HIPERPAV Software - Frequently Asked Questions
Validation Model Summary |
HIPERPAV Technical Memorandums |
The HIPERPAV Team has compiled a list of the most frequently asked questions about HIPERPAV and the corresponding answers. These questions and answers are grouped under the following subject headings:
Design Inputs |
Mix Design Inputs |
Environmental Inputs |
Construction Inputs |
Software Interface |
Questions on Design Inputs
1. How do I measure slab-base friction?
- By performing a push-off test (see question 2).
2. What is a push-off test?
- A push-off test is a procedure to measure the friction, bearing, adhesion, and bonding conditions at the slab-subbase interface. This test consists of casting a slab with an approximate thickness of the projected pavement on top of the same subbase type. A range of slab sizes have been used in the past, but we have generally used a 3 x 5' slab. A horizontal force is applied on one side of the slab with a hydraulic jack and a load cell is used to measure the applied load. Dial gages are installed at the opposite side to measure the slab movement. The friction force per unit area is determined from the applied force divided by the area of the test slab. The testing history of friction force and slab movement during the push off procedure is recorded and the friction force at free sliding is evaluated.
3. What value of tensile strength do I use - the spec value?
- HIPERPAV requires the mean values of tensile strength and modulus of elasticity. The 28-day laboratory measured values should be input in HIPERPAV. Construction specification values should not be used here since they are often much lower than mean values at 28 days. ASTM specifications C31 or C192 should be used in defining the curing conditions for the 28-day tensile strength and modulus values required by HIPERPAV.
4. What if I have only flexural or compressive strength?
Many correlations for estimating the tensile strength from flexural or compressive strength are available and may be used with caution in HIPERPAV. A good correlation from a number of studies is that the tensile strength may be predicted by taking 72% of the flexural strength. Also, tensile strength (ft) can be obtained from compressive strength (fc) with the following relationship:
ft = 0.72*2.3*(fc)2/3
Many engineers use the ACI Building Code Formula for predicting the tensile strength from the compressive strength, but it should not be used since it under predicts the flexural strength. However, given that the primary mode of failure during early-age behavior is in tension, the testing of concrete in tension is highly desired because it would produce more reliable results.
5. What if I don't have information on concrete's modulus of elasticity?
- The sensitivity of the stress calculation on modulus of elasticity is relatively low, when compared to most other variables. However, coarse aggregates such as lightweight, high calcium contents, or high silica contents may affect concrete's modulus significantly. If the modulus of elasticity used in HIPERPAV is higher than the actual modulus of elasticity of the concrete, the stresses calculated will be slightly higher than the actual stresses developing in the concrete, thus, a conservative high value of modulus of elasticity (i. e. 5,000,000 psi) may be used in a preliminary analysis if desired.
6. How is reliability considered?
- The stress and strength internally computed by the HIPERPAV system are mean values. Based on these mean values HIPERPAV uses a probabilistic approach to calculate a critical stress and critical strength as a function of the variability associated with the materials and construction procedures for a typical concrete placement as well as the reliability or level of risk selected. As the selected reliability level increases, the critical stresses will be higher and the critical strength will be lower.
7. Why is reliability an input for the user?
- A large reliability has higher associated construction costs due to the additional precautions that must be taken to minimize early-age damage. Therefore, reliability as an input gives the user the ability to assign the level of risk he/she is willing to take based on the importance of the project and regional policies. Generally, the higher the risk, the higher the reliability the user should assign.
Back to the Top
Questions on Mix Design Inputs
8. Does the program check mix proportions?
- The current version does not verify the adequacy of the mix proportions. The user has to ensure that the mixture proportions entered will yield the design strength entered in the "Design Inputs". Future versions of HIPERPAV will most likely include a Mix Design Module for mix proportioning.
9. How are chemical admixtures accounted for since there are no inputs for dosage rates?
- Only moderate adjustments to the hydration of the cement are considered in HIPERPAV when chemical admixtures are selected. These adjustments are based on average dosage rates. Online help is provided for the use of admixtures in HIPERPAV which describes the primary effects that are considered when chemical admixtures are used. Future versions of HIPERPAV will better characterize the effect of chemical admixtures as accurate models become available.
10. Can differences in cement chemistry from the default cement types be modeled?
- Yes, HIPERPAV allows the user to enter a different chemistry in terms of the Bogue compounds by selecting the "Cement Chemical Composition Data" option within the "Mix Design Inputs" Module. In recent years, the chemistry of cements has changed to produce a more rapid strength gain to help expedite construction.
11. Does the program consider cement grind (fineness)?
- Not in the current version, although there are plans for including this parameter in future versions. Suppliers have developed finer grind cements in recent years that produce higher strengths at early ages.
12. Why is cement type IV not included in the default cement types?
- Type IV cement is not considered as a common cement type for paving applications, although it may be easily included in a future version if a demand for this cement type is observed.
13. Does the software consider blended cements like cement type IS?
- The current version of HIPERPAV considers the use of blended cement type IP only. It does not have an option for Type IS or other blended cements. A percent replacement of cement by Fly Ash, Silica Fume, or Slag admixtures may be entered to approximate blended cements. As more information on the hydration of other cement types comes available, it will be incorporated into HIPERPAV.
Back to the Top
14. Could concrete heat of hydration information from adiabatic calorimetry tests be used as input for the software?
- Yes, but not in its current form - a more advanced user input screen may be incorporated in a future version of HIPERPAV.
15. Is air entrainment considered in HIPERPAV?
- Not in the current form, however, it may be added in a later version of HIPERPAV.
16. Is aggregate shape or size considered?
- Currently none of the models in HIPERPAV requires the shape or size of the aggregates as an input, however, this may be considered in future versions.
17. Does HIPERPAV consider blended aggregates?
- Not at this moment. However, an average CTE can be determined based on aggregate volumetrics that are input to HIPERPAV.
18. Can the program consider shrinkage-compensating cements?
- The models in HIPERPAV are applicable for any cement type, however, in its current form only cements type I, IP, II, III and V are considered. As more information is gathered on the properties and performance of other cement types, they will be included in HIPERPAV.
19. How do you take into account aggregate CTE during the design phase when there is yet no information on their source?
- HIPERPAV can help in developing specifications for aggregates with a range of CTE's. The intent is not to show that aggregates with low CTE, such as many limestone aggregates, are good and aggregates with high CTE, such as many siliceous aggregates, are bad. You may have to work with whatever material is available. However, you should be aware of their effect on early age behavior. And remember, aggregates have other properties such as durability, strength, etc. that HIPERPAV does not consider.
Back to the Top
Questions on Environmental Inputs
20. Can wind speed be varied over the 72 hour period?
- This is being considered as an improvement to the HIPERPAV software in the future.
21. Where should wind speed be measured?
- It is recommended that wind speed be measured as close to the surface as possible. The height and location where wind speed is measured is important since it varies significantly with height and proximity to obstacles. However, the weather information provided by local weather stations may be used as a reasonable estimate. The user should recognize that the Weather Bureau generally measures the wind speed approximately 10 feet (or more) above the ground.
Back to the Top
Questions on Construction Inputs
22. What would be the best time of placement based on ambient temperature conditions, noting that early placement during the day might be a dangerous time for hot weather concreting?
- The answer depends on the specific value for all the inputs including cement type, thickness, base type, etc. HIPERPAV has the ability to integrate all these concepts for specific situations.
Questions on the Analysis
23. Why is the program limited to 72 hours of analysis?
- Based on experience, the great majority of failures or distresses in pavement due to early age behavior occur in less than 72 hours after placement.
24. How often does HIPERPAV calculate stresses?
- HIPERPAV calculates stresses in 6 minute increments during the 72 hour period. The output presented is shown for every hour.
25. Why doesn't the stress drop when a crack is predicted?
- The software in its current form does not actually model the distress development. Instead, it provides a warning for potential damage to the pavement structure. Due to the associated variability of materials and construction procedures, HIPERPAV indicates the chances of excessive stresses in the pavement. It does not necessarily indicate that a failure will occur.
26. Can HIPERPAV determine what the pattern of cracking would be for any given concrete age at saw cut?
- The current version of HIPERPAV predicts the potential for early-age cracking - it does not predict the optimum patterns or timing of saw cutting operations.
27. Is the strength input to the program affected by environmental conditions?
- Yes, it depends on the input field conditions.
28. Why did the predicted strength not change when I increased my Cement Content?
- HIPERPAV adjusts the strength gain curve based on the 28-day design strength provided by the user. If the mix design inputs are changed, the 28-day design strength determined from lab tests for the new mix has to be modified by the user as well.
29. HIPERPAV performs the analysis for unloaded conditions. What happens for loaded conditions?
- Early traffic loading models have been validated in only a limited number of situations. They are not included in this version, but may be in the future. It was found that early traffic loads have no significant effect on early age cracking in most cases. The impact of traffic loading is a factor in long-term performance.
30. Why don't the program results change significantly with different curing compound dosage rates (single, double and triple)?
- A small change is considered by HIPERPAV based on the heat loss estimated for single, double, or triple compounds. However, at the present time there is little information available on moisture loss as a function of curing compound application. HIPERPAV can be easily modified to account for such effects as soon as validated research results become available.
Back to the Top
Questions on Modeling
31. How does HIPERPAV account for air temperature on concrete strength development?
- HIPERPAV uses maturity models (Arrhenius Method) to account for air temperature on concrete strength. The strength development deviates from that of specimens cured at normal lab temperatures.
32. What reference is taken for the temperature differential in the model's computation of stresses?
- Temperature at final set is the reference temperature. Set time is defined as the time when concrete changes from a plastic to a hardened state and stresses start to develop.
33. Is there a specification for initial and final set of concrete?
- There is an ASTM specification on initial and final set for mortars, but not for concrete.
34. At what point in the concrete slab's profile is the tensile strength predicted by HIPERPAV?
- The strength predicted is the average strength through the slab thickness.
35. Is bleed rate accounted for in HIPERPAV?
- Not in the current version, a more robust moisture model is being developed and may be added in a later version.
36. How is cement variability considered?
- At this moment, the models do not consider variability of the cement. When models that account for this are available and validated, they can be incorporated into HIPERPAV.
37. Before joint sawing, what slab length does HIPERPAV consider in the analysis?
- Before joint sawing, HIPERPAV considers an infinite slab length to simulate the restraint conditions. If time at saw cutting is entered as zero, HIPERPAV will simulate the development of stresses for a slab with the length entered in the design inputs. In this form, it is assumed that joint sawing is performed at a time before excessive stresses develop (optimum time). On the other extreme, if it is desired to know what is the latest that joint sawing can be performed, the sawing time can be entered as 72 hrs and the moment at which stresses exceed the strength will indicate the latest time for joint sawing.
38. What is the maximum thickness that can be modeled?
- Since most of the models in HIPERPAV are of a mechanistic nature and the core of the program is a 2-D finite element model, the maximum thickness that can be modeled depends on model limitations. Currently, HIPERPAV has been validated for thicknesses up to 12 inches. It is believed that it will provide good results for thicker pavements although further validation will be needed if significantly thicker pavements are modeled.
39. Can HIPERPAV be used for fast setting cements?
- HIPERPAV has the capability of simulating the behavior for these types of cements. Using calorimetry data to predict heat development, these types of cements can be modeled with HIPERPAV. Although there is currently no input in HIPERPAV for calorimetry information, it may be included in future versions for advanced users. In the mean time, information on maturity can be used instead.
Back to the Top
Questions on Software Interface
40. Is it possible to input mixed U.S. and Metric units?
- Yes, the software gives the flexibility to use a combination of both units systems.
41. Is there online help?
- There is limited online help for some of the more sensitive parameters such as cement chemistry, mineral and chemical admixtures and curing methods. An improved online help system will be developed for the next HIPERPAV version.
Questions on Availability
42. When will HIPERPAV be available?
- It's available NOW! Check the hiperpav.com website for details.
Back to the Top
43. Why should I trust this software? It seems like a black-box.
- HIPERAV has undergone extensive validation. All its models were tested for accuracy during a recent series of experiments. Real pavements throughout the US were instrumented to monitor their early age strength and stress development during this validation effort. The software was then used to predict these measured properties. It yielded reliable results for a range of design, environmental and construction conditions. In addition, a number of experts including academics and practitioners have reviewed the software for validity.
44. Can HIPERPAV predict cracking of bridge-decks?
- Some modifications of the program would be required for this purpose, but basically the core of HIPERPAV can be used in many other concrete applications such as bridge-decks.
45. Does this software tell the state or contractor what to do?
- HIPERPAV is only a tool to help the state and contractor make a better decision based on the factors present. In the past, a lot of decisions have been made arbitrarily. HIPERPAV can act as a guide for better decision making.
46. Are we looking at working with a computer to make decisions in the field?
- This may be the direction of the future. By using the real time input in HIPERPAV, early age pavement performance can be predicted with improved accuracy.