Strength evaluation of concrete structures can be done by using physical load test method. Strength evaluation may be required from time to time for various applications of concrete structure. Procedure for concrete structure physical load test is discussed in this article.
Strength Evaluation of Concrete Structures with Physical Load Test
The procedure of load test on concrete structures presented here depends on ACI -2008 Chapter 20. In case there is doubt about the safety requirements of a structure, licensed design professional or building official can ask for a strength evaluation.
In the start, methods simpler than the load test are considered and load test can be avoided if all involved parties are satisfied with the result of such evaluation.
A load test on concrete structure is required to determine the serviceability of the structure when the presence / effect of the strength deficiency and its remedial measures are not fully known or when the required dimensions and material properties for analysis are not available.
A load test is usually not made until the portion of the structure to be subjected to load is at least 56 days old. The test can only be performed at an earlier age if the owner of the structure, the contractor, and all involved parties agree.
Physical load test is more suitable to clarify the doubts about the shear or bond strength but it can also be used to check deficiencies related with flexure or axial capacity. It is preferable to compare the results of the load test with the results of the analysis.
If a load test is decided as a means of the strength evaluation process for a particular project, the first step is that all the involved parties decide and agree upon the region to be loaded, the magnitude of the load, the physical load test procedure, and acceptance criteria.
For a structure with significant deterioration, periodic reevaluations are recommended to be conducted even if the structure passes a load test.
If the doubt about safety of a part or all of a structure involves deterioration, and if the observed response during the physical load test satisfies the acceptance criteria, the structure or part of the structure is permitted to remain in service for a specified time period.
Periodic reevaluations are usually conducted at the end of each specified period. Another option for maintaining the structure in service is to limit the live load to a level determined to be appropriate.
The time period between successive inspections is based on the nature of the problem, environmental effects, nature of loading, and service history of the structure, repair and maintenance program and scope and extent of the inspection. After each evaluation, the building is declared serviceable for a specified period only.
Sometimes a concrete structure believed to be deficient passes a load test. This confusion or misunderstanding is due to the conservative design of concrete structures, extra reinforcing steel to control shrinkage, cracking and thermal effects, conservative design theories, overestimation of the loads, extra concrete strength and multi-directional sharing of the loads not considered in ordinary designs.
1. Load Arrangement for Physical Load Test on Concrete Structures
a) The spans and panels having more doubt during survey are considered.
b) The number and arrangement of spans or panels loaded are selected to maximize the deflection and stresses in the critical regions of the structural elements to be tested.
c) More than one test load arrangement is used if a single arrangement does not simultaneously produce maximum values of the force effects required to be studied for the adequacy of the structure.
d) The load is applied at locations where its effect on the suspected defect is a maximum. However, it is better to apply same type of load (point load or uniformly distributed load) as is expected on the structure under examination.
e) The pattern loading expected for the structure must also be considered in deciding the loading to produce maximum load effect in the area of the structure being tested. This includes use of checkerboard or similar type pattern loads.
2. Load intensity for Physical Load Test
The total test load is taken larger of the following three values:
(a) 1.15D + 1.5L + 0.4(Lr or S or R) – De
(b) 1.15D + 0.9L + 1.5(Lr or S or R) – De
(c) 1.3D – De
Where, D is the total dead load, L is the live load on floors, Lr is the roof live load, S is the snow load, R is the rain load and De is the dead load already in place. The live load L can be reduced as allowed by the building code. The load factor on the live load L in (b) is allowed to be reduced to 0.45 except for garages, areas occupied as places of public assembly, and all areas where L is greater than 4.8 kN/m2.
3. Loading criteria
1. Preliminary approximate analytical evaluation is performed before the load test to determine the location and magnitude of the test loading and to plan the test.
2. Before conducting the test, it must be made sure that the structure does not fully collapse under the test load and sufficient safety measures must be taken to save the working people and other parts of the building in case of an unexpected sudden failure.
Safety measures adopted for the testing must not interfere with load test procedures and these must not affect results.
3. Establish failure criteria before the test. This means at which stage of the loading the test is to be stopped and the structure is to be declared unsafe.
4. Carefully think about the types of expected cracks, method of measuring the width and length of expected cracks, expected locations where the cracks will be measured, and approximate limits for opening and development of cracks.
5. Deflection gages are installed on all critical sections supported by staging that remains stable throughout the duration of the test. Measurements must be made at locations where maximum response is expected. Additional measurements can be made if required.
6. The initial value for all applicable response measurements (such as deflection, rotation, strain, slip, crack widths) are recorded not more than 1 hour before application of the first load increment.
7. A load equal to service dead load D that is not already present, such as for partitions, false ceilings and ducts, must be applied and it should remain in place until the completion of the load test. Deflection readings are taken immediately after the application of this additional load.
The test can be started after an interval of 48 hours. After dead load deflections have stabilized, existing cracks and other defects must be observed, marked, and recorded.
8. Test load defined above is applied in approximately four or more equal increments. It is better to carry out visual inspection of the structure after each load increment.
9. Uniform test load is applied in a manner to ensure uniform distribution of the load to the portion of the structure being tested. The loading units placed on the surface must not have bridging or arching between them because this may make the load non-uniform with reduction of load near the mid-span. 10. All response measurements are made after each load increment.
11. If the measured deflections exceed expected values, the test must either be stopped or a written permission must be taken from the supervising engineer.
12. After each increment of the load, the formation or worsening of cracking and distress, and the presence of excessive deformations, rotations, etc., must be carefully observed. The investigator must approximately analyze the observed data and determine whether it is safe to proceed with the next increment.
It is advantageous that load-deflection curves are developed during the load test for all critical points of deflection measurements.
13. Maintain the full test load on the structure for at least 24 hours and record all response measurement after this time interval.
14. Total test load is removed in the least possible time after all response measurements are made in the above step.
15. Wait for 24 hours after the removal of the entire test load. All the response measurements are made after 24 hours again.
16. Physical load testing is basically used only for evaluating the strength of a structure for vertical gravity loads. Leaving some exceptions, in-situ load testing is not used for evaluating the strength of a structure against lateral loads.
4. Acceptance criteria of Physical Load Test on Concrete Structure
1. A general acceptance criterion for a structure under the test load is that it should not have any evidence of failure. Spalling and crushing of compressed concrete is considered an indication of failure.
Other evidences of failure include obviously excessive cracking or deflection of such magnitude and extent that the safety requirements of the structure are violated. Definite rules can not be developed for all types of structures and conditions identifying failure.
2. If sufficient damage occurs during a test, putting the structure into service even at a lower load rating and retesting is generally not allowed.
3. Casting imperfections are not considered while determining the stability of the structure.
4. Local spalling or flaking of the compressed concrete in flexural elements may not indicate overall structural distress.
5. Crack widths, lengths and number are good indicators of the state of the structure. However, visible cracks are developed at very early stage of loading or due to temperature and shrinkage, which must be distinguished from the potentially dangerous cracks.
The tested structural members must not have any indication of shear failure. The shear forces are resisted across a shear crack plane by a combination of aggregate interlock, by clamping action of transverse stirrup reinforcing and by dowel action of stirrups crossing the crack.
When the yielding of the stirrups occur indicated by widening of the cracks and diagonal extension up to full depth of the member, the member is considered to approach shear failure. A great care is required to study inclined cracks in the regions of no transverse reinforcement that may lead to brittle collapse.
In regions of anchorage and lap splices, the appearance along the line of reinforcement of a series of short inclined cracks or horizontal cracks must be evaluated. Cracking along the axis of the main reinforcement in regions of anchorage and lap splices is indicator of brittle failure of the element.
6. Measured deflections must satisfy any one of the following two equations:
Where = measured maximum deflection during first load test, mm
= difference between initial and final (after load removal) deflections for load test or repeat load test, mm
= span of the member under load test, taken as smaller of the distance between centers of supports and clear distance between supports plus overall height (h) of the member. Shorter span is taken for two-way slab systems and twice the distance from face of support to free end is taken for cantilevers, mm
h = overall height of member, mm
The deflection limits and the retest option follow past practice. If the structure shows no evidence of failure, recovery of deflection after removal of the test load is used to determine whether the strength of the structure is satisfactory.
7. If the measured maximum and residual deflections, and , do not satisfy equations in the previous step, it is allowed to repeat the load test.
8. The repeat test can not be conducted earlier than 72 hours after removal of the first test load. The portion of the structure tested in the repeat test is considered acceptable if
Where = measured maximum deflection during second load test relative to the deflected shape of the structure at the beginning of the second load test, mm
If the structure does not show clear onset of failure but does not satisfy prescribed conditions or criteria, the structure is allowed to be used at a lower load rating. This decision and the reduced load level must be approved by the building official based on the test results and other observations.
9. Retesting of a structure that has previously failed a load test is not permitted until appropriate repairs and strengthening is carried out to upgrade the structure.
5. Important considerations for Physical Load Test on Concrete Structures
- The separate pieces used to apply the test load, such as iron bars, bricks, concrete block, etc., must be separated by a clear lateral distance of at least 100 mm to prevent arching action. The individual units must have length less than one-sixth of the span of the structural element being tested. The pieces must be of uniform shape and weight and the weight of individual pieces must not differ by more than 5 percent from the average weight. The average weight is determined by weighing at least 20 pieces taken at random.
- In case water, loose sand, or other similar materials are used as a test load, these must be contained within small compartments to prevent shifting of the test load during significant deformation of the structure. The total accumulated test load should be within 5 percent of the intended value.
- The loading units must be such that their weight is easily measurable, these must be easy to place and readily removable and must not contain hygroscopic materials. These units used for sloping surfaces must be securely anchored to prevent their shifting.
- The test load can preferably be applied with hydraulic or pneumatic devices because of the ease of application and speed of removal. These loading devices must continue to function in a uniform fashion even with significant deformation of the structure and their reaction must be safely transferred to separate system.
The instrumentation installed for monitoring the performance of a structure during a load test must satisfy the following requirements:
- During the load test, deflections, lateral deformations, support rotations, support settlement or sliding, etc., must be monitored.
- Strain measurements can also be made on flexural members at critical locations.
- Duplicate devices must be used to record deflection and strain measurements in critical areas. The maximum acceptable error for measuring displacements must not be more than 5 percent of the calculated theoretical deformation or 0.13 mm.
- It must be possible to determine relative changes in the shape of the structure or structural element during the test.
- All instrumentation must be protected during the load test from environmental influences such as direct sunlight, significant temperature variations, and wind.
- Deflection of structural members is allowed to be measured with electronic or mechanical devices, or with conventional surveying equipment. Large deflections can be easily measured by suspending graduated scales from critical points.
- Measurement for deflection may also be taken at supports to detect column shortening, if suspected for a particular case.
- All cracks must be marked as they develop and expand. Crack lengths can be measured by using graduated magnifying glasses, or “crack comparators”. Crack opening or closing can be measured with dial gages, displacement transducers, demec gages or mechanical strain gages.
- Temperature readings must be taken in all areas of a structure that are affected by the physical load test. Variations of sunlight must be monitored for roof slabs and other areas of the structure which are exposed to direct sunlight during performance of a physical load test.
Before applying load for a physical load test, shoring must be provided to support the structure with all existing loads, test loads and impact effects in case of failure during the test and must prevent the working people.
For horizontal members, the gap between shoring and the underside of the structure must be equal to the maximum expected deflection plus 50 mm. Shoring must not interfere with the free movements of the structure under the test load.