Foam concrete is a kind of concrete with lightweight and has ideal strength. Because of the hollow structure in foam concrete, it has the function of absorbing heat and isolating sound.
The density of foamed concrete is 300-1200 kg/m3, and the thermal conductivity is between 0.08-0.3W/ (m ·K).
Foam concrete is widely used in CLC blocks, lightweight partition panels, roof insulation, floor cushion construction, floor heating backfilling, and other occasions.
At the same time, foam concrete is also a good sound absorption material, which can be used in highway sound insulation boards, sound absorption boards, and other fields.
Generally speaking, foam concrete is a kind of building material with multi-function, environmental protection, and economy, which has a wide application prospect.
What is concrete early strength?
Some research on the performance of the maturity method in cold weather was reported; however, this research is mainly based on heat curing. The method's performance for predicting the early age strength of concrete in cold weather without heat curing still needs to be investigated. Furthermore, no research in the open literature investigated the correlation between in situ strength and strength estimated using the maturity method at an early age (1–3 days) and in cold weather (less than 10°C), hence the need for this research. Both the conventional concrete cylinder method and maturity method are compared with the core strength of concrete to clearly understand their performance in estimating the strength of cold weather concrete. The current study was conducted in Canberra, Australia, and was designed to simulate the winter conditions of Canberra. The city is known for its cold winters and warm summers. In winter, the temperature at night can drop below 0°C while the mean maximum daytime temperature is about 14°C. A standard curing method in this area is keeping the form in place: moisture preservation. Standard cylinders and maturity methods based on cylinders kept in an insulated box are commonly used to estimate strength. As stated by Tank and Carino, the Nurse-Saul method was based on empirical observations and is adequate only under certain conditions. When different samples of a given concrete experience dissimilar early-age temperatures, the method does not correctly represent the effect of curing temperature on strength development. Arrhenius's maturity method, which accounts for the nonlinear rate of hydration of cement caused by the effect of temperature, was later developed by Hansen and Pedersen. Due to its advantages, the maturity method has been the center of research for early-age in-situ strength assessment of concrete. Various methods, standards and variations of the method have been proposed and used worldwide.
The determination of the strength of in situ concrete
Critical construction activities, such as formwork removal, post-tensioning, lifting precast members, and termination of cold weather protection, require the determination of the strength of in situ concrete. Waiting too long to perform these activities can be expensive, but acting too early can harm the structure. A classic example of the latter is the progressive collapse of a portion of a multi-story building in Fairfax County, Virginia, USA, in 1973, which claimed 14 lives and resulted in 34 injuries and a failure of a cooling tower being constructed in Willow Island, West Virginia, USA, in 1978, which claimed 51 lives. These failures initiated research in the area of estimating the in situ strength of concrete during construction. The maturity method estimates the concrete strength based on the combined effect of time and temperature. The method's origin can be traced back to a series of papers dealing with accelerated curing methods. McIntosh investigated the electrical curing of concrete and found that the amount of hydration is proportional to the area under the temperature–time curve above the datum temperature of 30°F (the temperature at which hydration virtually ceases). McIntosh termed this area the ‘basic age” of the concrete and measured it in °F × hours. The nurse investigated steam curing of concrete and plotted the product of time and temperature against the strength of concrete expressed as a percentage of the strength of the concrete after 3 days of storage at average temperature. The nurse used the curves to estimate the minimum steaming conditions to achieve a target and concrete strength after a specific steam curing scenario. Saul investigated the principles underlying the steam curing of concrete at atmospheric temperature. He defined maturity as the age of concrete multiplied by the average temperature above freezing, which the concrete has maintained. For a concrete mix, Saul stated that the strength of concrete stays constant whatever combinations of temperature and time go to make up the maturity.
Determination of the in-situ strength of concrete: Core strength
The core strength is an effective way of determining the strength of in-situ concrete. Thus, it is often used for calibration of other methods. However, the compressive strength of the core samples is generally lower than that of the conventional cylinders due to the drilling process. Factors such as length-to-diameter ratio, diameter, and core treatment affect the core samples' strength. This study compares the core strength with the strength from the maturity method, which is based on standard cylinders. The comparison of the core strength with the maturity strength should be performed after modifying the core strength to account for factors such as diameter, length-to-diameter ratio, and drilling operation effects. Standards, such as AS 1012.14 and ACI 214.4R, give such procedures for modifying the core strength. This report follows the ACI 214.4R method as it accounts for the strength reduction coming from the core drilling process. The worst estimate was observed in the case of 24 h-300 mm specimens, while the best estimate was observed in the case of 72 h-150 mm specimens. The standard cylinders underestimated the concrete strength by 76% at T1 exposure for 24 h-300 mm specimens, while it gives almost the same strength as the core for 72 h-150 mm specimens at T2 exposure. The standard cylinders could not correctly estimate the strength of 24 h-300 mm specimens due to the concrete's relatively early age and the slab's higher thickness. For Portland cement, the maximum rate of heat evolution occurs within the first 24 hours. Thus, more heat is generated in the concrete in this period. This higher heat generation creates a relatively higher difference between the cylinders and the slabs than the concrete tested at 72 h. For instance, for the B3-72T1-300 slab, a maximum temperature of 35°C was observed around the 9.5 h mark. At the 24th hour, the slab had a temperature of 26 °C while the cylinders had a temperature of only 8°C. At the 70th hour, the slab has a temperature of 5.5°C while the cylinders have a temperature of 7°C, showing a much lower difference between the slab and the cylinder. The concrete in the slab takes advantage of the higher temperature at an early age, resulting in higher strength. In comparison, the rate of hydration in the cylinders is adversely affected by the lower ambient temperature.
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What is concrete early strength?
When we ask what affects the strength of concrete, the answer is—just about everything. Among the factors are type, quality and amount of cement. Quality, cleanness and grading of the aggregate. Quality and amount of water. Presence or lack of admixtures. Methods were followed in handling and placing the concrete. Age of the concrete when placed in the forms. Temperature. Curing conditions. Age of the concrete when tested. Foreign materials may find their way into the concrete, affecting its strength. Finally, the indicated strength of the test specimens may or may not represent the strength of the concrete in the structure. Some of the variables and their effects. In using a table of this type, the first step is to eliminate the items that do not apply and then consider those that might be significant. In many cases, it will be found that more than one factor was acting at the time. There may be one or more factors of high significance or several of minor significance that, when acting together, become highly significant. In the following discussion, we will assume that test specimens truly represent the concrete from which they had been sampled. One problem with strength tests is the time lapse between making the concrete and testing specimens. When strength results become available, there is still time to do something about the concrete already placed. Still, the information at least warns us to avoid troublesome materials or practices in future work.
Appreciable variations in the strength of concrete
Appreciable variations in concrete strength result from using different brands intermittently or can even be caused by variations between shipments of cement from the same mill. Variations in raw materials, processing, age, fineness and temperature contribute to these variations. Undetected differences in cement types will affect strength, especially at early ages. If a batching plant has facilities for more than one type of cement, the wrong cement is always dangerous. There have been cases of accidental substitution of Type I for Type III. Also, accidental use of pozzolan instead of cement has occurred. Adequate inspection will minimize these incidents. In-place concrete tests can be made with instruments that measure the velocity of a small mechanical pulse through the concrete. Known as the pulse velocity method, the apparatus consists of two vibration pickups (phonograph pickups), a hammer device to apply a blow to the concrete and an electronic circuit to measure the velocity of the sound generated by the hammer blow as the sound travels through the concrete from one pickup to the next. Considerable experience and expertise are required to interpret results correctly. The second instrument is based on the principle that the penetration of a probe gauge into the concrete is inversely proportional to the compressive strength of the concrete. A Windsor probe (ASTM C803) uses a power-actuated device to drive the probe into the concrete. Accuracy is about the same as that of the Swiss hammer, but small indentations are left on the surface of the concrete, which might be unsightly in some exposed concrete.
Determining the strength of concrete in place
Two simple instruments are available for determining the strength of concrete in place. One of these, a Swiss hammer (ASTM C805), operates on the principle that the rebound of a springloaded steel plunger striking the concrete's surface is proportional to that concrete's strength. It is a quick, nondestructive test that can determine the approximate compressive strength of concrete in place. Still, it cannot replace properly conducted cylinder or core tests. Samples sawed or cored from hardened concrete are generally not required; their use is confined to those cases in which some question or dispute has developed regarding the quality of the concrete as revealed by tests of molded specimens. The number, location, size and type of specimens are determined when sampling becomes necessary. Coring and sawing specimens from hardened concrete are expensive expedients and should be adopted only as a last resort. Both procedures leave scars on the surface of the concrete that are difficult to eradicate, a condition that must be considered if the concrete is exposed to view. The possibility of structural damage, especially damage to reinforcement, cannot be ignored. If an approximation of the strength will suffice, one of the following described instruments can be used.
This paper aims to report that high early-strength concrete can be produced.
This paper aims to report that high early strength concrete can be produced with high replacement of cement by fly ash for precast/prestressed concrete operations. Effects of fly ash content on water demand and workability are also reported. Test data from mixture proportioning reported in earlier publications 1 · 5 established that this source of fly ash can be used for structural-grade concrete in quantities of up to 60 percent cement replacement. Demonstration projects are also reported in these publications, which show that pavement construction and other projects have successfully used structural-grade concrete with up to 70 percent cement replacement. Tests were carried out on a nominal 5000 psi (34 MPa) 28-day compressive strength concrete, where fly ash was substituted for cement at up to 30 percent replacement on a 1.25 to 1.00 fly ash replacement for cement basis. A literature search was also conducted to study further the water demand, workability and strength characteristics of fly ash concrete. Rather than compiling an exhaustive annotated bibliography of the available literature, some critical publications were reviewed and are listed in the references.
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The concrete mix in foamed concrete insulation materials can gradually set and harden after pouring until the final strength is obtained due to cement hydration. The speed of cement hydration is related to the composition of the concrete itself and the mix ratio and mainly changes with the temperature.
When the temperature rises, the hydration is accelerated, and the strength increases rapidly. When the temperature is reduced to 0℃, a part of the water present in the concrete begins to freeze and gradually changes from the liquid phase (water) to the solid phase (water). At this time, the water involved in the hydration of the cement is reduced, so the hydration is slowed down, and the strength growth is correspondingly slower. The temperature continues to drop, and when the water in the concrete completely turns into ice, that is, entirely from the liquid phase to the solid phase, the hydration of the cement stops, and the strength will no longer increase.
When the water becomes ice, the volume increases by about 9%, generating an ice expansion stress of about 2,500 kg/mm2. This stress value is often more significant than the initial strength value formed inside the cement stone, so the concrete is damaged to different degrees (frozen damage during the drought period) and reduces the strength. In addition, when the water becomes ice, it will also produce large particles of ice on the surface of the aggregate and steel bars, weakening the bonding force between the cement slurry and the aggregate and steel bars, thus affecting the compressive strength of the concrete. When the ice melts, it will form a variety of voids inside the concrete and reduce its density and durability.
In the construction of foam concrete insulation boards in winter, the change of water form is the key to the growth of concrete strength. Many scholars at home and abroad have conducted a large number of experimental studies on the form of water in concrete, and the results show that there is a pre-conditioning period before freezing of newly poured concrete, which can increase its internal liquid phase, reduce the solid phase, and accelerate the hydration of cement. The experimental study also shows that the longer the pre-conditioning period of concrete before freezing, the smaller the strength loss.
1) The raw materials of antifreeze concrete must meet the requirements of winter application. The cement used should be preferred Portland cement or ordinary Portland cement; its label should not be lower than 425, and the use of high-aluminum cement is strictly prohibited.
2) When using antifreeze, pay attention to the mixing method. Salt containing insoluble matter or negligible solubility in antifreeze must be ground into powder and mixed with cement. When it is necessary to use the solution, it should be fully dissolved and stirred evenly, and the concentration and amount of each addition should be strictly controlled. If composite antifreeze is used, their co-solubility, if not co-soluble, should be separately mixed into a solution and then added to the concrete. To accelerate the dissolution, the solution can be prepared with hot water at 40℃- 60℃ and added to the concrete. The antifreeze added in powder form should be ground fine through the 0.63mm screen before use if there is moisture caking.
3) It is necessary to control the dosage strictly, according to the minimum temperature of -10℃, 15℃ and -20℃ during the construction period; antifreeze with the specified temperature of -5℃, -10℃ and -15℃ can be used respectively. The amount of different antifreeze is very different, and the inaccurate amount has a significant influence on the performance of concrete. Excessive will make the concrete set too fast, resulting in construction difficulties and serious salt out on the component's surface, affecting the quality of the exterior decoration. Adding too much will also reduce the strength of the concrete; the strength will not increase after the positive temperature curing. If the dosage is insufficient, the concrete structure will freeze. Now, it is gradually developing in the direction of low alkali and low content, which is conducive to preventing the harm of alkali-aggregate reaction to concrete.
4) The mixing time of concrete mixed with antifreeze should be 50% longer than that without antifreeze to ensure that the antifreeze is evenly distributed in the concrete so that the strength of the concrete is consistent. Minimize transportation and watering time. To improve concrete's early strength, the concrete's temperature entering the mold should not be below -5℃.
5) The concrete with compound antifreeze containing air entrainment agent should be cured at harmful temperatures and can not be cured by steam. The use of steam curing will not only reduce the strength of concrete but also reduce its durability.
After pouring the foam concrete, it should be covered immediately, not watered. The initial curing temperature shall not be lower than the prescribed antifreeze temperature. When the concrete temperature drops below the temperature specified by the antifreeze agent, the critical strength of the concrete antifreeze should reach 3.5MPa; otherwise, insulation measures must be adopted.
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What is concrete early strength?
The quality of concrete is judged largely on the strength of that concrete. Equipment and methods are continually being modernized, testing methods are improved, and means of analyzing and interpreting test data are becoming more sophisticated. Before the 2008 edition of the ACI 318 Standard, we relied almost exclusively on the strength of 6-by-12-inch cylinders made on the job site and tested in compression at 28 days of age for evaluation and acceptance of concrete. The use of 4-by-8-inch cylinders for strength evaluation was first addressed in ACI 318-08. See the discussion on strength specimens in Chapter 13, Section 13.5. This may involve drilling cores from the structure or testing with certain nondestructive instruments that measure the hardness of the concrete. Some specifications permit a small amount of noncompliance, provided it is not serious, and may penalize the contractor by deducting from the payments due for the faulty concrete. Statistical methods, now applied to the evaluation of tests as described in Chapter 26, lend a more realistic approach to the analysis of test results, enabling the engineer to recognize the normal variations in strength and to evaluate individual tests in their true perspective as they fit into the entire series of tests on the structure. Strength is necessary when computing a proposed mix for concrete, as the contemplated mix proportions are based on the expected strength-making properties of the constituents.
The specified strength of very high-strength concrete
Very high strengths, understandably, require a very high level of quality control in their production and testing. Also, for economy in materials costs, the specified strength of very high-strength concrete is based on 56- or 90-day tests rather than on traditional 28-day test results. To give some idea of the strengths that might be required, Table 3.1 is included as information only. Remember that the plans and specifications govern. Other properties of the concrete can be significant for concrete exposed to freeze-thaw conditions, sulfate exposure and chloride exposure (effects of chlorides on the corrosion of the reinforcing steel). Strength, however, remains the basis for judgment of the quality of concrete. Although not necessarily dependent on strength, other properties to improve concrete durability are related to strength. Concrete that fails to develop its expected strength must be improved in other respects. Generally, when we speak of the strength of concrete, it is assumed that compressive strength is under consideration. There are, however, other strengths to consider besides compressive, depending on the loading applied to the concrete. Flexure or bending, tension, shear and torsion are applied under certain conditions and must be resisted by the concrete or by steel reinforcement in the concrete. Simple tests available for testing concrete in compression and flexure are used regularly as control tests during construction. An indirect tension test is available in the splitting tensile test, which can easily be applied to cylindrical specimens made on the job. Laboratory procedures can be used for studying shear and torsion applied to concrete.
What is the relationship between the strength indicated by the test cylinders and the strength of the concrete in the structure
Consider the first question. Test specimens are made, cured and tested under certain standard conditions that are usually appreciably different from those existing in the structure. Temperature and curing conditions can be vastly different. The value of the test specimens is that they measure the strength potential and other properties of the concrete; they evaluate the materials and mix under certain standard conditions. If they indicate a low strength, then something is wrong with the materials or proportions. The actual strength of the concrete in the structure can be appreciably different. Besides temperature and curing, other variables are moisture content, size, shape, quality of consolidation, possible presence of defects such as rock pockets, restraint, and combinations of loading in the structure. Because of these unknowns, the structural engineer must consider a factor of safety when the structure is designed. In answer to the second question, the variations in cylinder strengths are only sometimes reflected in the structure. If three specimens are made from one batch of concrete under identical conditions throughout the test, there is no assurance that they will all fail at the same strength. The probability is that they will each break at a different strength. These are normal variations.
The strength of concrete can be determined by any one of four methods
The strength of concrete can be determined by any one of four methods: by molding specimens from the fresh concrete on the job, by testing cores removed from the hardened concrete, by applying specific impact and rebound instruments to the hardened concrete and by sonic and electronic measurements applied to the hardened concrete in place. Specimens molded from fresh concrete are universally used for control and acceptance testing. Tests by other methods are used to check the results of molded specimens, especially in low-strength indications or disputes, and for research involving existing structures. Samples sawed or cored from hardened concrete are generally not required; their use is confined to those cases in which some question or dispute has developed regarding the quality of the concrete as revealed by tests of molded specimens. The number, location, size and type of specimens are determined when sampling becomes necessary. Coring and sawing specimens from hardened concrete are expensive expedients and should be adopted only as a last resort. Both procedures leave scars on the surface of the concrete that are difficult to eradicate, a condition that must be considered if the concrete is exposed to view. The possibility of structural damage, especially damage to reinforcement, cannot be ignored. If an approximation of the strength suffices, one of the following described instruments can be used.
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