Study, Wilshire Grand, Los Angeles.?Turner is the general contractor for the Wilshire Grand Hotel in Los Angeles.?Upon completion in June 2017, it will be the tallest building west of the Mississippi River.?In February 2014, the project completed a 21,000 cubic yard continuous raft foundation pour, which included the use of cooling pipes, as well as various other techniques to overcome the logistical challenges of undertaking such a large pour in downtown Los Angeles.??

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To achieve the relevant design requirements, a temporary 680 ton chilled water system was installed at a cost of approximately USD 1.5 Million.?This cooling system consisted of two (2) temporary mechanical chillers, two (2) 20,000 gallon water tanks, six (6) recirculation pumps, 27,400 meters of “PEX” piping, and 3,000 meters of header piping.?Temperature sensors were placed at the surfaces and at the center of the raft to measure absolute temperature differentials between the core and the surface.??

Traffic Management. Traffic?management for the pour was planned and meticulously coordinated with the local authorities to ensure that the concrete trucks could get from the ready mix batching plants to the site without delay.?In the heavily congested downtown Los Angeles area, detailed analysis and planning was paramount.?There was no room for error, whether within the site for the benefit of the pour or outside the site for the benefit of the general public.?Building information modeling (BIM) was used to plan the site logistics, which had a total of 17 pumps and 2 placing booms.?The pre-planning activities proved worthwhile, with Turner successfully completing the 21,200 cubic yard, continuous mass concrete pour for the mat foundation over an 18 ? hour span from 15 February 2014 to 16 February 2014. Predicted temperatures correlated with actual results, and the entire pour proceeded with minimal disruptions.??

Stay tuned for part 2, where we going to share about?Concrete Pumping, Formwork Systems, and Structural Steel Erection