Mass concrete or massive pours is defined as any volume of concrete with dimensions bigger enough to require measures to deal with the generation of heat from the hydration of cement & adjustments within the attendant quantity to reduce cracks.
Concrete of large or massive structures has always posed a great challenge to civil engineers in the past. However, life is slightly easier now, as excellent and reliable equipment is available to produce, transport, place, compact, and finish concrete. In addition, various types of mineral admixtures and chemical admixtures are available to modify the properties of concrete both in plastic and hardened form to meet the demanding situations of a massive concrete pour.
Large volumes of concrete or mass concrete are required to be placed in dams and water retaining structures. They have their own problems & challenges to face. However, the large building rafts and densely reinforced deep equipment foundations are equally challenging even though the volume of concrete required is relatively less than in massive dams.
Large concrete pours or mass concrete has to be executed considering the ambient temperatures and their location. The most important factor is planning which has to start from the design stage itself. During the concrete pour, reliability of equipment, consistent quality of materials and their adequate quantity, speed of concrete production, and teamwork between workmen play an important role.
Planning for Mass Concrete
It is extremely important to design the reinforcement and steel layout taking into consideration the type of equipment proposed to be mobilized. This is necessary to avoid segregation of concrete at certain locations, due to dense reinforcement. It is also important to ensure proper compaction of concrete in deep and congested areas.
The grade of concrete specified for the structure also plays an important role. Cement content in the mix can cause a serious problem with the heat of hydration. Generally, the high workability of concrete is preferred in densely reinforced foundations. This requirement along with high strength has to be very carefully examined vis-à-vis the cement content of the mix. Optimizing cement content to avoid excessive heat generation without sacrificing strength or workability is therefore important.
The ambient temperature at the time of concrete is extremely important. Hot or warm weather can cause a serious drop in workability or hardening of concrete resulting in difficulty while placing. These can result in undesirable cold joints and a high-temperature gradient between the concrete surface and the core. In extremely cold weather, it is necessary to maintain concrete temperatures above 10oC, at the time of placing and later covering/protecting the concrete surface from extreme cold. It is preferable that the temperature gradient between the core and concrete surface should not exceed 25oC. However, higher gradients are permitted depending on the structural design.
Concrete Mix Parameter for Mass Concrete
The concrete mix design must be finalized after considering the following factors:
- The thickness or depth of the structures being cast.
- The mode of placing concrete and the type and number of equipments available.
- The capacity and speed of concrete production plant.
- The ambient temperature.
- Availability of adequate work force and supervision staff.
The following concrete mix parameters, considering the above factors, will have to be seriously considered:
- Type of cement and mineral admixtures to be used to meet the strength and heat of hydration requirement.
- The maximum temperature at the time of placing of concrete to prevent high temperature gradient between the concrete core and surface.
- The pre-cooling methods required to be adopted for various concrete ingredients. Lowering the temperature of various ingredients specially water reduces the rate of cement hydration and thereby reduces temperature at the time of placing. This in turn reduces the temperature differential and prevents possibility of thermal stresses at a later stage. Use of ice flakes as a partial substitution of water further helps in temperature reduction.
- Workability of concrete at the time of placing has to be fixed considering the congestion of reinforcement steel and availability of type and number of compaction equipment and their capacity. Workability of concrete will also depend on the type of placing equipment used. e.g. Concrete pumps, crane and bucket conveyors etc.
- Workability retention will also play an important role if Ready Mix Concrete (RMC) is supplied. Drop in slump has to be minimized considering the optimum cement content of the mix and the minimum workability requirement at the site. Use of chemical admixtures which are compatible with cement and mineral admixtures is extremely important.
- Mass concrete is generally placed in steps, layer after layer. It is therefore important that when the subsequent (upper) layer is placed, the layer below should be still compactable. Use of retarding admixtures in warm/hot water, with proper dosage, needs to be considered. This helps in preventing cold joints.
- Selection of maximum size of the aggregate is required to be finalized considering the clear cover and spacing between the reinforcement. Generally in mass concrete it is preferable to use bigger size maximum aggregate as this can also help to minimize the cement content. The designer should consider use of larger diameter bars to reduce congestion and/or allow bunching of bars in congested areas.
- Use of plasticizer in mass concrete helps in two ways. Firstly, it improves the workability and secondly, it reduces the cement content thereby reducing the heat of hydration. Hence it is important to consider both these factors while selecting the chemical admixture and its dosage.
- Use of high strength cement in mass concrete could be advantageous as less quantity of cement is required and also it can permit use of higher percentage of mineral admixture thereby reducing the overall heat of hydration of the concrete mix.
- In cold freezing weather, it is necessary to have slightly higher cement content along with air entraining additive.
Factors Need to be Consider for Mass Concrete Production
The following factors have to be considered.
- The concrete production should be continuous and consistent in quality.
- Adequate quantity of consistent quality material should be available prior to the commencement of the pour. Cement silos should be fully loaded. Silos containing mineral admixtures and cement should be clearly identified.
- Concrete production plant should run preferably on fully automatic mode to avoid human errors.
- Production equipment should be properly examined prior to the pour and essential spares should be easily available along with the maintenance crew throughout the pour. This is extremely important especially when the production of concrete is planned from only one batching mixing plant.
- If it is planned to have a standby concrete production plant it is advisable to have the standby plant also working throughout the pour.
- Proper temperature control on all ingredients is required to obtain the consistent temperature of the mix. However, variations in ambient temperature throughout the day will require some adjustments. In hot afternoons ambient temperature can suddenly shoot up, than originally thought of, resulting in concrete temperature at the time of placing exceeding the requirements. It is therefore important to plan the concrete pour in such a manner that concrete in the central core layer is placed during the night when the ambient temperature is likely to be the lowest and any problem due to sudden temperature rise does not pose a serious problem. It is also advisable to consider the worst ambient temperature while fixing the temperature parameters of each ingredient.
- In warm/hot weather conditions, it is always necessary to spray water on the aggregate to prevent absorption of water by the aggregates during transportation which causes a drop in workability. The other advantage is that water on the surface of the aggregates, during storage, evaporates in the hot sun. This in turn cools the aggregates and therefore prevents slump drop and also helps in lowering the concrete temperature at the time of placing.
Factors Need to be Consider for Mass Concrete Transportation
Concrete transportation whether in hot or cold weather has to be done in the shortest possible time. The following factors need to be considered while transporting the concrete;
- Quick and continuous supply of concrete is necessary throughout the massive pour. In warm/hot weather quick transportation helps in preventing loss of workability and rise in temperature at the time of placing. In hot weather, a wrapping of wet hessian cloth or a protective hood with an air gap between the drum and itself is provided to prevent evaporation losses and a rise in concrete temperatures.
- Generally, transit mixers and concrete pumps are used for mass concrete works. However, if transportation is to be done only by manual means then it is essential to mobilize large labour and supervisory force. Co-ordination, safety, and proper staging with walkways will have to be given additional weightage.
- The concrete transporting equipment should be in good working condition to ensure trouble-free continuous concrete supply. Necessary spares and maintenance crew should be available throughout the pour. In case of emergency due to unforeseen failure of equipment necessary arrangements should be made to mobilize replacement equipment at short notice or to quickly repair the breakdown.
Factors Need to be Consider for Mass Concrete Placements
Proper planning is necessary to ensure systematic placement at a fixed rate without cold joints or segregation. The following factors need to be considered:
- For massive equipment and building foundations it is recommended to place concrete in a stepwise manner with layer thickness not exceeding 500mm.
- Concrete must be placed in proper sequence and in a stepwise manner ensuring that the previous layer is compactable till the subsequent layer over it is placed and compacted. This prevents cold joints. If the low layer has set and hardened, then the concrete layer placed over it will not bond properly leaving a cold joint that can be vulnerable to ingress of moisture and unwanted chemicals.
- Concrete of mass pours are generally required to be placed in deep sections and through layers of reinforcement. If concrete is placed through the reinforcement, segregation takes place, and mortar from concrete sticks to the reinforcement steel and coarse aggregates hit the reinforcement and get scattered and fall below. This causes honeycombs and porosity which result in loss of strength and durability of concrete.
- When mass concrete is commended in the deep areas, which are densely reinforced a thick layer of mortar (having the same water to cement ratio of the concrete mix) is placed. Concrete is then placed over it and compacted. Even if there is segregation, the thick mortar placed initially will rise while compacting the upper layer of concrete and act as a void filler and prevent the formation of honeycombs. Compacted concrete spreads and moves forward. Therefore, subsequent layers of concrete must be placed only on the compacted concrete below and not directly on the dry set lean concrete or on the formwork below.
- Concrete should be placed, as far as possible, at the location where it is supposed to ultimately rest. Concrete should never be transported using compaction equipment or by any other method. This can cause segregation of concrete mix.
Compaction in Mass Concrete
Concrete compaction using needle vibrators is generally done in mass concrete works. However, self-compacting concrete if used would be ideal and would not require any vibration at all. Assuming normal concrete will be used, the following factors need to be considered.
- If needle vibrators are used, it is important to take the following precautions to avoid equipments failure:
- In mass concrete work utmost care has to be taken so that the needles do not stop functioning repeatedly and cause delay in compaction.
- Needle should be held by the flexible hose at least two feet above the rigid part. Holding flexible hose too close to the rigid end causes failure of needle vibrator within a short time.
- Needle vibrator should not be used to vibrate reinforcement steel, inserts and/or formwork. Often the operators are careless and the needle hits the steel or inserts or formwork accidently and gets damaged. At times, there is a tendency to vibrate formwork, steel or other embedment to drop the concrete sticking to it.
- Needle vibrator motors are often pulled by the operators using the flexible hose. This can cause damage to the needle vibrator and can cause its failure.
- In reinforced mass concrete pours the most important precaution is the prevention of concrete mix segregation and assurance for full compaction. At times steel congestion is of a very high order and segregation can occur. This can be prevented as explained in the previous point of concrete placement.
- Adequate arrangements have to be provided to repair the vibrator needles and motors or to replace the non-functional ones.
- The speed of concreting operations depends on the speed of compaction to a great extent. Hence mass concrete placed in layers must be thoroughly compacted layer by and all entrapped air driven out to achieve full compaction.
- Adequate care must be taken to vibrate and consolidate concrete in areas where newly placed concrete is being placed at the location of a construction joint.
- Adequate care must be taken to vibrate concrete along the perimeter of the pour. During the planning stage itself adequate care must be taken to have predetermined locations close to the cover for inserting the needle vibrator up to the required depth. Generally, 60mm needle vibrators are recommended for use in mass concrete.
- Adequate care is also required for compacting concrete around embedded parts and corners. There should be adequate space to insert the needle close to the corners and around the embedded part to ensure proper compaction.
- While compacting the upper fresh layer of concrete, the needle should penetrate into the lower previously placed layer. This improves bonding between the previously placed layer and fresh layer.
- In case the lower layer has hardened and does not permit vibration, it is suggested to spread cement mortar of same water to cement ratio of the concrete mix and then fresh layer and compact it. Pipes for grouting the cold joints at a later stage, can be placed at the location where concrete layer had set hard.
- If the workability of the concrete mix is between 75mm to 100mm and 60mm diameter needles are proposed to be used then openings must be kept at a spacing of at least 500mm for the needle to be inserted upto the desired depth in the reinforcement cage for compacting concrete without any obstruction.
- In thick concrete rafts, there are locations which are not clearly visible from the top wherein concrete may not flow or may not get compacted to the required degree. In such cases, it is necessary to send the needle vibrator operator within the reinforcement cage to ensure proper compaction in the densely reinforced area not otherwise clearly visible.
- Observation windows and cutouts are required to be provided in critical area for proper placement, compaction and inspection.
- Since concreting of massive pours are done round the clock, it is extremely necessary to have proper lighting at night and additional provision of powerful hand-lamps and torches to help proper working and inspection during compaction.
- It is advisable to distribute vibrator motors in different locations covering a considerable area. Shifting of vibrator motors from one location to another repeatedly is not recommended to other areas. This gives rest to the equipment and helps the equipment to function more efficiently and prevents stoppages due to overworking.
In massive pours, the exposed surface area will need curing or protection immediately depending on the ambient conditions. In warm/hot climatic conditions, it is recommended to cover the freshly cast concrete with a plastic sheet or alternatively cover it by spraying a curing compound. Using chilled water for surface curing in warm/hot climatic conditions is not advisable. This can aggravate the cracking of concrete due to the development of thermal stresses. It is also advisable to retain the side formwork for at least 7 days or till such time the temperature in the core areas of the pour starts dropping.
In cold climatic conditions, the top concrete surface needs to be covered with an insulating cover to maintain the warmth from the concrete’s heat of hydration.
It helps in the faster development of strength. This will help in reducing the temperature differential between the core and the surface and therefore reduce the chances of thermal stresses developing resulting in the cracking of concrete.
If evaporation losses are prevented during the 7 day curing period, then moist curing or wet curing may not be necessary under any ambient conditions. If mineral admixtures or blended cement are used, the curing period will have to be extended by additional 7 days.