Placing and Finishing Concrete - Case Study Example

PLACING OF CONCRETE Name: Instructor: Course: Date: PART 1: POSSIBLE PROBLEMS AND DIFFICULTIES. SCENARIO 1: CASTING OF CONCRETE FOUNDATION. The following problems and difficulties may arise during the casting of the concrete for the foundation: 1. Soil settlement may occur as a result of soil consolidation. Soil consolidation arises if the clay is not properly compacted when preparing the foundation site. Care must therefore be taken because clay is exceptionally hard to compact. 2. Concreting will be done during hot weather this will create a problem. The elevated temperatures will cause increased rate of hydration of cement. It will also cause an increase in the evaporation thus fast loss of moisture from freshly mixed concrete. Other problems are increased demand of water, increased loss of slump, difficulty in handling and compaction, problems of removing entrained air and tendency of cracking as a result of plastic shrinkage. 3. Limited access to the site means that it will not be possible to use a crane. Other equipment will have to be used such as wheelbarrows and pumps. 4. All ready-mix suppliers in the surrounding area are located 45 minutes travelling distance of the site. This means that only 15minutes are left for pouring of the whole batch in order to ensure the concrete maintains its properties. This is a challenge because there is no crane on site. 5. If concrete is not poured continuously, there are chances of formation of joints in the concrete slab. These joints will allow entry of ground water into the foundation and thus affect the durability of the slab. SCENARIO 2: CASTING OF CONCRETE COLUMNS. The following problems and difficulties may arise during the casting of the concrete for the columns: 1. The cold weather will create the following problems: Protection of the poured concrete until it has attained a minimum strength of 3.5MPa or the first 24 hours. The time required for the concrete in the columns to reach its design strength will be extended thus care has to be taken when loading the columns. There is need to protect the heat produced during hydration from escaping into the atmosphere. This may require provision of supplementary heat in order to obtain good quality concrete. Concrete will set very slowly in cold weather. Below 40F, hydration of concrete stops and therefore the concrete will not achieve its design strength. There is a danger of the column reinforcement being covered with frost thus preventing placing of concrete. Cold temperatures result in the formation of ice crystals in concrete, which reduces its design strength and leads to cracking. 2. The concrete columns are heavily reinforced this will create a problem when compacting the poured concrete. SCENARIO 3: CASTING OF CONCRETE BRIDGE ABUTMENT. The following problems and difficulties may arise during the casting of the concrete for the bridge abutment: 1. The cold weather will create the following problems: Protection of the poured concrete until it has attained a minimum strength of 3.5MPa or the first 24 hours. The time required for the concrete in the columns to reach its design strength will be extended thus care has to be taken when loading the columns. There is need to protect the heat produced during hydration from escaping into the atmosphere. This may require provision of supplementary heat in order to obtain good quality concrete. Concrete will set very slowly in cold weather. Below 40F, hydration of concrete stops and therefore the concrete will not achieve its design strength. There is a danger of the column reinforcement being covered with frost thus preventing placing of concrete. Cold temperatures result in the formation of ice crystals in concrete, which reduces its design strength and leads to cracking. 2. The Abutment will be cast in two separate halves thus; care has to be taken when pouring concrete on the joint to ensure that no cracks are left. PART 2: SELECTION AND SPECIFICATION OF CONCRETE QUALITY AND COVER. SCENARIO 1: CASTING OF CONCRETE FOUNDATION. Concrete required 40m*30m*1.5m = 1800m3 Concrete Strength: Using Table 1 class C20/25 was selected. Aggregates should be of normal weight and the minimum characteristic cube strength at 28 days is 25N/mm2. Working life: According to BS EN1990 50 years for building structures. Cover to Steel : Using Table A.10 of BS8500-1:2002, cover to reinforcement is taken as 15+∆c mm Exposure : The classes selected according to BS8500-1:2006 are XCI , ACEC From the above requirements, the concrete specified: According to Table A.6 of BS8500-1:2600 the concrete selected to be used is Designated concrete RC30 with 15+∆c mm nominal cover , exposure classes XCI and ACEC ,strength class C20/25 and water /cement ratio of 0.70. Use IIB-V Portland cement containing 21% -35% fly ash. The reason for choosing this concrete mix is because the concrete will be placed during the hot weather and fly ash will help keep the temperatures of the mixed concrete down thus preventing loss of its consistency. The concrete will therefore be able to attain its required strength. SCENARIO 2: CASTING OF CONCRETE COLUMNS. Concrete required: Radius of column = 500mm/2 = 250mm = 0.25m Volume of concrete required for 1 column = πR2 x Height of columns = 3.142(0.25)2*8 Volume of 20 columns = 3.142(0.25)2*8*20 = 31.4m3 Concrete required = 31.4m3 Concrete Strength: Using Table 1 class C28/35 was selected. Aggregates should be of normal weight and the minimum characteristic cube strength at 28 days is 35N/mm2. Working life: According to BS EN1990 50 years for building structures. Cover to Steel : Using Table A.10 of BS8500-1:2002, cover to reinforcement is taken as 30 +∆c mm Exposure: The classes selected according to BS8500-1:2006 are XC3, XC4 and XF1 According to Table 2 of ENV206:1990 environmental conditions are of exposure class is scenario 3. Water / Cement ratio : Using Table 3 of ENV206:1990 and exposure class 3, the water /cement ratio is 0.5 From the above requirements, the concrete specified: Designated concrete RC28/35 is selected from Table A.3 from BS8500-1:2006, with nominal cover 30+Δc mm, exposure classes XC3, XC4 and XF1, with strength class C28/35, Water /Cement ratio 0.5 Use Portland CEM I This cement was chosen because it does not contain fly ash, which would result in slow setting and generation of very little internal heat. This is because concreting is being done during cold weather. It is recommended that the water used when mixing concrete be hot. SCENARIO 3: CASTING OF CONCRETE BRIDGE ABUTMENT. Concrete required Width of abutment at the center = (1500mm+ 800mm)/2 =1150mm Volume of concrete for bottom half = (1.5+1.15)/2 *4*30 =159m3 Volume of concrete for top half = (1.15+0.8)/2*4*30 =117 m3 Total volume of concrete for abutment = 276 m3 Concrete Strength: Using Table 1 class C28/35 was selected. Aggregates should be of normal weight and the minimum characteristic cube strength at 28 days is 35N/mm2. Working life: According to BS EN1990 50 years for bridges. Cover to Steel : Using Table A.10 of BS8500-1:2002, cover to reinforcement is taken as 30 +∆c mm Exposure: The classes selected according to BS8500-1:2006 are XC3, XC4 and XF1 According to Table 2 of ENV206:1990 environmental conditions are of exposure class is scenario 3. Water / Cement ratio : Using Table 3 of ENV206:1990 and exposure class 3, the water /cement ratio is 0.5 From the above requirements, the concrete specified: Designated concrete RC28/35 is selected from Table A.3 from BS8500-1:2006, with nominal cover 30+Δc mm, exposure classes XC3, XC4 and XF1, with strength class C28/35, Water /Cement ratio 0.5 Use Portland CEM I This cement was chosen because it does not contain fly ash, which would result in slow setting and generation of very little internal heat. This is because concreting is being done during cold weather. It is recommended that the water used when mixing concrete be hot. PART 3: METHODOLOGY OF PLACING AND CURING CONCRETE. SCENARIO 1: CASTING OF CONCRETE FOUNDATION. The following procedure will describe how to pour and cure concrete for a deep foundation excavation into clay in the centre of London. The contractor should consider using ready mix concrete from any one of the available suppliers this is because the hot weather would prove to be a challenge if he is going to do manual mixing. He should request the supplier to use set retarders and water reducers so as to modify the concrete. This will result in moderate production of heat during the hydration process. Request the supplier to use ice chips when mixing the concrete, this is because as the ice melts it will absorb the heat from the concrete and thus lower the overall temperature of the concrete. Another option is injecting liquid nitrogen in the mixer. The amount of cement used in the concrete should be limited to only what is required to meet durability and strength requirements; this is because the higher the amount of cement in the concrete mixes the higher the temperatures will rise as a result of hydration. Cool the formwork immediately before placing of concrete so as to prevent unwanted increases in temperature. Avoid pooling of water in the form work. Since the site is not easily accessible and a crane can therefore not be used when pouring the concrete the contractor needs to acquire the services of many laborers so as to ensure quick placing of concrete when it is delivered. During curing, avoid moisture loss by immediately covering the placed concrete with a moisture retaining material for example burlap or a curing compound. This will help optimize the hydration process, which will result in the concrete reaching its full potential. SCENARIO 2: CASTING OF CONCRETE COLUMNS. The following procedure will describe how to pour and cure concrete for columns of a large reinforced concrete underground reservoir is being constructed in South Yorkshire: Since the columns will be cast in cold weather, remove all ice, snow and standing water that could be mixed up with the concrete during placement. Everything that will come into contact with the concrete should be warmed. The concrete can also be heated by adding accelerators. Admixtures can be added to the concrete so as to accelerate the rate of setting and strength gain. This can also be achieved by use more Portland cement or using high early strength cement. To reduce bleeding and setting time concrete should be placed at the lowest applicable slump. Materials and equipment for protection of concrete should be put in place. Their purpose is to protect the concrete from freezing and enable the concrete to retain the heat it generates during cement hydration. Insulated tarps, blankets and straw covered with plastic sheets are the commonly used measures. The contractor should monitor the temperatures in days prior to the placing of concrete and before removing any cold weather protection. This is because shocking the concrete would result in a large drop of temperatures, which would hinder development of strength and durability. Shocking could also result in cracking. The columns are heavily reinforced; this creates a problem during the compaction of concrete after it is placed. The contractor should ensure that adequate compaction using a poker vibrator is done to prevent honey combing. SCENARIO 3: CASTING OF CONCRETE BRIDGE ABUTMENT. The following procedure will describe how to pour and cure concrete for abutment to a motorway under bridge under construction in the Midlands, close to Birmingham: The contractor should monitor the temperatures in days prior to the placing of concrete and before removing any cold weather protection. This is because shocking the concrete would result in a large drop of temperatures, which would hinder development of strength and durability. Shocking could also result in cracking. Since the abutments will be cast in cold weather the contractor should make sure to remove all ice, snow and standing water that could be mixed up with the concrete during placement. Everything that will come into contact with the concrete should be warmed. The concrete can also be heated by adding accelerators. Admixtures can be added to the concrete so as to accelerate the rate of setting and strength gain. This can also be achieved by use more Portland cement or using high early strength cement. To reduce bleeding and setting time concrete should be placed at the lowest applicable slump. Materials and equipment for protection of concrete should be put in place. Their purpose is to protect the concrete from freezing and enable the concrete to retain the heat it generates during cement hydration. Insulated tarps, blankets and straw covered with plastic sheets are the commonly used measures. The abutments will be concreted in two halves thus; care should be taken to ensure proper jointing. REFERENCES. Cadman Heidelberg Cement Group (2011) “Hot Weather Concreting” Accessed on 9th April 2011. http://www.cadman.com/section.asp?pageid=3149 Domeier Tammy (2011) "Cold Weather & Concrete Curing Problems" Accessed on 9th April http://www.ehow.com/info_8097755_cold-weather-concrete-curing-problems.html Portland Cement Association (2011) “Concrete Technology” .Accessed on 9th April http://www.cement.org/tech/faq_cold-weather.asp Braggs Ashleigh (2008) “Seasonal Planning: Concrete in the winter”. Accessed on 9th April http://www.lowesforpros.com/seasonal-planning-concrete-in-winter Read More